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STARTING HIGH INERTIA LOADSCopyright Material IEEE

Paper No . PCIC-97-27

Robbie McElveen

Member, IEEEReliance Electric

Rockwell Automation

101Reliance RoadKings Mountain, NC 28086

A b s t r a c t : Common methods used to start AC inductionmotors driving high inertia loads include across-the-linestarting and reduced voltage starting by auto-transformer,

wye-delta, or resistor/reactor. Application of thesemethods is generally well documented and understood.

However, with the increased use of electronic soft starters

and variable frequency drives, applications can bemarkedly different. This paper reviews the

aforementioned methods and explains the benefits andlimitations of each. The effect of high inertia loads onboth acceleration time and motor heating is examined. Ac6se history of starting a centrifuge with an electronic soft

starter is studied. Finally, a comparison of all of themethods is presented with recommendations on choosing

the proper starter given.

Key woids: inertia, motors, soft-start, starting, variablefrequency drives

Entr o d u c t o n

Many methods are used to reduce the current draw during

startup of high inertia applications such as centrifuges,hammermills, or large fans. Reduced current conditionsare desired not only to lessen the burden on the electricalsystem and avoid power company penalties, but also to

decrease the strain on both the motor and the connectedmechanical system. This reduction in starting current,

however, leads to a corresponding reduction in the

starting torque available from the motor. For the purposesof this paper, starting torque is considered to be theaverage torque produced by the motor which is availableto accelerate the load. This reduction in torque leads tolonger acceleration times and the potential for increasedheating during startup. The goal of this paper is to

evaluate the "conventional" methods of starting, to explainhow soft starters and variable frequency drive (VFD)

starting works, and to compare each of the methods fortemperature rise, acceleration time, and economicalconsiderations.

First, a brief overview of starting methods (including soft-

start and VF D starting) is presented. Next, the effects of

inertia on both acceleration time and motor heating arediscussed. Comparisons for starting time, temperature

Mike ToneySenior Member, IEE EAmoco Corporation

3700 Bay Area Blvd.

Houston, TX 77058

rise, and other factors are made. An actual case historyof a soft-started centrifuge is presented, including theproblems and solutions associated with this application.

Following this case history, the calculation methods used

for both the soft-start and VFD starting are explained. Aspecial emphasis is placed on the motor losses andheating for each of these starting methods. Finally,

factors which should be considered when choosing a

starter are discussed and recommendations given.

M e t h o d s of St a r t i ng I nduc t i on M o t o r s

Methods of starting AC induction motors can be brokendown into four basic categories: Full Voltage (across-the-line) starting, electro-mechanical reduced voltage starting,

solid-state reduced voltage starting, and variablefrequency drive starting. Electro-mechanical reduced

voltage starting has been in existence nearly as long asthe induction motor itself. This starting method

encompasses auto-transformer starting, wye-delta (star-delta) starting, and resistorkeactor starting. Each of thesemethods requires the use of some type of mechanical

switch or contact. Electromechanical starting is the mostcommon method of reduced voltage starting used inindustry today.

Solid-state starters, on the other hand, have only been inexistence since the early 1970s. This method of starting

uses programmable logic controllers in combination withsophisticated power electronic circuits to provide reducedvoltage and/or torque. Advances made in the electronicsindustry with new high-power diodes and S C R s have ledto the development of both electronic soft starters as wellas inverter controlled variable frequency drives. In eachof these cases, smooth, electronically controlled startscan be achieved with a high degree of process control.

