INDUCTION MOTOR STARTING METHODS AND ISSUES

Copyright Material IEEEPaper No. PCIC-2005-24

John LarabeeMember IEEE,

iohn.larabee(siemens.com

Brian PellegrinoMember IEEE

brian.oelleqrinoasiemens.com

Benjamin Flick

beniamin.flickCZsiemens.com

Siemens Energy & Automation, Inc.4620 Forest Ave.

Norwood, OH 45212USA

Abstract - Many methods can be used to start large ACinduction motors. Choices such as full voltage, reducedvoltage either by autotransformer or Wye - Delta, a softstarter, or usage of an adjustable speed drive can all havepotential advantages and trade offs. Reduced voltagestarting can lower the starting torque and help preventdamage to the load. Additionally, power factor correctioncapacitors can be used to reduce the current, but caremust be taken to size them properly. Usage of the wrongcapacitors can lead to significant damage. Choosing theproper starting method for a motor will include an analysisof.the power system as well as the starting load to ensurethat the motor is designed to deliver the neededperformance while minimizing its cost. This paper willexamine the most common starting methods and theirrecommended applications.

Index Terms: motor starting. reduced voltage start,autotransformer, wye-delta, power factor correction

I. INTRODUCTION

There are several general methods of starting inductionmotors: full voltage, reduced voltage, wye-delta, and partwinding types. The reduced voltage type can include solidstate starters, adjustable frequency drives, andautotransformers. These, along with the full voltage, oracross the line starting, give the purchaser a large varietyof altematives when it comes to specifying the motor to beused in a given application. Each method has its ownbenefits, as well as performance trade offs. Properselection will involve a thorough investigation of any powersystem constraints, the load to be accelerated and theoverall cost of the equipment.

In order for the load to be accelerated, the motor mustgenerate greater torque than the load requirement. Ingeneral there are three points of interest on the motor'sspeed-torque curve. The first is locked-rotor torque (LRT)which is the minimum torque which the motor will developat rest for all angular positions of the rotor. The second ispull-up torque (PUT) which is defined as the minimumtorque developed by the motor during the period ofacceleration from rest to the speed at which breakdowntorque occurs. The last is the breakdown torque (BDT)which is defined as the maximum torque which the motorwill develop. If any of these points are below the requiredload curve, then the motor will not start. See Fig. 1.

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The time it takes for the motor to accelerate the load isdependent on the inertia of the load and the marginbetween the torque of the motor and the load curve,sometimes called accelerating torque. In general, thelonger the time it takes for the motor to accelerate theload, the more heat that will be generated in the rotor bars,shorting ring and the stator winding. This heat leads toadditional stresses in these parts and can have an impacton motor life.

PERCENT SYNCHRONOUS SPEED

Fig. 1 - Typical Speed Torque and Current Curves with50% Centrifugal Type Load Curve

II. FULL VOLTAGE

The full voltage starting method, also known as across theline starting, is the easiest method to employ, has thelowest equipment costs, and is the most reliable. Thismethod utilizes a control to close a contactor and apply fullline voltage to the motor terminals. This method will allowthe motor to generate its highest starting torque andprovide the shortest acceleration times.

This method also puts the highest strain on the powersystem due to the high starting currents that can betypically six to seven times the normal full load current ofthe motor. If the motor is on a weak power system, thesudden high power draw can cause a temporary voltagedrop, not only at the motor terminals, but the entire powerbus feeding the starting motor. This voltage drop willcause a drop in the starting torque of the motor, and adrop in the torque of any other motor running on the powerbus. The torque developed by an induction motor variesroughly as the square of the applied voltage. Therefore,

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depending on the amount of voltage drop, motors runningon this weak power bus could stall. In addition, manycontrol systems monitor under voltage conditions, asecond potential problem that could take a running motoroffline during a full voltage start. Besides electricalvariation of the power bus, a potential physicaldisadvantage of an across the line starting is the suddenloading seen by the driven equipment. This shock loadingdue to transient torques which can exceed 600% of thelocked rotor torque can increase the wear on theequipment, or even cause a catastrophic failure if the loadcan not handle the torques generated by the motor duringstaring.

A. Capacitors and Starting

Induction motors typically have very low power factorduring starting and as a result have very large reactivepower draw. See Fig. 2. This effect on the system can bereduced by adding capacitors to the motor during starting .

The large reactive currents required by the motor lag theapplied voltage by 90 electrical degrees. This reactivepower doesn't create any measurable output, but is ratherthe energy required for the motor to function. The productof the applied system voltage and this reactve powercomponent can be measured in VARS (volt-amperereactive). The capacitors act to supply a current that leadsthe applied voltage by 90 electrical degrees. The leadingcurrents supplied by the capacitors cancel the laggingcurrent demanded by the motor, reducing the amount ofreactive power required to be drawn from the powersystem.

