FIRSTEDITION

VARIABLE SPEED DRIVESAND MOTORSVARIABLE SPEED DRIVESAND MOTORS

Motor Insulation Voltage StressesUnder PWM Inverter Operation

Developed by the joint GAMBICA/REMA Working Group

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This Technical Report has been produced tomeet a demand for an authoritative guide ongood practice in the application of motors onPWM inverter supplies with respect to voltagestresses. It is the result of a study carried out byGAMBICA and REMA taking note of wellestablished fundamental theory, technical papersand carrying out specific investigations.Theinformation given, while it can be applied tomotors and inverters in general, is specific tocurrent generation products of membercompanies.The Technical Report principally considersvoltages developed by the voltage source PWMinverter, supply line effects and motor insulationsystems. It supplements IEC 60034-17: 1998(Guide for the application of cage inductionmotors fed from converters) which providesadditional information on other importantaspects including: torque de-rating, additionallosses, noise and maximum safe operating speed.The report covers motors and inverters installedwith a separate cabling system connecting thecomponents together, it is not applicable to drivesintegrated into a motor design, generally nowavailable up to 7.5kW - see section 7.2.This information supplements theGAMBICA/REMA Technical Guide and a usefulreference list for further reading is provided inSection 9 of the report.

This Report provides information on the basicoperating principles of PWMinverters with anemphasis on the type using IGBT switchingdevices, and the interface with the motor. It dealswith transmission line effects in the supply cablesbetween the inverter and the motor and theimpact of voltage peaks at the motor terminals.Advice is given on the capability of motors towithstand inverter supplies and whenpreventative measures to control peak voltageand/or excessive capacity currents should betaken.Supplies considered are low voltage up to 690Va.c.

(i)

◆ Motor winding insulation experiences higher voltage stresses when used with an inverter than when connected directly to the a.c . mains supply.

◆ The higher stresses are dependent on the motor cable length and are caused bythe interaction of the fast rising voltage pulses of the drive and transmission line effects in the cable.

◆ For supply voltages less than or equal to 500V a.c., most standard motors are immune to these higher stresses.

◆ For supply voltages over 500V a.c., amotor with an enhanced winding insulation system is required.Alternatively, additional components can be added to limit the voltage stresses to acceptable levels.

For nominal supply volta ges less than or equal to 500V a.c .

Select a standard motor from REMA manufacturers. No further considerations are necessary forPWMinverters.For motors from other suppliers, where the motor cable length exceeds 10m, the pulsewithstand capability should be established by reference to the supplier.The permittedvoltage/rise-time characteristic should equal or exceed curve A shown in Figure 17.

For nominal supply volta g es greater than 500V and up to 690V a.c .

Select a motor with an enhanced insulation system available from REMA manufacturers.Theadditional cost of the enhanced insulation is typically 10-20% of the standard motor cost.Motors with enhanced insulation systems may be slightly de-rated compared to standardmotors. No further insulation considerations are necessary for PWM inverters.For motors from other suppliers, the pulse withstand capability should be established byreference to the supplier.The permitted voltage/rise-time characteristic should equal or exceedcurve B shown in Figure 17.

Notes:i) Motor frame selection must be appropriate for the application and duty.ii) Insulation requirements may be affected by the application specific notes in section 7.iii)The pulse withstand requirements of curves A and B in Figure 17 both exceed the minimum capability

specified in IEC 60034-17:1998,which is also shown in Figure 17.

(ii)

1. INTRODUCTION 12. PRINCIPLES OF PWM DRIVES 2

2.1 General 22.2 Terminal voltage 32.3 Winding voltage 6

3. THE INDUCTION MOTOR AND INSULATIONSYSTEMS 73.1 General

3.2 Standard motor insulation systems 73.3 Enhanced motor insulation systems 9

4. INSULATION BEHAVIOUR 95. PRACTICAL INSULATION REQUIREMENTS 116. ALTERNATIVE PREVENTATIVE METHODS

6.1 General 126.2 Output reactors 126.3 Voltage limiting filter (dv/dt filter) 126.4 Sinusoidal filter 126.5 Motor termination unit 136.6 Relative characteristics of preventative measures 136.7 Cost comparisons for preventative measures 14

7. APPLICATION SPECIFIC NOTES 147.1 Cable capacitance effects 147.2 Combined inverter/motor solutions 147.3 Applications with frequent or continuous braking duties 147.4 Active front end (sinusoidal rectifier) considerations 157.5 Drives with special control strategies 157.6 Explosive atmosphere approval 157.7 Applications with ‘very long’ cable lengths 15

8. CONCLUSIONS 159. REFERENCES 1610. FURTHER READING 16

(iii)

Variable speed a.c . drives as illustrated below areused in ever-increasing numbers because of theirwell-known benefits for energy efficiency andflexible control of processes and machinery usinglow-cost maintenance-free a.c. motors.Virtuallyall a.c. drives use power switching techniques andgenerate high rates of change of voltage.Most modern a.c. drives use voltage-sourcePWM inverters with very fast-switching powersemiconductor devices such as Insulated GateBipolar Transistors (IGBT).The fast-changing voltage generated by such adrive causes some increase in the voltage stresson the motor winding insulation - although thisreport will show that in most cases the additionalstress is well within its capability. For some yearsthere have been occasional reports of motor

winding insulation failures which appear to havebeen caused by use with a PMWdrive [1].Virtually all of the failures reported have been insystems using a.c. supply voltages in the region of525V to 575V and above, which are most widelyused in North America and South Africa.There islittle evidence of such effects occurring withstandard European LV supplies.The number offailures reported is small, but sufficient to causeconcern in some application areas.As a resultsome users and consultants may be specifyingcounter measures which are costly and are oftenunecessary.This report aims to give a clearexplanation of the effects involved andstraightforward advice on what precautions arerequired to avoid problems.