Fu/l Vo l taue (Across- the -L ine ) S ta r t ing : Of the manymethods used to start induction motors, full voltage (oracross-the-line) starting is typically used unless there is

either an electrical or mechanical constraint which makesthis option unsuitable. With this method, full voltage is

applied to the motor at the instant the "switch" is thrown.This method of starting results in a large initial currentsurge, known as inrush, which is typically 600% to 700%

ISBN: 0-7803-4217-8 97-CH36128-6/97/0000-0257 $10.00 8 1997 IEEE- 57 -



RMS of the full load current drawn by the motor. In reality,

the first half-cycle current is considerably higher inmagnitude, but is short in duration. This large inrush can

cause problems for the connected electrical system.Power companies may apply restrictions as to how much

current draw is allowed. These restrictions are typicallyspecified as the maximum allowable voltage drop at theincoming power connection point or the maximum

allowable kVA which may be drawn by the plant. Theserestrictions may limit when and how many times aparticular motor can be started. By limiting the inrush, the

corresponding voltage drop will be reduced. Another

problem can be with in-plant buss capacity where thegiven system simply cannot handle this large current

draw. Brownout or other associated problems may beexperienced if the voltage dips too much. Furthermore,this inrush current induces large magnetic forces in thestator windings which actually try to force the windings to

move and distort. This transitory force can eventually

lead to deterioration of the insulation between thewindings, especially if adequate coil head bracingtechniques are not employed.

Full voltage starting produces the greatest amount ofstarting torque. High starting torque is generally desired

when trying to start a high inertia load in order to limit theacceleration time. However, in certain cases, this high

starting torque may damage the mechanical system.Gears or chains might be broken or damaged. Belt lifemay be reduced by strain or slippage. Gearboxes arealso put under a greater stress and are subject to moreabuse. Voltage drop on the system must be carefullystudied and the breakerlrelays need to be coordinatedwith upstream devices to prevent nuisance tripping ofthese devices during startup. If the voltage drop limitation

for :he system is exceeded, other methods of starting

should be considered. Beyond the initial shock of inrush

current and torque, this type of starting does result in asmooth acceleration characteristic with the shortestacceleration time, which offers an advantage over someof the other available methods of starting.

Electro-Mechanical Reduced Voltaae Startinq:

Another popular method of starting which is used to limitinrush current is reduced voltage starting. With any typeof reduced voltage starting, the theoretical current drawnby the motor decreases linearly with decreasing voltage.(The exception is VFD starting where the frequency

changes as well as the voltage. This is a special topicand is discussed more thoroughly in a later section.)Similarly, the torque is theoretically reduced by the squareof the percent voltage ratio, (i.e., 80% reduced voltageresults in (0.8)' =0.64 or 64% of nominal motor torque).Although the motor torque and current are dependentupon many factors such as saturation, deep bar effect,and skin effect, a good approximation of the current andtorque at any speed and reduced voltage condition maybe found by using the following equations:

Ired =(VredNraled)i * /rated (11

Tred =(redNrated)* * * Trated

where

Ired =current at reduced voltagelraied=rated currentVred=reduced voltage

V =rated voltageTred=reduced torque

Trated=

rated torque

Author 's Note: The proportion constants used as

exponents in the above equations are factors developed

from locked rotor saturation testing and are applicable tothe majority of induction motors.

Electromechanical reduced voltage starting can beachieved in three ways:

1)Auto-transformer2) Wye-Delta3) Primary resistorlreactor

With auto-transformer starting, a tapped transformer is

used to supply reduced voltage to the motor. Typically, asthe motor gains speed, the taps are changed to increase

the voltage to the motor terminals. However, thisswitching of voltage can result in a high spike of current

during this transitory period of operation. The magnitude

of this spike is dependent upon the motor speed andcurrent when the switching occurs. There is a torquetransient associated with this current peak which againmay cause problems for the driven equipment. Note thatthis phenomenon is not present if closed transition startingis used where the circuit is never actually opened duringthe switching operation. One big advantage from thepower system standpoint is that the line current on the

distribution side of the autotransformer is reduced by the

square of the voltage ratio at the power system input. Forthe other methods mentioned in this section, the linecurrent varies directly. However, autotransformer startingis a more costly method than either wye-delta orresistorheactor starting.

