Disclosure to Promote the Right To Information

Whereas the Parliament of India has set out to provide a practical regime of right to information for citizens to secure access to information under the control of public authorities, in order to promote transparency and accountability in the working of every public authority, and whereas the attached publication of the Bureau of Indian Standards is of particular interest to the public, particularly disadvantaged communities and those engaged in the pursuit of education and knowledge, the attached public safety standard is made available to promote the timely dissemination of this information in an accurate manner to the public.

इंटरनेट मानक

“!ान $ एक न' भारत का +नम-ण”Satyanarayan Gangaram Pitroda

“Invent a New India Using Knowledge”

“प0रा1 को छोड न' 5 तरफ”Jawaharlal Nehru

“Step Out From the Old to the New”

“जान1 का अ+धकार, जी1 का अ+धकार”Mazdoor Kisan Shakti Sangathan

“The Right to Information, The Right to Live”

“!ान एक ऐसा खजाना > जो कभी च0राया नहB जा सकता है”Bhartṛhari—Nītiśatakam

“Knowledge is such a treasure which cannot be stolen”

“Invent a New India Using Knowledge”

है”ह”ह

IS 15880 (2009): Three phase cage induction motors when fedfrom IGBT Converters - Application guide [ETD 15: RotatingMachinery]

--_.._--------------------------,

is 15880 : 2009

Indian Standard

THREE PHASE CAGE INDUCTION MOTORS WHEN FEDFROM IGBT CONVERTERS -APPLICATION GUIDE

res 29.160.30

© BIS 2009

BUREAU OF lND!AN STANDARDSMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEWDELHI 110002

June 2009 Price Group?

.. -- - -- --- --------

IS 15880 : 2009

Indian Standard

THREE PHASE CAGE INDUCTION MOTORS WHEN FEDFROM IGBT CONVERTERS -APPLICATION GUIDE

1 SCOPE

2 REFERENCES

The following standards .contain provision, whichthrough reference in this text, constitute provisionsof this standard. At the time of publication, theeditions indicated were valid. All standards aresubject to revision and parties to agreements basedon this standard are encouraged to investigate thepossibility of applying the most recent editions ofthe standards indicated below:

This standard deals with the steady-state operationof cage induction motors, when fed from IGBTconverters. It covers the operation over the wholespeed setting range, but does not deal with startingor transient phenomena.

The standard is applicable for low and mediumvoltage (up to 690 V) three phase motorsmanufactured up to 400 frame, and having a slip atrated load less than 5 percent.

4 FREQUENCY SPECTRUM OF VOILTAGEAND/OR CURRENTS '

With respect to the necessary torque derating andto the oscillating torques excited by harmonics, it isimportant to know the relative harmonic content of .motor voltage and/or currents compared with thoseduring operation on a sinusoidal supply voltage.

Figure 1 shows the typical waveform of the motorline-to-line voltage foroperation with a synchronizedpulse pattern where the harmonics are of the ordern=5; 7; 11; 13.

4.1 lHlarmonic VoltageFactor (HVlF')

The harmonic voltage factor (HVf) is defined asfollows:

the wide frequency range of the most importantharmonics (band width from 0 up to 30 kHz), agenerally valid motor equivalent circuit cannot bespecified. As a rule it is not admissible to use thequantities from the equivalent circuit for steady stateoperation at system frequency (for example withleakage inductances for normal running) in order tocalculate torques and losses due to harmonics. Themotor manufacturer may provide appropriate valuesof the equivalent circuit only, if the frequencyspectrum of currents and/or voltages generated bythe converter is known.

Title

Permissible limits of noise levelfor rotating electrical machines

Mechanical vibration of rotatingelectrical machines with shaftheights 56 mm and higher ­Measurement, evaluation andlimits ofvibration severity

12075: 1987

IS No.

12065: 1987

3 ClHfARACTElRJISTllCS Olf TIHlE MOTOR

In the case of IGBT converters, knowledge of themotor equivalent circuit is not normally importantfor the design of the commutating circuit, but theharmonic impedances ofthe motor greatly influencethe losses caused by harmonics.

The above condition is relevant for the basicoperation capability of the drive. If details arerequired of the additional torques (in particularoscillating torques) and of the additional losses,which occur during converter operation, thenknowledge of. the equivalent circuit parameters ofthe motor covering the harmonic spectrum will benecessary.

Due to the existing design variants ofcage inductionmotors (for example copper deep bar rotors andaluminium double-cage rotors are used) and due to

where

n = order of odd harmonic, not including thosedivisible by three; and

Vn

= per unit magnitude of the voltage at the ,n th

harmonic frequency. .

Example: With per unit voltages of 0.10,0.07,0,045, and 0.036 occurring at the '.5,7,11, and 131h

harmonics, respectively, the value of the HVF is:

0.1020.0720.04520.0362.

+++=0.0546

5711 13

4.2 Derating for Harmonic Content

Harmonic currents are introduced when the line

r------------------------- -- .--------

IS 15880 : 2009

,r

,>.

I

II I

II

! I 2400 3000 3600

600 1200 18b~ II

I II

I

5 ADDllnONAL LOSSES

is assumed that any voltage unbalance or any evenharmonics, or both, present in the voltage arenegligible,

Harmonics ofvoltage and current in a cage inductionmotor supplied from a converter cause additional ironand winding losses in the stator and the rotor.

