DC & AC Machines and Speed Control

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Electric Motors Introduction: Electric motors are used to efficiently convert electrical energy into mechanical energy. Magnetism is the basis of their principles of operation. They use permanent magnets, electromagnets and exploit the magnetic properties of materials in order to create these amazing machines. There are several types of electric motors available today. The following outline gives an overview of several popular ones. There are two main classes of motors: AC and DC. AC motors require an alternating current or voltage source (like the power coming out of the wall outlets in your house) to make them work. DC motors require a direct current or voltage source (like the voltage coming out of batteries) to make them work. Universal motors can work on either type of power. Not only is the construction of the motors different, but the means used to control the speed and torque created by each of these motors also varies, although the principles of power conversion are common to both. They range in power ratings from less than 1/100 hp to over 100,000 hp. The rotate as slowly as 0.001 rpm to over 100,000 rpm. They range in physical size from as small as the head of a pin to the size of a locomotive engine.

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Classification of motors:

D.C Motors

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Construction: A DC machine can operate as a motor or as a generator. This kind of machine is usually realized as an internal rotor/external pole machine. The ring coat shaped housing of the machine is also used as a magnetic yoke for the magnetic field through the armature and poles. The excitation winding (field winding) is located directly on the main poles of the stator. A current that flows in this winding generates the main field. Since the machine is operated with DC current, the magnetic field in the stator is constant and so all iron parts of the stator can be made of massive material. Nevertheless the main poles and the commutating poles are often laminated because of easier manufacture. Modern DC machines, used in closed-loop controlled drives, with a fast change in armature current and main field consist of one completely laminated magnetic circuit. A massive iron construction would strongly influence the dynamics and the efficiency of the machine due to the appearance of eddy currents. The rotating part of the machine holds on its shaft the armature with the commutator. Since the alternating flux flows through the armature, iron parts must be built from laminated, mutually insulated and slotted magnetic steel sheets. The coils of the

armature winding are placed in the slots; their ends are connected to the commutator segments. The current is fed into the commutator by carbon brushes. As the rotor revolves, conductors revolve with it. The brushes contact the commutator segments.

♦Basic Construction The relationship of the electrical

components of a DC motor is shown in the following illustration. Field windings are mounted on pole pieces to form electromagnets. In smaller DC motors the field may be a permanent magnet. However, in larger DC fields the

field is typically an electromagnet. Field windings and pole pieces are bolted to the frame. The armature is inserted between the field windings. The armature is supported by bearings and end brackets (not shown). Carbon brushes are held against the commutator.

♦ Armature The armature rotates between the poles of the field windings. The armature is made up of a shaft, core, armature windings, and commutator. The armature windings are usually for Wound and then placed in slots in the core.

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Brushes ride on the side of the commutator to provide supply voltage to the motor. The DC motor is mechanically complex this can cause problems for them in certain adverse environments. Dirt on the commutator, for example, can inhibit supply voltage from reaching the armature. A certain amount of care is required when using DC motors in certain industrial applications. Corrosives can damage the commutator. In addition the action of the carbon brush

against the commutator causes sparks which may be problematic in Hazardous environments.

♦ Basic DC Motor Operation: • Magnetic Fields

You will recall from the previous section that there are two electrical elements of a DC motor, the field windings and armature. The armature windings are made up of current carrying conductors that terminate at a commutator. DC voltage is applied to the armature windings through carbon brushes which ride on the commutator. In small DC motors, permanent magnets can be used for the stator. However, in large motors used in industrial applications the stator is an electromagnet. When voltage is applied to stator windings an electromagnet with north and south poles is established. The resultant magnetic field is static (no rotational). For simplicity of explanation, the stator will be represented by permanent magnets in the following illustrations.

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Magnetic Fields A DC motor rotates as a result of two magnetic fields interacting with each other. The first field is the main field that exists in the stator windings. The second field exists in the armature. Whenever current flows through a conductor a magnetic field is generated around the conductor.

• Right-Hand Rule for Motors A relationship, known as

the right-hand rule for motors, exists between the main field, the field around a conductor, and the direction the conductor tends to move. If the thumb, index finger, and third finger are held at right angles to each other and placed as shown in the following illustration so that the index finger points in the direction of the main field flux and the third finger points in the direction of electron flow in the conductor, the thumb will indicate direction of conductor motion. As can be seen from the following illustration, conductors on the left side tend to be pushed up. Conductors on the right side tend to be pushed down. This results in a motor that is rotating in a clockwise direction. You will see later that the amount of force acting on the conductor to produce rotation is directly proportional to the field strength and the amount of current flowing in the conductor.