The second type of electro-mechanical reduced voltagestarting which is used extensively is wye-start, delta-run.This is particularly true for motor voltages of less than1000 volts. With this type of starting method, a normally

delta-connected stator is connected in wye during the

initial startup phase. It is most common for the motor toreach full speed before the transition to the deltaconnection is made. However, it is possible for theconnection to be switched from wye to delta as the motorapproaches 50-60% of full load speed. This essentiallyapplies full voltage to the motor at this point. Theadvantage of connecting the stator in wye is that only 1/43

times rated voltage is applied to the phase windings. Thisresults in only 1/3 of nominal current draw, but reduces

the starting torque by factor of three as well. When thestator connection is switched from wye to delta, a



transitory current arises which can often be equal to or

greater than the peak current seen with across-the-linestarting. Again, this current and resulting torque transientis present only if open transition is used and is not aproblem for closed transition switching. A drawback ofthis method is that it requires the neutrals of the motor, inaddition to the normal line leads, to be externallyconnected (six leads). Thus, this starter is not an optionfor use with a motor that was originally constructed withonly three leads.

Following are charts illustrating the relative magnitudes oftorque and current for both wye and delta configurations.Both NEMA Design B and Design C type motor curves areshown for comparative purposes. Note the very low

amount of torque available on the wye connection for theDesign B characteristic. This low amount of available

starting torque is an important issue to keep in mind whenthis type of starting method is being considered.

700 zs n1 l l l l l l I l l

. . . . . . . .

0 in i n 311 111 50 60 70 80 90 ~ n n

Percent Synchronous Speed

FIGURE la. Comparisonof torque and amps for wyeand delta connections (NEMA DesignB)

,W T .............. ..........., ... , ....., ....... . , .............. . . .

cable runs to the field have to be installed in order to

perform wye-delta switching and starters must be

interlocked in order to prevent catastrophic failure. Again,there is additional heat generated which must be takeninto account at the starter, the lead cable, and at themotor. This system is relatively simple to operate, butadjusting the starting characteristics is not an option oncethe system is installed. Overload protection needs to bedesigned for both the wye and delta connections.

Primary resistor/reactor starting is achieved by placing aresistance or inductance in series with the motor leads inorder to reduce the inrush current. The torque is againreduced by the voltage ratio as shown in equation (2),

while the line current decreases per equation (1). Again,when the motor is nearly up to speed, the resistor orreactor may be switched out of the circuit, causing

transitory currents with their corresponding torquepulsations. Energy is wasted as heat is dissipated in the

resistor during each startup cycle. Less energy is wastedwhen using a reactor, but the magnitude can still besignificant.

Elect ronic Soft-Startinq:There are two basic categoriesof soft-starting methods: current limit starting and voltageramp starting. Most of today's units offer both options (orvarious combinations) in one starter package so that thestarting characteristic can be optimized to provide a morehealthy start for both the motor and connected equipment.

Voltage ramp starting is the simplest form of soft startingin which a microprocessor is used to control the firing

angle of pairs of SCRs, thus progressively increasing thevoltage supplied to the motor. In order to understand theeffect the firing angle of the SCRs has on the voltagewaveshape and harmonic content, consider the followingexample for a single phase voltage source. Figure 2

below shows the percentage I"(fundamental), 3rd,5'h,and7'h harmonics for a phased back sine wave. If the phasedback angle were go", the percent content of each

harmonic would be as follows:

% Fundamental=100

FIGURE 1 . Comparisono f torque and amps forwyeand delta connections (NEMA Design C)

Wye-Delta starting requires additional contactors in thestarter which drives up the cost of the equipment. Two

YOFundamental =58.5

% 3rd =31.7

O h 5th =10.7Yo 7th =10.5

FIGURE 2. Waveshapes for 0" and 90 " Firing Angles

Please note that the above analysis is only applicable fora resistive load. Due to the fact that the current lags the

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voltage for an inductive load (such as a motor), the

voltage waveshape will be somewhat different. In order to

turn off an SCR, the current through the SC R must pass

through zero. The resultant voltage waveshape will

resemble:

FIGURE 3. Voltage Waveshape for Induc t ive Load(90° Fi r ing Angle)

Harmonics produce pulsating torques whose net total iszero. These harmonics contribute to additional losses(heating) in the motor. Useable motor torque is producedonly by the fundamental component of the voltage. Theusable fundamental component of the voltage can beconsiderably less than the RMS value, depending uponthe complexity of the circuit used to fire the SCRs. This

factor must be considered when designing a motor for use

with this type of starter.

