05-02-2008

CONCEPTS, CONFIGURATIONS, & BENEFITS OF MOTOR STARTING AND OPERATION WITH MV AC ADJUSTABLE SPEED DRIVES IEEE-IAS Cement Industry Committee William J Horvath PE, MIEEE Senior Application Engineer, TMEIC GE Automation Systems Abstract Modern MV AC Adjustable Speed Drives [MV ASDs] are frequently used for industrial process optimization and to provide energy savings over fixed speed solutions or mechanical ASD solutions. However for over 20 years MV ASDs have also be used to accelerate motors and their connected loads to line frequency equivalent speed and then connect them to the power grid. When properly done, this synchronized starting causes little impact on utility current or voltage beyond normal running values for the motor and load. This paper reviews the power system impact of starting large motors, and the broad topic of ASD synchronized starting. Other topics covered are system performance, equipment selection, relative economics and complexity. Index Terms—AC ASD, VFD, drives, motor starting, synchronized starting I. Introduction The starting of large motors and their associated loads has always presented an electrical and equipment challenge. Large motor starting can disrupt processes, affect other connected utility users, and even prevent a successful start. The trend in recent years has been an growing increase in the use of large electric motors, sometimes at very remote sites with little ability to support high-current starts. MV AC Adjustable Speed Drive [MV ASD] technology has advanced as well. Using MV drives to accelerate large motors to speed and connecting them without power surges has become more economical. This paper outlines the motor, drive and system technical areas and tradeoffs to enable proper consideration and selection of MV ASDs for synchronized starting applications. II. Starting Large Motors Starting a large motor induction or synchronous motor by connecting it directly across its utility source can create large voltage disturbances. This voltage disturbance results from the large initial current required at standstill, called “locked rotor current”. For most induction motors this starting current is about 600% of the motor’s running amperes. Figure 1 shows a typical induction motor characteristic at start, including both per unit torque and current. Notice that the line current level remains high for most of the acceleration period. Once the motor speed is past the speed where its maximum [breakdown] torque point occurs, the current falls to a running level determined by the load and the design slip.

Figure 1 Full voltage start of single or multiple induction motors

978-1-4244-2081-0/08/$25.00 © 2008 IEEE

III. Effects and Impact of Across the Line Motor Starting The starting-induced voltage disturbance has several undesirable results in several areas. Consider the following: a. Remote locations on long power lines – the overhead line utility feeds to these sites are highly reactive in their impedance characteristics. Since induction motor starting power factor is typically 0.20 lagging, voltage drop at the utility feed can be high. b. Many new motors applications are very high power – since plant production is increasing, loads such as ID fans and mills are growing. In other industries such as petrochemical large compressors are moving from gas turbine and internal combustion prime movers to electric motors. Since many of these applications are located in remote areas or where the existing power system is at or near capacity, the voltage drop issue compounds to become especially critical. c. Processes in rest of facility may suffer from voltage drop - any time the voltage feeding motors or plant loads falls to less than 90% of rated levels the result can be a major disruption, from misoperation to process shutdown. d. Utility company restrictions - utility providers may either forbid or restrict high-current starts, or charge high rates for a long period following such starts. Such regulations and restrictions can come from commitments to other nearby customers to provide power at contractually guaranteed voltage levels. Reducing Starting Current Motor inrush current varies directly with voltage applied to the motor. To reduce system voltage disturbance, the starting current can be reduced by using one of various types of starting equipment that temporarily reduce the motor terminal voltage. Figure 2 shows such an arrangement.

Figure 2 Reduced voltage start of induction motors

Reduced voltage starters have several effects beyond current reduction. First, the torque available to start and accelerate the connected load decreases by the inverse-square of voltage. At 60% voltage, for example, available torque is 60% x 60% = 36%. While this might be sufficient for such loads as fans or pumps, a mill load requiring 100% motor torque would not start. Second, the current and start torque keep the same shape as the motor applied at full voltage. The minimum torque [pull-up torque] also reduces with applied voltage reduction, and may not be high enough to accelerate some loads to past the critical breakdown [maximum design torque] point. Third, incoming amps are still highly reactive, so voltage drop to the motor will roughly be reduced in proportion to the reduction in starting current.