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CEMF Whenever a conductor cuts through lines of flux

a voltage is induced in the conductor. In a DC motor the armature conductors cut through the lines of flux of the main field. The voltage induced into the armature conductors is always in opposition to the applied DC voltage. Since the voltage induced into the conductor is in opposition to the applied voltage it is known as CEMF (counter electromotive force). CEMF reduces the applied armature voltage. The amount of induced CEMF depends on many factors such as the number of turns in the coils, flux density, and the speed which the flux lines are cut.

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Armature Field An armature, as we have learned, is

made up of many coils and conductors. The magnetic fields of these conductors combine to form a resultant armature field with a north and South Pole.

The north pole of the armature is attracted to the south pole of the main field. The south pole of the armature is attracted to the north pole of the main field. This attraction exerts a continuous torque on the armature. Even though the armature is continuously moving, the resultant field appears to be fixed. This is due to commutation, which will be discussed next.

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Commutation In the following illustration of a DC

motor only one armature conductor is shown. Half of the conductor has been shaded Black, the other half white. The conductor is connected to two Segments of the commutator. 

In position 1 the black half of the conductor is

in contact with the negative side of the DC applied voltage. Current flows away from the commutator on the black half of the conductor and returns to the

positive side, flowing towards the commutator on the white half.

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In position 2 the conductor has rotated 90°. At this position the conductor is lined up with the main field. This conductor is no longer cutting main field magnetic lines of flux; therefore, no voltage is being induced into the conductor. Only applied voltage is present. The conductor coil is short-circuited by the brush spanning the two adjacent commutator segments. This allows current to reverse as the black commutator segment makes contact with the positive side of the applied DC voltage and the white commutator segment makes contact with the negative side of the applied DC voltage.

As the conductor continues to rotate from

position 2 to

Position 3 current flows away from the commutator in the white half and toward the commutator in the black half. Current has reversed direction in the conductor. This is known as commutation.

♦ Wiring types: The dynamic behavior of the DC machine is mainly determined by the type of the connection between the excitation winding and the armature winding including the commutation and compensation winding: 1.

Separately excited DC machine:

Excitation and armature winding supplied at separate voltages 2. Shunt DC machine: Excitation and armature winding are connected in parallel (i.e. fed by the same source) 2.

Series-wound machine:

The excitation and the armature winding connected in series; if the stator is laminated, series-wound machines can operate at AC current 3.

Compound machine:

This is a combination of 2 and 3 (both shunt and series winding are available)

Types of DC Motors The field of DC motors can be a permanent magnet, or electromagnets connected in series, shunt, or compound. 1. Permanent Magnet Motors are use permanent

magnets rather than windings in the field section. DC power is supplied only to the armature. Permanent magnet motors are not expensive to operate since they require no field supply. The magnets, however, lose their magnetic properties over time and this effect less than rated torque production. Some motors have windings built into the field magnets that remagnetize the cores and prevent this from happening. Permanent magnet motors produce high torque at low speed, and are self-braking upon disconnection of electrical power. Permanent magnet motors cannot endure continuous operation because they overheat rapidly, destroying the permanent magnets.

2.

Series Motors In a series DC motor the field is

connected in series with the armature. The field is wound with a few turns of large wire because it must carry the full armature current. An increase in load results in an increase in both armature and field current. As a result, the armature flux and field flux increase simultaneously. Since the torque developed in DC motors is dependent upon the interaction of armature and field flux, torque increases by the square of current increase. Characteristic of series motors is the motor develops a large amount of starting torque. However, speed varies widely between no load and full load. Series motors cannot be used where a constant speed is required under varying loads. Additionally, the speed of a series motor with no load increases to the point where the motor can become damaged. Some load must always be connected to a series-

connected motor.

V= Ia*(Ra+Rf) + E E= K*Φ*ω = K*Ia* ω T= K*Φ*Ia = K*Ia^2

If=Ia

3. Shunt Motors

In a shunt motor the field is connected in parallel (shunt) with the armature windings. The shunt-connected motor offers good speed regulation. The field winding can be separately excited or connected to the same source as the armature. An advantage to a separately excited shunt field is the ability of a variable Speed drive to provide independent control of the armature and field. The shunt-connected motor offers simplified control for reversing. This is especially beneficial in regenerative drives.