To avoid over voltage and motor damage, great careshould be used to make sure that the capacitors areremoved as the motor reaches rated speed, or in theevent of a loss of power so that the motor will not go into agenerator mode with the magnetizing currents providedfrom the capacitors. This will be expanded on in the nextsection and in the appendix.

CALCULATED PF vs. PER UNIT SPEED

0.

-x

SPEED (pu)

Fig 2 Typical Power Factor vs. Speed for 2 Pole Motor

B. Power Factor Correction

Capacitors can also be left permanently connected toraise the full load power factor. When used in this mannerthey are called power factor correction capacitors. Thecapacitors should never be sized larger than the

magnetizing current of the motor unless they can bedisconnected from the motor in the event of a power loss.

The addition of capacitors will change the effective opencircuit time constant of the motor. The time constantindicates the time required for remaining voltage in themotor to decay to 36.8% of rated voltage after the loss ofpower. This is typically one to three seconds withoutcapacitors.

With capacitors connected to the leads of the motor, thecapacitors can continue to supply magnetizing currentafter the power to the motor has been disconnected. Thisis indicated by a longer time constant for the system. Ifthe motor is driving a high inertia load, the motor canchange over to generator action with the magnetizingcurrent from the capacitors and the shaft driven by theload. This can result in the voltage at the motor terminalsactually rising to nearly 50% of rated voltage in somecases. If the power is reconnected before this voltagedecays severe transients can be created which can causesignificant switching currents and torques that canseverely damage the motor and the driven equipment. Anexample of this phenomenon is outlined in the appendix.

Ill. REDUCED VOLTAGE

Each of the reduced voltage methods are intended toreduce the impact of motor starting current on the powersystem by controlling the voltage that the motor sees atthe terminals. It is very important to know thecharacteristics of the load to be started when consideringany form of reduced voltage starting. The motormanufacturer will need to have the speed torque curveand the inertia of the driven equipment when they validatetheir design. The curve can be built from an initial, orbreak away torque, as few as four other data pointsthrough the speed range, and the full speed torque for thestarting condition. A centrifugal or square curve can beassumed in many cases, but there are some applicationswhere this would be problematic. An example would bescrew compressors which have a much higher torquerequirement at lower speeds than the more commoncentrifugal or fan load. See Fig. 3.

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PERCENT SYNCHRONOUS SPEED

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Fig. 3 - Sample Speed Torque and Current Curve withScrew Compressor type load curve

By understanding the details of the load to be started themanufacturer can make sure that the motor will be able to

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generate sufficient torque to start the load, with thestarting method that is chosen.

A. Autotransformer

The motor leads are connected to the lower voltage sideof the transformer. The most common taps that are usedare 80%, 65%, and 50%. At 50% voltage the current onthe primary is 25% of the full voltage locked rotor amps.The motor is started with this reduced voltage, and thenafter a pre-set condition is reached the connection isswitched to line voltage. This condition could be a presettime, current level, bus volts, or motor speed. The changeover can be done in either a closed circuit transition, or anopen circuit transition method. In the open circuit methodthe connection to the voltage is severed as it is changedfrom the reduced voltage to the line level. Care should beused to make sure that there will not be problems fromtransients due to the switching. This potential problemcan be eliminated by using the closed circuit transition.With the closed circuit method there is a continuousvoltage applied to the motor. Another benefit with theautotransformer starting is in possible lower vibration andnoise levels during starting.

Since the torque generated by the motor will vary as thesquare of the applied voltage, great care should be takento make sure that there will be sufficient acceleratingtorque available from the motor. A speed torque curve forthe driven equipment along with the inertia should be usedto verify the design of the motor. A good rule of thumb isto have a minimum of 10% of the rated full load torque ofthe motor as a margin at all points of the curve.

Additionally, the acceleration time should be evaluated tomake sure that the motor has sufficient thermal capacity tohandle the heat generated due to the longer accelerationtime.

B. Solid State or Soft Starters

These devices utilize silicon controlled rectifiers or SCRs.By controlling the firing angle of the SCR the voltage thatthe device produces can be controlled during the startingof the motor by limiting the flow of power for only part ofthe duration of the sine wave. See Fig 4.

The most widely used type of soft starter is the currentlimiting type. A current limit of 175% to 500% of full loadcurrent is programmed in to the device. It then will rampup the voltage applied to the motor until it reaches the limitvalue, and will then hold that current as the motoraccelerates.