1

Figure 1: Essential elements of a PWM inverter drive

2

Voltage source PWMinverter drives are the mostcommon type of low voltage inverter driveswhich are currently in use.The process ofobtaining the required frequency involvesconverting the incoming alternating voltage todirect current by means of a rectifier, smoothingthe DC in an intermediate DC link with capacitiveenergy storage, then inverting back to analternating current. Standard texts [2] providedetailed explanations, but Fig 1 above illustratesthe basic principles:The pulsed output voltage is applied to the motorand the resultant current, modified by thesignificant motor inductance, consists mainly of thefundamental sinewave at the required operatingfrequency with a superimposed low magnituderipple component based on the switchingfrequency. Both voltage and current over onecycle are illustrated in simplified form with

deliberately reduced switching frequency inFigure 2 below.

Figure 2.: PWM inverter output voltage and currentwaveforms.

2.1 General

where L (Henries) and C (Farads) are theinductance and capacitance per metre,respectively.The velocity of propagation of apulse in a typical PVC-insulated cable is about 1.7x 108m/s (i.e. in 100ns the pulse travels only17m). It varies little over the variety of cabletypes in general use, since it is determined mainlyby the permitivity of the internal insulatingmaterial.The essential features of how a pulse propagatesalong the motor cable are illustrated in Figure 4and Figure 5 on the following pages. Moredetailed analysis is given in reference [1].

Drive designers generally aim to use the highestpractical switching frequency, since this has avariety of benefits including reducing the audiblenoise from the motor.This means they areconstantly seeking to use faster power switching

devices giving shorter rise-times, which iscontributary to steep wavefronts.The valueslisted in Table 1 illustrate the relative values ofpulse rise time compared to the powerfrequency and switching frequency period.

Fr equency (Hz) Period/time

Power frequency 50 20ms

Pulse switching frequency 3000 333us

Pulse rise-time - 100ns

Table 1:Typical frequencies and times

The PWMpulse rise-times are so short that theirpropagation along the motor cable to the motorcan change the pulse shape and may produce avoltage overshoot.The cable can be consideredas a transmission line, i.e. a long string ofdistributed series/parallel connected, inductor-capacitor sections as shown in Figure 3. Forsimplicity only one phase is represented.At each pulse edge , the drive has to charge theinductance and capacitance of the cable, so apulse of energy is delivered into the cable.Transmission line theory shows that the pulsetravels at a speed equal to

( 1 )√LC

m/s

2.2 Terminal Voltage

Figure 3: Distributed inductance and capacitance of cable

3

Each pulse represents one ‘edge’ in the PWMwaveform.The pulse enters the drive end of the cable at timet=0 and rises to Ud in time t r. In this idealisedexample tr is smaller than the cable propagationtime tp, corresponding to a case where the cablelength exceeds about 30m.

(a) Time t=tr (ie at the end of the rise-time of the pulse)

The pulse travels from the drive along the cable tothe motor.When it reaches the motor it is reflected,because the motors’ high frequency impedance ishigher than that of the cable.This causes the voltageto rise towards twice its original peak voltage. Thevoltage can be represented as having twocomponents, the forward pulse and the reflectedpulse, each having magnitude +U d.

(b) Time t=tr+tp (ie after one cable propagation time)

The reflected pulse returns to the drive and becausethe drives’ impedance is very low, the pulse isreflected in a negative sense. This reflection does notappear in the voltage waveform at the driveterminals because the drive clamps the voltage toUd. A negative current pulse results, which istransformed into a negative voltage pulse as itreturns along the cable.

(c) Time t=2tr+2tp (ie after two cable propagation time)

The second reflection, which returns from the drive inthe reverse polarity, is also reflected as in stage (b)and is doubled at the motor. It counteracts theoriginal motor voltage increase. If the cable is shortso that 2tp is less than t r, the voltage never reaches2Ud. However with a longer cable as illustrated here,the reflection ar rives too late to reduce the peak.

(d) Time t=2tr+3tp (ie after three cable propagation time

Figure 4: Idealised pulse propagation in motor cable

MOTOR

Voltage

DRIVE

Voltage pulseleaving drive

+Ud

tr

tr is the pulserise-time

Voltage pulsereflected at motor

+Ud

tr + tp

tp is the time of propagationof the pulse along the linelength

tr + tp

+2Ud

Returned pulsereflected at drive

+Ud

2tr + 2tp 2tr + 2tp

+2Ud

Reflectedcurrent

Equivalentreflected voltageUd

Second reflectioncancels the first

+Ud

2tr + 3tp 2tr + 3tp

+2Ud

4

In an idealised case the reflections would causethe voltage to oscillate indefinitely. In practice, thevoltage rise-time is increased due to highfrequency losses in the cable , and the waveformsbecome rather rounded and less clear-cut thanthe idealised waveforms illustrated in Figure 4.Also due to high frequency losses, the peakvoltage oscillations over one pulse cycleprogressively decay and stabilise at the d.c. linkvoltage. Figure 5 shows the waveform with 42mof cable and derives its main features from theprocess described in Figure 4.

