Electrical motor 2
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Engi ngine nee erin ring g Ency ncyclo clope pedia dia Saudi Sa udi A ramco DeskTop Standards
Selecting Se lecting A Motor Type
Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.
Chapter : El Electrical File Reference: EEX20302
For additional information on this subject, contact W.A. Roussel on 874-1320
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C O NT E NT S Operating Characteristics and Applications of Three-Phase AC Motors and Single-Phase AC Motors
P AG E 1
Operating Characteristics and Typical Applications of DC Motors
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Selecting the Appropriate Types of Three-Phase AC Motors
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Selecting the Appropriate Types of Single-Phase AC Motors
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Selecting the Appropriate Types of DC Motors
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WORK AID: Work Aid 1: Procedure Procedure and Technical Technical and Economic Economic Factors Factors From SADP-P-113 for Selecting the Appropriate Types of Three-Phase AC Motors
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W ork Aid 2: Procedure and Technical and Economic Factors from Established Engineering Practices for Selecting the Appropriate Types of Single-Phase AC Motors
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Work Aid 3: Procedure Procedure and Technical Technical and Economic Economic Factors Factors from Established Engineering Practices for Selecting the Appropriate Types of DC Motors
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GLOSSARY
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OPERATING CHARACTERISTICS AND APPLICATIONS OF THREE-PHASE AC MOT ORS AND AND SINGLE-PHASE SINGLE-PHASE AC MOTO RS The operating characteristics of three-phase and single-phase alternating current (AC) motors are very different from from each other and will be separately separately discussed. Each type of AC motor also has a typical application that is based on the motor's operating characteristics. The following topics will be discussed: •
Operating Characteristics of Three-Phase AC Motors
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Typical Applications of Three-Phase AC Motors
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Operating Characteristics Characteristics of Singe-Phase AC Motors
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Typical Applications of Single-Phase AC Motors
Oper ating Char acteristics acteristics of Thr ee-Phase ee-Phase AC AC M otor otor s All three-phase three-phase AC motors can be supplied from from the same power network. The difference difference in three-phase motors is in the characteristics that the motor displays when the motor is in operation. The following types of three-phase AC motor operating characteristics characteristic s will be discussed: •
Squirrel-Cage Induction Motors
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Wound Rotor Induction Motors
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Synchronous Motors
Squirr el-Cage el-Cage Induction Motors
The squirrel-cage induction motor is the simplest and most rugged of the three-phase AC motors. The squirrel-cage squirrel-cage induction motor can be used used in a variety of applications applications due to the motors design. The following topics of squirrel-cage induction motors will be discussed in this section: •
Uses and Classifications
•
Operating Characteristics
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Uses and Classifications - The standard squirrel-cage motor is a general-purpose motor.
The squirrel-cage induction motor is for use in driving loads that require a variable torque at a relatively constant speed and a high full-load efficiency. There are different types of squirrel-cage induction motors. The main difference between types of squirrel-cage induction motors is the construction of the rotor. A change in the construction of the rotor causes a change in the resistance characteristics of the rotor; a change in the resistance characteristics of the rotor causes a change in the torque and current characteristics of the motor. The National Electrical Manufacturers Association (NEMA) classifies squirrel-cage motors in accordance with the motor's electrical characteristics. Squirrel-cage motors have the following classifications: •
NEMA Class A motors are the most popular motors. Class A motors have a normal starting torque, a normal starting current, and a low slip.
•
NEMA Class B motors are built to develop a normal starting torque with a relatively low starting current.
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NEMA Class C motors have a high starting torque, a low starting current, and a low slip.
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NEMA Class D motors are special purpose motors. Class D motors have a very high starting torque, a high slip (15-20%), a low starting current, and a low efficiency.
Operating Char acteristics - Figure 1 shows the basic torque/speed characteristics of an
induction motor. Figure 1 shows two curves: curve 1 represents the load torque and curve 2 represents the motor torque. The following specific torques that are associated with the operating characteristics of an AC induction motor are identified on Figure 1: •
Locked-rotor torque - The minimum torque that is developed by a motor at the instant that rated power is supplied to the motor terminals. Locked-rotor torque also is called breakaway or starting torque. The motor must have enough lockedrotor torque to start turning the load. A motor cannot start a connected load when the connected load has a higher torque rating than the motor.
•
Breakdown torque - The maximum torque that a motor can develop when the motor is supplied with its rated input power.
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Accelerating torque - The torque that a motor develops between zero speed and full rated speed when the motor is supplied with its rated input power. Accelerating torque is the net difference between the motor torque and the load torque. Accelerating torque determines the rate at which the motor can accelerate a load to full rated speed.
•
Full-load torque - The torque that a motor can develop when the motor is at rated speed and the motor is supplied with its rated input power. The previous motor torques normally are expressed as a percent of the full-load torque value.
The torque of a motor is a rudimentary operating characteristic. Analysis of torque fluctuations during the stages of motor operation will provide an understanding of the types of conditions in which a particular type of motor can be utilized. The torque that a motor produces depends on a set of operational variables. The following proportion shows how the operational variables relate to motor torque. The operating characteristics of a squirrel-cage induction motor can be derived through an analysis of the operational variables. where:
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_ I V L N R
= = = = = =
Motor torque Motor current Motor terminal voltage Load on the motor Motor speed Resistance of the motor windings
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Basic Torque/Speed Characteristics of an Induction Motor Figure 1
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The best way to analyze how each of the variables effect the torque of the squirrelcage induction motor is to look at the different phases of motor operation. There are three phases of motor operation to analyze: _How the motor responds at starting. _How the motor responds to changing loads. _How the motor responds to an overload. A motor at standstill must produce enough starting torque to cause rotation of the motor and the connected load. The development of motor start torque can be seen through an analysis of the variables in the following torque relationships: The relative values of the variables in the torque relationship at the moment a squirrelcage induction motor start are as follows: _I
Current - When the motor is energized, the starting current that is drawn will be high. The high starting current is due to the fact that no counter electromotive force (CEMF) is being produced in the motor.
_V
Voltage - The applied voltage will equal line voltage and will not fluctuate.
_L
-
_N
Speed - The motor speed at the instant of start is zero. The speed of the motor at start is as low as possible, which causes torque to be high.
_R
Resistance of the Rotor - This value will not vary with a particular motor. The only way that the resistance will vary is to design the motor differently.
Load - The load is constant at this point.
As the variables change, so does the motor torque. At the moment of start, the torque is high because current is high and speed is low. The starting torque that is developed by a motor must be larger than the torque that is required by the load. Starting torque that is equal to or less than load torque will not cause rotation of the motor and load. Figure 2 shows the minimum starting torque for a squirrel-cage induction motor as a percentage of full load torque for various numbers of motor poles. The minimum starting torques are established by the National Electrical Manufacturers Association (NEMA).
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Minimum Starting Torque for a Squirrel-Cage Induction Motor Figure 2
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After the motor has been energized and the motor develops starting torque, the rotor will start to rotate. The rate at which the motor accelerates depends on the motor's developed torque and the torque that is required by the load. The difference in these torques is known as net accelerating torque. The change in motor torque from standstill to full rated speed can be analyzed through reference back to the torque proportion. _I
Current - The motor current will continue to increase initially until there is sufficient rotation to produce CEMF that will limit the current flow. The increase in current will cause an increase in torque.
_N
Speed - The speed will continue to rise as the motor accelerates. The rise in speed will tend to lower torque, but until the motor reaches about 80-85% of full rated speed, the rise in current will greatly outweigh the rise in speed.
_V-L-R -
Voltage, load, and resistance will all remain relatively constant during motor acceleration.
The net accelerating torque of a squirrel-cage induction motor will be large at the moment of starting. Net accelerating torque will continue to increase until the motor reaches about 80-85% of the motor's rated speed. After reaching 80-85% of rated speed, the net accelerating torque of the motor will start to decrease. The next phase of motor operation is how the motor responds to a change in load. Analysis of how torque and the variables of torque vary during a change in load will explain the operating characteristics of a running squirrel-cage induction motor. The torque proportion for use in this analysis remains the same as during starting. A motor that is running at rated speed will develop just enough torque to maintain the load rotation at a predetermined speed. As the variables of torque change during operation, the changing variable will cause other variables to change, which keeps the proportion balanced and the load running. Figure 3 shows a graphic representation of how the variables of torque change during operation.