With voltage ramp starting, it is possible for the user to

set the initial voltage which will be applied at time zeroand to specify the ramp time (how long it will take thevoltage to rise from its initial to final value). By starting themotor at reduced voltage, starting current and startingtorque are reduced, thus lessening the stress on both themechanic31 and the electrical system. This method ofstarting results in a smooth acceleration of the load fromzero speed to full load speed. Below is a graph showing

typical voltage and current characteristics for a fifty

second voltage ramp. Notice how the current tends to

ramp up at a rate similar to the voltage. This

characteristic is due to the fact that the current magnitudedoes not decrease dramatically until the motor reachesbreakdown speed. In the graph below, breakdown rpm isreached at approximately 48 seconds.

FIGURE 4. Voltage and Current Character is t ics for aTypical Voltage Ramp Soft-Start

The following graphs (Figures 5 & 6) show acceleration

times and motor heating results for various voltage ramps.

In each graph, the first three bars are the calculated data

for an initial voltage of 2080, a final voltage of 4160, and

various ramp times from 15-60seconds. The full voltagestarting results are also given in column four forcomparison.

2080 V 2080 V 2080 V Full

15 Sec 30Sec 60 See V oltage

Initial Voltage / Ramp Time

FIGURE 5. Accelerat ion Time vs. Voltage Ramp

160.,I I ,

I 40

F I 20-IO0 E

Ig 80

5 60

E 40

20

0

;

2080 v, 2080 V , 2080 V, Full

1 5 Sec 30 Sec 60 S ec Voltage

In it ial Voltage / R a m p T i m e

FIGURE 6. Temperature Rises vs . Voltage Ramp

The second general type of soft starting is current limitstarting. In this case the user can set a pre-defined

maximum current that will be supplied to the motor(usually given in percentage of full load amps). The starter

control circuit will sense the load current or motor backEMF and alter the firing angle of the SCRs in order to

adjust the voltage at every point to whatever value isnecessary in order to maintain the current at the desiredlevel. As mentioned before, this reduced current results in

a torque reduction by an exponent of 2.2. Care must betaken not to set the current limit to a value which willreduce the starting capability of the motor too much. Ifthis happens, excessive motor heating will result, whichmay leadto premature failure.

On the following page is a graph showing typical voltage

and current characteristics for a current limit type start.Notice how data from the feedback circuit is used to

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adjust the voltage in order to maintain the current at a

constant level.implementing any type of reduced voltage starting the

application engineer must be sure not to reduce the motortorque to a point which would result in the motor stalling.In this case, motor damage would be likely to occur.

FIGURE 7. Voltage and Current Characteristics for a

Typical Current Limit Soft-Start

Below are graphs showing acceleration times and motorheating results for various current limits. Figure 8

demonstrates how the acceleration time increases withdecreasing the current limit to the motor. As less currentis supplied to the motor, less starting torque is produced,which results in increased acceleration time.