Motor peak voltage is therefore a function ofboth cable length and rise time. For example,with 20m of cable with a velocity of 1.7 x108m/s, any pulse with a rise-time less than235ns can be expected to increase by nearly100%. Figure 6 shows some typical measuredvoltage waveforms (based on a 460V test supply)which show this effect in practice. Even with 4mof cable some overshoot is apparent.With 42mthe overshoot is virtually 100%.

The pulse rise-time is an important factor in thestudy of these effects. Since the pulses whicharrive at the motor terminals are not atrapezoidal shape, there is no self-evidentdefinition for the rise-time . Unfortunately the twostandard bodies, IEC and NEMA, have chosen touse different definitions. Figure 7 illustrates the

application of each method to the same samplewaveform whereby it can be seen that the IECmethod gives a value of approximately twice thatcalculated using the NEMA definition.All values inthis report are given in accordance with the IECmethod as defined in IEC60034-17 1998.

Figure 5: Features of a typical pulse waveformat the motor terminals - cable length 42m

Figure 6: Motor terminal voltage waveforms forvarying cable lengths

a) Cable Length = 0.5m b) Cable Length = 4m c) Cable Length = 42m(Note scale changes)

5

The voltage overshoot has little effect on themain motor insulation systems between phasesand from phase to earth, which are designed towithstand large over voltages. However, becauseof its short rise-time the voltage overshoot alsostresses the insulation between turns, andespecially between randomly touchingconductors within a coil or between coil ends.The front edge of the voltage pulse with itssuccession of voltage peaks (Figure 5), will travelaround the motor winding as it does along themotor cable, with a measurable propagationtime. Figure 8 illustrates how this may result in alarge proportion of the pulse appearing betweenturns, at random points within a coil or between

2.3 Winding voltage

coil ends.This effect progressively decays to auniform voltage distribution in subsequent coilsdue to high frequency inductive and capacitivelosses.Dependent on motor and winding parameters(e.g. motor rating, type of winding, number ofturns, size of coil, turn propagation time etc) andthe time between reflected peaks in the incidentterminal voltage, the voltage appearing betweenturns or randomly within a coil may briefly reachbetween 30% and 90% of the incident peakvoltage. Figure 9 shows the possible variations infirst coil voltage plotted against the rise time ofthe incident peak voltage at the motor terminals.With a sinusoidal supply voltage (uniformlydistributed), the coil ends only experience afraction of the phase voltage, as determined bythe number of coils.With a variable speed drivetherefore, there can be a considerable increase inthe voltage stress within a coil.

a) IEC 60034-17 1998 b) NEMA MGI 1993 part 31Figure 7: Different definitions of rise time

Figure 8: Propagation of a voltage pulse throughmotor windings

Figure 9: First coil voltage distribution againstincident voltage rise time

6

us

The development of the squirrel cage inductionmotor, with its associated insulation system, hasgenerally been for sinusoidal supplies. Its design iswell proven and inherently robust leading to longreliable service with minimum maintenance.Practical life of insulation systems, and hencemotor life, can be many years with ultimate failurelikely to be through thermal and mechanicaldegredation of the insulating materials, not byshort-term direct electrical breakdown.Requirements for motors with standard suppliesare established internationally in the IEC 60034

series of standards. These cover aspects ofperformance, starting characteristics, thermalclassifications, mechanical protection, safety,insulation level by dielectric test etc.Developments in the materials and varnishes usedin motor insulation systems have improved thethermal, mechanical and dielectric characteristicsconsiderably beyond the minimum requirementsof those standards and overall, the standardinduction motor is well able to withstand thevoltage waveforms encountered with the majorityof inverter drives.

For low voltage motors up to 690V, there aretwo main types of winding, broadly classed asrandom and form. Lower power motors aregenerally random wound, i.e. with coils in whichthe turns of round section wire are randomlylocated in the coil forming process as illustratedin Figure 10. For larger powers, form windings areoften utilised where the pre-formed coils arelayered up uniformly - usually with rectangularsection conductors. Coils for both types ofwindings are shown in Figure 11, with typical slotcross sections for random and form windingsdepicted in Figure 12.

3.2 Standard Motor Insulation Systems

3.1 General

Figure 10:Random coil forming

7

Figure 12:Typical slot cross-section for Random winding and Form winding

Typical phase to earth and phase to phaseinsulation will be polyester film/meta aramid papercomposites with inter-turn insulation provided bymulti-layer polyester/polyamide-imide enamel onthe conductor or alternatively mica/polyester

wrapped film in the case of rectangular formwound turns. Figure 13 shows the insulation of arandom winding in an intermediate stage ofmanufacture and Figure 14 a partially wound formwinding.