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Speed of a squirrel-cage induction motor will vary as load is added or subtracted from the motor. A squirrel-cage induction motor's operating range is from approximately 90% synchronous speed to 100% synchronous speed. A load (L) increase will cause motor speed (N) to decrease. The decrease in motor speed will cause the current (I) of the motor to increase. The resultant increase in current will cause torque of the motor to increase to a level that is high enough to support operation of the added load. This relationship between load, speed, and torque will continue until the point of breakdown torque is reached. The changes in speed on the curves of Figure 3. The bottom axis shows that as speed decreases, both current and torque will increase. The amount of motor slowdown for a load increase is a characteristic of a particular squirrel-cage induction motor design. Torque and current of a squirrel-cage induction motor will decrease when speed increases as load is removed. Because the torque of a squirrel-cage induction motor also varies with the square of the terminal voltage that is applied to the motor, low terminal voltage will significantly reduce a squirrel-cage induction motor's torque. The final phase of motor operation is how a squirrel-cage induction motor responds to an overload. All squirrel-cage induction motors are designed to operate under a certain amount of overload; however, the overload cannot exceed the breakdown torque of the motor. The breakdown torque is the point at which the torque that is required to run the load at overload exceeds the maximum torque that the motor can produce. A squirrel-cage induction motor will react to the increase in load (overload) as previously discussed. Every time more load is added to the motor, the motor's speed will decrease and the motor's current will increase. The resultant change will be an increase in motor torque. Torque will continue to increase as load is added to the maximum value of torque that the motor can produce. A motor that operates at maximum torque will operate just to the right of the breakdown torque point on Figure 3. Any more load that is added to the motor will cause the motor's speed to drop more and will cause current to increase. The torque that is produced by the motor will not be large enough to continue operation of the motor, and the motor will stop.
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Typical Relationship Between Current, Torque, and Speed in a Squirrel-Cage Induction Motor Figure 3
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Figure 4 shows the comparative torque speed characteristics of the different classifications of squirrel-cage induction motors. Typically, starting torque is 150% to 250% of the full load torque. All of the NEMA classes of motors will respond to operational changes in the same manner. The difference in the four types of squirrelcage induction motor classes is the construction of the rotor. A change in the squirrelcage induction motor's rotor construction will change the resistance of the motor's rotor circuit. A change in the rotor circuit's resistance will cause the motor's torque characteristics to change.
Comparative Torque-Speed Characteristics of Different Classifications of Squirrel-Cage Induction Motors Figure 4
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In addition to torque, the following operating characteristics are important to an understanding of the operation of a squirrel-cage induction motor: _Slip _Power factor _Efficiency For a given motor, slip is the difference between synchronous speed and motor speed. Slip is expressed as a percentage of the synchronous speed. The amount of slip of the motor depends on the amount of load. The slip of the motor will increase and the motor will run slower when the load is increased. At full load, the motor only slows slightly, which amounts to one to four percent of synchronous speed. Because of the small changes in speed from no load to full load, a squirrel-cage induction motor is considered to be a constant speed motor. The actual speed of the motor rotor will never reach the motor's synchronous speed. A difference between the speed of an induction motor and synchronous speed is necessary because of the way the rotor field is developed in an induction motor. The most common method for calculation of slip in induction motors is through use of the following formula: The synchronous speed of a motor is found through use of the following formula: where:
Ns
=
Synchronous speed
The following is an example of how to determine the slip of an induction motor. A three-phase, squirrel-cage induction motor with four poles is operating on a 60 Hz, AC power circuit at a motor speed of 1,728 rpm. The slip of this squirrel-cage induction motor can be determined through substitution of the following values into the previous formulas: Because of the natural slip characteristic of squirrel-cage induction motors, the conclusion can be made that the squirrel-cage induction motor is not suitable in industrial applications where a great amount of speed regulation is required. The reason for non-selection of a squirrel-cage induction motor is that the speed only can be controlled by a change in frequency, the number of poles of the rotor, or the motor slip. Speed of a motor is seldom changed through change of the frequency. The number of poles can be changed either through use of two or more distinct windings or through reconnection of the same winding to establish a different number of poles. Slip is an inherent characteristic of the motor's design.
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A squirrel-cage induction motor will operate most efficiently when the power factor range is maintained in the design range of the motor. A squirrel-cage induction motor's power factor will vary as the load on the motor changes. The power factor of the squirrel-cage induction motor will be lowest at no load and will increase to the highest value at rated full load of the motor. Load that is added to the motor beyond full load will cause the power factor to start to decrease. The power factor of a squirrel-cage induction motor also is a factor of the motor's design speed. The power factor of a slow-speed squirrel-cage induction motor will be lower than the power factor of a squirrel-cage induction motor that operates at a higher rated speed. The change in power factor over the range of motor speed is due to the high leakage reactance of the squirrel-cage induction motor at lower speeds. The efficiency of a squirrel-cage induction motor is the last characteristic that must be discussed. Efficiency is the ratio between the input and the output of a motor. The efficiency of a motor can be described by the following equation: This equation can be restated as: The losses of a squirrel-cage induction motor will vary with the exact construction and application of the motor. Some examples of the losses that are experienced by a squirrel-cage induction motor are: _I2R _Winding _Bearing friction _Hysteresis _Eddy currents The efficiency of a squirrel-cage induction motor also will vary with the load on the motor. A lightly-loaded squirrel-cage induction motor will have a lower efficiency than an identical squirrel-cage induction motor that is supplied with rated load. Because a motor is more expensive to operate when the efficiency is lower, each motor that is installed should be selected so that the actual load and the rated load of the motor are as close as possible. Figure 5 shows a comparison of AC squirrel-cage induction motor curves. The comparison shows how the values of power factor, amps, watts, and efficiency of the motor vary as a percent of the motor load. Efficiency and power factor of a squirrelcage induction motor are maximum at full load. Note how rapidly efficiency and power factor decrease when the motor is operated at less than 100% motor load.
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Comparison of AC Induction Motor Curves Figure 5
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Wound Rotor Induction M otors
The wound rotor induction motor is another form of the three-phase induction motor. The wound rotor induction motor has operating characteristics that are similar to the squirrel-cage induction motor. The only real difference in the operating characteristics of the two types of induction motors is that some of the operating characteristics of the wound rotor induction motor can be varied. The operating characteristic that can be varied are torque, current, speed, and efficiency. These characteristics are varied through a change in the amount of external resistance that is connected in series with the wound rotor windings. Figure 6 shows typical torque, current, and speed relationships of wound rotor induction motor with different amounts of external resistance added. Curve 1 shows the torque speed characteristics of the wound rotor motor with no external resistance added to the rotor. Curve 2 is the torque speed characteristics of the wound rotor motor with 10% external resistance added to the rotor. The external resistance is given as a percentage of the external resistance value required to give full load torque at standstill. The starting torque of the wound rotor induction motor with no external resistance adds is approximately 90% of full load torque. Through addition of 10% external resistance to the rotor circuit, the starting torque produced by the wound rotor induction motor can be raised to approximately 200% of full load torque. The starting torque required by the load can be achieved through change in the amount of external resistance that is added to the circuit. Also, the addition of the resistance in the rotor circuit will cause the starting current of the motor to drop. During operation, the wound rotor induction motor will produce the necessary running torque that is required to support the operation of the load. The variations in running torque of the wound rotor induction motors shown in curve 1 and curve 2 are the rate of change in running torque as compared to speed. A wound rotor induction motor with no external resistance added to the rotor circuit will develop more running torque for a given drop in speed than a wound rotor induction motor with 10% external resistance added. The difference in the rate of development of running torque is due to the current change between the motor in curve 1 and the motor in curve 2. The resistance added to the rotor circuit of the motor in curve 2 will limit the rise in current as motor speed decreases. The lower increase in current will cause less running torque to be produced.
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As mentioned above, the breakdown torque of a motor is the maximum torque that a motor can produce when the motor is supplied with its rated input power. Through change of the amount of external resistance added to the rotor circuit of a wound rotor induction motor, the value of breakdown torque can be varied and the speed at which the motor reaches breakdown torque can be varied. The breakdown torque of the wound rotor induction motor with no external resistance shown by curve 1 has a breakdown torque of approximately 250% of full load torque; the breakdown torque of the motor is reached at approximately 83% of synchronous speed. Addition of 10% external resistance the rotor circuit will cause the motor's breakdown torque to change as shown in curve 2. The value of the breakdown torque will only slightly vary by a few percentage points of full load value. The biggest change is the speed of the motor when breakdown torque is reached. The motor in curve 1 reached breakdown torque at approximately 83% of synchronous speed, but, if 10% external resistance is added to the rotor circuit, the motor will not reach breakdown torque until the motor slows to approximately 50% of synchronous speed.
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Typical Torque, Current, and Speed Relationship of Wound Rotor Induction Motors With Different Amounts of External Resistance Figure 6
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The speed and efficiency of the wound rotor induction motor are dependent upon each other. The speed of the wound rotor motor can be varied by about 50 to 75 percent. To change the speed of the wound rotor induction motor under a constant load condition, resistance is added or removed from the rotor circuit. The speed of the motor is decreased through the addition of resistance to the rotor circuit. The resistance will cause the current flow to drop in the rotor; the torque produced will be reduced; and the speed of the motor will slow. Conversely, the speed of the wound rotor induction motor is increased through a removal of resistance from the rotor circuit. The wound rotor motor is not designed to run at speeds that are slower than rated speed for extended periods of time. The addition of resistance to the rotor circuit to lower speed will generally only be done for short duration duties. A consequence of the addition of resistance to the wound rotor induction motor is the change in the motor's efficiency. The addition of resistance to the motor rotor circuit to lower the motors speed will cause the efficiency of the motor to drop. Operation of a wound rotor induction motor with external resistance added for extended periods of time will significantly add to the operating cost of the motor due to the drop in efficiency of the motor. With all the external resistance removed from the motor's rotor circuit, the wound rotor induction motor's overall efficiency will be about 2 to 3% less than the overall efficiency of a comparable squirrel-cage induction motor because of a difference in the motor's construction. The power factor of a wound rotor induction motor is a factor of the motor's design. The power factor of the motor will vary over the load of the motor just as the power factor varied on the squirrel-cage induction motor. Synchr onous Motors
Synchronous motors have many of the same relationships and characteristics as induction motors; however, there are differences. Figure 7 shows the relationship between the speed, torque, and current in a synchronous motor. Note the location of the torque points on Figure 7 and the high starting current and low running current at different percents of synchronous speed. The high starting current and low running current at different percents of synchronous speed are typical of the following torques in a synchronous motor: _Starting Torque is the torque that is developed when full voltage is applied to the armature windings and when there is no motion of the motor rotor. Because the synchronous motor itself has very little starting torque, an alternate starting method must be used to develop a large enough starting torque. _Pull-In Torque is the torque that is developed during the transition from slip speed to synchronous speed. Pull-in torque is the maximum constant torque with which the motor will pull its connected load into synchronism, with rated power input, when field excitation is applied.