0 200 400 600 800 IO00

Current L i m i t ( YO ul l L oad Amps)

FIGURE 8 . Acceleration Time vs. Current Limit

Figure 9 in the next column illustrates how both the rotorand stator temperature rises are affected by decreasingthe current limit. Because there is less available torqueunder limited currznt conditions, the tendency is to believethat increased motor heating may result. However, due to

the fact that the current is reduced, both the rotor andstator i2R losses are decreased. In addition to thisdecrease in losses, the acceleration time is extended,

allowing for more of the heat generatedto be dissipated tothe frame and surrounding atmosphere. Thus, thereduced current leads not to increased heating, but ratherto a cooler acceleration for the motor. However, when

140..e 1 3 0 . -

1 2 0 - -

G

r

IVI.-EL 110.-

0LI0 100..K

44

42 i^36 5

h34 z

- 32

! 30

0 200 400 600 800 1000

Curr ent L imit (%FW)

-Rotor

+Stator

FIGURE 9. Rotor and Stator Temperature Rises for

Various Current Limits

Electronic soft starters are typically more expensive thanthe other starting means discussed previously, but mayprovide a lower total cost of ownership over the life of themotor. There are limitations to the distance that the

starters can be installed from the motor and harmonics doexist as described above. However, most soft starters are

bypassed once the motor reaches full speed, so exposure

to the harmonics is limited.

Author‘s Note: The previous graphs are included to

demonstrate typical characteristics of soft-started loads.Individual data will depend upon motor design, startingload curve and load inertia.

Variable F reauencv Drive Startinq: Starting a motorusing a variable frequency drive provides maximum

control over the starting characteristic. Because thefrequency is varied, the motor operates only on the rightside of breakdown on the speed-torque curve. Thus, any

torque value from full load to breakdown can be achieved

across the entire speed range from zero speed to basespeed assuming that the drive has the necessary currentcapability. The load can be accelerated as slowly asdesired, thus virtually eliminating mechanicai stress.Further, full voltage inrush is never “seen” by either themotor or the connected power system. Typically,

maximum drive currents are 150-200% of the full load

current. This results in a relatively long acceleration timedue to the fact that the motor will produce only 150-200%

accelerating torque. However, motors can generally carrythis much current for an extended period of time withoutthermal stress (overheating) becominga problem. If fasteracceleration times are desired, the drive must be suitableto supply higher current during the startup period.

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Rotor losses are proportional to the difference in the

rotational speed of the rotor and synchronous speed. For

inverters, the motor operates at low slip (nearsynchronous speed) due to the fact that the frequency is

varied. Thus, the rotor losses associated with slip speed

are dramatically reduced when using an inverter to start ahigh inertia load. In turn, this results in relative little rotorheating during startup.

The variable frequency drive offers the most control forstarting any type of load. However, the VFD is

considerably more expensive than the other methodsdiscussed and takes up more space than the other

electronic starter option (soft start). The VFD is generallynot bypassed after the motor reaches full speed which inthe past has caused some reliability concerns. However,

with the increasing dependability of power eiectronicdevices and circuits, this is becoming less of an issue.Maintenance of the VFD is more involved which will addto the life cost of the equipment. Harmonics and

mounting distance from the motor are issues that must beaddressed and can add to the complexity of the

installation. Relay coordination with a VFD is simplified

due to the protection afforded by the electronics and tothe controlled starting of the motor itself.

R = (Wk2 AN) (N, - N,) e T

(2.17 x IO‘) T,

where

R =rotor energy during speed interval (kW-sec)

Wk2=total connected inertia (lb-ft2)AN =speed change during given interval (rpm)

N, =synchronous speed (rpm)N, =

average speed during given interval (rpm)T =average motor torque during interval (Ib-ft)

T, =average accelerating torque during giveninterval (Ib-ft)

An alternate method would be to use the equivalent circuitin order to calculate the total i2R losses in the rotor duringstartup. By multiplying these total losses by the

acceleration time, the energy (kW-sec) can be obtained.

The energy which is input to the stator can also bedetermined by either of two methods. A simple approach

is to use the ratio of the stator resistance to the rotor

resistance as follows:

whereI ne r t ia and M o t o r Hea t ing

S =(r,/r2) R

Inertia is defined as a body’s resistance to a change invelocity. This velocity can either be linear or rotational innature. An object’s moment of inertia (commonly referredto as Wk2) is the product of the weight, W, of the objectand the square of the object‘s radius of gyration, k. The

radius of gyration is a measure of how the object’s mass

is distributed about the center of rotation and is commonly

expressed in units of feet.