The essential elements of both random and form wound insulation systems consist of:

● Phase to earth insulation - slot liner and closure.● Phase to phase insulation - slot separator and end-winding.● Inter-turn insulation - slot and end-winding.● Impregnating varnish - slot and end-winding

Figure 13: Partially wound stator core withrandom winding

Figure 14: Partially wound stator core withform winding

8

Figure 11: Random and form wound coils

Random Form

Impregnating the winding, typically with class F or Hpolyester resin, provides mechanical strength withoverall electrical insulation and resistance toenvironmental contamination. Figure 15 shows acompleted random wiring.

To withstand the higher stresses on suppliesgreater than 500V and up to 690V, an enhancedrandom wound motor insulation system willinvolve further reinforcement of slot liners, slotclosures, slot separators, inter-phase barriers, end-winding bracing, etc, and possibly the use of

3.3 Enhanced Motor Insulation Systems

special winding wire.This is completed by amultiple impregnation regime. In the case of formwinding, standard windings, having mica/polyesterwrapped conductors, would meet enhancedinsulation requirements.

There are three suggested possibilities forinsulation damage:-● Breakdown between coil and stator coil

➯ Normally not a problem as slot liners are used

● Phase to phase failure - in the slots or end-windings➯ Normally not a problem as motors use

inter-phase barriers (or are form-wound)● Inter-turn failure between adjacent

conductors in the stator winding➯ The most probable cause of failure due to

the non-uniform distribution of voltage along the stator windings, associated with the short rise times of the incident voltagepulses as described in section 2.With formwound motors, this is a less significant problem because the turns are evenlydistributed within the slot.

Depending upon the homogeneity of the statorwinding impregnation, there may be voids in theimpregnating resin. It is in such voids that the

failure mechanism in the inter-turn insulationoccurs.The failure mechanism is a complexphenomenon called partial discharge (PD).PD is a low energy discharge that occurs whenboth the following conditions apply:● The peak value of the applied voltage is

lower than the actual breakdown voltage of the insulation system

● The local electric field intensity that is created in a void or cavity is sufficient to exceed the breakdown strength in air (Partial Discharge Inception Voltage)

When subject to continuous partial discharges, theinsulation system progressively degrades,prematurely ageing the insulation material.Theageing process results from an erosion of theinsulation material, reducing its thickness at thedischarge sites until its breakdown voltagecapability is reduced to below the level of theapplied voltage peak, at this stage insulationbreakdown occurrs.Recent investigations, particular ly at Dresden

9

Figure 15: Completed random winding

University [3] have produced relationships formodel insulation systems between the appliedpeak voltage, rise times, the probability of PD andthe insulation lifetime.The results are shown inFigure 16 for a reference temperature of 20˚Cwith a typical standard induction motor insulationsystem that is rated for operation with a nominalsupply voltage up to 500V a.c. on an invertersupply.

Figure 16(a) which is based on the Dresdenresults, shows the cumulative number of pulses(0.1µs rise time , 5µs duration) that the insulationshould survive whilst Figure 16(b) shows the

probability of PDoccurring, both plotted againstthe pulse amplitude of the applied voltage.PDinception voltage is influenced by temperature.Temperature increases occur from the normallosses in the motor, compounded to some extentby the losses associated with the high frequencynature of the applied voltage pulse. Reports [3]indictate that an increase in temperature of 80Kmay reduce the PD inception voltage by

approximately 10%. In circumstanceswhere partial discharges are occurringthis reduction in the inception voltagewill result in an acceleration of theageing of the insulation system.If the motor insulation system isoperated such that the applied peakvoltage is less than the PD inceptionvoltage, or at a voltage where theprobability of PDis low and thenumber of pulses to breakdownexceeds 1012 in Figure 15, then noreduction in lifetime is expected.In relating the unipolar curve of Figure16 to standard insulation windings, it isnecessary to apply correction factorsfor temperature and the first coilvoltage (Figure 9). For the normalclass B temperature rise (80K), these

factors effectively equalise each other indicating apermissable terminal voltage peak of 1.3kV with arise time of 0.1us for a projected lifetime of 10pulses.This equates to the practical situation ofstandard motors from REMAmanufacturers andsupply voltages up to and including 500V a.c. (seecurve A in figure 17).

Figure 16: Relationship between peak Voltage,insulation lifetime and PD probability

Bipolar Switching MethodsBipolar switching where the polarity of the pulse may be reversed in successive switching operations(thereby producing alternating pulses) may be found in inverters which use hysteresis switching -generally described as direct torque injection schemes. In these cases, a PWM modulator is notused, and the output offers a non deterministic switching pattern. In these special control schemes, it ispossible to generate pulses which change from +Ud to -Ud in one transition. Note that in this case, it ispossible to obtain peak voltage reflections (described in Figure 4) of up to 4Ud.That is, twice thevoltage stress compared to PWM inverters - see note 7.5.The frequent polarity reversals provide a further increase to the insulation stress and the predictedfailure time is therefore significantly worse than with the more typical unipolar PWM inverters.