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_Pull-Out Torque is the value of the torque when the rotor will fall out of synchronism with the rotating stator field. With increases in the motor load, the rotor will fall behind the rotating stator field but not out of synchronism. If the load is increased beyond the pull-out torque point, the motor will "slip a pole" or pull-out of synchronism. The mechanical pull-out point of a synchronous motor is approximately half of the distance between adjacent poles.
Synchronous Motor Speed/Torque/Current Curves Figure 7
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The speed of a synchronous motor is determined through the frequency of the power supply and the number of poles of the motor. The operating speed of a synchronous motor will be constant for a given frequency and the number of poles. The following formula is for use in the determination of synchronous motor speed in revolutions per minute (RPM). Because synchronous motor speed is controlled by the number of poles in the motor, a synchronous motor can be designed for a specific speed application. Figure 8 shows synchronous motor speeds in rpm for motors that are designed with different numbers of poles for different supply frequencies (Hertz).
Synchronous Motor Speeds (rpm) Figure 8 Saudi Aramco DeskTop Standards
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The speed of a synchronous motor must always remain constant no matter how the load changes. The angle between the rotation of the field and the rotation of the rotor will increase as load is increased on a synchronous motor. This increase in angle will cause torque to increase, but the speed of the synchronous motor will remain constant. Load can be added to the synchronous motor until the developed torque of the motor reaches pull-out torque. The addition of any more load to a motor that is operating at pull-out torque will cause the motor to lose synchronism and stall. Power factor and power factor correction are important aspects of a synchronous motor's operation. Power factor is defined as the ratio of real power to apparent power and is usually expressed as a percent leading when the current in the circuit leads the voltage in the circuit, or as a percent lagging when the current in the circuit lags the voltage in the circuit. Power factor is a measure of the efficiency of a circuit. Power factor takes into account inductive and capacitive reactance that dissipates power that is not available to do real work. The capacitive reactance in a capacitive circuit causes the current in the circuit to lead the voltage in the circuit; therefore, capacitive circuits have leading power factors. The inductive reactance in an inductive circuit causes the current in the circuit to lag the voltage in the circuit; therefore, inductive circuits have lagging power factors. Power factor is expressed as a unitless fraction. Power factor equals one for a purely resistive circuit (no inductance or capacitance) and is less than one for circuits with inductive or capacitive reactance. Power factor is essentially a ratio of the pure resistance of a circuit to the circuit's total impedance. The power factor of a synchronous motor is controlled by the amount of field excitation that is supplied to the motor. In a synchronous motor that is pulling a constant load, a variation of the stator current is accomplished through variation of the field current. Figure 9 shows synchronous motor "V" curves for no load, 1/2 load, 3/4 load, and full load conditions. The V curves describe the relationship between stator current and field current. The curves are called V-curves because of their shape. For any given load and any given motor, there is a single value of field current that will give a unity power factor at the motor terminals. An increase in the field current above the point for unity power factor (moving right) will cause a corresponding increase in stator current that will cause the power factor to become increasingly leading. A decrease of the field current from the unity point will cause the stator current to increase, and the power factor will become more lagging (moving left).
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Synchronous Motor "V" Curves Figure 9
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The motor field current is set at the value that is stamped on the nameplate and is kept at this point for all loads during operation. Maximum pull-out torque is maintained through sustenance of rated field current. Sustenance of rated field current provides the maximum level of power factor correction. In a motor that is operated at reduced load for a long period of time, reduction in the field current may be desired. Such a reduction of the field current would increase the motor efficiency. For a motor that operates at part load with a unity power factor, the field current can be adjusted until the stator current is at a minimum value. The following equation is for use in the determination of the required stator current for a given pf: A motor that operates at other than unity power factor will supply the system with either leading or lagging kVA. The amount of kVA that is supplied to the system can be determined, but first the correct stator current to achieve a desired power factor must be determined. The amount of kVA that is supplied to a system by a synchronous motor must be known to allow protective devices and operating mechanisms to be set. A synchronous motor that operates at full load and rated excitation delivers to the power system a leading kVA equal to: where: hp rating Eff cos _
= = =
The horsepower of the synchronous motor The efficiency of the synchronous motor Power factor
Electrical Engineers should note that more leading kVA is supplied at partial loads and rated excitation. The curves in Figure 10 show the reactive kVA for synchronous motors at four different power-factor ratings and at varying load conditions. These curves are based on maintenance of full-load rated field current at all loads. For example, a 100 hp (74.6 kW) 80% power factor synchronous motor operated at 75% load supplies a leading reactive kVA equal to approximately 66 percent of the motor's horsepower rating, or 66% reactive kVA. The unity power factor synchronous motor (100% pf motor), whose curve is shown in Figure 10, only supplies a leading reactive kVA when the load is less than 100%. The unity power factor synchronous motor, although providing no leading reactive kVA at full load, still improves the system power factor through addition of kilowatt load without increase to the system reactive-kVA load. A synchronous motor that operates at 90, 80, or 70% of power factor will provide a leading reactive kVA at all loads.
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Reactive kVA for Synchronous Motors Figure 10
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The efficiency of a motor is a ratio of the input power to the output power of the motor. Because a synchronous motor has no slip as load is added, the synchronous motor will have a higher efficiency than a corresponding induction motor. The full load efficiency of a synchronous motor is generally one to three percent higher than that of an induction motor. Typical Applications of Thr ee-Ph ase AC M otors The squirrel-cage motor is one of the most widely used machines because the squirrel-cage motor can be built with electrical characteristics to suit almost any industrial requirement. Another reason the squirrel-cage induction motor is widely used is the motor's simplicity of construction. Squirrel-cage motors are not suitable in situations where a high starting torque is required, but, when the starting-torque requirements are of a medium or low value, the squirrel-cage induction motor is very suitable. Typical applications of the squirrel-cage induction motor include blowers, centrifugal pumps, and fans. Because of the absence of any exposed electrical connections, the squirrel-cage induction motor is suitable for use in areas with hazardous environments. The wound rotor induction motor is very similar to the squirrel-cage induction motor in application, but the wound rotor induction motor has the ability to start extremely heavy loads. The following are specific applications of the wound rotor induction motor: _To drive various types of machinery that require development of considerable starting torque to overcome friction. _To accelerate extremely heavy loads that have a flywheel or inertial effect. _To overcome back pressures set up by fluids and gases in the case of reciprocating pumps and compressors. _When motors must be started frequently without overheating the motor. The advantages of a wound rotor motor over a squirrel-cage induction motor are: _High starting torque. _Moderate starting current. _Smooth acceleration under heavy load. _No excessive heating during starting. _Good running characteristics. _Adjustable speed control.
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The main disadvantage is that both initial and maintenance costs of a wound rotor motor are greater than those costs of the squirrel-cage rotor motor. Also, the efficiency of the wound rotor induction motor is lower than the efficiency of a squirrel-cage induction motor. A synchronous motor can be used for almost any application for which a squirrel-cage induction could be used. The main applications of synchronous motors fall into three areas: _Power-factor correction _Constant-speed, constant-load drives _Voltage regulation
Synchronous motors have two advantages over AC induction motors: _A constant speed with no variation due to changes in load. _An ability to improve power factor when operated with high DC excitation. Another factor that must be taken into account in the decision between an induction or synchronous motor is cost. The cost of the higher-speed, low-horsepower, squirrel-cage induction motor and control is lower than the cost of the corresponding synchronous motor. The motor costs are reversed for higher horsepower and lower speeds; the synchronous machines are less costly. Running cost also must be considered in selection between a synchronous motor and an induction motor. The full-load efficiency of an induction motor is generally one percent to three percent lower than that of a synchronous motor of the same horsepower and speed rating. The greater efficiency of the synchronous motor over the induction motor can pay cost dividends over the life of the motor operation. The synchronous motor should not be used where fluctuations in torque are violent. As a general rule, synchronous motors also are not used in small sizes (under 50hp) because they require DC excitation and are more difficult to start than induction motors. Synchronous motors also fall out of step quite readily when system disturbances occur.