One final note is appropriate. Most synchronous motors started across the line develop torque using a cage-type rotor winding. The resulting sync motor starting current inrush characteristics are of a magnitude and shape quite similar to a squirrel cage induction motor of the same size. IV. Some Effects Of Across the Line Starts On The Motor Every time a motor is started across the line, it produces several areas of high stress in the motor. 1. The very high starting currents put mechanical stress on the stator windings, Such stress cycles contribute to winding fatigue that can lead to stator insulation failure. 2. The high current through the stator quickly raises the temperature of the windings. Each acceleration cycle takes some life from the motor windings insulation. The longer the cycle of acceleration, the more heat is built up in the stator. Protection is needed to prevent successive starts from causing irreparable damage to the windings. Motors are specified and designed with an allowable limit on the number of starts per hour. 3. The rotor also carries very high currents during the full voltage, high-slip acceleration cycle. The magnitude of the rotor heat is roughly equal to the amount of energy contained in the connected load at its final velocity. For very high inertia loads, such as a cement kiln ID fan, a motor that must start the fan across the line must be designed with much more mass to absorb the heat from the start. Such designs can be very expensive. Using a reduced voltage starter does reduce stator heating, but does nothing to reduce the amount of heat induced in the rotor during a direct online start. A reduced voltage start does tend to lengthen the start cycle and could possibly allow some of the rotor heat to be dissipated during the start cycle. However, the total amount of heat injected into the rotor for a given start is the same as if it were controlled by a full voltage starter. V. Starting motors Using Adjustable Speed Drives [ASDs] Slip is the difference in actual rotor speed and the synchronous speed of the motor. The synchronous speed depends on motor design and applied frequency. This relationship is shown by the formula below: Sync RPM = 120 * frequency / number of poles For example, a 4-pole motor run on a 60 Hz supply would have a synchronous frequency of 120 x 60/4 = 1800 rpm. At 30 Hz, the same motor’s sync RPM would be 900 RPM, and so forth. Figure 3 shows a general induction motor speed-torque characteristics, the shape of which was shown in the earlier starting current discussions. Peak [Breakdown] Torque, BDT

Torque

Locked Rotor Tq Pull Up Torque Rated Torque Rated SlipRPM = Sync - Rated RPM

RPM

Rated RPM Sync RPM

Figure 3 General speed torque characteristics of an induction motor at a particular applied frequency

Referring to Figure 3, motoring torque is produced only when the motor turns at a small RPM difference

below the synchronous speed. This is typically on the order of about 1% of synchronous speed. The rated torque of the motor is the design value on the motor nameplate that is developed with rated voltage and rated frequency at the rated slip RPM below the synchronous RPM. In English units, this rated torque can be calculated as Rated Torque in lb-ft = Rated HP x 5252 / Rated RPM For example, for a 7500 HP 900 RPM synchronous speed and a rated speed of 892 RPM, Trated = 7500 x 5252 / 892 = 44159 lb-ft All the across-the-line induction motor starting characteristics discussed so far depend on the motor current and torque design starting at 100 percent slip. It has already been shown that a motor designed for 60 Hz will turn at half its rated RPM on a 30 Hz supply. The induction motor characteristic then becomes a family of curves as frequency increases from near zero, to rated speed. Figure 4 shows this family of curves.

AC Adjustable Speed Drive Motor Amps & Torque vs Speed

Controlled, Increasing Volts and Frequency

RPM,RPM FREQ

Figure 4 Speed torque characteristics of an induction motor on an increasing [ASD]

Torque, Amps

Torque

Ideal Family Of Speed Torque Curves as Function of RPM [Holding Volts/HZ Ratio Constant]

Motor AvailableTorque

ne y Li Utilit s C A Amp

Note – line amps shown for PWM VSI type drive

Full Load Torque Level

Frequency, RPM

Figure 5 Load acceleration and line amps of for a Voltage Source ASD feeding and induction motor

Figure 4 Speed torque characteristics of an induction motor on an increasing [ASD] applied frequency

During the acceleration, frequency and voltage are generally controlled such that full voltage is reached at the design rated RPM of the motor. Motor voltage and current are regulated to provide speed and torque control to the load. The voltage to frequency ratio is held roughly proportional. In practice torque developed at very low frequencies falls off and extra voltage above the value of the volts / HZ ratio is automatically applied to regain the lost torque. An AC ASD would accelerate a constant torque load such as a conveyor or cement mill by advancing frequency as shown below in Figure 5. Several things can be noted from the simplified drawing in Figure 5. 1. First, the motor delivers torque to the load from the part of the torque curve to the right side of the peak [breakdown] torque point, at low slip, as if it were operating at its rated running electrical conditions. This means that the stator or rotor currents are no higher than under normal operation. Therefore there is no longer any limit on the number of starts in any time period. 2. For a VSI [Voltage Source Inverter] topology drive using DC link capacitors, the AC utility amps reflect the kW actually being delivered to the load. Figure 5 shows the relation of utility amps vs. load RPM for a VSI ASD for a constant torque load. Utility amps start out very low [transformer magnetizing amps plus initial low kW] and increase as speed and kW increase.