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Compound Motors Compound motors have a

field connected in series with the armature and a separately excited shunt field. The series field provides better starting torque and the shunt field provides better speed regulation. However, the series field can cause control problems in variable speed drive applications and is generally not used in four quadrant drives.

Hint: To reverse the direction of rotation of d.c motor, it is necessary to reverse the direction of current through the armature with respect to the current of field circuit. This is simply done by reversing either the armature circuit connection with respect to the field circuit or vise versa. Reversal of both circuit connections will produce the same direction of rotation. Usually armature circuit selected for several reasons: First: the field is highly inductive circuit and frequent reversal induces undesirable high emf.

Second: if the shunt field is reversed the series field must also reversed, otherwise the motor will be differential compounded. Third: if the reversing switch is defective and field is fails to close, the motor may "run away".

Advantages and disadvantages of D.C machines Advantages:  Easy to understand design  Easy to control speed  Easy to control torque  Simple, cheap drive design

Disadvantages:  Armature reaction  Commutation process  Expensive to produce •

High maintenance

Speed Control Of D.C Motor ♦ Introduction: The speed of a DC motor is directly proportional to the supply voltage, so if we reduce the supply voltage from 12 Volts to 6 Volts, the motor will run at half the speed. How can this be achieved when the battery is fixed at 12 Volts? The speed controller works by varying the average voltage sent to the motor. It could do this by simply adjusting the voltage sent to the motor, but this is quite inefficient to do. A better way is to switch the motor’s supply on and off very quickly. If the switching is fast enough, the motor doesn't notice it, it only notices the average effect. When you watch a film in the cinema, or the television, what you are actually seeing is a series of fixed pictures, which change rapidly enough that your eyes just see the average effect - movement. Your brain fills in the gaps to give an average effect.

The Motor drive divided into two categories: 1. D.C-D.C converters

1.1. Rheostat 1.2. Choppers 1.2-1.Single quadrant 1.2-2.Two quadrant 1.2-3.Four quadrants 2. A.C-D.C converter (Thyristor Rectifiers)

2.1. Single quadrant

2.2. Two quadrant 2.3. Four quadrants

• Methods for adjusting the machine speed: 1. Varying the flux, i.e. the excitation current (concerning the saturation in the excitation circuit, only a weakening of the flux is possible) the regulation of the rotational speed at a constant armature voltage is possible only to speed values above the rated rotational speed, i.e. beyond the rotational speed at maximum flux. Maximum permitted excitation current. Limit: mechanical stress (centrifugal force) and commutation (brush fire, sparking). 2. Reducing the armature voltage Right arrow the regulation of the rotational Speed is possible only to speeds below the rated rotation speed, to avoid Possible fire on the brushes at higher voltages; (voltage switching, e.g. from 220 V to 110 V or supply at DC motor controller, Leonard set). 3. Increasing Rtot with an additional series resistance R (starter) in the armature Circuit. This possibility is rarely used due to the additional losses and strong load Dependency of the speed.

(D.C-D.C

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converters (Chopper

Definition: A DC-to-DC converter is a device that

accepts a DC input voltage and produces a DC output with a desired voltage level. In addition; DC-to-DC converters are used to provide noise isolation, power bus regulation, etc. •

General block

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:diagram

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General out lines:

Vavg = (1/T).∫v(t)dt = (td/T).Vm =D.Vm Where D is the duty cycle defined as (td/T)

 single quadrant chopper:

We will start off with a very simple circuit (see the figure below). The inductance of the field windings and the armature windings has been lumped together and called La. The resistance of the windings and brushes is not important to this discussion, and so has not been drawn.

Q1 is the MOSFET. When Q1 is on, current flows through the field and armature windings, and the motor rotates. When Q1 is turned off , the current through an inductor cannot immediately turn off, and so the inductor voltage drives a diminishing current in the same direction, which will now flow through the armature, and back through D1 as shown by the red arrow in the figure below. If D1 wasn’t in place, a very large voltage would build up across Q1 and blow it up.