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Oper ating Ch ar acteristics of Single-Phase AC Motors Single-phase motors were one of the first types of motors developed for use on AC circuits. Single-phase motors have been perfected over the years from the original repulsion type into many improved types. The following are the types of single-phase motors that will be covered: _Split-phase motor _Repulsion induction motor _Capacitive start motor _Universal motor Split-Phase Motor
The split-phase induction motor is the most popular of all the single-phase motors. The splitphase motor consists of a squirrel-cage rotor and two stator windings, a main winding, and a starting winding. Current that is applied to the motor will cause both windings to produce a magnetic field. The magnetic fields that are produced by the main winding and the starting winding will be mechanically and electrically displaced. The mechanical displacement is produced through position of the windings in the stator. The electrical displacement is produced through the use of windings with different electrical properties. The main winding is produced to have a low resistance and a high inductance. The starting winding will have a high resistance and a low inductance. The different characteristics of the two windings produce a weak rotating electric field. The interaction of the two fields that are produced by the windings produce the motor's starting torque. In a split-phase motor, the starting torque is 150 to 200 percent of the full-load torque, and the starting current is six to eight times of the full-load current. Figure 11 shows the speed torque characteristics of a split-phase induction motor. Upon energization of the motor, the combined windings produce the rotating magnetic field that will produce the necessary torque to start the motor. The motor accelerates to 75 to 80 percent of synchronous speed. At this speed, a starting switch (usually centrifugally operated) opens to disconnect the starting winding, and the motor operates with the main winding only. The function of the starting switch is to prevent the motor from drawing excessive current from the line and to protect the starting winding from damage due to overheating. The motor may be started in either direction through reversal of the connections to either the main or the starting winding, but not to both.
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Speed-Torque Characteristics of a Split-Phase Induction Motor Figure 11
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Repulsion In duction Motors
The repulsion induction motor has a combination of a squirrel-cage and a repulsion winding on the rotor. Because of the combination windings, occasionally the motor is referred to as a squirrel-cage repulsion motor. A repulsion induction motor can be designed to have either a constant speed or a variable speed characteristic. In the repulsion induction motor, the desirable starting characteristics of the repulsion motor (such as high starting torque) and the constant speed characteristics of the induction motor are obtained. Unfortunately, the two types of motors are impossible to combine and obtain only the desirable characteristics of each. Because the combination of both windings will cause the running torque of the repulsion induction motor to be less than a comparative split phase induction motor, a larger repulsion induction motor would be necessary for the same load rating. Figure 12 shows the torque-speed characteristics of a typical repulsion induction motor. The rotating magnetic field of the repulsion induction motor is produced in the same way as in the split phase induction motor. The construction of the repulsion induction motor was discussed in Module EEX 203.01. The torque-speed curve of the repulsion induction motor is very similar to that of a repulsion motor. The repulsion induction motor has a high starting torque (approximately 300-350% full load torque) and can operate at relatively high speeds under light loads. The similarity between the repulsion induction motor curve and the curve of a repulsion motor is due to the dominance of the commuted repulsion winding when the repulsion induction motor is started.
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Torque/Speed Characteristic of a Typical Repulsion-Induction Motor Figure 12
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Brush position of the repulsion induction motor is very important in determination of the motor's operating characteristics. Figure 13 shows the characteristic curves of a repulsion induction motor that illustrates the effects of adjustment of brush position. Through adjustment of the brushes, the direction of rotation of the motor can be changed from clockwise to counterclockwise or vice versa. The other main effect of a shift in the brush position is the effect on motor starting torque. When the brushes are at the 0 brush position, the repulsion induction motor will produce zero starting torque. Starting torque can be maximized through shift of the motor brushes to 25 degrees off center. The torque graph is a quantative analysis of how the motor's torque will change. The line of zero torque shows the relative amount of motor starting torque as compared to other brush positions. The shift in brush position also will lower the motor's current.
Characteristic Curves of a Repulsion Induction Motor Showing the Effect of Adjusting Brush Position Figure 13
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Capacitive Start Motors
The capacitive-start motor is another form of split-phase induction motor that has a capacitor that is connected in series with the auxiliary winding. The auxiliary circuit of a capacitive start motor is opened when the motor has attained a predetermined speed. The net effect of the capacitor in the auxiliary circuit is to give its motor a starting torque of about four times the motor's rated torque. Once the capacitive start motor has come up to speed and the starting winding has been disconnected, the motor will have the same running characteristics as the split-phase motor. The rotating magnetic field is produced identically to the way in which this field is produced in the split-phase motor. The larger starting torque comes from the addition of a capacitor in series with the starting winding. The addition of the capacitor will cause the electrical displacement of the two fields to increase. This increase in the displacement of the electrical fields produces the larger torque. Figure 14 shows a comparison of the torque slip curves for a capacitor start and a split-phase motor. Curves are shown for both types of motors to show the comparison. Various starting capacitor values (200 _F, 300 _F, 400 _F, and 500 _F) also are shown for comparison. Through change of the value of the starting capacitor, the starting current will be greatly effected. An increase in the capacitor size will cause an increase in the motor's starting torque. The capacitor start type of motor has certain advantages over the other single-phase AC motors in that the motor has a considerably higher starting torque that is accompanied by a high power factor. Notice how the torque of the capacitive start motor drops at the point where the centrifugal switch opens. The capacitive start motor operates in the same speed range as a split-phase induction motor.
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Torque-Speed Curves for Capacitor-Start and Split-Phase Motors Figure 14
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Universal Universal Motor s
A universal motor is a series wound motor that may be operated on direct current (DC) or single-phase single-phase alternating current current (AC). Because a universal motor is a series series wound motor, the universal motor's operating characteristics are very similar to those of a DC series wound motor. The main difference in the operating characteristics of the universal motor and the series DC motor is that the the universal motor will will have a no load speed. The no load speed of the universal motor will be quite high but not high enough to damage to motor. Universal motors are very susceptible to changes in speed and these changes in speed must be considered whenever whenever a universal motor is used. The following three factors factors change the speed of a universal motor: _A change in load. _A change in frequency of the power supply. _A change in applied voltage. When a load is placed on a universal motor, torque torque will increase and speed will will decrease. The speed of the universal universal motor will continue continue to decrease as as load and torque are added. added. Figure 15 shows the torque-speed torque-speed characteristics characteristics for a typical universal universal motor with a change in the frequency of the the power supply. The power supply frequencies frequencies that are shown are of 25 Hz AC, 60 Hz AC, and DC power. The curves show that that at a 25 Hz supply, the the universal motor motor will develop the maximum torque and that the minimum starting torque will be developed at 60 Hz. Adjustment of the speed speed of a universal motor is very easily easily accomplished. accomplished. The speed of the universal motor can be adjusted through adjustment of the input voltage to the motor. Adjustment of the input voltage to the motor is accomplished through use of a variable resistor. Adjustment of the value of the variable resistor allows the speed of the universal motor to be adjusted at will.
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Torque/Speed Characteristics of a Typical Universal Motor Figure 15
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Typical Applications of Single-Pha se AC Motor s The split-phase induction motor is the most popular of the fractional-horsepower motor types. The split-phase motor is most commonly used in sizes that range from 1/30 hp (24.9 W) to 1/2 hp (373 W) for applications such as fans, business machines, automatic musical instruments, and buffing machines. The split-phase motor has the advantage of a very low initial cost. A disadvantage of the split-phase motor is that the motor has a relatively low starting torque. The capacitive-start motor is made in sizes from 1/4 hp (150W) to 10 hp (7.5 KW). The starting capacitor is a dry-type electrolytic cell made for AC use. Typical values of the capacitors are from 200 to 600_F. The major advantage of the capacitor start motor is the increase in starting torque. The starting torque of a capacitive start motor can be about four times the rated torque of the motor. This increase in starting torque makes the capacitive start motor very useful. The disadvantage in the capacitive start motor is the increased cost over the split-phase motor. Typical applications of the capacitive start motor would be a compressor or a pump drive because of the large starting torque that is developed by the capacitive start motor. The repulsion induction motor is especially suitable to drive frequently started devices such as compressors, air pumps, and water systems. The two advantages to the repulsion induction motor are its low starting current and its constant speed. The low starting current of the repulsion induction motor is what makes this motor so suitable for applications that require frequent starting. The motor's constant speed characteristics add to the motor's efficiency. The only disadvantage to the repulsion induction motor is the increased cost of the motor over the split-phase induction motor. The repulsion induction motor will generally cost about twice as much as a split-phase induction motor. The universal motor is often preferred because of this motor's ability to operate on direct current (DC) or on alternating current (AC). In areas where both AC and DC are available, use of a universal motor increases the flexibility of the motor's application. Most universal motors are used in high speed applications (such as portable tools) because of the difficulty in obtaining similar performance from AC and DC power supplies at low speeds. The ability to adjust the speed of a universal motor at will by adjustment of the resistance is an advantage in uses where speed must be adjusted over a large range. The disadvantage of a universal motor is the increased cost. The increased cost is due to the increased winding insulation requirements. The winding insulation requirements increase over a comparable series wound DC motor because of the peak voltage to which insulation is exposed in an AC supply. Saudi Aramco DeskTop Standards
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OPERATING MOTORS
CHARACTERISTICS
AND
TYPICAL
APPLICATIONS
OF
DC
Each type of DC motor, although it contains the same basic parts as discussed in EEX 203.01, has very different operating characteristics. The operating characteristics of the different types of DC motors will be determined by how the windings of the motors are employed. The following topics will be covered in this section: _Operating Characteristics of DC Motors _Typical Applications of DC Motors Oper ating Char acteristics of DC Motors Different types of DC motors have different operating characteristics. Because of these differences, the proper type of DC motor should be selected when the load to be driven is known. The following types of DC motors are discussed below: _Series Motors _Shunt Motors _Cumulative Compound Motors _Differential Compound Motors. Series Motor s
The series motor has the highest starting torque of all DC motors and is ideal for applications (such as hoists, cranes, and locomotives), that require high torque and slow speeds. The speed of a DC series motor is controlled by the size of its load. Figure 16 shows the torque/speed characteristics of a DC series motor. Note that the motor speed varies greatly with respect to the torque. At point 1, there is no load on the motor, and the motor will overspeed and destroy itself from excessive speed. At point 2, 50% of the load has been applied to the motor. The torque increases, and speed will be about 150% of full load speed. At point 3, 100% load has been applied to the motor; the torque has increased to 100% full load torque; and speed is at 100% of full load speed. As load is increased past 100% full rated load, the speed of the series motor will drop rapidly. A heavy load must always be applied to a series DC motor otherwise, these motors will speed out of control and destroy themselves.