10

IEC(60034-17:1998) and NEMA (MG1 part 31)have both revised their respective standards givingwithstand characteristics for motor insulation

systems when inverter fed. In both cases, thesecurves are not now representative of present daypractice - see figure 17 below.

Figure 17: Limit curves of admissable motor terminal peak voltage for a.c. motors up to 500V a.c. (Curve A)and from 500V a.c . to 690 a.c . (Curve B).

Notes:i) Motor pulse withstand requirements on 400/415V supply generally exceed the minimum capability

specified in IEC 60034-17.ii) The pulse withstand requirements detailed in MG1 part 31 for definite purpose inverter fed motors are not

adequate for all cases of modern PWM inverter operation.iii) Pulse rise times are normalised in accordance with the IEC 60034-17 definition.iv) These curves are based on the practical experience of GAMBICA and REMAmembers.

The example measurements shown are typical oflower power motor and are for illustrativepurposes only as the actual peak voltages aredependent on a series of factors including -motor rating, winding configuration, connectiondetails (star or delta) and cable type/size,However, the test results plotted in Figure 17illustrate in principle the effect of lengthening themotor cable.The rise time increases steadily withincreasing length, whilst the peak voltageovershoot tends to reduce after a peak at about50m.The voltage stress on the motor thereforeusually declines above quite moderate cablelengths (except in the special case of very longmotor cables described in Section 7.7).From the test results given in Figure 17 above, itcan be seen that standard PWM drives with cablelengths of 20m or more can produce peak

voltages at the motor terminals that are outsidethe IEC60034-17:1998 profile, even whenoperating from a 400/415V a.c. supply. Curve Aindicates that REMA motor manufacturer sproduce, as standard, motors whose capabilitysubstantially exceeds the requirements ofIEC60034-17:1998, and the enhanced insulationsystems developed by REMA manufacturersexceeds the NEMA cur ve requirements andcomfortably meet the 690V peak voltagerequirements.Figure 17 can be used in discussions with nonREMA motor manufacturers to indicate the peakvoltage/rise-time withstand profile that is requiredfor reliable operation (Cur ve A or B depending onsupply voltage) and to ensure that the expectedoperating life is achieved.An alternative motorsupplier should be asked to confirm this capability.

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Curve A - standard insulation

Curve B - enhanced insulation or form wound

IEC60034-17:1998

NEMA MG1 pt31:1993

Example of test results withsteel wire armour cable at thelengths indicated:

415V a.c.measurements

690V a.c.measurements

Effects of increasing motor rating

Pulse Withstand Curves

≤ 500V a.c. (Curve A)

≥ 500-690V a.c . (Curve B)

IEC 60034 -17:1998

NEMAMG1 pt 31:1993

11

Voltage Pulse Rise Time (µs)

5m

20m30m

50m 100m

In applications where it is not feasible to employmotors which meet the withstand capabilityachieved with standard or enhanced insulationgiven in Figure 17 curve A or B respectively, someform of alternative solution is required. Exampleswhere these alternative solutions may be requiredinclude:

● Undefined motor characteristics

● Retrofit application of VSDs to ‘old’ motors

● Motors with inadequate pulse withstand capabilities

In the above cases, some form of motor terminalvoltage modification technique is necessar y.Thesetechniques essentially involve placing additionalapparatus between the motor and the inverter tolimit the rate of rise of the pulse, reduce thereflection coefficient and thereby reduce the peakvoltage level. Some of the devices are also usedto compensate for large capacitive cable chargingcurrents.These techniques may be summarised asfollows:

● Output Reactors

● Output dv/dt Filters

● Sinusoidal Filters

● Motor Termination Units

These solutions should be correctly matched tothe application and the basic characteristics are asdescribed below.

6.2 Output Reactors

These are specially designed reactors which canaccommodate the PWMwaveform withoutcausing undue reactor heating and can alsoprovide the necessary inductance values over thefrequency spectrum needed.They are used toreduce the dv/dt and peak voltage. However, careis needed as reactors can theoretically extend theduration of overshoot if incorrectly selected -particular care is needed with ferrite corereactors. In the case shown in Figure 18(a), whichcompares to Figure 6(c) (42m case), the additionof the reactor has extended the rise time toaround 5us and reduced the peak voltage to

792V - acceptable to most standard motors.Normally, the output reactor is mounted with theinverter cabinet and of course leads to theacceptance of extra space , cost and reducedefficiency (less than approx. 0.5%). Output reactorscan also be used to compensate for cable chargingcurrents (balances cable capacitance) and may beused for motor cable lengths up to many hundredmetres on larger drives (check technical data).

6.3 Voltage Limiting Filter (dv/dt Filter)

In this case, a design consisting of capacitors,inductors and diodes or resistors may be used tolimit the dv/dt to typically less than 500V/µs (tocomply with the 1992 version of IEC34-17),drastically reducing both the amplitude and therate of rise of the peak voltage. In the exampleshown in Figure 18(b), the peak voltage is reducedto 684V with a dv/dt of 40V/µs. Such filters allowthe use of most motors without problem and aretherefore recommended if the data of a motor isunknown (e.g. in the case of a retrofit), particularlyon higher voltage supplies (›500V). Increasedlosses of 0.5 - 1.0% must be accommodated.