Reversing To reverse a DC motor, the supply voltage to the armature must be reversed, or the magnetic field must be reversed. In a series motor, the magnetic field is supplied from the supply voltage, so when that is reversed, so is the field, therefore the motor would continue in the same direction. We must switch either the field winding’s supply, or the armature winding’s supply, but not both. One method is to switch the field coil using relays:

When the relays are in the position shown, current will flow vertically upwards through the field coil. To reverse the motor the relays are switched over. Then the current will be flowing vertically downwards through the field coil, and the motor will go in reverse. However, when the relays open to reverse the direction, the inductance of the motor generates a very high voltage which will spark across the relay contact, damaging the relay. Relays which can take very high currents are also quite expensive. Therefore this is not a very good solution. A better solution is to use what is termed a full-bridge circuit around either the field winding, or the armature winding. We will put it around the armature winding and leave the field winding in series.

 The bridge power converter: As described in the previous, the speed of a series DC motor can be altered by varying the voltage applied to its terminal. One way of varying the applied voltage is by using the pulsewidth modulation (PWM) technique. Using this technique, a fixed frequency voltage signal with varying pulse-width is applied to the motor terminal. The following Figure shows an example of a PWM signal where T is the signal period, td is the pulse-width, and Vm is the signal amplitude. The average voltage can be calculated from:

Vavg = (1/T).∫v(t)dt = (td/T).Vm =D.Vm Where D is the duty cycle defined as (td/T) From the previous equation it can be seen that the average (DC component) of the voltage signal is linearly related to the pulse-width of the signal, or the duty cycle of the signal since the period is fixed. Therefore, varying the duty cycle of the signal can alter the voltage applied to the motor terminal.

The PWM voltage waveforms for the motor can be obtained using a special power electronic circuit (DC chopper). A DC chopper basically uses power switching devices to switch a constant DC voltage on and off according to a specified switching scheme in order to obtain the required voltage and current waveforms. There are various types of DC chopper configurations. In this section, we will discuss the DC chopper configuration which called: Bridge power converter also known as H-bridge converter. The schematic diagram of this converter is shown in following Figure, T1 to T4 are controlled switches that can be

implemented using power semiconductor devices such as Power MOSFET. These devices provide low resistance for the current flow when they are turned on and very high resistance when turned off. Diodes D1 through D4 provide a path for preserving the continuity of the current flow when one or more of the switches are turned off. This is necessary to protect the power switches from excessive voltage spike due to the inductive load presented by the DC motor. These diodes are also known as Freewheeling diodes. The DC voltage supply Vm can be obtained from a rectified ac signal or a DC voltage source such as a car battery. A full bridge circuit is shown in the diagram below. Each side of the motor can be connected either to battery positive, or to battery negative. Note that only one MOSFET on each side of the motor must be turned on at any one time otherwise they will short out the battery and burn out!

To make the motor go forwards:

Q4 is turned on, and Q1 has the PWM signal applied to it. The current path is shown in the diagram below in red. Note that there is also a diodes connected in reverse across the field winding. This is to take the current in the field winding when all four MOSFETs in the bridge are turned off.

Q4 is kept on so when the PWM signal is off, current can continue to flow around the bottom loop through Q3's intrinsic diode:

To make the motor go backwards: Q3 is turned on, and Q2 has the PWM signal applied to it:

Q3 is kept on so when the PWM signal is off, current can continue to flow around the bottom loop through Q4's intrinsic diode:

For regeneration: when the motor is going backwards for example, the motor (which is now acting as a generator) is forcing current right through its armature, through Q2's diode, through the battery (thereby charging it up) and back through Q3's diode:

Four Quadrant Operation

A.C-D.C Converter(Thyristor rectifiers):

1- One quadrant

2- Two quadrant

3- Four quadrant

Closed Loop Control

A.C motors Three phase induction motor •Introduction The Induction motor is a three phase AC motor and is the most widely used machine. Its characteristic features areo

Simple and rugged construction

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Low cost and minimum maintenance

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High reliability and sufficiently high efficiency

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Needs no extra starting motor and need not be

synchronized An Induction motor has basically two parts – Stator and Rotor

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Construction

The Stator is made up of a number of stampings with slots to carry three phase windings. It is wound for a definite number of poles. The windings are geometrically spaced 120 degrees apart. Two types of rotors are used in Induction motors - Squirrel-cage rotor and Wound rotor.