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Operating Characteristics of A DC Series Motor Figure 16
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Shunt Motors
The shunt motor, in comparison to the series motor, has a very low starting torque that requires the shaft load to be relatively small. A DC shunt motor has a no load speed point and can be operated without a connected load. Operation of a DC shunt motor without load will not cause the motor to speed out of control. Figure 17 shows the torque/speed characteristics of a DC shunt motor. Figure 17 shows that this motor will run at nearly the same speed at any load within the motor's capacity and that the motor will not slow very much even when it is greatly overloaded. There is only a slight drop in speed from no load (point 1) to full load (point 2). The slight difference in speed is called the droop of the motor. Figure 17 also shows the development of linear torque through addition of load to the motor. The linear addition of torque allows for very smooth operation of the motor over a varying load.
Operating Characteristics of a DC Shunt Motor Figure 17
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The shunt motor's speed can be varied through variance of the amount of current that is supplied to the shunt field. Control of the current to the shunt field allows the rpm to be changed by 10 to 20 percent when the motor is at full rpm. A shunt motor's speed control usually is accomplished through placement of a rheostat in series with the shunt field. Change in the position of this rheostat will increase or decrease the voltage that is applied to the field. This change in the voltage that is applied to the field results in a corresponding change in field current and strength. When the field current is decreased, the motor speed will increase. Motor speed will increase because of the following chain of events: _When motor field decreases, CEMF decreases (F C _ N ¢ f ). _When CEMF decreases and applied EMF stays the same, armature current increases. _When armature current increases in a shunt motor, torque increases (_ a _ _f Ia). _When torque increases with constant load, speed increases (N _ _). In the above explained sequence: Fc N _f Ia Ea Ec Ra
_=
= Force of the CEMF = Speed of rotor = Flux of the field = Armature current = Armature voltage = CEMF voltage = Armature resistance Torque
From this sequence, the net effect of a decrease in the shunt motor field current is an increase in the shunt motor's speed. The opposite is true when shunt motor field current is increased. The shunt motor's rpm also can be controlled through regulation of the voltage that is applied to the motor armature. If such regulation is applied, and if the motor is operated on less voltage than is shown on its nameplate, the motor will run at less than full rpm. The shunt motor's efficiency will drastically drop off when the shunt motor is operated below the motor's rated voltage. The drop in motor efficiency is caused by an increase in heat loss in the motor windings. Because the motor will tend to overheat when operated below full voltage, motor ventilation must be provided. The motor's torque also is reduced when the motor is operated below the full voltage level.
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Cumulative-Compound Motors
In a cumulative compound motor, the series and shunt windings are connected so that the flux that is produced by the windings aid each other. The DC cumulative compound motor will have a combination of the operating characteristics of a series DC motor and a shunt DC motor. The cumulative compound DC motor will have more starting torque than a shunt DC motor but not as much starting torque as a series DC motor. The cumulative motor will have larger speed droop than a shunt DC motor but not as much speed droop as a series DC motor. The characteristics of the cumulative motor will be determined by the amount of turns in the series field. The more turns there are in the series winding, the more closely the operating characteristics will emulate those of a series DC motor. When a cumulative compound DC motor has few turns in the series field DC motor, the motor will more closely resemble the operating characteristics of a DC shunt motor. Figure 18 shows the torque/speed characteristics of a cumulative compound DC motor. Notice how the speed droops as the load is increased. Although the droop is greater than that of a shunt DC motor, the cumulative compound motor does have a no load speed and will not runaway. The torque development of a cumulative DC motor is relatively linear. Because speed is not easily controlled in a cumulative compound DC motor, the cumulative compound DC motor is not suitable for applications that require adjustable speed.
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Torque/Speed Characteristics of a Cumulative Compound Motor Figure 18
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Differential Compound M otors
A differential compound DC motor is of a very similar design to a cumulative compound DC motor. The only difference in the two types of compound motors is that in the differential compound DC motor the series and shunt windings will be connected so that their individual flux will be in opposition to each other. A change in the connections of the fields in the differential compound DC motor will cause the field fluxes to oppose each other. The differential compound DC motor will have a lower starting torque but a more constant speed characteristic than the cumulative compound motor. Figure 19 shows the torque/speed characteristics of a differential compound DC motor. Notice that the torque will raise in an approximately linear manner as in the cumulative compound DC motor, but the speed will not droop as much as it will in the cumulative compound DC motor. Figure 19 also shows that the differential compound DC motor will have a rising speed characteristic when load is added beyond the full load of the motor.
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Torque/Speed Characteristics of a Differential Compound Motor Figure 19
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Typical Applicat ions of DC Motor s A series DC motor is used in applications where a high starting torque is required but where running speed regulation is of little concern. The advantages of the series DC motor are the motor's high starting torque and relatively low initial cost. One disadvantage of the series motor is that the speed will decrease steadily as more load is applied to the motor. Another disadvantage of the series DC motor is that the motor must be hard-connected to a load because the series DC motor has no "no load" speed. Loss of load on a DC series motor will cause the motor to speed out of control and destroy itself. A shunt DC motor, as compared to the series DC motor, will have a lower starting torque but much better speed control. Because of the motor's speed control, the shunt DC motor is very useful in applications where speed accuracy is required but where a large starting torque is not required. A good example of where speed control would be necessary is on machine tools or lathes. Because a shunt DC motor also has a no load speed, runaway is not a concern of the shunt DC motor. A no load speed makes the shunt motor very useful in running belt drive equipment such as a conveyor belt. The main advantage of a shunt DC motor is speed control. The main disadvantage of a shunt DC motor is the low starting torque. The use of the cumulative and differential compound DC motors are very similar. Because both the cumulative and differential compound DC motors are the same except for electrical connections, cost is not an issue in selection of the motor type. Both of the compound DC motors cost more than the series or shunt motors. The cumulative compound DC motor would be used where a higher starting torque is required but where speed control is not a vital issue; examples of this application would be in some hoisting and conveying machinery. The differential compound DC motor would be used in situations where a high starting torque is not required but where speed regulation is more important. Examples of typical applications of the differential compound DC motor would be in pumps or paper cutting machines. The main advantage of a compound DC motor is that these motors can be designed to combine the desired characteristics of the shunt DC motor and the series DC motor. The main disadvantage of compound DC motors is that these motors cost more than the shunt DC motor or the series DC motor. Figure 20 shows an overall composite of the speed, torque, and current (load) characteristics for compound, shunt, and series DC motors of equal size. This figure provides a comparison of the torque and speed characteristics that each DC motor type will exhibit. The cumulative and differential compound motors are shown as one line that is called compound because the curve that is shown is typical. Figure 20 shows that the series motor has the highest torque, followed by the compound motors, and by finally the shunt motor. If starting torque is the most important consideration, the series motor would be the best selection.
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Figure 20 also shows that the series motor speed droop is much greater than that of the compound or shunt motors. If speed control is the most important consideration, the shunt motor is the best selection. The DC compound motors are good examples of a compromise in both torque and speed characteristics. The selection of this compromise will cause costs to increase due to the complexity of the motor.