3. For a current source topology drive [with DC link inductors], the start cycle utility line amps are higher than for a VSI drive. In some cases, line current is roughly equal to stator current, and is approximately constant for a constant torque load. Stator amps are determined by the load torque as a ratio of rated amps and torque. Essentially, the stator amps and line amps do not exceed the actual load amps, This eliminates all the utility supply problems described above. If the utility can sustain the final loaded motor amps then it can always sustain the ASD line amps during a start. VI. Synchronized Motor Starting Using the ASD The preceding information provides a basis to apply the advantages of the ASD to what is called “synchronized starting”. In synchronized starting, ASDs are used to accelerate a motor and its connected load to line frequency equivalent speed and then connect them to the power grid. This synchronized starting causes little impact on utility current or voltage beyond normal running values for the motor and load. The arrangement shown in Figure 6 is typical of a VSI ASD connected to a single motor for the purpose of synchronized starting. A review of the circuit and a synchronized start cycle follows.

Figure 6 Typical one-line of a single VSI ASD synchronized start system for an induction motor

The major components included in the system of Figure 6 are summarized below. 1. M Induction motor to be started 2. Converter-Inverter – ASD variable frequency rectifier and inverter. 3. M1 & M1A Input disconnect and pre-charge contactor 4. PT Voltage sensing transformers for input and output 5. Drive control microprocessor- based drive control with synchronizing logic 6. L-1 Output isolation inductor for closed transition of motor to utility 7. M2 Drive isolation contactor and switch – open during utility operation 8. M3 Bypass contactor controlled by drive synchronizing microprocessor 9. CTO Current sensing transformer to assure utility current flow after synchronizing. 10. Relay 25 Independent synchronizing check relay used as verification of synchronize conditions.

VII. Sequence Of Operation For Synchronizing Figure 7 shows some traces of voltages and currents taken from an actual ASD synchronizing system like that shown in Figure 6. The steps below outline the operational sequences. 1. Drive is made operational by proper startup sequence, and contactor M1 and M1A are closed. 2. Drive is accelerated to speed by user process input. 3. A contact closure requesting synchronous Utility Volts transfer to line is received. 4. The ASD accelerates the motor on a predetermined ramp to voltage and frequency exactly equal to line voltage and Drive Volts frequency. 5. Using the potential transformer on the DRIVE ACCELERATES -> utility side and output of the drive, shown Drive is matched in A frequency, phase and as PT in Figure 6, the drive control volts to li ne and continuously monitors the voltage, Motor Volts bypass co ntactor is frequency, and phase of the drive input clo sed and output. 6. When drive input and output is matched in voltage level, frequency, and phase, the Utility Amps drive control activates a digital output, closing the bypass contactor M3, if the UTILITY PICKS UP AMPS -> B redundant synchronizing check relay marked as 25 is closed. The 25 relay is not used to synchronize the drive and Drive Amps motor, but serves as a permissive in series with the drive control to ensure accurate DRIVE AMPS END -> operation. This is Point A in the voltage and current chart of Figure 7 7. Output inductor L-1 is included to provide Motor Amps isolation between the utility source and the drive during the instant of the closed MOTOR AMPSAMPS ANDAND TORQUE ARE MOTOR TORQUE IS SMOOTH transition between the inverter and the SMOOTH THRU WHOLE PROCESS! THROUGH THE WHOLE PROCESS power line. The size of the inductor is selected dependent upon the total Figure 7 Traces of drive, motor, and utility voltages and impedance of the utility source in currents during VSI ASD synchronized start cycle. comparison to the connected motor load. 8. The drive control monitors current flowing into the motor through the newly established bypass utility power feed through contactor M3. When the proper levels of utility line current are flowing into the connected motor, the synchronizing process is complete, and the output contactor M2 is opened. This is shown at Point B. 9. The inverter power switch gating circuits are blocked, and the inverter output stops. 10. If multiple motors are to be started, the process is repeated. Each motor will have its own bypass contactor, and feedback CT. A dedicated output contactor is supplied for each motor. A second drive may be incorporated for redundant control of the group of motors. 11. Note how the bottom trace of Figure 7 shows the motor amps to be smooth throughout the whole process. No noticeable torque or current disturbances are produced.