6.4 Sinusoidal Filter

A special design of low pass filter allows the highfrequency currents to be shunted away.Thisresulting waveform at the motor becomessinusoidal, the voltage and current are, for onecycle of the waveform, as shown in Figure 18(c).These types of filters are the most expensive andalso have other limitations.They prevent themotor voltage from exceeding 90% of the supplyvoltage (thereby de-rating the inverter).They alsowill not be suitable for applications that requirehigh dynamic performance. However, they havethe following additional advantages:

● Reduced motor noise

● Reduced motor losses

● Simplifies hazardous area motor certification

● Allows use of standard motors and long motorcables (eliminates capacitive charging currents)

12

6.1 General

6.5 Motor Termination Unit

Some manufacturers produce seriesresistive/capacitive filters which may be locallyconnected at the motor terminals (usually as anextra box mounted on the motor).The fast risingincident pulse sees the capacitor as a short circuitand the resistive element is temporarilyconnected across the end of the cable. If thisresistor approximates to the characteristicimpedence of the cable , overvoltages will notoccur. As the capacitor charges, the currentthrough the circuit reduces - therefore the lossesin the resistor are limited to the rising edgeduration.Typically, these filters add around 0.5 -1.0% losses.For example illustrated in Figure 18(d) the peakvoltage is now only 800V with a rise time of 2µswhich should be suitable for most motors.To date, these devices have not been popular.One concern is that the parallel connection

would be compromised thereby subjecting themotor to the high transients without any warning.Some users [4] have reported potential difficultiesin matching the inverter current rating to themotor rating to obtain the inherent l2t protectionfacilities available on many drives (presumably dueto the terminator capacitor changing current).Termination units must not be used on ‘EX’motors..

6.6 Relative Characteristics ofPreventative Measures

The relative motor terminal voltage characteristics[5] of the preventative measures discussed aboveare shown in Figure 18 and should be comparedwith the earlier Figure 6(c).The mitigation methods below are specific to themotor/inverter installation and the actual selectionshould therefore be advised by the drive supplier.

13

Note scale changes

Figure 18: Relative characteristics of alternative preventative measures.

6.7 Cost Comparisons for Preventative Measures

In considering the relative merits of thecompeting solutions, the issue of costs should alsobe considered.The table below gives someindications.

7.1 Cable Capacitance Effects

In addition to the peak voltage effects, totalmotor cable length should additionally beconsidered in the context of instantaneouscurrent peaks.At each inverter output pulse, thedistributed cable capacitance (C1 - Cn in Figure3) must be charged and discharged. For smallmotors with ‘long’ cables, the cable chargingcurrents may be of the same order as the motorrated current! Cable charging currents may causenuisance inverter overcurrent tripping.This leadsto recommendations for each inverter frame sizeregarding the maximum cable lengths for bothshielded (braided or armoured) or unshieldedcables and may vary from 10m on very smalldrives to above 250m on high power drives.Mitigation measures such as additional reactors,transformers or filters may be used to extend themaximum cable length - refer to the technicalcatalogues of the particular inverter type.

7.2 Combined Inverter/Motor Solutions

In the mid 1990’s manufacturers introduced thecombined motor inverter topologies in which theinverter is integrally mounted within the motorenclosure, typically in the terminal box orsometimes as an extension to the motor casing.The very short cable length between the inverteroutput connections and the motor windings limitsthe reflections and therefore the peak voltageproblems do not exist. Taken in the context of theadditional benefits of simplified installation,reduced EMC problems and lower overall costs,this solution is well suited to lower powerapplications and is now rapidly gaining marketacceptance.

7.3 Applications With Frequent orContinuous Braking Duties

For applications such as powered unwind standson web handling machines, the motor may spenda large part of its operating time in the brakingmode.The braking energy is transferred through

14

T Y P I C A L R E L ATIVE COSTS - DRIVES AND PREVENTATIVE MEASURES( M O TOR = 100%)

P R E V E N TATIVE MEASURE

R AT I N G D R I V E O U T P U T OUTPUT dv/dt S I N U S O I D A L M O TO RI N D U C TO R F I LT E R F I LT E R T E R M I N ATION UNIT

2 . 2 k W, 415V 3 5 0 % 7 5 % 4 4 0 % 3 3 0 % 1 7 0 %

7 5 k W, 415V 2 2 0 % 1 5 % 1 0 0 % 1 5 0 % 1 0 %

2 5 0 k W, 415V 1 2 0 % 5 % 6 5 % 11 0 % 3 %

1 6 0 k W, 690V 1 5 0 % 1 5 % 4 0 % N / A 4 %

2 5 0 k W, 690V 1 4 0 % 1 5 % 4 0 % N / A 2 %

5 0 0 k W, 690V 1 4 0 % 1 5 % 3 5 % N / A 1 %

POWER CIRCUITO U T L I N E

Table 2:Additional preventative methods and costs

flywheel diodes back on to the intermediate DClink, thereby giving a 15-20% increase in the DClink voltage (and also therefore the peak motorvoltage).The effect is similar to increasing thevoltage supply by up to 20%; this should be takeninto consideration - e.g. treat a 400V applicationas if it was supplied with 480V (i.e. standardmotor would still be suitable).