Speed-Torque and Current Characteristic Curves for Compound, Shunt, and Series DC Motors of Equal Size Figure 20
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SELECTING THE APPROP RIATE TYPES OF THREE-PHASE AC MOTORS Selection of the appropriate type of three-phase motor for a Saudi Aramco application, requires that all the motor selection factors to be considered. Early failure may occur in a three-phase AC motor that does not completely fit the intended application. The selection of a type of 3_ AC motor basically involves a choice between an induction and a synchronous motor. The subselection of a squirrel-cage induction versus a wound rotor motor is based on torque requirements and cost. The selection of a motor also will take into account the area into which the motor will be installed. Motor Selection Factors A designer must weigh all of the factors that bear on the selection of the type of 3_ AC motor for a particular application. The following factors must be considered: _Preferred Voltage and Horsepower Ranges _Load Characteristics _Motor Starting Characteristics _Speed _Power Output Required _Limitations of the Supply Network _Cost Pr eferr ed Voltage and H orsepower Ra nges
Three-phase AC motors come in a variety of standard voltage and horsepower ratings. A special order for a specific rating that is not in a standard rating would cause the cost of the motor to increase. Saudi Aramco allows a limited choice of voltage and horsepower ranges. The actual table of the allowed voltage and horsepower ranges for use in Saudi Aramco installations is in Work Aid 1. The second table in Work Aid 1 shows the preferred voltage and horsepower ranges of Saudi Aramco installations. Column 1 of the table in Work Aid 1 shows the nominal system voltages. Column 2 of the table in Work Aid 1 shows the nameplate voltage or utilized voltage. The European practice is to quote the nameplate voltage, but the American practice is to quote nominal system voltage. To avoid confusion of voltage, all motors for use in Saudi Aramco will be specified by the nameplate voltage only. Column 3 of the table in Work Aid 1 gives the number of phases in the motor. Column 4 of the table in Work Aid 1 shows the available Hp ranges for the specified voltage and phases. Column 5 of the table in Work Aid 1 shows the type of motor to be selected to meet the necessary requirements.
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The table shows that synchronous motors are only used on high horsepower applications unless the operating speed is required to be less than or equal to 1200 rpm. Load Char acteristics
Load characteristics refer to the following: _Load horsepower _Required load starting torque _Speed at which the load must operate _Steadiness or unsteadiness of a load Load horsepower will determine the size of the motor that is required for the application. In actuality, any type of motor can be designed for a specific horsepower requirement. Column 4 of the second table in Work Aid 1 gives the approved Saudi Aramco horsepower ranges for motors. Required Load Starting Torque of a load will vary with the type of load. For loads that obey a square-law characteristic, a motor's required load starting torque should be at least 60 percent of the full-load torque for liquid pumps, and 40 percent of full-load torque for gas-handling pumps. Pumping requirements over 11,000 kW (15,000 Hp) should be referred to Consulting Services Department. A constant speed motor must be applied for loads that must operate at constant speed. The synchronous motor by design must always run at a constant speed. The speed of a synchronous motor was designed by its construction and the supply frequency. Saudi Aramco uses 60 Hz input frequency for all motors. The synchronous motor would be the best choice for a load that must be operated at a constant speed. The steadiness of a load also plays a large role in the selection of a type of 3_ AC motor. Induction motors account for varying loads through adjustment of the amount of slip. A synchronous motor does not compensate for a varying load. Where the variations in load are excessive, such as a compressor application, a synchronous motor may slip out of synchronism and stall. The pole slippage could be for a short duration such as one pole or a complete stoppage of the motor. Slippage or stoppage depends on the amount of load variation.
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Figure 21 shows typical load speed/torque curves for Saudi Aramco equipment. The figure shows that the profile of the curve for the centrifugal pump can be changed through variance of the load (valve shut or valve open). Reduced load starting (valve shut) should be done only when absolutely necessary because of excess heat build up in the motor. The axial compressor load torque drops initially after the load commences to move because of the inertia of the load. The axial compressor will develop a smooth torque build up.
Typical Load Speed/Torque Curves Figure 21
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Motor Starting Char acteristics
All motors that are used in Saudi Aramco applications must be self-starting. Induction motors are self-starting by design and always can be used in Saudi Aramco. Synchronous motors must be designed with a method of self-starting. The most common way to accomplish the self-starting of a synchronous motor is to build a squirrel-cage into the motor's rotor. A squirrel-cage that is built into the rotor of a synchronous motor provides the necessary starting torque to start the rotor rolling. At the point where the motor reaches synchronous speed, there will be no relative motion between the rotating magnetic field and the squirrel-cage; no voltage, therefore, will be induced into the squirrel-cage. Motor starting torque is a major consideration in the selection between a squirrel-cage induction motor and a wound rotor induction motor. The addition of the external resister in a wound rotor induction motor causes starting torque to increase. Also, motor run up time can be safely increased because the resisters will limit the amount of current flow. Limitation of the amount of current flow will prevent damage to the motor's windings when the load takes a long period of time to accelerate. Speed
A motor's speed generally is dictated by the need of the driven equipment, except in cases where the driven equipment is connected to the motor through use of gear boxes. For example, if the driven equipment must rotate at 1800 RPM, the motor must rotate at 1800 RPM or the driven equipment must be connected to the motor through use of a gearbox that changes the speed of the motor to 1800 RPM. Such specifications are referred to the relevant Project Engineers. Occasionally the motor application requires that speed be optional: e.g., 600 rpm, 900 rpm or 1800 rpm will perform the job to an equal level of satisfaction. When the speed of the motor is optional, the 1800 rpm (4-pole) motor should be chosen. Generally, 1800 rpm motors are lighter in weight and less expensive than lower speed motors and 3600 rpm motors. Because the speed of a synchronous motor is constant at synchronous speed, and because the speed of an induction motor will vary slightly with load, a synchronous motor is preferable when constant speed is required. Power Output Required
The second table in Work Aid 1 shows the acceptable Hp ranges of motors for Saudi Aramco applications. The table shows that a motor can be selected for almost any necessary output range and that synchronous motors are used only for motors that are larger than 15,000 Hp or for motors that are between 670 - 4000 HP and that operate at 1200 RPM or below.
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Motors should not be oversized for an application. An oversize motor will cause the motor to run at less than maximum output and, consequently, at less than maximum efficiency. Limitations of the Supply Network
Motor placement and application depend on the kind of power that can be supplied to the location. Where the application needs a 3_ AC power and only a single-phase AC power is available, either the installation must be reconsidered or a 3_ AC power supply must be supplied to the system. Consideration must given to the strain that starting of the motor will place on the supply network when large motors are to be selected. This strain will increase as the size of the motor increases. Voltage dips upon starting will be discussed in more detail later in this course. Cost
All costs of the motor must be taken into account prior to selection of a squirrel-cage induction motor, wound rotor motor, or a synchronous motor. The following costs must be taken into account to determine the total cost: _Initial cost _Running cost _Maintenance cost Initial cost is lowest in the squirrel-cage induction motor. The lower initial cost is due to the simple design of the motor. The wound rotor induction motor and synchronous motor are more complicated in design and construction and cost more to buy and install. Maintenance cost is again lowest in the squirrel-cage induction motor. The lower maintenance cost can again be attributed to the simple construction. The wound rotor motor has the addition of slip rings and brushes that require a certain amount of maintenance. The added maintenance for the brushes and slip rings of the wound rotor motor causes the maintenance cost to rise. Synchronous motors have the highest maintenance costs because they have not only brushes and slip rings but also some source of excitation that requires a certain amount of maintenance. The running cost of a motor will depend on the size of the motor. All motors improve in efficiency as their size increases. The efficiency of synchronous motors tends to be higher than the efficiency of induction motors. The increase in efficiency of running a synchronous motor can sometimes justify the added initial and maintenance costs of the synchronous motor. For Saudi Aramco purposes, the critical size for justification of a synchronous motor
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selection is approximately 11,000 kW (15,000 Hp), but the economic power rating may be lower if there is a need to control the power factor of an installation.
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Motor Selection for Hazar dous Areas Motors that will not ignite a flammable atmosphere can be constructed in various forms to meet the different classifications of hazards and can be constructed with different types of protection. The "Ex d" (flameproof or explosion-proof) type of protection, is usual for Zone 1 (Division 1) applications. The "Ex n" (non-sparking type of protection) of totally enclosed motors is usual for Zone 2 (Division 2) locations. Because different types of protection are permitted in Zone (Division) 1 and 2 areas, the final choice must be based on economic considerations. The economic considerations should take into account the operating costs, reliability, maintenance, and capital cost. The following guidelines highlight the main cost parameters of each type of protection. Other factors, however, may apply to particular installations or motor requirements. _Zone 1 (Division 1 ) Areas The Ex d type is preferred in sizes up to several hundred kW because this type is simple and rugged and requires no special motor protection. Above 750 kW, the price becomes prohibitive because it is difficult to produce a sufficiently strong enclosure to withstand an internal explosion. Large motors should not be located in Zone 1 areas to minimize costs. _Zone 2 (Division 2) Areas In Zone 2 areas, Ex n motors should be used because they are cheaper. Ex d type motors should not be used due to their higher cost. Figure 22 shows a cost comparison for different Ex type motors. The cost of each of the Ex type motors is compared to the cost of a standard industrial motor. At lower ratings, Ex d motors that are explosion-proof are relatively inexpensive. But as motor rating goes up the cost of an Ex d motor rapidly increases. The increase in cost of Ex d motors prohibit their use above about 750 kW. The cost of Ex n motors that are non-sparking starts out just slightly above the cost of a standard industrial motor and decreases as the motor rating increases. The Ex n motor is the most economical motor to use. Ex p motors are pressurized motors. For small motors, the cost of the Ex p motor is prohibitively large. The cost of the larger Ex p motors drops rapidly as the motor size increases. At large motor ratings, the cost of an Ex p motor is feasible.