There is no simple method to calculate the additionallosses, and no general statement can be made abouttheir value. Their dependence upon the differentphysical quantities is very complex. Also there is agreat variety both of converters (for example withdifferent pulse frequencies and pulse patterns) andofmotors (for example kind ofwinding, skewing, slotgeometry).

FIG. I WAVEFORt;J OF LINE-TO-LINE VOLTAGE U".FOR [aST CONVERTER SUPPLY

WITH PULSE FREQUENCY1;, = 30 x J; (EXAMPLE)

voltages applied to a three phase induction motorinclude voltage components at frequencies otherthan nominal (fundamental) frequency ofthe supply.Consequently, the temperature rise of the motoroperating at a particular load and per unit voltageharmonic factor will be greater than that for the motoroperating under the same conditions with onlyvoltage at the fundamental frequency applied.

When a motor is operated at its rated conditions andthe voltage applied to the motor consists ofcomponents at frequencies other than the nominalfrequency, the rated power of the motor should bemultiplied by the factor shown in Fig. 2 to reduce thepossibility of 'cJamage to the motor. This curve isdeveloped under the assumption that only harmonicsequal to add multiples (except those divisible bythree) of,the fundamental frequency are present. It

1.0

II0.9

r~0I- 0.8

to~

Ii ~ 0.7

Il ~Ii ffi 0.6Ii\i 0,

I--.~

<, :--...r-,

I'-...-;

I\...

a 0.02 0.04 0.06 0.08 0.10

HARMONIC VOLTAGE.FACTOR (HVF)

• FIG. 2 DERATING CURVES FOR HARMONIC VOLTAGES

2

.0.12

rs 15880 : 2009

The columns in Fig. 3 show, as an example, thecalculated loss composition ofa specific motor (framesize 315 M) when supplied both from sinusoidalsupply and IGBT converter. The comparison cannotbe transferred to other converter-fed cage inductionmotors and other types of converters (with differentpulse frequencies). To facilitate comparison inFig. 3, the fundamental voltages and currents duringconverter operation are assumed to be the same as .under rated conditions.

5. t Efficiency

Efficiency will be reduced when a motor is operatedwith [GBT converter, The harmonics present willincrease the electrical losses which, in turn, decreaseefficiency. This increase in losses will also result inan increase in motor temperature, which furtherreduces efficiency.

5.2 Temperature

When the motor is tested with converter supply at

a- 26 %

1-1 % 1-0.5%

H-1 %G-2%

F-3%F· 0.5 %

0-2% 0-2% 0-2%

Sinusoidal voltage Current sdurce .Voltage source converter with

converter optimized pulse pattern(pulse frequency '" 3 kHi)

C\ Time dependence of

V the impressedquantity

(,100% 125 % 115 % Losses

I

~ 95.3% 89% 94,5% Efficiency

Losses caused by harmonics Lossescaused by fundamental frequency

J - Commutation losses E ~ Frictional Losses

I - Additional load losses . D - Additional load losses

H - fron losses C - Iron losses

G - Rotor winding losses B - Rotor winding losses

F - Stator winding losses A - Stator winding losses

FIG. 3 iNFLUENCE OF CONVERTER SUPPLY ONTHE LOSSES OFA CAGE INDUCTION MOTOR (FRAME SIZE 315 M)

WITH RATED.VAWES OF TORQUEANDSPEED

..,

is liS880 : 2009

rated load, base voltage and base frequency, thepermissible temperature rise win be within the limitsofinsulation system used. For example when a motorhaving Class F insulation system is tested withsinusoidal supply at rated voltage, frequency andrated load, may have temperature rise within Class Blimits. However, when the same motor is tested withconverter supply under same test conditions, thetemperature rise will be within Class F limits. It isassumed that while testing with converter supply,the HVF factor of converter output supply is maximum3 percent.

It is recommended that the HVF factor is determinedprior to temperature rise test.

6 1I'OIRQlUE

6.1 Motor1I'orque DurilllgOperatioll1BeHowBaseSpeed

To develop constant torque below base speed bymaintaining constant air gap flux, the motor inputvoltage is varied to maintain approximately ratedvoltslhertz. The fun line curve in Fig. 4 refers to aconverter producing the same fundamental motor fluxas a sinusoidal supply.

Frequently, in practice, the converter rating does notimply that the.fundamental flux at rated frequency isthe same as on sinusoidal voltage. The consequenceis an additional torque deviation, the values of'whichdepend on the individual parameters.

Within -the speed setting range below thesynchronous speed at motor rated frequency,applying aU/I,. = constant law leads to a constantpull-out torque if the stator winding resistance isnegligible in comparison with the motor reactances.To compensate for the effect of the motor statorresistance, and to maintain constant air gap flux (that

is constant torque), some converters are designedto have a characteristic in accordance with the dashline in Fig. 4. Effectively, this is an increase in thevolts per hertz ratio (boost voltage) and is normallyprovided at frequencies below' approximately 50percent of rated frequency. At low speeds highertorques are generated than in the absence of suchcompensation. Also, for applications that requireless than rated torque below base speed, systemeconomics may be improved by operation at reducedvolts per hertz ratio.