VIII. Sequence of operation for capturing a utility fed motor and reconnecting it to the drive Once a motor is connected to the line by the drive and is powered entirely by utility power, it can be “captured” smoothly by the drive and brought back into variable speed operation or to a smooth stop. Here is a simple sequence of operation for this “de-synchronizing process”: 1. Again referring to Figure 6 , consider that the motor is connected to the utility line through contactor M3 and is turning its connected load at a Utility Volts speed equivalent to its nameplate RPM. Figure 8 shows traces of de-sync process. 2. A CAPTURE command contact is received from the user Drive Volts requesting the motor to be captured from the line [desynchronized]. A DRIVE ACCELERATES -> 3. The drive control prepares the inverter to gate the output switches to produce voltage of Motor Volts the correct magnitude, phase, and frequency point A in Figure 8. 4. The output contactor M2 is BYPASS CONTACTOR closed while the bypass Utility Amps Drive is matched in contactor M3 is closed. frequency, phase 5. The bypass contactor M3 is and volts to line & UTILITY AMPS END -> bypass contactor is commanded to open. opened 6. After a precisely set time delay, B Drive Amps the inverter gates the output switches to produce voltage of DRIVE PICKS UP AMPS -> the correct magnitude, phase, CURRENT TRANSITION and frequency. Within approximately 1/10 of a power cycle, current flows from the Motor Amps drive to the motor, giving smooth transfer. Refer to Point B of MOTOR AMPS AND TORQUE ARE Figure 8. The disturbance in SMOOTH THRU WHOLE PROCESS motor current shown is brief, so no significant torque disturbance Figure 8 Traces of drive, motor, and utility voltages and is introduced in the driven load. currents during VSI ASD de-synchronizing cycle 7. Operation of the motor is now completely where motor is captured from the utility to the ASD. under the control of the drive.

IX. Notes on ASD Controlled vs Synchronized Motor Operation The preceding information has been essential background in discussing synchronized starting of motors using any technology. However, it will be noted that ASDs are often, if not usually, used for their broader benefits of providing variable speed operation to the driven processes. Production Related Issues 1.

Recalling that an ASD-started motor has no limits on the number of allowable starts per hour, if anything in the process stops the motor, it can be restarted immediately. For a line-started motor, it might be necessary to wait for the protection relays to determine it is safe to restart the affected large motor, or risk motor damage.

2.

While on the ASD, the output can be varied to match the needs of the downstream process.

Energy Related Issues 1.

If the process requires operation at less than 100% output, [flow, speed, etc] the energy difference between max and running points provides the opportunity to save energy.

2.

A typical ASD system operates at a total overall efficiency of around 97%. For those times when the process requires operation at 100% output, synchronizing to the line provides a direct connection of the motor to the utility, saving the 3% losses represented by the ASD. During the time when the motor is synchronized, the ASD is left in standby mode to allow the motor to be captured and smoothly connected to the ASD.

Maintenance Related Issues 1.

Following the discussion of motor starting above, synchronized starting reduces motor stress and could extend motor life and reduce maintenance.

2.

Operating the industrial process at less than top speed on the ASD decreases the mechanical wear on the driven components. This is particularly obvious on such applications as conveyors, crushers, etc. This, of course is contingent on the production needs at the time being met at a reduced speed. X. Starting And Using Synchronous Motors with ASD Systems To this point we have been discussing the use of ASDs for the synchronized-to-utility starting of induction motors, as opposed to synchronous motors. It has also been noted that a synchronous motor, when starting across the utility line, actually starts as an induction motor, with the rotor construction including an amortiseur winding in the rotor poles. Synchronous motors on today’s ASD systems are operated synchronously motors from standstill to running speed, and do not make use of the starting rotor winding to make torque. The rotor field must then be energized from standstill. Figure 9 shows a single motor ASD system with the field excitation system shown. Note that exciter and controls included must be able to transition from being controlled by the drive to being controlled by the demands for reactive power and shaft torque under utility mode operation. Functions such as contactor interlocking, sync field reference control, synchronizing supervisory logic, operator control / HMI interface and overall DCS or plant interface are included in the two blocks labeled Drive Control and System Sequencer. The functions will likely be spread between the drive and an external PLC or equivalent.