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Cost Comparisons for Different Ex Type Motors Figure 22 Saudi Aramco DeskTop Standards
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Electrical Selecting Motor Type
SELECTING THE APPROPRI ATE TYPES OF SINGLE-PHASE AC MOTOR S Selection of a single-phase AC motor is much simpler than the selection of a 3_ AC motor. Fewer factors must be considered because of the size and applications of single-phase AC motors. Motor Selection Factors The following factors are taken into account in the selection of a single-phase AC motor: _Load Characteristics _Motor Starting Characteristics _Speed _Power Output Requirements _Cost Load Char acteristics
The load characteristics must be taken into account in selection of a 1_ AC motor just as with a 3_ AC motor. The amount of torque that is required to start a load in motion must be known. The starting torque of the load must be met so that the correct motor can be selected. A motor with a starting torque that is too low will cause the run up time to increase and can cause the motor to overheat. Duty cycle also must be considered. The length of time that the load will be required to run and the frequency with which the load will be cycled are important factors in the selection of a single-phase AC motor. The more often a load is cycled, the more times the motor windings will be subjected to starting current. The starting current also is a concern with long load run up times. A long load run up time will result in the application of the starting current for a longer period of time. Motor Starting Char acteristics
For proper selection of a single-phase AC motor, the motor starting characteristics must be matched with the required load characteristics. The starting torque that is required by the load must be met by the selected single-phase motor. The split-phase motor has the lowest starting torque. The repulsion induction motor has a large amount of starting torque. The capacitor start and universal both have about the same amount of starting torque. The motor to be selected must have the necessary amount of starting torque to operate the load. If the starting torque is too low, damage to the motor can occur through excessive starting current. The repulsion induction motor will have the lowest starting current. This
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characteristic makes the repulsion induction motor more suitable for a load with a harshcyclic duty.
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Speed
The split-phase, capacitor start, and repulsion induction motors can be designed for a wide variety of speeds. When operated, these types of single-phase motors will exhibit approximately the same speed droop characteristics. The universal motor is generally only designed for speeds of 3500 rpm or higher. Most universal motors will operate between 8,000 and 10,000 rpm. Universal motors operate at high speeds because it is difficult to obtain similar performance on AC and DC at low speeds. Power Output Required
The second table in Work Aid 1 shows the accepted ranges of Hp for single-phase AC motors. Each of the four types of single phase AC motors is manufactured for a range of power output requirements. The HP ranges for each type of single-phase AC motor is shown in Work Aid 2. The power output of the motor must be matched with the power required of the load. A motor is not to be oversized. An oversize motor will cause motor efficiency to drop and cost to increase. Cost
The cost of a single-phase AC motor is much simpler to determine than the cost of a 3_ AC motor. No maintenance costs are associated with single-phase AC motors. Because of smaller sizes and relative cheapness of the single-phase AC motors, it is more cost effective to replace them than to perform maintenance on them. Because all four types of motors are designed to operate at about the same efficiency, running costs are about the same. The only difference in cost is the initial cost of the single-phase AC motor. The split-phase motor is the cheapest single-phase AC motor due to the simplicity of the motor's design. The capacitive start motor is a little more expensive because of the addition of the starting capacitor. The repulsion induction motor is still more expensive because of the dual rotor (discussed earlier). The most expensive is the universal motor. The cost of the universal motor increases due to the different types of voltages that this motor is designed to withstand.
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SELECTING THE APPROPRIATE TYPES OF DC MOTOR S Selection of the appropriate type of DC motor is very important for the protection of the motor and the load. Improper selection of the motor can cause significant damage or failure of the motor or load. Motor Selection Factors The following factors should be considered in the selection of a type of DC motor: _Load Characteristics _Motor Starting Characteristics _Speed _Power Output Required _Cost Load Char acteristics
Load characteristics of the DC motor are very similar to the load characteristics of the AC motors that were previously discussed. The motor must meet the starting torque of the load. If the motor does not meet the starting torque of the load, the motor can be damaged. Variations in load also are a large concern in the selection of a DC motor because of the effect the load has on the speed of the DC motor. Speed will be discussed later. One load characteristic that is unique to the selection of a DC motor is the type of drive that is used. The two methods that are used to connect a load to a motor are direct drive or through a belt drive. All DC motors can be used on direct drive systems. Belt drive systems add an extra factor to the selection of DC motors. The possibility that something will go wrong with the belt and that the motor will be left with no load must be considered in the selection of a DC motor. A belt failure on a shunt, cumulative compound, or differential compound motor would not adversely affect the motor because each motor has a no load speed point. Conversely, a series motor does not have a "no load" speed point. On a series DC motor, a belt failure would cause the motor to be under no load and to speed out of control. This condition would damage the motor. Because of the no load runaway of a series DC motor, the series DC motor never should be used except in a direct drive set up.
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Motor Starting Char acteristics
Motor starting characteristics again must match those of the load or damage to the motor will occur. The series DC motor has the largest starting torque of all DC motors. The series DC motor has a very large starting torque because this torque, will vary in proportion to the square of the armature current. The cumulative compound motor has a high starting torque due to the cumulative properties of the series and shunt windings. The starting torque of the cumulative compound motor is high but is not as high as the starting torque of a series DC motor. In the differential compound motor, the series and shunt winding fluxes oppose each other. This opposition will lower the starting torque of the differential compound motor lower than the starting torque of the cumulative compound motor. The shunt DC motor will have the lowest starting torque. Speed
All DC motors can be designed to operate at any given full load speed. Analysis of how motor speed varies over a range of load is more important in the selection of the type of motor. Where the load does not require a set speed as the load varies, a series motor could be used. The series motor speed will vary up and down as the load is changed up and down. When very small variations in load speed are allowable, a shunt DC motor should be applied. The shunt motor, because its speed has little variation over the load range, is very useful for constant speed applications. The cumulative compound and differential compound DC motors are a combination of a series and shunt motor. The speed regulation will fall between the two extremes. The importance of speed regulation vs added cost to the motor must be considered in the selection of a DC motor. Power Output Required
Power output requirements are a small concern in themselves. DC motors can be designed for almost any value of power output without concern for the type. The power output concerns are more accurately described as cost concerns. As the power output that is required is increased, the size of the DC motor also increases. The differences in the cost of the different types of DC motors increases as the motor size increases.
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Cost
In several instances, cost can be the deciding factor in the selection of a DC motor. Cost becomes a deciding factor because of the closeness in the operating characteristics of the four types of DC motors. The cheapest type of DC motor would be the series motor due to its simple construction. The shunt motor is only slightly more expensive and is still relatively simple in construction. There is a large jump in cost when the compound motors are selected. Because the cumulative and differential compound motors are the same except for connections, the cost of these motors would be the same. The large jump in cost with compound motors, is due to the use of two sets of windings (series and shunt) instead of a single set. The difference in cost of the four types of DC motors will increase in percentage as motor size increases. This increase in the percentage difference in cost is due to the more complex and difficult construction of the compound motors. Cost will be the overriding factor in DC motor selection only if two different types of motors are equally capable of performing the specified task.
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WORK AID 1: PROCEDURE AND TECHNICAL AND ECONOMIC FACTORS FROM SADP-P-113 FOR SELECTING THE APPROPRIATE TYPES OF THRE E-PHASE AC M OTORS Wor k Aid 1A: Pr ocedur e Use this procedure to fill in the chart to compare the requirements of the load with the supplied ratings in the description and to select the appropriate type of three-phase AC motors. 1.
Determine the motor output requirements and the motors that can meet this requirement.
2.
Determine the speed requirement of the load and the motors that can meet this requirement.
3.
Determine classification of the area in which the motor is to be installed and the motors that can meet this requirement.
4.
Determine the level of speed regulation that is required by the load and the types of motors that can meet this requirement.
5.
Determine the types of power supplies that are available in the installation and the types of motors that can use these power supplies.
6.
Determine if cost is a major concern. requirements, choose the cheaper motor.
7.
Compare the motors that will meet the requirements of the installation and choose the most appropriate three-phase AC motor. The most appropriate three-phase AC motor will be the least expensive motor that can meet the requirements of the load.
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WOR K AID 1 (Cont' d)
Question 1. 2. 3.
4. 5.
6.
Requir ement of the Loa d
Allowable Motor for this Equipment
Output-load required Speed Classification of the location the motor is to be installed Level of speed regulation required Available types and values of power (AC,DC, 1_,3_, voltage) Cost comparison
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WOR K AID 1 (Cont' d)
Nominal System Voltage 120 208 240 208 240 480 4160 4160 6900 13800 13800
Motor Nameplate Voltage 115 200 230 200 230 460 4000 4000 6600 13200 13200
Number of Phases 1 1 1 3 3 3 3 3 3 3 3
kW (HP) up to 0.25 (0.34) up to 0.25 (0.34) up to 0.25 (0.34) 0.18 (0.24) to 3.7 (5) 0.18 (0.24) to 7.5 (10) 0.18 (0.24) to 300 (400) 150 (200) to 3000 (4000) 500 (670) to 3000 (4000) 1000 (1340) to 6000 (8000) above 1000 (1340) above 10 (15000)
Type ---Induction Induction Induction Induction Synchronou s Induction Induction Synchronou s
Notes 1 1 1 1 2 3 4
Note: 1.