6.2 'Iorque Derating Basedon Reduction i~ Cooling

Induction motors to be operated in converterapplications should be derated due to the reductionin cooling resulting from any reduction in operatingspeed. This derating should be in accordance withFig. 5. This derating may be accomplished by or

. inherent in the load speed-torque characteristics, ormay require selection of an oversized motor. Thecurves are applicable only to the standard frame sizesand standard designs and as noted, additionalderating for harmonics may be required. For largerframes or special design types, consult the motormanufacturer.

NOTES

1 Deration curve is based on a sinusoidal wave shape,rated air-gap flux. Additional derating for harmonicvoltages should be applied as a multiplier to the abovelimits.

2 Curve is. applicable only to motors up to 400 framesizes and standard designs. For larger frames or other designtypes consult the motor manufacturer..

6.3 Torque Derating IDlllrilmg Centro] Operation

Induction motors to be operated on adjustable-speeddrive applications should also be derated as a resultof the effect of additional losses introduced by

1,21.•00,80,60,40,2

/

//

//

Voo 1,4

./VfN

FIG. 4 FUNDAMENTAL VOLTAGE U. AS A FUNCTION OF OPERATING FREQUENCYI,.

1,0

0.2

0,4

0,6

0,8

4

IS 15880 : 2009

, I

~:-----~---/V

//'

)~",

w:::>oa::~ go~s...J

:::I:::>u, 0a cow

~u,00~cow(J~wa..

oFREQUENCY

FIG. 5 THE EFFECTOF REDUCED COOLING ON THETORQUE CAPABILITY AT REDUCED '

SPEEDSOF 50 Hz STANDARD DESIGN MOTORS

harmonics generated by the converter. The torqueavailable from the motor for continuous 'operation isusually lower than on a sinusoidal voltage source.The reduction results from tile additional temperaturerise due to harmonic losses and also from the voltage­frequency characteristics of converters.

The temperature rise at any load-speed pointdepends on the individual motor design, the type ofcooling, the effect of the reduction in speed on thecooling, the voltage applied to the motor, and thecharacteristics ofthe converter (for exampleharmonic

. spectrum of the converter and HVF factor). Whendetermining the derating factor, the thermal reserveofthe particularmotor is important. Takingall ofthesematters into account, the derating factor at ratedfrequency ranges from 0 to 20 percent.

Figure 6 shows example of a derating curve for atypical motor for which the thermal reserve of themotor at rated frequency is less than the additionaltemperature rise resulting from operation on aconverter. It is not possible to produce a curve whichapplies to all cases. Other motors with different

TIITN

1.0

0.8

{ '

0.6

0,4

0.2

(\

-- ;.-'-

I/~ <,':--

oo 0.2 0,4 0.6 0.8 1.0 1,2 1,4

•.111ft-!FIG. 6 EXAMPLES OFTORQUE DERATING OF MOTORSWHEN USED W0.51.0( p.u.) WITH CONVERTORS

5

ns 15880 : 2009

"

thermal reserve, different methods of cooling (self­circulationcoolingor independentcooling),and usedwith other types of controls will have differentderating curves.

In general, however, motors supplied from IGBT(pulse frequency in the kHz range) require smallertorque reductions than those supplied from blockconverters. The derating is normally reduced as thepulse frequency increases.

There is no established calculation method fordeterminingthe derating curve for a particular motorused with a particular control that can be used byanyonenot familiarwith all of the detailsof the motorand control characteristics. The preferred methodfordetermining the derating curve for a crassof motorsis to test representative samples of the motor designunder load while operating from a representativesample of the converter design and measure thetemperature rise of the winding.

6.4 Motor TorqueDuringOperationAboveBaseSpeed

Above base speed, as frequency increases, a motor

with constant input voltage will result in constantpoweroperation (torque reducing with reduced voltsper hertz). This constant input voltage is having afundamental componentequal to rated motor voltage(which may be limited by the converter and its inputpower).This is also called as operation in fieldweakening range. In the event of this occurringwithin the frequency operating range , then thederating factor will change with a rapid reduction inFig.4 abovefi/fN = 1,0. The maximum (breakdown)torque capabilityofthe motorwithinthis speed rangewilllimitthe maximum frequency (andspeed)at whichconstant power operation is possible.

The curves in Fig. 7 represent the load which thedefined motor is capable of carrying above basespeed. The curve represent operation at constantpower. The maximum frequency of t.5 times ratedfrequency is established based on the approximatepeak torque capability of greater than 175 percentfor standard design motors, assuming operation at aconstant level of voltage equal to ratedvoltage fromrated frequencyto 1.5 times rated frequency. For thecapability of motors for which the minimum

n l I ~

I l .