Tachometers can be used with solid state starters tocontrol acceleration time. Voltage output is adjusted asrequired by the starter controller to provide a constant rateof acceleration.

The same precautions in regards to starting torque shouldbe followed for the soft starters as with the other reducedvoltage starting methods. Another problem due to thefiring angle of the SCR is that the motor could experienceharmonic oscillating torques. Depending on the driven

equipment, this could lead to exciting the naturalfrequency of the system.

C. Adjustable Frequency Drives

This type of device gives the greatest overall control andflexibility in starting induction motors giving the mosttorque for an amount of current. It is also the most costly.

The drive varies not only the voltage level, but also thefrequency, to allow the motor to operate on a constant voltper hertz level. This allows the motor to generate full loadtorque throughout a large speed range, up to 10:1. Duringstarting, 150% of rated current is typical.

This allows a significant reduction in the power required tostart a load and reduces the heat generated in the motor,all of which add up to greater efficiency. Usage of theAFD also can allow a smaller motor to be applied due tothe significant increase of torque available lower in thespeed range. The motor should still be sized larger than

40

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Wave shape of 90° firing angle

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Fig 4 - Wave shapes for 00 and 900 firing angles.

the required horsepower of the load to be driven. TheAFD allows a great degree of control in the acceleration ofthe load that is not as readily available with the other typesof reduced voltage starting methods.

The greatest drawback of the AFD is in the cost relative tothe other methods. Drives are the most costly to employand may also require specific motor designs to be used.Based on the output signal of the drive, filtered or

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unfiltered, the motor could require additional constructionfeatures.

These construction features include insulated bearings,shaft grounding brushes, and insulated couplings due topotential shaft current from common mode voltage.Without these features, shaft currents, which circulatethrough the shaft to the bearing, through the motor frameand back, create arcing in the bearings that lead topremature bearng failure, this potential for arcing needs tobe considered when applying a motor/drive package in ahazardous envikronment, Division2/Zone2.

An additional construction feature of a motor used on anAFD may require is an upgraded insulation system on themotor windings. An unfiltered output signal from a drivecan create harmonic voltage spikes in the motor, stressingthe insulation of the motor windings.

It is important to note that the features described pertain tomotors which will be started and run on an AFD. If thedrive is only used for starting the motor, these featuresmay not be necessary. Consult with the motormanufacturer for application specific requirements.

D. Primary Resistor or Reactor Starting

This method uses either a series resistor or reactor bankto be placed in the circuit with the motor. Resistor startingis more frequently used for smaller motors.

When the motor is started, the resistor bank limits the flowof inrush current and provides for a voltage drop at themotor terminals. The resistors can be selected to providevoltage reductions up to 50%. As the motor comes up tospeed, it develops a counter EMF (electro-magnetic field)that opposes the voltage applied to the motor. This furtherlimits the inrush currents. As the inrush currentdiminishes, so does t>e voltage drop across the resistorbank allowing the torque generated by the motor toincrease. At a predetermined time a device will shortacross the resistors and open the starting contactoreffectively removing the resistor bank from the circuit.This provides for a closed transition and eliminates theconcerns due to switching transients.

Reactors will tend to oppose any sudden changes incurrent and therefore act to limit the current duringstarting. They will remain shorted after starting andprovide a closed transition to line voltage.

IV. INCREMENT TYPE

The first starting types that we have discussed have dealwith the way the energy is applied to the motor. The nexttype deals with different ways the motor can be physicallychanged to deal with starting issues.

A. Part Winding

With this method the stator of the motor is designed insuch a way that it is made up of two separate windings.The most common method is known as the half windingmethod. As the name suggests, the stator is made up oftwo identical balanced windings. A special starter isconfigured so that full voltage can be applied to one half ofthe winding, and then after a short delay, to the secondhalf. This method can reduce the starting current by 50 to60%, but also the starting torque. One drawback to thismethod is that the motor heating on the first step of theoperation is greater than that normally encountered onacross-the-line start. Therefore the elapsed time on thefirst step of the part winding start should be minimized.This method also increases the magnetic noise of themotor during the first step.

B. Wye Delta Starting

Induction motors that are set up for this type of starting arebuilt so that the leads from each end of each phase groupare brought out to the motor terminal box. This allows themotor starter to initially connect the leads in a wyeconnection for starting, and then reconnect them in thedelta configuration for running. By initially starting themotor in the wye connection, it reduces the voltage by a

factor of I / J This reduces the starting current and thestarting torque by roughly 2/3. Depending on theapplication the motor can be switched to the deltaconnection from about 50% speed to full speed. It mustbe noted that the same issues apply that were coveredearlier in regards to the switching method. If the opencircuit method is used, transients may be a problem.