The inertia of the given load is a major factor whendetermining both the acceleration time and the motorheating. The acceleration time may be calculated usingthe following formula:

t,=Wk2*AN

308 0 T

where

( 3)

ta =acceleration time (seconds)Wk2 =total connected inertia (lb-ft2)

A N =speed change during time t (rpm)

308 =constant

T, =average accelerating torque (lb-ft)(=average motor torque - average load torque

during startup)

The energy which must be dissipated by the rotor can

found by calculating the kinetic energy dissipated while

accelerating the given inertia, as shown in equation (4).

(4 )

S =stator energy during speed interval (kW-sec)(r1/r2)=ratio of stator to rotor resistance during speed

R =rotor energy during given interval (kW-sec)interval

This method can be derived from the motor equivalentcircuit, given that the magnetizing branch is ignored. The

losses are proportional to the relative magnitudes of the

rotor and stator resistances. Thus, once the rotor energyhas been calculated, the stator energy can be easily

obtained using equation (5).

The second method is to calculate the total i2R lossesassociated with the stator windings as shown in equation(6) below.

where

S =stator losses during speed interval (kW)I s=stator current during speed interval (amps)R =stator resistance during speed interval (ohms)

Again, by multiplying the total stator losses by theacceleration time, the total energy can be obtained.

Once the energy which must be dissipated by both therotor and stator has been determined, the corresponding

temperature rises are found by dividing the kW-sec by thespecific heat (C,) of the material and the total weight of

the material. It is important to note that temperature rises

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calculated in this fashion are by absorption only; that is,

all the heat is assumed to go into either the stator

windings or rotor bars and no heat dissipation is assumed.

For acceleration times of only a few seconds, this methodis satisfactory. For longer acceleration times, the actual

temperature rises will be lower since heat will bedissipated by conduction, convection, and radiation. Inthis case, it is common practice to correct the calculatedtemperature rise by applying reduction factors which havebeen determined experimentally.

Current

Following is a table summarizing the acceleration time,locked rotor torque, locked rotor amps, stator rise/start,and rotor risektart for each starting method given above.The motor used in each calculation is a 500 HP , TEFC,

High Torque (Design C), 4-pole, 4160 Volt motor. Theload inertia used was 30,000 Ib-ft2and the load curve wasassumed to be linear from 0 to 1800 rpm, with 190 Ib-ft oftorque required at full load speed.

Accel. Time (Sec) Stator Temp Rotor Temp

TABLE I

Various Starting Methods Summary

M e t h o d

FulIVoItage

r -SGrtia l-~CCel-r- RT 1 Mot or I L ine I Stator I R ot or ITime ( %FLT) LR A C u r re n t R i s e ( " C ) R i s e ( % )

( sec ) ( %FLA) (OAFLA)

533 2825 745 745 44 1 140 2

80% RV S

( resistor )

65%RV S

837 1685 5759 5759 43 0 128

134 8 1065 4642 2890 40 1 101 2

(auto-

t ransformer)

Wye-Delta

300%CL,210Se c

2080-4160,

120Se c

VF D

Author's Note: Design C type rotors are not suited forinverter duty applications due to the harmonic losses andexcessive heating in the small, upper cage. However, thisfact has been disregarded for the sake of comparison.Analyzing starting methods, not motor design, is the mainfocus of this paper.

1792 805 2306 2306 61 3 96 7

2100 344 300 300 37 9 95 5

1123 61 5 3478 3478 40 7 88 3

126 150 150 150 42 79

Case History (Centrifuge Startup)

The load was a chemical plant centrifuge with 30,000 Ib-f t2 of inertia. It was determined that belt slippage anddecreased life would be a problem if some type ofreduced voltage/torque starting was not used. Also,reducing the current draw during startup would be abenefit. The soft-start manufacturer specified a motorwhich would accelerate the centrifuge with a current limitof 225% of full load amps. It was critical to be able to

calculate the heating which would be experienced by boththe rotor and stator while accelerating this large inertiaunder reduced torque conditions.