Ii

\,

~~~

.........~ r--.... .... -r--r------ -

. .

..

oo~

oN~

W:::lo0: 0geea~..J..J..JO:::leou,

ow

~LLOQ'¢I-

ill-o0:WoQ.N

i,

.FREQUENCY

FIG. 7 TORQiJECAPABILITY ABOVEBASE SPEED

6, .

breakdown torque is less than 175 percent, consultthe motor manufacturer.

For operation above 1.5 times rated frequency at arequired power level, it may be necessary to utilize amotor with a greater kW rating at rated frequency.

However, the maximum speed at which a motor cansafely operate may be limited to some speed belowthe maximum speed related to its load carryingcapability because of mechanically considerations(see t4).

NOTES

1 Limit for Class B 80°C or Class F 105°C rise byresistance.

2 Curve is based on a sinuso idal wave shape, constantvoltage equal to rated voltage. Additional derating forharmonic voltages should be applied as a multip lier to theabove limits.

3 Curves are applicable to standard design motors havingbreakdown torques of not less than 175 percent at ratedfrequency.

4 See 14 for any additional limitations on the maximumoperating speed.

7 CURRENT

7.1 Running Current

Controls are generally rated in terms ofa continuousoutput current capability, a short term output current,and a peak output current. To properly choose thesize of control required in an application,consideration should be given to the peak andtransient values in addition to the rms value ofmotorcurrent, and the manner in which the system is to beoperated. Because some level of current will exist ateach of the harmonic frequencies characteristic ofthe particular type of control, the total rms sum ofcurrent required by the motor at full load may befrom 5 percent to 10 percent greater than that levelof current corresponding to operation on a sinusoidalpower source. The magnitude of the peak values ofthe current waveform may vary from 1.3 to 2.5 timesthe rms value of the current, depending on the typeof control considered and the motor characteristics.An additional margin from 10 percent to 50 percentin the current rating of the control should beconsidered to allow for possible overload conditionson the motor so as not to trip the control on suchshort time overcurrent demand. When the motor andcontroi are used in a system where sudden changesin load torque or frequency might occur, the controlshould be sized based on the peak value of thetransient current which results from the suddenchange. Also, when changing from one operatingspeed to another, if the rate of change in frequencyis greater than the possible rate of change in motorspeed and if the slip increases beyond the value ofslip at rated load, then the amou~t ofrms current or

IS 15880 : 2009

peak current required from the control may exceedthat of the steady state requirements.

7.2Btarting Current

In a stall condition, the magnitude and frequency ofthe applied voltage and the impedance of the motorprimarily determine the amount of current drawn byan induction motors. Under adjustable frequencycontrol, motors are normally started by applyingvoltage to the motor at a low frequency (less than 3Hz): The current drawn by the motor under thiscondition is mainlya function of the equivalentstatorand rotor resistances since the reactive impedanceis small because of the low frequency. In order toprovide sufficient starting torque, it is necessary toprovide an increase in voltage (voltage boost) at lowfrequencies in order to overcome this resistive dropin the motor. This voltage boost is the product of therequired phase current (for the level of breakawaytorque needed) and the stator phase resistance andthe square root of 3 (to convert phase quantity toline-to-line value).A wyeconnection isassumed.Forrated torque at start it will be necessary to adjust thevoltage boost to have at least rated current. Sincestator and rotor resistances vary with temperature,the actual starting current will be a function of themachine temperature.

Continued application of boosted motor voltage at. low frequencies under no load conditions will

increase motor heating. When voltage boost isrequired to achieve a breakaway torque greater than140percent ofrated torque, the motor should not beoperated under voltage boost condition atfrequencies less than 10 Hz for more than I minwithout consulting the manufacturer.

S IRJESONANCE AND OSCHlLlLATHNG TORQUES

The asynchronous (time-constant) torquesgenerated by harmonics have little effect on theoperation of the drive. However, this does not applyto the oscillating torques, which produce torsionalvibrations in the mechanical system.

When an induction motor is operated from a control,torque ripple at various frequencies may exist overthe operating speed range. Consideration should begiven to identifying the frequency and amplitude ofthese torques and determining the possible effectupon the motor and the driven equipment. It is ofparticular importance that the equipment not beoperated longer than momentarilyat a speed where aresonant condition exists between the torsionalsystem and the electrical system (that is the motorelectrical torque). For example,if the control is of thesix step type than a sixth harmonic torque ripple iscreated which would vary from 36 to 360 Hz whenthe motor is operated over the frequency range of 6

7

I,"

is 15880 : 2009

to 60 Hz. At low speeds, such torque ripple may beapparent as observable oscillations of the shaftspeed or as torque and speed pulsations (usuallytermed "cogging' ) . It is also possible that somespeeds within the operating range may correspondto the natural mechanical frequencies of the load orsupport structure and operation other thanmomentarily could be damaging to the motor and/orload and should be avoided at those speeds.