200 V rating only for operation on 208 V system, and 230 V only for operation on 240 V system (see NEMA MG 1-14.33).
2.
Only for application at operating speeds of 1200 rpm and below.
3.
Above 1000 kW (1340 HP), the additional level of 6.6 kV is permitted. The use of a 6.6 kV motor plus unit transformer must be compared with a 13.2 kV motor on the basis of cost.
4.
Synchronous motors smaller than 10000 kW (15000 HP) only for operating speeds of 1200 rpm and below.
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WOR K AID 1 (Cont' d) Wor k Aid 1B: Technical and Economic Factor s The application of synchronous motors is primarily reserved for: _Low speed, high power applications (typically reciprocating compressors) in which synchronous motors have high efficiencies and are generally smaller and lighter than an equivalent rated induction motor. _High speed, high power, high inertia applications (typically axial compressors or centrifugal pumps) in which motor efficiency, rotor temperature rise under starting, and system supply limitations dictate the choice of synchronous motors. _General high power applications in which the system supply limitations dictate the choice of synchronous motors. _Where speed control is a major concern for proper operation. Motors for applications in hazardous areas: _In a Zone 1 location, a type EX d (explosion proof) motor must be used. _In a Zone 2 location, the type Ex n (non sparking) motor can be used. _In a non-classified area, any type of motor can be used. Where either type of motor could be utilized, based on the foregoing technical considerations, motor choice should be dependent on first cost, running costs, and maintenance costs. A squirrel-cage induction motor will run at a constant speed (just less than synchronous speed). Because there are no mechanical connections to the rotor, the squirrel-cage motor is classified as non-sparking. A wound rotor motor has the ability to vary speed while running, but to incorporate this ability to vary speed, external connections must be made to the rotor. For this reason, a wound rotor motor cannot be classified as non sparking. A wound rotor motor will have a higher starting torque than a squirrel-cage induction motor.
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WORK AID 2: PROCEDURE AND TECHNICAL AND ECONOMIC FACTORS FROM ESTABLISHED ENGINEERING PRACTICES FOR SELECTING THE APPROPRIATE TYPES OF SINGLE-PHASE AC MOTORS Wor k Aid 2A: Pr ocedur e 1.
Determine the load output requirements of the motor. Determine what motors can meet these requirements.
2.
Determine the speed output requirement. Determine the importance of speed output.
3.
Determine the duty cycle for the load. Determine which motors are good for this operational duty.
4.
Determine the necessary value of starting.
5.
Determine the type of power that is available for the installation.
6.
Determine if the cost is the deciding factor in selection of motor type. If there is more than one type that can be used, the cheaper motor should be applied.
7.
Determine the appropriate single-phase motor to be selected through comparison of the requirements of the installation to the motors that meet the requirement. Compare all motors that can be used and determine what motor is most appropriate.
Use the answers to these questions to determine the most appropriate type of 1_ AC motor. Fill in the chart to determine the correct motor.
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WOR K AID 2 (Cont' d)
Question 1. 2. 3. 4. 5. 6.
Requir ement
Allowable Motor for this Equipment
Output - load required Speed Duty Cycle Starting torque Power available Cost comparison
Wor k Aid 2B: Technical and Economic Factor s Load Requirements: The motors are manufactured for different Hp ranges. Split-Phase 1/30 - 1/2 Hp Capacitive Start 1/4 - 10 Hp Repulsion Induction 1/2 - 15 Hp Universal 1/3 - 3/4 Hp The speed requirement of the motors: Split-phase, repulsion induction, and capacitive start are manufactured for any motor speed required. Universal is only designed for speeds greater than 3500 rpm. Duty Cycle: All 1_ AC motors can be continuously operated. Motor installations with a cyclic duty, the repulsion induction motor is more suited due to the motors low starting current.
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WOR K AID 2: (Cont' d) Starting torque of 10 Ac motors: Split-Phase Capacitive Start Repulsion induction Universal motor
about 200% Full load torque about 400% Full load torque about 300% Full load torque about 400% Full load torque
Power Requirements: All the motors can be designed to run on 1_ AC at 120 or 240 volts. The universal motor has the added advantage over the other single-phase AC motors of being able to run on AC and DC voltage. Cost Comparison: From the cheapest to the most expensive: Split-Phase Capacitive Start Repulsion Induction Universal
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WORK AID 3: PROCEDURE AND TECHNICAL AND ECONOMIC FACTORS FROM ESTABLISHED ENGINEERING PRACTICES FOR SELECTING THE APPROPRIATE TYPES OF DC MOTORS Wor k Aid 3A: Pr ocedur e Use the questions below to fill in the chart for selection of the appropriate type of DC motor. 1.
Determine the importance of speed control on the load.
2.
Determine the required starting torque.
3.
Determine the required output of the motor
4.
Determine the type of drive that the motor and load use.
5.
Determine the load variations that the motor will accept.
6.
Determine if cost of the motor is the deciding factor in the selection of the type of DC motor. Where more than one type of DC motor meets the requirements of the load, choose the cheaper motor.
7.
Determine the appropriate DC motor to be selected through comparison of the requirements of the installation to the available motors that meet the requirements.
8.
Select the appropriate DC motor through comparison of the motors that meet the installation requirements and select the most appropriate one.
Factor
Requir ement
Types of Motor for Use in Requirement
Speed Regulation Starting Torque Motor Output Motor Drive Type Load Variations Cost
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Wor k Aid 3B: Technical and Economic Factor s Speed Regulation: The series motor speed will continually vary with the load over a larger range. The series DC motor has no "no load" speed and will overspeed if no load is applied. The shunt motor has very small speed variations over load range. Shunt DC motor has no load speed with no load applied. The cumulative compound has more speed variation than the shunt motor but not as much speed variation as a series motor. The cumulative compound motor has no load speed. The differential compound motor has less speed variation than a cumulative compound motor but more than a shunt motor. The differential compound motor has no load speed. Starting Torque: The starting torque will vary with the type of DC motor used. The motor's torque compare as below: _The series motor has the highest starting torque of DC motors. _The cumulative compound motor has less torque than a series motor but still has relatively high torque. _The differential compound motor has less torque than a cumulative compound motor but more than a shunt motor. _The shunt motor has the lowest amount of starting torque. Relative Cost: The cost of a DC motor will vary with the type of DC motor used. The comparative cost of the different DC motors are: _The series motor is the cheapest due to simple design. _The shunt motor is slightly more expensive than the series motor.
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_The cumulative and differential compounds motors both cost the same and more than the series and shunt motors due to more complicated design.
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GLOSSARY altern ating cur r ent A periodic current, the average value of which over a period is 0. (AC) acceler ating torq ue The torque that is developed with rated power input between zero speed and full rated speed. br eakdown tor que The maximum torque that is developed with rated power input. counter electromotive for ce (CEM F)
The effective electromotive force that is within the system that opposes the passage of current in a specified direction.
capacitive start motor
A split-phase motor with a capacitor that is connected in series wit the auxiliary winding.
cumulative compound
Applied to a compound machine to denote that the magnetomotive forces of the series and the shunt field windings are in the same direction.
differential compound
Applied to a compound machine to denote that the magnetomotive forces of the series field winding is opposed to the magnetomotive force of the shunt field winding.
du ty cycle
The time interval occupied by a device on intermittent duty in starting, running, stopping, and idling.
Ex d
Explosive-proof motor.
Ex n
Non sparking motor.
full load t orqu e
The torque that is necessary to produce rated output at rated speed and at rated power input.
impedance
The total opposition to the flow of current in an alternating current (AC) circuit.
kVA
Kilovolt amperes.
locked-rotor torque The minimum torque that is developed by the motor at the instant that rated power is supplied to the terminal of the motor.
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NEMA
National Electrical Manufacturer's Association.
power factor
The ratio of the circuit power (watts) to the circuit volt-amperes.
pull-in torqu e
The torque that is developed during the transition from slip speed to synchronous speed.
pull-out torqu e
The value of torque where the rotor will slip out of synchronism with the rotating stator field.
repulsion-induction A motor with repulsion-motor windings and short-circuited brushes, motor without an additional device for short-circuiting the commutator segments, and with a squirrel-cage winding in the rotor in addition to the repulsion motor winding. resistance
Opposition to current flow.
series motor
A commutator motor in which the field circuit and armature circuit are connected in series.
Squirrel-cage induction motor
A motor in which the secondary circuit consists of a scroll-cage winding that is suitably disposed in slots in the secondary core.
shunt-wound motor A DC motor in which the field circuit and armature circuit are connected in parallel. slip
The quotient of the difference between the synchronous speed and the actual speed of a rotor.
starting torq ue
The torque that is developed when full voltage is applied to the armature windings and when there is no motion of the rotor.
universal motor
A series-wound or a compensated series-wound motor that is designed to operate at approximately the same speed and output on either direct or single-phase alternating current.
wound-rotor
An induction motor in which the secondary circuit consists of polyphase induction motor windings or coils whose terminals are either shortcircuited or closed through suitable circuits.
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