For this reason, a computer program was developed

which could simulate the torque produced and the currentdrawn by the motor at every point during the acceleration

time period. By knowing the torque output of the motor atthe reduced current conditions, the acceleration time and

motor heating can be calculated. Using this program, itwas determined that the motor would be suitable to startthis load with the specified current limit without

overheating either the rotor or stator. Data was takenduring the actual startup of the motor/load combination totest the accuracy of the program. Following is a table

summarizing the results of this startup. Unfortunately,due to plant limitations and the fact that this was a totallyenclosed machine, actual rotor temperatures could not berecorded.

TABLE I ITest Data Summary

1 ,.

\ I I

I Calc I Test I Calc I Test I Calc I Test775% I 175 I 11 7 I 74 I ?f i I R I I N l A

Torque Efficiency

The term "Torque Efficiency" (TE) has been used in thepast to describe a motor's torque per amp ratio. This ratiois a measure of how much output torque is supplied by amotor for a particular current level. For example, if motorA supplies 200% of full load torque at locked rotor anddraws 650% of full load amps, its torque efficiency wouldbe 200/650 =0.308 or 30.8%. On the other hand, if motorB supplies the same 200% of full load torque at locked

rotor and draws only 450% of full load amps, its torque

efficiency would be 200/450 =0.444 or 44.4%.

Torque efficiency can be a major factor when using

current limit starting. Special rotor bar shapes andmaterials and winding configurations are used to achievea high TE ratio. A motor with a higher value of torqueefficiency would produce more starting torque for a givencurrent limit than a motor with a lower TE value. Thus, if asoft-starter were being used on a high inertia or heavyload application, a motor with a high TE ratio may bedesired. Design C type motors generally have a high TEratio as compared to Design B type motors. Some easier

applications, such as pumps and compressors may notneed a special high TE motor. Whether or not a special

motor is needed is very application dependent.

Variable frequency drives offer the highest torque

efficiency available. Because VFDs operate on the righthand side of breakdown, the torque produced is roughlyproportionate to the current drawn. For example, to

produce 150% of full load torque, only about 150% of fullload current is needed. This results in a torque efficiencyof approximately 150/150 =1O or 100%. For this reason,

among others, VFD starting is the best method whencurrent is limited and maximum starting torque is required.

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Methods o f Calculation T A B L E 111Recommendation Summarv

The calculation procedure for both soft-starting andinverter duty starting was implemented utilizing a

numerical integration technique in which the acceleration

intervals were broken down into small sections. Usingequations (3), (4), and (5) and taking into account theharmonics produced by the starter, the calculations wereperformed for each individual period during acceleration

from zero to full load speed. Each parameter involved(i.e., losses, acceleration times, and heating calculations)was computed for each of the speed intervals and thentotaled to obtain the final result. By letting these speedintervals approach zero, a very accurate representation ofwhat is happening during the motor startup is obtained.

As mentioned earlier, the heating calculations wereperformed based on absorption only. Empirical factors

obtained from test data were used to correct fordissipation by convection, radiation, and conduction.

Evaluations and Recommendations

In order to give a recommendation on which type of

starter is best for use with high inertia loads, it isnecessary to develop a list of features that are to beconsidered and also to decide the importance of each

attribute to the final decision. The following table shows

four major factors to be weighed when choosing a starterfor a high inertia load. In parenthesis is the percentage

each factor was weighed in the decision making process.Values from 1 to 5 were assigned to each factor, with a 1given to the method considered to be the best option in agiven category and a 5 being given to the least desirable.The final column shows a weighted average for each ofthe starting methods. The best possible score is one.

The motor heating rankings are based upon the relative

temperature rises per start for both the rotor and statorusing different starting techniques. These temperaturerises depend upon several factors including

currentholtage limit and the driven equipment load curve

and inertia. The values given here are based upon atypical installation.