Single Motor Sync-Desync Bypass for Synchronous Motor M3

M1A

VSI AC Drive CTO

M1

L-1

Converter - Inverter VL PT

Sync Check

Drive Control

System Sequencer

PT

M-2

Sync Mot

Field Exciter

VO Sync Fld Ref

Figure 9 Typical one-line of a single VSI ASD synchronized start system for a synchronous motor Synchronous motors can be the technology of choice for large process loads for two main reasons. First, a synchronous motor is usually more efficient than an induction motor, by about 1 to 1.5 percentage points. Second, the synchronous motor’s line power factor can be changed by controlling its DC rotor field to allow net export of reactive power while supporting the load kW requirements. If sufficient design margin is included in the motor, this ability to run at leading power factor can be useful in providing voltage support for the plant and utility system. It is important to note that the synchronous motor shown in Figure 9 can only deliver its reactive power benefits to the power system if it is directly connected to the utility and disconnected from the drive, as when contactor M3 is closed and M2 is open. While connected to the drive, the sync motor is set to run at a reactive power level [usually between 0.90 leading and unity power factor] compatible with the drive type. During ASD controlled operation, the power line sees the reactive power level set by the drive incoming converter, not the motor. For a VSI system with diode rectifier AC-DC conversion, this line power factor is 0.95 to 0.98 lagging. This is of most importance when the drive will be used to control the motor for extended periods. When the ASD is used primarily or predominantly for start mode operation, the sync motor reactive power benefits could be realized most of the time. One final note on synchronous motors as applied to ASD operation. As mentioned above, a synchronous motor must have DC field available at zero speed to develop starting and accelerating torque. A DC brushless exciter type sync motor does not develop field at zero speed, because the exciter generator must turn to develop field current. This low-current brushless type synchronous motor is intended for across the line starting as an induction motor to provide the needed field exciter rotation prior to excitation as a sync motor. The two synchronous motor designs that are suitable for starting on an ASD either use DC slip rings, or an AC brushless type exciter to energize the field at standstill, during acceleration, and while running. While under ASD control, the field excitation level is set by the ASD control. If the motor is synchronized to the utility, control of the motor field excitation is transferred from the drive to an independent control means to set its level appropriate to the process and power system reactive power requirements.

XI. Equipment Selection for ASD Synchronous Starting Systems Selecting equipment ratings for starting motors using ASDs includes several key factors beyond the induction motor vs synchronous areas listed in the previous section. The list below does not cover every application aspect but should cover the majority of them. Where multiple motors must be started the motor and load-specific areas for each must be defined. Figure 10 is a more general diagram to highlight the areas to consider. M3 CTO

VSI Drive M1A M1

Motor Matching Drive Output Transformer

Incoming Transformer

Converter - Inverter

M2

M

T1

VL

Drive Control & Sequencer

PT

25

Figure 10

L-1

T2 PT

VO

General Single Motor Sync-Desync Bypass with possible transformer voltage matching transformer

A more detailed examination of the ASD components will help to understand how the ASD is applied for motor starting. Figure 11 below shows the major ASD components to be considered.

Figure 11 Main Sections of a VSI Inverter

Key ASD application areas Using Figures 10 and 11 we can explain how the system requirements affect equipment selection and rating. Table 1 below summarizes the inter-relationships. Application or Requirement Area

Section of ASD Start System [See Descriptive Figures 10 & 11]

Breakaway Torque Type of load, and speed torque profile from start to synchronizing

Torque vs Speed

11C, T2, L1

Motor Rating

Versus Synchronize & Run

Entire System

Motor Voltage

T1, 11B, 11C, M2, M3, T2

Nameplate kW or HP

Entire System

Motor Power Factor

11C, T2, L1

Nameplate & Peak Torque Currents

11C, T2, L2

Voltage

Entire System

Available Short Circuit

L1, M2, M3

One time start or repeated cycles and frequency

11C, L1, M2, T2

Utility system

Duty Cycle

Select output inverter with proper current rating to carry the total motor current including torque and excitation amperes over the entire torque speed curve. Inverter overload ratings may be applicable.

Max Power point Continuous Operation on ASD is Required

Affect on ASD Start System components and configuraion

Select all equipment [input to output] for the largest motor and that motor's continuous load Inverter voltage output must match motor or use an output matching transformer. High starting torque requires special design of output transformer. Select all equipment for the largest motor and that motor's continuous load Affects inverter output amp rating during start and running. Affects inverter input transformer, input and output switchgear & output voltage rating [see notes on motor voltage]. Higher short circuit capacity affects size of output reactor and switchgear. Affects inverter output rating and other output components. Depending on duty, inverter overload ratings may be able to be used.