In drives with IGST converters, the frequencies ofthe dominant oscillating torques are determined bythe pulse frequency while their amplitudes dependon the pulse width. Thus the oscillating torqueamplitudes may be as high as 15 percent, providedthat the pulse frequency exceeds 10 times thefundamental frequency, which is usually the case intoday's converters. With higher pulse frequencies(in the order of 21 x I.) the oscillating torques offrequencies 6 xI. and 12 xI. are practically negllglble,provided a suitable pulse pattern is applied (for

. example modulation with a sinusoidal reference waveor space-phasor modulation). Additionally,oscillating torques of twice the pulse frequency aregenerated. These, however, do not exert detrimentaleffects on the drive system since their frequency isfar above the cri tical mechanical frequencies.

9 MAGNETnCAlLLY EXCiTED NmSE

Due to harmonics, the excitation mechanism formagnetic noise' becomes more complex than foroperation on a sinusoidal supply. A reliableprecalculation ofmagnetic noise which occurs duringconverter operation of cage induction motors is notpossible at present.. In particular, resonance mayoccur at some points in the speed range.

Sound levels are highe r when using induction motorswith IGST converter. The sound level is dependentupon the construction of the motor, the number ofpoles, the pulse pattern and the pulse frequency,and the fundamental frequency and resulting speedofthe motor. The response frequencies of the drivenequipment should also be considered. Sound levelsproduced thus, are higher than published valueswhen operated above rated speed. ' At certainfrequencies, mechanical resonance or magnetic noisemay cause a significant increase in sound levels,while a change in frequency and/or voltage mayreduce the sound level. '

When supplying a motor. by IGST converter,additional magnetically excited tones are generatedby the voltage harmonics. The frequencies are closeto the pulse frequency .of the converter or multiplesof it and can be close to the natural frequencies ofthe active parts of the stator. The amp Iitudes of thenoise exciting forces vary much with the control

8

strategy of the converter. When the pulse frequencyis fixed, there are high spikes in the spectrum. Bycontrast, when the pulse pattern is controlled online,practically no spikes are visible in the spectrum.Therefore the A-weighted noise level increase variesmore widely compared with operation on a sinusoidalsupply at rated voltage and rated frequency than"forcurrent source converter supplied motors. Accordingto experience the increase at constant flux (at ratedvoltage and frequency) is likely to be in the range IdS to 15 dB. For other frequencies, the noise levelsmay be higher. .

to SERVJ[CE UFE OF WE llNSVlLAnON SYSfEM

The insulation system of the motor is subjected tohigher die lectric stresses than in the case of supplywith sinusoidal voltages and currents. The voltagegradients, which stress the interturn insulation,particularly, that of the line coils, are of importancein the case of supply from IGST converters.

The dielectric stress of the winding in insulation isdetermined by the peak voltage, rise time andfrequency ofthe impulses produced by the converter,the characteristics and the length of the connectionleads between the converter and motor, the windingconstruction and other system parameters.Especially the voltages between the different partsof the winding and the ground represent a significantbasis of evaluation.

Motors with random wound windings with enamelledround wires will typically endure the pulse voltagesofFig. 8 at the terminals without significant reductionoflifetime.

The combination of fast switching inverters withcables will cause peak voltages due to transmissionline effects. For motors rated at voltages less than orequal to 500 V ac the insulation system shouldtypically give satisfactory life when subjected to peakvoltages shown in Fig. 8.

For motors rated over 500 V ac upto 690 V ac,supplied from a fast switching inverter, an enhancedinsulation system and/or filters designed to limit the 'rise time and/or peak voltages may be required.

The term rise time is based on the foIlowing definitionwhich takes into account the transient phenomenawithin the winding (see Fig. 9) .

The voltages range ~u is the difference between theinstantaneous values of the voltage directly beforeand after the voltage impulse. 'T his impulse isfinished at the instant when the voltage has reachedits first maximum. The rise time t is defined as the•interval during which the voltage changes from 10percent to 90 percent ofthe whole voltage range ~u.

IS n5880 : 2009

~ t--

"""""~

~

~

1500

1000

ULL V

2000

500

oo 0,2 0,4 0,6 0,8 1,0 1,2 1,4

fa /.Is

FIG. 8 LIMITING CURVEOFADMISSIBLE IMPULSE VOLTAGE ULL (INCLUDING VOLTAGE REFLECTION AND

DAMPING) AT THEMOTOR TERMINALS ASA FUNCTION OF THERlSE!1ME ta

In view ofthe complex interrelations, a careful designof the complete drive is suggested. Sometimes theuse of filters at the converter output is necessary.

11 BEAR]NG CURRENTS

During converter operation bearing currents may becaused by two different kinds of voltages.

11.1 Shan Voltage

The term shaft voltage is applied to a voltage, which

is induced in the conducting loop comprising theshaft, the bearings, the end shields and the housing(see Fig , 10), by a ring flux in the stator yoke.Irregularities within the yoke (for example dovetailedpunchings to clamp 'the core, ventilation ducts,magnetic anisotropies of the laminations) may causea ring flux. The ring flux may be increased by a zero­sequence component ofthe stator currents (so-calledcommon mode curren ts), the magnitude of whichdepends on the earthing system of the motor. A

