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AC Drives and Soft StarterApplication Guideby Walter J Lukitsch PE, Gary Woltersdorf

Jeff Theisen, and John StreicherAllen-Bradley Company

Abstract:Tere are usually several choices or starting mo-

tors. wo o these, ac variable requency drives (VFDs) andsot starters, seem to have similar characteristics. erms and

descriptions used in product literature are nearly the same.

Even the list o possible applications is similar. However,

the technology and perormance are signicantly dierent.

When these dierences are understood, it becomes clear

when and where to properly apply each o them.

IntroductionTe objective o this paper is to provide the basic technical

inormation to understand the dierences. First covered arethe operating principles o the VFD and sot starter. Howmotor perormance is aected is the other key to selectiono the proper starting method. Finally, guidelines will thenbe presented.

Variable Speed DrivesTe VFD works on the principle that the ac line voltage

is converted to a dc voltage. Tis dc voltage is then invertedback to a pulsed dc whose rms value simulates an ac voltage.Te output requency o this ac voltage normally varies or0 up to the ac input line requency. On certain applicationsthe requency may actually go above the line requency.Tough high perormance current regulated ac drives ca-pable o operating in torque mode are available, the moreprevalent volts per hertz drive is addressed here.

Te most common VFDs manuactured today work usingpulse width modulation to create the output sine wave. Teconducting components used in drives are diodes, SCRs,transistors and IGBs. Tese inverters have three distinctand dierent sections to their power circuits as shown inthe typical inverter block diagram gure 1 below.

Te rst section uses a diode or SCR ull-wave bridgeto convert the ac line voltage to dc. Filtering o this dc isdone in the second section with a capacitor to supply theinverter bridge with a stable dc power source. A dc linkchoke is normally present on 10 horsepower and largerdrives. Te nal section uses a transistor or IGB bridge todeliver a pulse width modulated (PWM) dc voltage to themotor. Te eective rms voltage delivered to the motor isdependent on the undamental output requency that theinverter bridge is commanding.Tis is what leads to theterm volts per hertz drive.

Te control or logic section o the inverter and userprogrammed settings determine the requency output othe inverter. During acceleration, the requency will vary

according to a predetermined algorithm such as linear rampor s-curve, rom minimum or 0 Hz up to commanded speed.Te drive can also be programmed to skip over certain re-quencies that may cause a mechanical resonance.

Figure 1 Typical Inverter Block Diagram

Soft StartersTe sot starter operates on a dierent premise. Tis prin-

ciple is that by adjusting the voltage applied to the motorduring starting, the current and torque characteristics canbe limited and controlled.

For induction motors, the starting torque (LR) is ap-proximately proportional to the square o the starting cur-rent (LRA) drawn rom the line. LR I

2. Tis starting


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Figure 7 Voltage Output o Individual SCRSot Starter Voltage Wave Form

Figure 8 Sot Start Starting Current Wave

Te inverter output can be any requency below or abovethe line requency up to the limits o the inverter and/orthe mechanical limits o the motor. Note that the drive isalways operating within the motor slip rating

Operation of Soft Startersiming o when to turn on the SCRs is the key to

controlling the voltage output o a sot starter. During thestarting sequence the logic o the sot starter determineswhen to turn on the SCRs. It does not turn on the SCRsat the point that the voltage goes rom negative to positive,but waits or some time ater that. Tis is known as phasingback the SCRs. Te point that the SCRs are turned on isset or programmed by what is called either initial torque,initial current or current limit setting.

Te input voltage to the sot starter is the same as theVFD shown in gure 3. Te result o phasing back theSCRs is a nonsinusoidal reduced voltage at the terminalso the motor which is shown in gures 7. Since the motoris inductive and the current lags the voltage, the SCR staysturned on and conducts until the current goes to zero. Tis

is ater the voltage has gone negative.I compared to the ull voltage waveorm in gure 3, itcan be seen that the peak voltage is the same as the ullvoltage wave. However the current does not increase to thesame level as when ull voltage is applied due to the induc-tive nature o motors.

When this voltage is applied to a motor, the output cur-rent looks like gure 8.As the requency o the voltage isthe same as the line requency, the requency o the currentis also the same. As the SCRs are phased on to ull conduc-tion, the gaps in current ll in until the wave orm looks thesame as applying the motor directly across the line.

Motor Characteristics Using VFDsDuring acceleration, the inverter applies dierent re-

quencies to the motor. It also changes the voltage but indirect proportion to the requency. Tis is know as constantvolts per hertz and provides constant torque while the mo-tor accelerates.