Table 1 ASD start system requirements and effects on equipment selection

1. Type of load, and speed torque profile from start to synchronizing. During a starting cycle on ASD operation, the load torque as reflected at the motor shaft affects the ASD current output requirement. Figure 13 below is for a variable torque compressor with valves closed for starting but a large ID fan with blocked input flow might have the same characteristic.

C

B

A

Figure 13 Compressor Load vs Speed, showing starting breakaway Torque at point A, Reduced full speed torque for start cycle [valve closed] at B, and full load, full speed torque C

From the summary chart of Table 1, and the preceding figures, the breakaway torque at A will affect initial current drawn by the motor. As might be expected, it is very low for a fan or centrifugal compressor. This in turn affects the current load imposed on the ASD inverter output section. It is also important to realize that the current seen by the ASD inverter is the total vector sum of the current required to produce motor torque and the current required to magnetize the motor. The information in the simplified diagrams of Figures 14 and Table 2 and below illustrates this.

Stator X1s

ROTOR

R1

X2s

R2/slip

Motor Terminal LineVolts Volts V

Torque Producing Volts

Xm

EXCITATION AMPS

Figure 14 Illustration of Vector components of Induction Motor Current and simplified induction motor 1-phase model. Induction Motor Load Torque Current Component

No Load

100%

150%

175%

200%

Total Amps From Inverter

~169

246

318.3

357.5

398.0

Design Full Excitation Amps

167

167

167.0

167.0

167.0

ApproximateTorque amps

~20 [Friction & Windage]

180.6

270.9

316.1

361.3

Table 2 Example induction motor torques, real currents and magnetizing currents for a low speed 700 HP, 390 RPM 2300 volt crusher.

The important things to note is that the ASD inverter section must be rated to carry the maximum current required during the start, including both the torque component and excitation [magnetizing] component. This is true both at zero speed and top speed. ASD equipment usually includes an overload rating for 60 seconds, usually 115% or 150%. The extreme example motor characteristics of Table 2 were deliberately selected to illustrate the effect of a very high magnetizing component. This is typical of a low speed induction motor with low [lagging] power factor. In a more typical design, like a 6 pole 1200 RPM motor, this magnetizing component might be 25 to 30% of rated amps, where in this extreme example it is 167 / 246 = 67%! This high magnetizing current has a big impact on total current supplied by the inverter. It is important to note motor non-linearities require that the ASD also modify the level of excitation from the initial base level to obtain good torque performance. Returning to the more typical starting example of Figure 13 its corresponding motor data is as follows: Nameplate data: 4000 HP, 4160 volts, 3-phase, 60 HZ, 3390 RPM FL amps = 483, PF = 89% @100% load, 88% @ 50% load The magnetizing component of such a 2-pole, 3600 RPM motor would be in the order of about 20-25% of rated amps. The actual value is obtained either by measurement of no load current or from the manufacturer data sheet. At this more typical and moderate level of excitation current, the total current at top speed is only about 3% higher than required to support the load and will not significantly affect ASD inverter current output rating.

2.

3.

4.

5.