A squirrel-cage rotor consists of thick conducting bars embedded in parallel slots. These bars are short-circuited at both ends by means of short-circuiting rings. A wound rotor has three-phase, double-layer, distributed winding. It is wound for as many poles as the stator. The three phases are wired internally and the other ends are connected to slip-rings mounted on shaft with brushes resting on them. The brushes are connected to an external resistance that does not rotate with the rotor and can be varied to change the N-T characteristics. In fact an Induction motor can be compared with a transformer because of the fact that just like a transformer it is a singly energized device which involves changing flux linkages with respect to a primary (stator) winding and secondary (rotor) winding.

Basic equations and equivalent circuit diagram The stator and rotor of the induction machine both are equipped with a symmetrical Three phase winding. Because of the symmetry it is sufficient to take only one phase. Every phase of the stator and the rotor winding has an active resistance of R1 and R2, As well as a self-inductance of L1 and L2. The windings of the stator and the rotor are magnetically coupled through a mutual Inductance M. Since the current flowing in the stator winding has the frequency f1 and the current Flowing in the rotor winding has the frequency f2, then at the rotor speed n. • Currents induced from the stator into the rotor have f = f2 • Currents induced from the rotor into the stator have f = f1. According to this, voltage equations for the primary and secondary sides can be derived.

The equivalent circuit diagram after the conversion of the rotor parameters on The stator side is presented

The voltage and current equations are:

With this equivalent circuit diagram, the operational performance of an induction machine can be completely described. This diagram is purposely used for the operation with a constant stator flux linkage, as well as for the operation on network with constant voltage and frequency. For normal machines with the network frequency f1 = 50 Hz, the stator resistance R1Can be neglected: R1 = 0 At normal operation the windings of slip ring rotor are also short circuited through Slip rings and brushes like the squirrel cage rotor. As far as the skin effect in squirrel Cage rotor is neglected, the operational performance for both types of the rotor

Theory of operation As the stator connected to three phase balanced supply, a balanced current will flow; as a result a rotating magnetic field will be set up rotating at speed defined as

Ns=60F/p Where P= # of pairs of poles of stator winding This field travels past rotor conductors, inducing a voltage in each conductor. As the rotor winding is short circuited a current will flow in it. The interaction between stator rotating flux and rotor currents will set up a torque tending to rotate the rotor in the same direction of the stator flux rotation. The rotor will flow the stator flux at a speed Nr which must be kept lees than Ns to maintain torque. Practically Nr is near Ns during normal operating condition. For the observer on the rotor surface the stator flux will slip past him. The slip is defined as (Ns-Nr) and in per unit is

S = (Ns-NR)/Ns The frequency of the voltage induced in rotor winding depends on the difference between Ns and Nr and is given by

Fr = p (Ns-Nr)/60 = S*Fs Where Fr = frequency of induced rotor voltage Fs = frequency of applied stator voltage

Classes of Polyphase Induction motor The rotor of a polyphase induction machine may be one of two types; the squirrel cage-rotor, with alternatives for motor classes A, B, C, D and the wound rotor. The polyphase induction motor has a squirrel-cage rotor with a winding consisting of conducting bars embedded in slots in the rotor iron and short-circuited at each end by conducting end rings. The extreme simplicity and ruggedness of the squirrel-cage construction are outstanding advantages of this type of induction motor and make it by far the most commonly used type of motor in sizes ranging from fractional horsepower on up.

Design Class A: Normal Starting Torque, Normal Starting Current, Low Slip This design usually has a low-resistance, single-cage rotor. It emphasizes good running performance at the expense of starting. The full-load slip is low and the full-load efficiency is high. The maximum torque usually is well over 200 percent of full-load torque and occurs at a small slip (less than 20 percent). The high starting

current (500 to 800 percent of full-load current when started at rated voltage) is the principal disadvantage of this design.

Design Class B: Normal Starting Torque, Low Starting Current, Low Slip This design has approximately the same starting torque as the class-A design with but 75 percent of the starting current. Fullvoltage starting, therefore, may be used with larger sizes than with class A. The starting current is reduced by designing for relatively high leakage reactance, and the starting torque is maintained by use of a double-cage or deep-bar rotor. The full-load slip and efficiency are good, about the same as for the class A design. However, the use of high reactance slightly decreases the power factor and decidedly lowers the maximum torque (usually only slightly over 200 percent of full-load torque being obtainable).