This method is usually employed on motors rated below600V. Motors rated 2.3kV and higher are usually notsuitable for Wye-Delta starting

The following table summarizes the various starting methods and characteristics

Adjustable PrimaryFull Auto- Solid State Frequency Resistor I

Voltage transformer Starters Drives Reactor Part Winding Wye Delta

Initial System LOW MODERATE HIGHER HIGHEST MODERATE LOW LOWCost

Starting HIGHEST LOW MODERATE LOWEST MODERATE MODERATE LOWCurrent

Starting HIGHEST LOW LOW HIGHEST LOW LOW LOWTorque _

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V. CONCLUSION

There are many ways start an induction motor. Theselection of the best method to use will be based on theoverall power system constraints, cost for the equipment,and the driven equipment. While the full voltage method is

the easiest and least expensive from the point of theequipment, it may invoke cost penalties from the utility, orthe power system at the location may not be able tosupport the required level of energy draw. The reduced

voltage methods provide a solid means of easing herequired energy draw, but do so at the expense of motorgenerated torque during starting. These methods mayalso lead to having to increase the size of the motor inorder to generate the torque required for the load. TheAdjustable frequency drive will remove both of theseobstacles, but requires a price premium for the equipment.Understanding the utility limitations, as well as the startingtorque and speed requirements of the driven equipmentwill allow you to determine the best overall configurationfor your application.

VI. REFERENCES

[1] "Methods for the Control of Large Medium-VoltageMotors: Application Considerations and Guidelines"Kay, J.A., Paes, R.H., Seggewiss, J.G., Ellis, R.G.,IEEE Transactions On Industry Applications, Vol. 36,No. 6, November/December 2000.

[2] "Starting High Inertia Loads" McElveen, R.F., Toney,M.K., IEEE Transactions on Industry Applications, Vol.37, No. 1, January/February 2001.

[3] "Applying Capacitors at Motor Terminals", Moore, R.C.,Schwartzburg, W.E., Motor Application Reference, AllisChalmers Electrical Review, 1967.

[4] "Compensating for Low Motor Power Factor" Ader, E,Finley, W.R., Plant Engineering, June 17, 1993.

[5] "Starting of Large Medium Voltage Motors, DesignProtection and Safety Aspects", Bredthauer, J., Struck,N., IEEE PCIC-94-17 1994.

[6] NEMA MG1-2003

VIl. VITA

John A. Larabee received his BS Degree in ElectricalEngineering from Florida Intemational University, Miami,FL. Currently, he is Manager of Product Engineering forSiemens Energy & Automation, Inc., Cincinnati, OH. Hehas a background within Siemens of design engineering,process engineering, and information technology.

Benjamin D. Flick received his BS degree in ElectricalEngineering form the University of Cincinnati, Cincinnati,Ohio. Currently he is Supervisor, Electrical Engineeringfor Siemens Energy & Automation, Inc., Cincinnati, Ohio,and is currently active in NEMA working groups.

Brian Pellegrino received his BS Degree in ElectricalEngineering from the University of Cincinnati, Cincinnati,Ohio. He is currently a Senior Product Engineer forSiemens Energy & Automation, Inc. Cincinnati, OH.

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Appendix: Capacitor Calculations for a 700HP 4160Vmotor. Full Load Efficiency 94% Full Load PowerFactor 90%, No load current at full voltage 21.5A

kW = 700 * 746/1000 = 522.2kW

kVA = 522.2/0.90 = 580.2

kVAR = 58Q.2sin(25.84) = 252.9

This allows us to construct the power triangle for thismotor.

522.2 kW

252.9 kVAR580.2

Where E = cos-1 (0.90) = 25.84deg

To find the magnetizing kVAR use the no load current atfull voltage 21.5A

V *I*I 4160*21.5* 1 55kYkVAR= = l55kVAR1000 1000

The capacitance is given by:1000*kVAR~ =23.754

The capacitive reactance is given by:

xc = 106 =111.70hms

By assuming different values for the voltage we canconstruct a line labeled 155kVAR in Fig A.

In order to see the effect of 175kVAR on the system wecan perform the calculations again with a capacitivereactance value of 98.5 Ohms. This line is labeled175kVAR in Fig A. This indicates that the terminalvoltage will raise to approximately 119% of normal whenthe power is removed from the motor and no reduction inmotor speed with 175kVAR of capacitance applied to themotor terminals. The line 270kVAR is based oncalculations where the power factor is corrected to 100%In this case the voltage can rise to 145% of the rated valuewith no decrease in motor speed. If the power circuit isreclosed before this voltage has had an opportunity todecay severe switching currents and torques can result.

Fig. A

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No Load Saturation Curve

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