II (11) -100 %--li------__

90 % c=Y---li'

tl.1I = II (11)- II (fO)

10%

II (to) % - u.....~ _{=--~-~-~==C>-

fa

FIG. 9 DEFINITION OFTHERISE TIME t OFTHEVOLTAGE AT THEMOTORTERMINALS• a

9

rs 15880 : 2009

' +-'~---~---'- '- '-----._----I

o tPring (t)

icirc

FIG. 10 RfNG FLUX INCLUDfNd SHAFT VOLTAGE AND RESULTfNG CIRCULATING CURRENT i . BEARING VOLTAGEeire

I

I

Ii

~ I

current spike appears at all .instants, when one of thesemiconductor elements is switched, generally six­times per period of the pulse frequency.

The term bearing voltage is applied to a capacitivelycoupled voltage at the radial clearance of the bearing(see Fig. 11). The bearing voltage is caused originallyby an alternating voltage between the mean potentialof the stator winding and the grounded stator core(so-called common mode voltage), which is inherentto the control algorithm of IGBT converters. Thecommon mode voltage consists in particular ofcomponents of three-times the system frequency, ofthree-times the basic frequency at the outputterminals ofthe converter and Ofthe pulse frequency.Its peak value is in the range of 50 percent of the dcvoltage in the intermediate circuit of the converter.Depending on the capacitances between the statorwinding and the rotor, between the rotor and thehousing and the capacitance of the bearing itself, aspecific percentage of the common mode voltage can

be measured at the radial clearance of the bearings.Thus the time characteristic of the bearing voltage isa replication ofthe common mode voltage.

If it is intended to measure the shaft voltage or thebearing voltage during converter operation,appropriate precautions should be taken and specificinstrumentation and shielded measuring cables.haveto be used.

When operating a motor at sinusoidal voltage, thebearing voltage is practically zero. Ifthe shaft voltagedoes not exceed approximately 500 mV (peak), noprotective devices are necessary according toexperience of long standing. Shaft voltage aboveapproximately 500 mV (peak) may produce circulatingcurrents in the conducting loop indicated above,which may destroy the bearings within a relativelyshort period of time. Insulation Of one bearing,preferably at the non-drive end, is sufficient to avoidcirculating currents through both bearings andeventually through bearings of the driven equipment.

C

----~---I Ro Lo Csr

r

"om!A CSfIIsng

Crr

!... _---_._._-_._-

IIbrg

B

I-~----------IIIIIIIII'

:2CsII

: iEDMIt

: isI

FIG. II ~OMMONMODE CIRCUIT MODEL AND BEARING VOLTAGE ~brg

to

The insulation of bearings is neither necessary norcustomary in case ofmotors within the scope of thisspecification, when operated at sinusoidal supplyvoltage and fabricated according to the current stateof the art.

During lOBT converter operation, shaft voltage andbearing voltages exist simultaneously. Highgradients of the bearing voltage may result incapacitive bearing currents (so-called du/dtcurrents). In addition short-time discharging currents(so-called EDM currents) arise if.the bearing voltageexceeds its breakdown value. The repetition rate ofthe EDM currents increases with increasing valuesof bearing voltage and pulse frequency. Both kindsof currents caused by bearing voltage flow alongthe path of the capacitances indicated aboverespectively along the grounding system-of themotor, but not through both bearings(EDM = electrostatic discharge machining).

Depending on the pulse frequency. ' the rise time ofthe pulses and the motor rating, the shaft voltagecontains peak of high frequency, possibly in excessof 10 V which may cause the puncture of the'lubrication film of the bearings. But the componentof fundamental frequency produces an emf. Thusthe frequency spectrum of the circulating currentspredominantly contains the fundamental frequencyand its 3rd harmonic. The current flow may besustained by the component of fundamentalfrequency, even when its amplitude is less than 500mV (peak), in so far as a motor with low shaft voltageat sinusoidal supply is advantageous to avoidcirculating currents at converter supply.

According to measurements, typically the bearingvoltage is in the range of 10 V to 30 V (peak). Aninterrelation between circulating currents and bearingvoltage could not be recognized.

Experience shows:

a) Motors within the scope of this specificationand with frame numbers up to and including

.3 15 seldom experience-bearing failure causedby converter operation. Nevertheless thedielectric stress on the bearings varies widelywith the type of the control algorithm of theconverter. However when using converters

.having a pulse frequency greater than 10kHzand an output voltage greater than 400 V rms,consideration should be given to the use ofbearing insulation. '

b) For machines within the scope of thisspecification and with frame numbers above315 , it is recommended either;

II

IS 15880 : 200,9

,Q To use a converter with a filter designedto reduce the zero-sequence. To use aconverter with a filter designed to reducethe zero-sequence; or

ii) to reduce the du/dt of the voltage; and/or

iii) to insulate the motor bearing(s).

c) The insulat ion of a ball bearing can beachieved by replacement with an insulatedbearing of the same dimensions. The need to .insulate both motor bearings is seldomnecessary. In such a case the examination ofthe whole drive system by an expert is highlyrecommended and should include the drivenmachine (insulation of the coupling) and thegrounding system (possibly use ofan earthingbrush).

12 LONG CABLES