A series o speed torque curves is shown in gure 9.Tese relate to speed torque curves at various requencies.Te constant torque line represents the ull load or ratedtorque o the motor.

Tis constant torque line is actually the ull load pointon a locus o curves representing the speed torque curves othe motor rom 0 to ull speed. Te inverter produces ratedmotor torque rom 0 to rated speed. It will produce ull

load torque while drawing much less than ull load currentrom the power line during starting. Tis is due to the actthat the motor is eectively always running at speed or theapplied requency.

When ull voltage starting, the slip o the motor at 0speed is 100 percent and the motor is highly inductive. Tisresults is the very high inrush current, 600800 percent, andrelatively low starting torque, 150180 percent o ull loadtorque, compared to the current draw. Almost all o themotor current here is reactive. Reactive current, by nature,does not produce torque.

When a motor runs at speed the slip is typically in thearea o one to three percent. Under this condition the reac-tive current is much less and the motor produces rated torqueat rated current. With a VFD the motor runs virtually atspeed during acceleration. Since the voltage is reduced atlow speeds, the input current can be 10 percent or less withmore than 150 percent torque.


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Figure 9 Inverter Speed Torque Curves

Figure 10 Sot Start Speed Torque Curves

Since the motor always runs at speed, or within ratedslip, the acceleration time is dependent on the ramp timesetting. Tis assumes that the drive has been properly se-lected or the load.

Motor Characteristics Using Soft Starters

Unlike the ac drive, the line current and motor currentor a sot starter is always the same. During starting thecurrent varies directly with the magnitude o the appliedvoltage. Te motor torque varies as the square o either theapplied voltage or current.

Te most critical actor when evaluating a sot starter isthe motor torque. Standard motors produce approximately180 percent o the ull load torque at starting. Tereore,a 25 percent reduction in voltage or current will result inthe locked rotor torque equal to the ull load torque(180%*(.75)

2= 101%). I the motor draws 600 percent o

the ull load current on starting, then the current in thisexample will reduce the normal 600 percent starting currentto 450 percent o the ull load current.

able 1 below gives more examples o the eects o reduc-ing the voltage or current on a motors locked rotor torque.Tis data is valid or sot start and series impedance starting.Tey do not apply to other types o reduced voltage startingsuch as autotransormer and wye-delta starting.

Table 1 Locked Rotor Torque Vs LockedRotor Amperes for Soft Starters

percent Current

or Voltage

percent Full Load

Current

percent Full Load

Torque100 600 180

90 540 146

80 480 115

75 450 101

70 420 88

60 360 65

50 300 45

40 240 29

When applying sot starters, the same constraint aselectromechanical reduced starters applies. Tat constrainis will the motor be able to produce enough torque to getthe load started with the current the sot starter is allowingto ow to the motor?

Sot starters do have an advantage over conventionareduced voltage starting. Tey are able to adjust voltagecurrent, and, thereore, torque over a wide range instead osingle or a ew xed values. Tis can be seen in Figure 10

When voltage or current is held to a constant value, thespeed-torque curve labeled Current Limit is producedTis curve would move up or down depending on the current limit setting. Te upper boundary o this adjustmenis the Full Voltage curve.

Te sot starter can also ramp the voltage rom an adjustable initial value up to ull voltage over an adjustable timerame. Tis is represented by the Sot Start curve. A step-less transition, which is designed to eliminate current/torquetransients, is produced by this ramp.

Te operating speed o the motor cannot be varied be-cause the sot starter only adjusts the voltage to the motorand not the requency. Te requency applied to the motois always the line requency. Because o this, the accelerationtime is more dependent on the load than the ramp time.

Application DifferencesWith the knowledge o VFD and sot starter principle

o operation and motor perormance with each, applicationdierences can be reviewed. With the list o applicationbeing very similar, the general application parameters wil

be covered along with several application examples.Motor speed is a parameter where a VFD has an ad-vantage over sot starters. First, and most obvious, is wherethe speed o the motor needs to be varied rom 0 to linerequency and sometimes higher than line requency. Tesot starter applies line voltage and requency; thereore, theoperating speed is xed.

Te second speed-related advantage to which an inverterelates is processes that require a constant speed. I a xedrequency is applied to a motor, the actual speed o thatmotor is not precisely regulated by the input requency. Teoutput speed is actually regulated by the load applied to the


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motor. So i a process requires very tight speed regulation,the requency applied to the motor must be changed inrelation to the load that is applied. With the use o eed-back to the VFD this can be accomplished. Again the sotstarter only applies line requency so any speed regulationis not possible.