7.4 Active Front End (SinusoidalRectifier) Considerations

For drives with PWMactive front ends(regenerative and/or unity power factor), specialconsiderations may be required.As a function ofthe operation of active front end drives, the DClink voltage is continuously between 10-15%higher than for standard inverters.The effect issimilar to increasing the supply voltage by up to15%; this should be taken into consideration - e.g.treat a 480V application as if it was supplied with550V (i.e. enhanced insulation or otherpreventative measure now required). Refer to theinverter supplier for further guidance

7.5 Drives with Special ControlStrategies

Some drive types use control strategies without aconventional PWM modulator - e.g. direct torqueinjection schemes.These systems could double

7.6 Explosive Atmosphere Approval

The application of inverters to ‘Ex’ motors mayinvalidate the hazardous area certification - referto the motor manufacturer.

7.7 Applications With ‘Very Long’ CableLengths

The definition of ‘very long’ depends on the driverating and type, and may vary between 250m forlower power drives and 500m for higher powerratings - refer to manufacturers technicaldocumentation. For these applications, newfactors which could influence the voltage stress,are introduced and the drive supplier should beconsulted.

the motor peak voltage stress compared to driveswith conventional modulators and furtherprecautions may be required. GAMBICAmanufacturers do not produce this type ofinverter and both the drive and motor suppliersshould be consulted to confirm any specificmeasures required.

• A combination of fast switching transistors and ‘long’ motor cables can cause peak voltages upto twice the DC link voltage (2.7 times the supply voltage) due to transmission line effects. Inextreme cases, this high peak voltage and the uneven voltage distribution in the motor windingscan cause a low energy partial discharge between turns of the first coil. Partial discharge cancause premature ageing effects of the winding insulation system until failure occurs.

• By selecting the correct motor, or by the use of appropriate preventative measures, damagingpartial discharge can be avoided, thereby ensuring the maximum intended motor lifetime isachieved.

Following the recomendations described in this report will ensure thatmotor insulation life is not compromised.

15

[1] Persson E, “Transient Effects in Application of PWM inverters to Induction Motors”IEEE-IAS 28.1095-1101, 1992

[2] Mohan, Undeland & Robbins, “Power Electronic Converters,Application & Design”Wiley 1989

[3] Kaufhold M, Borner G, Eberhardt M, Speck J,“Failure Mechanisms of the Interturn Insulation of LowVoltage Machines Fed by Pulse Controlled Inverters”

IEEE Electrical Insulation Magazine Vol 12, No. 5, 1996

[4] Doherty, K G, “Investigation of Voltage Reflections Associated with PWMInverter Installations”IEE Submission 1996

[5] Finlayson P T, “Output Filter Considerations for PWM Inverter Drives With Induction Motors”IEEE, Industry Application Magazine Jan 1998

[a] GAMBICA/REMA, “Motor Insulation & PWM Inverter Drives” Shortform Guide 1999

[b] IEC60034-17:1998 Rotating electrical machines - cage induction motors when fed from converter s- application guide

[c] NEMA MG1-1993: Motors and Generators - parts 30 - Application considerations for constant speed motors used on a sinusoidel bus with harmonic content and general purpose motors used with variable-voltage or variable-frequency controls or both.

[d] NEMA MG1-1993: Motors and Generators-parts 31 “DEFINITE - PURPOSE INVERTER - FED MOTORS

16

LIST OF MEMBERSFIRST

EDITION

THE GAMBICA ASSOCIATION LIMITEDWESTMINSTER TOWER3 ALBERT EMBANKMENTLONDON SE1 7SWTel: +44 (0)20 7793 3050Fax: +44 (0)20 7793 7635Email: assoc@gambica.org . u kWeb: www. g a m b i c a . o rg . u k

R O TATING ELECTRICAL MACHINES ASSOCIAT I O NWESTMINSTER TOWER3 ALBERT EMBANKMENTLONDON SE1 7SLTel: +44 (0)20 7793 3041Fax: +44 (0)20 7582 8020

GAMBICA is the Association for Instrumentation, Control, Automation and has a product group for suppliers of Variable SpeedDrives. A GAMBICA guide to the Power Electronic Variable Speed Drives and Systems and their suppliers is available.

REMA is the Rotating Electrical Machines Association re p resenting manufacturers of rotating electrical machines, other than turbinetype machines, traction motors or machines for the use in airc r a f t .

The greatest care has been taken to ensure the accuracy of the information contained in this guide, but no liability can be acceptedby GAMBICA, REMA or their members, for errors of any kind.

Always refer to your Drive and Motor Suppliers if in doubt about correct matching.