When the motor is at a long distance from theconverter, the cable connecting' the motor to theconverter acts like a transmission line with theequivalent circuit shown in Fig. 12. The inductanceis the phase inductance per unit length, and thecapacitance is the line-to-ground capacitance perunit length.

From transmission line theory, when the lineimpedance is less than the load impedance, voltageand current Waves are reflected , and the voltage islargest at the load. This is the case for driveapplications. The motor surge impedance is oftenseveral times the value of cable characteristicimpedance. Therefore the reflection coefficientapproaches to unity and reflected wave has peak oftwo times DC bus voltage. Although the mismatchbetween cable and motor impedance is highest forsmall motors, in all cases, voltage is greatest at themotor.

The reflection,coefficient is:

p = Zm -Zc

z; +Zc

where

Zm .= motor surge impedance, a~d

Zc = rmrge impedance ofthe cable to travellingwaves.

V~ = (1+ P) x Va

The voltage at the motor is the sum of the incidentand reflected waves. For motors smaller than about18.5 kW, the reflection coefficient is 1.0. For longcables, voltage doubling occurs at the motorterminals .

IS 15880 : 2009

CABLE

FIG. 12EQUIVALENT T~SMISSION LINE CIRCUIT

)I

13 CRlTJCAL.CAlBLE LENGTH

The critical cable length is the maximum cable lengthat which voltage amplification does not occur. It isthe length at which the sum of the reflected andincident waves is equal to the peak value of theincident wave. Voltage at the motor terminalsincreases as cable length increases beyond criticalcable length.

If the propagation speed of the voltage wave 'is Sand the rise time of the IOBT converter wave front(defined as the time taken for the output to go from10 percent to 90 percent of its peak value) is T, thenthe distance traveled by the wave front during itsrise time is S x T. If the motor is at a position wherethe incident wave has just reached 50 percent of itsfull value and ifthe reflection coefficient is 1.0, thenthe sum ofthe reflected and incident waves will yield100 percent of the peak value of the incident wave.Any distance greater than this critical cable lengthwould result in a voltage greater than 1.00 per unit.So the critical cable lengthis given by:

SxT

2

The propagation speed over a conductor dependson its inductance and capacitance per unit lengthand can be expressed as:

1S = ..fLe

-Typical values for propagation speed range from 100to 150 m/us,

Table 1 lists critical cable lengths'with a propagationvelocity ofl 00 m/us for rise times ranging from2 ~IS to .05 ~s. The rise time of the voltage wave is

12

controlled by how quickly the power semiconductorsin the converter are turned on. Typical rise times forpower semiconductors used in converters range from.05 to .50 us,

Table 1 Critical Cable Length for Various Rise Times

81 No. Rise Time -Crttlcal Lead Length

J.ls m(I) (2) (3)

i) 2.00 100

ii) 1.0 50

iii) 0.50 25

iv) 0.10 5

v) 0:05 2.5

When using higher cable lengths, it is advisable touse a lower switching frequency and dvldt filters.

14 MAXIMUM SAFE OPERATING SPEED

Ifa motor is intended to be operated at speeds aboveits rated speed, the maximum safe operating speedof a direct coupled motor at 0° to 40°C ambienttemperature should not exceed the values given inthe following table. Depending on the motor design,the operation at higher speeds for conditions otherthan stated may be allowable, but this possibilityshould be verified by the motor manufacturer.

When operating at speeds above rated speed, noiseand vibration levels will increase. It may also berequired to refine the balance for acceptableoperation above rated speed.

Operation at speeds close to the maximum safeoperating speed for extended periods of time maycause considerable shortening of the service life ofthe bearings. Moreover, the shaft seals and/or theregreasing intervals (or the grease service life in thecase of greased-for-life bearings) may be affected.

NOTES

1 The permissible overspeed value is 20 percent abovevalues in Table 2 (not to exceed 2 min in duration) .

2 The values in Table 2 are based on mechanicallimitations. With'in the operating limits noted in Table 2,the motor is capable of constant power from ratedfrequency to approximately 1.5 times rated frequency.Above approximately 1.5 times rated frequency, themotor may not provide sufficient torque based on specifiedvoltage to reach stable speeds while under load.

3 Operation above nameplate speed may require refinedbalance.

<3 Considerations:

a) Noise limits as per IS 12065 and vibration limits asper IS 12075 are not applicable.

b) Bearing life will be affected by the length of time themotor is operated at various speeds.

15 POWER lFAcrOR. COlRRlEC1flION

The use of power capacitors for power factorcorrection on the load side of an electronic powersupply connected to an induction motor is notrecommended. The proper application of suchcapacitors requires an analys is of the motor,electronic power supply an,4 J!>~9 .9P'~r9pt~r:j ;>~jc~ asa function of speed to avoid potential over-excitationofthe motor, harmonic resonance and capacitor overvoltage. For such applications the drive manufacturershould be consulted.

][6 OPERATION iN ]l-JAZAR]I)OUS (ClLASSJiFiliED)JLOCATIONS

.Motors operated from adjustable frequency oradjustable voltage power supplies or both, should

lIS 15880 : 2009

not be used in any Division I hazardous (classified)locations unless the motor is identified on thenameplate as acceptable for such operation whenused in Division I hazardous (classified) locations .

For motors to be used in any Division 2 hazardous(classified) locations, the motor manufacturer shouldbe consulted.