Design Class C: High Starting Torque, Low Starting Current. This design uses a double-cage rotor with higher rotor resistance than the class-B design. The result is higher starting torque with low starting current but somewhat lower running efficiency and higher slip than the classA and class-B designs.

Design Class D: High Starting Torque, High Slip This design usually has a singlecage, high-resistance rotor (frequently brass bars). It produces very high starting torque at low starting current, high maximum

torque at 50 to 100 percent slip, but runs at a high slip at full load (7 to 11 percent) and consequently has low running efficiency. On the other hand, a wound rotor is built with a polyphase winding similar to, and wound with the same number of poles as, the stator. The terminals of the rotor winding are connected to insulated slip rings mounted on the shaft. Carbon brushes bearing on these rings make the rotor terminals available external to the motor

Modes of operation

An induction machine has three operation modes: • Motor (the rotor rotates slower than the rotation field): M > 0, n > 0, 0 < s < 1 • Generator (the rotor rotates faster than the rotation field): M < 0, n > n1, s < 0 • Braking operation (the rotor rotates in reverse direction to the rotating field: M > 0, n < 0, s > 1

Efficiency By neglecting the copper losses in the stator R1 = 0 the efficiency of an induction Machine at rated operation is:

To obtain a higher rated efficiency, the rated slip Sn should be as small as possible. In Practice, under the consideration of the stator copper losses and the iron losses, the Efficiency reaches a value between 0.8 - 0.95.

Single-Phase Induction Motor Single-Phase Theory Because it has but a single alternating current source, a singlephase motor can only produce an alternating field: one that pulls first in one direction, then in the opposite as the polarity of the field switches. A squirrel-cage rotor placed in this field would merely twitch, since there would be no moment upon it. If pushed in one direction, however, it would spin. The major distinction between the different types of singlephase AC motors is how they go about starting the rotor in a particular direction such that the alternating field will produce rotary motion in the desired direction. This is usually done by some device that introduces a phase-shifted magnetic field on one side of the rotor.

Split-Phase Motors The split phase motor achieves its starting capability by having two separate

windings wound in the stator. The two windings are separated from each other. One winding is used only for starting and it is wound with a smaller wire size having higher electrical resistance than the main windings. From the rotor's point of view, this time delay coupled with the physical location of the starting winding produces a field that appears to rotate. The apparent rotation causes the motor to start. A centrifugal switch is used to disconnect the starting winding when the motor reaches approximately 75% of rated speed. The motor then continues to run on the basis of normal induction motor principles.

Capacitor-Start Motors Capacitor start motors form the largest single grouping of general purpose single phase motors. These motors are available in a range of sizes from fractional through 3HP. The winding and centrifugal switch arrangement is very similar to that used in a split phase motor. The main difference being that the starting winding does not have to have high resistance. In the case of a capacitor start motor, a specialized capacitor is utilized in a series with the starting winding. The addition of this capacitor produces a slight time delay between the magnetization of starting poles and the running poles. Thus the appearance of a rotating field exists. When the motor approaches running speed, the starting switch opens and the motor continues to run in the normal induction motor mode.

This moderately priced motor produces relatively high starting torque, 225 to 400% of full load torque. The capacitor start motor is ideally suited for hard to start loads such as conveyors, air compressors and refrigeration compressors. Due to its general overall desirable characteristics, it also is used for many applications where high starting torque may not be required. The capacitor start motor can usually be recognized by the bulbous protrusion on the frame where the starting capacitor is located

Capacitor start capacitor run These motors have a run capacitor and an auxiliary winding permanently connected in parallel with the main winding. In addition, a starting capacitor and a centrifugal switch are also in parallel with the run capacitor. The switch disconnects as the motor accelerates. It should be noted that the capacitor startcapacitor run motor utilizes the same winding arrangement as the permanently split capacitor motor when running a full load speed and the same winding arrangement as a capacitor-start Motor during startup.

The advantage of the capacitor start-capacitor run design is derived from the fact that the start winding and capacitor remain in the circuit at all times (similar to PSC type motor) and produce an approximation of two-phase operation at the rated load point, plus with an additional capacitor in series with the start winding circuit (similar to the capacitor-start type motor), the starting current now leads the line voltage, rather than lagging as does the main winding, dramatically increasing starting torque. Capacitor Startcapacitor run motors feature a low running current due to an improved power factor caused by the run capacitor. This results in better efficiency, better power factor, increased starting torque and lower 120 Hz torque pulsations than in equivalent capacitor-start and split-phase designs. The capacitor start-capacitor run motor is basically a combination of the capacitor-start and PSC motor types and is the best of the single-phase motors.