Point A of Figure 13 represents full speed, open-valve or closed-inlet running at 3600 RPM, and about 3900 HP. Point B, with valve or inlet vane closed for reduced load acceleration, is at about 48% torque and 1872 HP and requires about 48% of torque current. Once the motor and load reaches point B, the synchronizing function connects the motor to the utility, and the drive is released to standby mode. Since the motor power factor at 50% load is very nearly the same as the power factor at full load, a reasonable estimate of the total inverter current demand at 48% load would be 50%, including a bit of safety margin. So for this case a good selection of the output amp rating of the ASD for starting duty only would be 243 amps. With the proper use of a particular drive’s short time ratings, it is often possible to use an equivalent drive rated at 40 to 50% of the rating required for continuous duty. Continuous Operation requirements in terms of motor power, torque and amps. Inherent in the discussion of the system of Figure 13 was that the ASD system was to be used only as a synchronized starter. If the process could benefit from variable speed operation, then the drive must be rated for continuous duty. In the example in Figure 13, the load was reduced during start, to 48% of maximum at point B. For continuous operation at Point C, all ratings, including input switchgear, transformer, converter, inverter, output reactor and contactor would have to carry the power and corresponding current for the Point C load continuously. Motor rating in voltage, no load and full load current Much has already been said about the motor current demand on the inverter output. Motor nameplate voltage, which matches the utility voltage, also plays a key role. At the moment of synchronized transfer, motor voltage from the ASD must match the utility. For very high horsepower applications, motor rated voltages of 12.4 to 13.2 kv are often applied. Drive technology at this higher voltage can be quite expensive, large and contain many components to achieve voltage rating, affecting system reliability. At this level, the most costeffective solution can be the use of an output matching transformer, shown as T2 in Figure 10. Again, if the ASD will ever control the load on a continuous basis, T2 will need to be rated to carry the full KVA load of the motor, with the effect of power factor considered. Special design constraints also apply to ASD out transformers. They must be able to support the load at low frequency during starting, have proper insulation for the waveforms applied, and include some tolerance of dc voltage offset from zero in the phase voltages. Utility system voltage and short-circuit levels Part of this issue was discussed above under motor voltages. System voltage does also affect the selection of the input, bypass and output switchgear. The ASD isolation transformer primary also must match the system voltage. The utility system short circuit current capacity affects the corresponding rating of the switchgear. Duty Cycle – Even if the determination is made that the ASD will be used as a start system, if repeated cycles or multiple motors are sequentially started, the ASD rating will be affected. Repeated cycles [within 5 minutes of the last start] will likely mean that the ASDs short time overload rating cannot be used. If cycles are frequent enough, then the other components must be rated on a more continuous basis.

XII. ASD Synchronous Starting of Multiple Motors When multiple large motors at a location must be started, it is possible to use the straight-forward approach of using one ASD for each motor. However, using combinations of ASDs, controls and switchgear can allow one drive to selectively start and synchronize multiple motors. The obvious advantage is sharing the cost of the ASD across multiple motors, but other advantages also present themselves. These motors do not have to be of the same power rating, as long as the ASD is selected to support the motor and load with the highest load demand.

Figure 15 shows a typical two motor VSI ASD, very similar to the single motor diagram of Figure 9. In Figure 15 a second set of bypass contactors and output contactors is included to select which motor will be connected to the ASD and then, if desired, to connect to the utility. The motors do not have to be of the same power rating. Either motor can be brought online & synchronized and the ASD switched to the other for either synchronizing or operation at variable speed. All the equipment selection criteria in the previous section apply. The “System Sequencer” is either a PLC, relay logic, a portion of drive control logic or a combination of these. In practice, with so many possible combinations of possible connections and operations, PLC sequencing is used for systems of 3 or more motors. The system of Figure 14 can be extended to as many motors as practical. Table 3 later in this paper shows the relative costs of adding multiple motors to an ASD synchronized start system.

Figure 15 Single drive two motor VSI ASD synchronized start system

Figure 16 below shows an arrangement with two ASDs to control any of 4 motors. In this arrangement, either drive can start and control any motor. Normal operation would have drive A controlling Motors 1 and 2, while drive B would control Motors 3 and 4. The tie contactor is normally open. If either drive is out of service for any reason, the tie contactor can be closed and the out-of-service drive disconnected. The remaining drive can then bring any of the 4 motors up to speed and synchronized it to the utility. If sized to do so, this drive can then control the speed of one of the 4 motors continuously.

Figure 16 Dual drive 4 motor VSI VSD synchronized start system with flexible ASD backup operation

Tradeoffs Complexity, flexibility and costs tradeoffs are done to select the best system arrangement for a particular application is selected. • Complexity – Switchgear and control, feedbacks and PLC increase in complexity and cost as the system control modes and number of motors grows • Flexibility – more modes of operation allow possible higher availability due to 100% backup of critical ASD equipment if more than one drive is used. • Costs – as will be shown in a later section, the ASD system costs are affected by performance and system configuration. XIII. Example System Cement raw material supply at the quarry is usually both a batch process and a continuous one. As such, trucks carry material to a hopper, an apron feeder conveyor operates at variable speed to feed the primary crusher that prepares rough-sizes the rock. Another fixed-speed conveyor takes the material to the plant. At the plant, raw mills further reduce the material size. In one such an actual application, the apron feeder to the crusher is powered by a 2300 volt motor M1 as shown in Figure 17 below [derived from Figure 15 above] and operates at variable speed. The motor powering the crusher is a very high torque low speed [400 RPM] 2300 volt 700 HP motor [M2 in the diagram below] whose torque and ampere characteristics were listed in Table 2 above. These motors operate on a weak power system, and reducing current inrush at start is very important. The sequence of operation first has the ASD accelerate and synchronize the M2 crusher motor to the utility line. Then the ASD accelerates the apron feeder conveyor motor M1 to a speed that matches the ability of the crusher to process the M31 material without bogging down. The ASD itself must be selected to provide the starting demands of either motor, plus the continuous power demand of the 1200 HP CTO1 M1A VSI Drive CONVEYOR feeder conveyor. M1 M-21 L-1 This means: Converter - Inverter M1 • The entire drive from utility feed V System Drive Control CRUSHER Sequencer to motor input must be rated for PT PT M-22 the 1200 HP continuous power. V L