GAMBICA MEMBERS- Variable Speed Drives

ALSTOM Power ConversionBoughton RoadRugby CV21 1BUTel: +44 (0)1788 563625Fax: +44 (0)1788 563756E m a i l :g e n e r a l . d r i v e s @ i n d . a l s t o m . c o mWeb: www. a l s t o m . c o m

Claude Lyons LtdB rook Road, Waltham Cro s sH e rt f o rd s h i re EN8 7LRTel: +44 (0)1992 768888Fax: +44 (0)1992 788000Email: info@claudelyons.co.ukWeb: www. c l a u d e l y o n s . c o . u k

Control Techniques plcSt Giles Technology ParkNewtown, Powys SY16 3AJTel: +44 (0)1686 612900Fax: +44 (0)1686 612999Web: www. c o n t ro l t e c h . c o . u k

Danfoss LtdPerivale Industrial ParkHorsenden Lane SouthG re e n f o rd, Middlesex UB6 7QETel: +44 (0)20 8991 7000Fax: +44 (0)20 8991 7171Web: www. d a n f o s s . c o . u k

Eaton Ltd, Cutler-Hammer DivisionElstow RoadB e d f o rd MK42 9LHTel: +44 (0)1234 267433Fax: +44 (0)1234 267607Email: ch_help_uk@eaton.comWeb: www. c u t l e r- h a m m e r. c o m

HID Ltd / HitachiS h u t t l e w o rth Close, Gapton HallInd Estate, Great Ya rm o u t hN o rfolk NR31 0NQTel: +44 (0)1493 442525Fax: +44 (0)1493 442323Email: sales@hid.co.ukWeb: www. h i d . c o . u k

Hill Graham Controls LtdLincoln Road, Cre s s e xHigh Wy c o m b eBucks HP12 3RBTel: +44 (0)1494 440121Fax: +44 (0)1494 438810Email: hillgraham@compuserv e . c o m

Mitsubishi ElectricEurope B.V.Automation Systems DivisionTravellers Lane, HatfieldH e rts AL10 8XBTel: +44 (0)1707 276100Fax: +44 (0)1707 278695Web: www. i n d u s t r i a l . m e u k . c o . u k

Moeller Electric LtdPO Box 35,Gatehouse CloseAy l e s b u ry, Bucks HP19 3DHTel: +44 (0)1296 393322Fax: +44 (0)1296 421854Email: support @ m o e l l e r. c o . u kWeb: www. m o e l l e r. c o . u k

R-R Industrial ControlsLtdKingsway Team Valley Trading EstateGateshead Tyne & Wear NE11 OQJTel: +44 (0)191 487 0811Fax: +44 (0)191 482 0006Email: pgen@rr i c . d e m o n . c o . u kWeb: www. ro l l s - ro y c e . c o m

Rockwell Automation LtdPitfield, Kiln FarmMilton Keynes MK11 3DRTel: +44 (0)1908 838800Fax: +44 (0)1908 261917Email: emcourt @ r a . ro c k w e l l . c o mWeb: w w w. a u t o m a t i o n . ro c k w e l l . c o m

Schneider Electric LtdUniversity of Wa rwick Science ParkSir William Lyons RoadCoventry CV4 7EZTel: +44 (0)24 7641 6255Fax: +44 (0)24 7641 7517Web: www. s c h n e i d e r.co.uk

Siemens plcAutomation & DrivesSir William Siemens HousePrincess RoadManchester M20 2URTel: +44 (0)161 446 6400Fax: +44 (0)161 446 5 7 5 1Email: info@plcman.siemens.co.ukWeb: www. s i e m e n s - i n d u s t ry. c o . u k

Toshiba International(Europe) LtdAlbany House, 71-79Station Road, West DraytonMiddlesex UB7 7LTTel: +44 (0)1895 427400Fax: +44 (0)1895 449493Email: sales@toshibai.co.ukWeb: www. t o s h i b a - e u ro p e . c o m

Yaskawa Electric EuropeGmbHUnits 2/3 Centurion CourtBrick Close, Kiln FarmMilton Keynes Bucks MK11 3JATel: +44 (0)1908 565874Fax: +44 (0)1908 565891Web: www. y a s k a w a . d e

REMA MEMBERS- Motors

ALSTOM ElectricalMachines LtdLeicester Road, RugbyWa rw i c k s h i re CV21 1BDTel: +44 (0)1788 542121Fax: +44 (0)1788 541280

ALSTOM Energy LtdLichfield RoadS t a ff o rd ST17 4LNTel: +44 (0)1785 223221Fax: +44 (0)1785 274176

Electric MotorDevelopments (Halstead) LtdKings Road, HalsteadEssex CO9 1HLTel: +44 (0)1787 473461Fax: +44 (0)1787 477311

Invensys BrookCromptonSt. Thomas Road,H u d d e r s f i e l d ,West Yo r k s h i re ,HD1 3LJTel: +44 (0)1484 42215Fax: +44 (0)1484 548718w w w. b ro o k c ro m p t o n . c o m

Lothian Electric MachinesLtdHospital Road, HaddingtonEast LothianScotland EH41 3PDTel: +44 (0)162 082 3611Fax: +44 (0)162 082 5412

Morley ElectricalEngineering Co LtdB r a d f o rd Road, LeedsWest Yo r k s h i re LS28 6QATel: +44 (0)113 257 1734Fax: +44 (0)113 257 0751Web: www. m e e c . c o m

SEM LtdFaraday Wa yOrpington Kent BR5 3QTTel: +44 (0)1689 884700Fax: +44 (0)1689 884884

Siemens plcAutomation & DrivesSir William Siemens HousePrincess RoadManchester M20 2URTel: +44 (0)161 446 6400Fax: +44 (0)161 446 5471

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