Permanent-Split Capacitor Motors The capacitor of this motor is left in series with the starting winding during normal operation. The starting torque is quite low, roughly 40% of full-load, so low-inertia loads such as fans and blowers make common applications. Running performance and speed regulation can be tailored by

selecting an appropriate capacitor value. No centrifugal switch is required.

Shaded-Pole Motors The shaded pole motor is the simplest of all single phase starting methods. In the shaded pole motor, the stator poles are notched and a copper short circuiting ring is installed around a small section of the poles. As a result of the alteration of the filed pole configuration, the build-up of the magnetic field is delayed in the portion of the pole surrounded by the copper shorting ring. From the rotor's point of view, this makes the magnetic field seem to rotate from the main pole toward the shaded pole. This slight appearance of field rotation is adequate to start the rotor moving and, once started, it will accelerate up to full speed. The shaded pole motor is simple and inexpensive, but has low efficiency and a very low starting torque. Speed regulation is poor, and it must be fan-cooled during normal operation. Shaded-pole motors are thus used in shaft-mounted fans and blowers, and also small pumps, toys, and intermittently used household items.

Advantages & Disadvantages •

Advantages:

- Simple & robust construction - Can run directly from the main supply - Power electronic may be applied to improve the performance of the motor

- Brushless - Low cost and minimum maintenance - High reliability and sufficiently high efficiency

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Disadvantages:

- Difficult model to understand and complicated to compute simulation - Cogging & crawling phenomenon - Its complicate to apply speed control

Speed control of induction motor techniques Pulse Width Modulated (PWM) Figure shows a block diagram of the power conversion unit in a PWM drive. In this type of drive, a diode bridge rectifier provides the intermediate DC circuit voltage. In the intermediate DC circuit, the DC voltage is filtered in a LC low-pass filter. Output frequency and voltage is controlled electronically by controlling the width of the pulses of voltage to the motor. Essentially, these techniques require switching the inverter power devices (transistors or IGBTs) on and off many times in order to generate the proper RMS voltage levels.

This switching scheme requires a more complex regulator than the VVI. With the use of a microprocessor, these complex regulator functions are effectively handled. Combining a triangle wave and a sine wave produces the output voltage waveform.

The triangular signal is the carrier or switching frequency of the inverter. The modulation generator produces a sine wave signal that determines the width of the pulses, and therefore the RMS voltage output of the inverter.

AC drives that use a PWM type schemes have varying levels of performance based on control algorithms. There are 4 basic types of control for AC drives today. These are Volts per Hertz, Flux Vector Control, and Field Oriented Control. V/Hz control is a basic control method, providing a variable frequency drive for applications like fan and pump. It provides fair speed and torque control, at a reasonable cost. Sensor-less Vector control provides better speed regulation, and the ability to produce high starting torque. Flux Vector control provides more precise speed and torque control, with dynamic response. Field Oriented Control drives provide the best speed and torque control available for AC motors. It provides DC performance for AC motors, and is well suited for typical DC applications. Volts/Hertz Volt/Hertz control in its simplest form takes a speed reference command from an external source and varies the voltage and frequency applied to the motor. By maintaining a constant V/Hz ratio, the drive can control the speed of the connected motor.

Typically, a current limit block monitors motor current and alters the frequency command when the motor current exceeds a predetermined value. The V/Hz block converts the current command to a V/Hz ratio. It supplies a voltage magnitude command to the voltage control block. The angle of this tells the voltage where it should be with respect to current. This determines flux current to the motor. If this angle is incorrect, the motor can operate unstable. Since the angle is not controlled in a V/Hz drive, low speeds and unsteady states may operate unsatisfactorily. An additional feature in newer drives, a “slip compensation” block, has improved the speed control. It alters the frequency reference when the load changes to keep the actual motor speed close to the desired speed. While this type of control is very good for many applications, it is not well suited to applications that require higher dynamic performance, applications where the motor runs at very low speeds, or applications that require direct control of motor torque rather than motor frequency. V/Hz Speed vs. Torque