25

M2

O

•

The output inverter section must be rated for the high amperes for the 200% peak torque that M32 the crusher requires if it must start with material inside it. Figure 17 Single drive two motor VSI VSD synchronized crusher After study of these two and conveyor start system requirements, an ASD frame was selected that met both needs reliably. The system selection process allowed the best combination of equipment and sequencing. It is also interesting to note that the VSI drive in Figure 17 replaced an existing older [current source] technology [installed around 1991] that performed the same synchronized starting on the same motors. The site’s switchgear was re-used and a customized interface created to work with the ASD and system sequencer of the new drive. CTO2

XIV. ASD Synchronized Starting Configuration Economic Comparisons The previous discussions show how ASD synchronized start systems are configured, applied and some arrangements for using drives flexibly in starting or controlling multiple motors. Actual system costs are directly tied to technology, manufacturer, and many site-specific factors. However, it can be useful to see the economic impact of combining ASDs, switchgear and controls on a relative basis. Table 3 below shows such a comparison for a target system: •

4160 volts input and output

• •

VSI MV-IGBT PWM technology with integral isolation transformer and incoming switch 3000 HP, 4160 volt, 1200 RPM motor

• •

Variable torque driven load as in Figure 13 The choice of either full power ASD [point C of Figure 13] capability or a reduced starting duty ASD rating to match a load that is mechanically [valve or damper] limited to 50% of motor torque [and kw] at max RPM [point B of Figure 13].

Single ASD Synchronized Start System Shared Between Multiple Motors Number of Motors

Continuous ASD No Sync Start

COLUMN A

COLUMN B

Synchronized Start ASD Rated for One motor at a time at Full 3000 HP

Synchronized Start Torque Mechanically Limited to 50% of 3000 HP

1

100%

132% 154% 171% 189%

86% 102% 113% 125%

2 3 4

Table 3 Target ASD relative costs comparison versus number of drives, motors and load conditions

Here are some key observations from Table 3. •

The 100% cost of Table 3 is for a single, fully rated 3000 HP 4160 volt ASD without synchronized start capability.

•

COLUMN A shows that the extra cost to add synchronized one-motor starting to full 3000 HP ASD would be 32%. Refer to Figure 6 for this single motor ASD start system.

•

COLUMN A continues to show the extra cost to add synchronized starting of 2, 3 or 4 motors at full 3000 HP. This ranges from 54% to 89% over the cost of a single, full-time 3000 HP nonsynchronized drive. Refer to Figure 14 for a two motor ASD start system. The Figure 14 system can be extended to more motors by adding more switchgear and sequencing equipment.

•

COLUMN B is similar to COLUMN A except uses a 50% rated drive in a starting duty mode only, with max HP and torque also limited to 50% of 3000 HP. The torque-speed curve of Figure 13 applies. The numbers above could possibly be used to do an initial relative cost analysis for system planning. Note that at ASDs ratings, the output voltage and current levels increase, changing the type of switchgear required and the cost per horsepower [kW] for the ASD. This will change the relative costs for incrementally adding motors as in Table 3

XV. Conclusion and Summary This paper has covered many aspects of controlling large motors during starting and operation. We have reviewed impact of starting large motors on industrial power systems. ASD systems have been successfully applied to bring large motors and their loads to full speed and connect them to the power line. How these systems operate and how their components are applied is crucial knowledge in configuring and selecting synchronized systems. Various aspects of equipment rating and selection were covered. The reduced impact in plant and power system can often offset the costs involved. The relative costs were presented to show the additional economies that might be gained by sharing a single ASD by switching it between multiple motors. References: [1]

TMEIC GE e-news: Soft-Start Multiple Dura-Bilt5i MV® Drives www.tmge.com/upload/library_docs/english/Multi-Motor_App_Article_1142532580.htm

[2]

TMEIC GE MV ASD Systems & Motors School, Sessions 2-A ASD Fundamentals, and 3-A Drive Applic Overview.