Adjustable acceleration deals with whether or not theacceleration time can be controlled and to what degree.Full voltage starters obviously allow for no control andthus are ranked worst in this category. Electo-mechanicalreduced voltage starting methods do allow for a minordegree of control (adjustable taps, etc.), but still arerelatively fixed in their starting characteristics. Solid-state

soft-starters and inverters, on the other hand allow forvirtually unlimited control of the start. The current limit forthe inverter determines the minimum acceleration time,

but the load can be brought up to speed as slowly asdesired. Soft-starter currentholtage limits can beadjusted as well to either increase or decrease theacceleration time if desired.

Author's Note: The weighting factors were chosen to tryand match the importance the industry places on each ofthese features. If the reader places a different value onany of the factors, the table can be easily modified to fill a

specified need.

Obviously, if there are no power company or applicationrestrictions, most users will purchase a full voltage starterbased on price alone. Similarly, if variable speedcapability is needed an inverter is the only option.However, when reduced voltage is required for eitherelectrical or mechanical reasons, there are a few optionsavailable. Among the reduced voltage options, electronicsoft-starters appear to be the best choice.

Electronic soft-starters offer the most versatility for their

price and are a very good selection for starting high inertia

loads. The low currents drawn by the motor coupled with

the extended acceleration time lead to relatively lowtemperature rises even with very high inertia loads. Thislow rise is beneficial for both motor reliability andlongevity. Many motors that are used to drive high inertialoads must be sized for startup temperatures rather thanfull load operating temperature. When using a soft-starter, the motor design engineer may have the ability touse a smaller motor with less active material (steel,copper, etc.). The fact that both the voltage and currentare adjustable offer the process engineer a great deal offlexibility and control over the startup process as well. If

the process changes for any reason and more startingtorque is required, the current limit of the soft-starter cansimply be adjusted to increase the starting capability.This flexibility is not readily available with the otherreduced voltage starting methods discussed. Eventhough soft-starters are somewhat more expensive thancomparable wye-delta starters, work is being done tobring down their price and make them more cost

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competitive. This fact, coupled with the other benefits ofsoft-starters previously mentioned make them a superb

choice for starting high inertia loads.

Conc lus ion

Various methods of starting are available for use with highinertia loads. Motor heating, acceleration time, and totalcosts of ownership are all factors that must be consideredwhen choosing a specific starter for a high inertiaapplication. No matter which starting method is used, it is

vital that the motor manufacturer be advised so that theproper design steps can be taken to be certain the motor

will perform as desired. More work needs to be done toinvestigate further the impact that the harmonics have onboth motor heating and usable motor torque. However,

test data shows that the program developed for analyzingsoft-starters is reasonably accurate for predictingtemperatures and acceleration times for soft-started

motors. Using the weighting factors chosen and basedupon the test results, soft-starters are an excellent choiceto start high inertia load applications when some type of

reduced voltagekurrent is required.

Acknowledgments

The authors wish to thank Frank Heredos for his guidanceand technical assistance. Thanks are also extended toJ ohn Koehler for his help implementing the programs forboth the soft-start and VFD start calculations.

References

[I ] W. McMurray, “A Comparative Study of Symmetrical3-Phase Circuits for Phase-Controlled A.C. Motor Drives”.

[2] Allen Bradley Company, Bulletin 150Application and

Product Guide, 1995.

[3] A. N. Eliasen, “High-inertia drive motors and theirstarting characteristics”, IEEE Trans. PAS, Vol. 99, NO.

4 p 1472-1482, JUlyIAUg 1980.

[4] J. F. Heidbreder, “Induction motor temperaturecharacteristics”, Paper 55-761 AlEE Fall Meeting 1955.

[5] J . H. Dymond, “Stall time, Acceleration time,Frequency of starting: The Myths and the Facts”, Paper

NO. PCIC-91-03, 1991.

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starting high inertia loads

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