The plot above shows the steady state torque performance of a Volts/Hertz drive. A torque transducer directly on the motor shaft supplied the data that is plotted. The drive is given a fixed speed/frequency reference. Then load on the motor is increased and actual shaft torque is monitored. Notice that the ability of the drive to maintain high torque output at low speeds drops off significantly below 3 Hz. This is a normal characteristic of a Volts/Hertz drive and is one of the reasons that the operating speed range for Volts/Hertz drives is typically around 20:1. As the load is increased, the motor speed drops off. This is not an indication of starting torque. This only shows the ability of the drive to maintain torque output over a long period of time. Sensor-less Vector Sensor-less Vector Control, like a V/Hz drive, continues to operate as a frequency control drive, with slip compensation keeping actual motor speed close to the desired speed. The Torque Current Estimator block determines the percent of current that is in phase with the voltage, providing an approximate

torque current. This is used to estimate the amount of slip, providing better speed control under load.

The control improves upon the basic V/Hz control technique by providing both a magnitude and angle between the voltage and current. V/Hz drives only control the magnitude. V-angle controls the amount of total motor current that goes into motor flux enabled by the Torque Current Estimator. By controlling this angle, low speed operation and torque control is improved over the standard V/Hz drive Flux Vector The flux vector control retains the Volts/Hertz core and adds additional blocks around the core to improve the performance of the drive. A “current resolver” attempts to identify the flux and torque producing currents in the motor and makes these values available to other blocks in the drive. A current regulator that more accurately controls the motor replaces the current limit block. Notice that the output of the current regulator is still a frequency reference. The early versions of Flux vector required a speed feedback signal (typically an encoder) and also detailed information about

the motor in order to properly identify the flux and torque currents. This led to the requirement for “matched motor/drive” combinations. While there is nothing inherently wrong with this approach, it does limit the users motor choices and does not offer independent control of motor flux and torque. Flux vector control improves the dynamic response of the drive and in some cases can even control motor torque as well as motor speed. However, it still relies on the basic volts/Hertz core for controlling the motor.

Recently, flux vector control has been enhanced to allow the drive to operate without the use of a speed feedback device, relying instead on estimated values for speed feedback and slip compensation. Again, the basic Volts/Hertz core is retained.

Field Oriented Control What distinguishes a product using Field Oriented Control from a traditional vector product is its ability to separate and independently control (or regulate) the motor flux and torque. Notice that in the definition of Field Oriented Control we did not say “currents in an AC motor”. That’s because the concept applies equally well to DC motors and is the reason we can demonstrate “DC like” performance using Field Oriented Control on AC drives. Force Technology uses patented, high bandwidth current regulators in combination with an adaptive controller, to separate and control the motor flux and torque. This is a fundamental difference between Force Technology and other vector control techniques.

A high bandwidth current regulator that separates and controls the components of stator current replaces the Volts/Hertz core. The high bandwidth characteristics of this control eliminate nuisance trips due to shock loads and continuously adapt to changes in the motor and load characteristics. A separate adaptive controller uses information gained during auto tuning, actual reference information, and motor feedback

information to give independent torque and flux control. This allows continuous regulation of the motor speed and torque. Also notice that Force Technology generates separate flux and torque references to improve the overall control of those quantities. Sensor-less Field Oriented Control As with flux vector products the newest versions of Force Technology allow users to control the motor without the use of a speed-sensing device. A major difference is that the drive continues to operate with Field Oriented control, instead of reverting back to Volts/Hertz control. This provides significant benefits with dynamic performance, trip less operation, and torque regulation.

Below is a plot of a drive using the Sensor-less version of Force Technology. Notice that the torque output is consistent from no load to full load over a very wide speed range. You can also see that the motor has a speed/torque characteristic that is very similar to its DC counterpart, even when operating above base speed.

Performance Comparison The graph below shows a drive using Force Technology operating with and without an encoder, and a Volts/Hertz drive. Notice that there is very little difference in operation with or without an encoder. You can clearly see the response to the step load and the recovery time. The same can be seen when the load is removed.

Comparison between D.C &A.C Drives D.C

A.C

Weight

Heavy

Light

Size

Large

Small

Cost

Expensive

Less expensive

Starting torque

High

Low

High speed

Not used

Suitable

Control

Simple

Complex

Feed back signal

Available

Complicated

Types

Separately,

Induction,

series,

Synchronous

shunt