Failure to comply with this warning could result inan unsafe installation that could cause damage. toproperty or serious injury or death to personnel, orboth.

Table2 MaximumSafeOperatingSpeedsforDlrect-CoupledMotors Used0111

JGB'f Converters

(Clause 14)

81 No. Flame 2 Pole <3 Pole 6 PoleNumber

(I) (2) (3) (4) (5)

i) 112 5200 3 600 2400

ii) 132 4500 2700 2400

iii) 1.6Q 4500 2 ?QO 21,1)9. .. .~

jv) 180 4500 2700' 2400

v) 200 4500 2300 2400

vi) 225 3600 2300 1 800

vii) 250 3600 2300 1 800

viii) 280 3600 2300 1 800

ix) 315 3600 2300 1 800

NOTE - The above value may have to be reduced to meetthe requirements of flameproof motors.

13

------------------------------ .. _ .. -

GMGIPN-153 BIS/ND/09-300

Bureau of Indian Standards

BIS isa statutory institutionestablishedunderthe Bureau ofIndian Standards Act, 1986to promoteharmoniousdevelopment of the activities of standardization, marking and quality certification of goods and attending toconnected matters in the country.

Copyright

BiS has the copyrightof all its publications. No part of thesepublications maybe reproduced in anyform withoutthe priorpermission in writingofBIS. Thisdoesnotprecludethe freeuse, incourseof implementing the standard,of necessary details, such as symbols and sizes, type or grade designations. Enquiries relating to copyrightbeaddressedto the Director (Publications), BIS.

Review of Indian Standards

Amendments are issued to standards as the need arises on the basis of comments. Standards are also reviewedperiodically; a standard along with amendments is reaffirmed when such review indicates that no changesareneeded; if the review indicates that changes are needed, it is taken up for revision. Users of Indian Standardsshould ascertain that they are in possessionof the latestamendments or editionby referringto the latest issue of'BIS Catalogue' and 'Standards: MonthlyAdditions',

This IndianStandardhas beendevelopedfromDocNo. :ETD 15 (5787).

Amendments Issued Since Publication

Amendment No. Date of Issue

BUREAU OFINDIAN STANDARDS

TextAffected

.Branches : AHMEDABAD. BANGALORE. BHOPAL. BHUBANESHWAR. COIMBATORE. FARIDABAD.GHAZIABAD. GUWAHATI. HYDERABAD. JAIPUR. KANPUR. LUCKNOW. NAGPUR.PARWANOO. PATNA. PUNE. RAJKOT. THIRUVANATHAPURAM. VISAKHAPATNAM.

,I

II

J

III

Headquarters:

ManakBhavan, 9 BahadurShahZafarMarg,New Delhi r10002Telephones: 2323 0131,23233375,23239402 Website: www.bis.org.in

Regional Offices:

Central : ManakBhavan, 9 Bahadur ShahZafar MargNEWDELHI 110002

Eastern : 1/14, c.t.t; Scheme VIIM,V,I.P. Road, KankurgachiKOLKATA 700054

Northern : SC0335-336,Sector34-A, CHANDIGARH 160022

Southern : C.LT. Campus, !VCrossRoad, CHENNAI 609113

Western : Manakalaya, E9 MIDC,Marol,Andheri (East)MUMBAI 400093

Telephones

{2323761723233841

{2337 8499,2337856123378626,23379120

{26038432609285

{2254 1216,2254 144222542519,22542315

{28329295, 2832 785828327891,28327892

I...enwOJenZoa<0

. -P'RINTiED By THE GENERAL MANAGER, GOVT. OF INDIA PRESS, NASHIK-422 006

~ II

J\"11\ ,

I,III"

Rotating Machinery Sectional Committee, ETD 15.'

FOREWORD

This indian Standard was adopted by the Bureau of Indian Standards, after the draft finalized by the RotatingMachinery SectionaL Committee had been approved by the Electrotechnical Division Council.

The performance characteristics and operating data for drives with converter-fed cage induction motors areinfluenced by the complete system, comprising supply system, converter, induction motor, mechanical shaftingan« control equipment. Each of these components exists in numerous technical types. Any values quoted inthis standard are thus indicative only.

. '"

In view ofthecomplex technical interrelations within the system and the variety ofoperating conditions, it isbeyond the scope and object of this standard' to specify numerical or limiting values for all the quantities,which are of importance for the design of the drive.

,

To an increasing extent it is practice that drives consist of components produced by different manufacturers.The object of this standard is to explain and quantify, as far as possible, the criteria for the selection ofcomponents and their influence on the performance characteristics of the drive.

Motor Categories

There are two categories of cage induction motors, which can be applied in variable speed electric drivesystems:

a) Standard cage induction motors, designed for general-purpose applications. The design andperformance of these motors are optimized for operation on a fixed frequency sinusoidal supply.Nevertheless, they are generally also appropriate for use in variable speed drive systems.

b) Cage induction motors specifically designed for converter operation. The design and construction ofsuch motors may be based on standard motors with standardized frame sizes and dimensions, but withmodifications for converter operation

Guidance on this field of application is given in this standard.

This category is covered by IS 15881 : 2009 'Three phase cage induction motors specifically designed forIGBT converter supply - Specification'.

While preparing this standard, necessary assistance has been derived from IEC 60034-17 'Rotating electricalmachines - Part 17: Cage induction motors when fed from converters - Application guide' .

For the purpose ofdeciding whether a particular requirement ofthis standard is complied with, the final value,observed or calculated, expressing the result of a test or analysis, shall be rounded off in accordance withIS 2: 1960 'Rules for rounding offnumerical values (revised)'. The number ofsignificant places retained inthe rounded off value should be the same as that of the ,specified value in this standard.