On applications where acceleration time needs to beconsistent, an inverter should be used. Tis is due to the actthat acceleration time or a sot starter is more dependent

on the load than the selected ramp time. I accelerationtime is not an issue and controllingthe torque or current iswhat is needed, then a sot starter is a good candidate orthe application. (Note: some sot starters use eedback, suchas tachometers. Tese units can provide timed accelerationwith varying loads. It should be noted that current duringeedback acceleration could reach the same level as startingat ull voltage 600 800 percent o ull load).

With regard to stopping, a VFD will bring the motor toa rest in a specied time. Tis may be built into an inverteror may require a dynamic braking optional unction or highinertia and overhaulingtype loads. Te sot starter with asot stop eature can only extend the stopping time, andjust like acceleration, the stopping time is dependent on theload. I stopping time and stopping characteristics are notcritical then a sot stop may t the application.

Some specially designed sot starters can also providebraking.Tese are designed to reduce stopping time wherecoast to rest is very long. I the load is not a pure inertia andcan vary, the stopping time will also vary.

Where limiting current is the prime reason or not start-ing at ull voltage, the rst method to be considered todayis usually sot starters. Tis is due to the cost dierentialbetween a sot starter and a VFD at the ampere ratings thatcurrent limiting becomes a actor. In most instances the sot

starter is an appropriate choice.Tere are applications where the additional cost o aninverter is appropriate. Tese cases are where the motorcannot provide sufcient torque to start the load with theampere limitations imposed by the distribution system.able 1 shows the motor torque provided at various levelso sot starter current limit. Unlike sot starters, drives canaccelerate a motor to ull speed at ull load torque withline current that does not exceed the ull load amperes othe motor. Keep in mind that the power into the VFD isequal to the power out plus the losses. Tereore, or thoseloads that require higher torque than the sot starter canprovide with the limits imposed by the distribution system,

an inverter may be the required solution.I starting torque is a concern when selecting a drive or

starter, keep in mind the drastic dierence in the amount otorque that can be developed or a given amount o line cur-rent. Te drive has a much higher torque per ampere ratio.

Sample ApplicationsProvided here are our sample applications. wo will be

or pumps, and two will be or conveyors. Tese examplesdo not require variable speed or precise speed regulation,so a VFD or sot starter could be used.

Application 1) A pump is being started on ull voltage.Tere is signicant water hammer and the pipe bracingneeds constant maintenance.

Answer: A sot starter will t the application. It providescontrolled torque during acceleration and has been shown tominimize and in many cases eliminate water hammer. Tereis no concern about current limitations as the application isnow being started on ull voltage.

Application 2)A new irrigation pump is being installedin a rural location. Because o this, the maximum currentdraw rom the utility line without signicant voltage drophas been calculated as 200 percent o the motor nameplatereading.

Answer: An inverter is preerred over a sot starter. In someinstances sot starters can accelerate pumps with as littleas 200 percent current. Application experience indicatesthat more oten 250 300 percent current is required. TeVFD can provide the torque required to accelerate thepump within the current limit restrictions o the distribu-tion system.

Application 3) An overland conveyor requires 100 percenttorque to accelerate when starting ully loaded. Te maxi-mum current draw rom the utility is limited to 500 percento the motor ull load amperes. Te conveyor will normallybe started unloaded; however, on occasion it may need tobe started when it is loaded. Rate o acceleration is criticalto prevent the conveyor belt rom being damaged

Answer:Initially a sot starter seems to be the correct choice.Te sot starter can provide 101 percent torque with 450percent current (table 1). However the rate o acceleration,which equates to starting time is critical. Te load also variesrom unloaded to ully loaded. In this case a VFD wouldbe the correct solution.

Application 4) A 20 horsepower motor drives an overheadplastic chain conveyor through a gearbox. It starts and stopsrequently. Full voltage starting could be used, but i theconveyor starts too quickly the product will swing and maybe damaged or the chain may break.

Answer: A sot starter would t the application. Tere is notime constraint and no limitation on current. Ramp startwould typically be used to allow or minor load variationsreected back to the motor. I the gear reduction is highenough, a current limit start could provide a smoother

start.

ConclusionTese examples were designed to show how slight ap-

plication variations can change the type o motor startingthat is required. Each application must be evaluated on itsown merits. Neither sot starters nor VFDs are the perectsolution or all situations.

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