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A SHRAE JOURNAL

Insulated Gate BiPolar Junction Transistors (IGBTs) have been appliedin variable speed drives since the early 1990s. These devices changedthe characteristics of wave forms applied to the motors due to the

speeds at which they cycle on and off. Variable speed drives have used atechnique known as Pulse Width Modulation (PWM) for nearly 30years for controlling power transistors. However, as the switching speedsincreased to allow higher carrier frequencies from 1 or 2 kHz to 8, 15or as high as 20 kHz, new concerns arose over phenomena previouslyseen only in wave transmission devices like antennae and broadcast signalequipment. The basic reliable motor control did not change in theory,but faster transistor switching time changed the application variables such asdrive to motor lead length.

characteristics stable while operating atvarying speeds. Additionally, when con-trolling IGBT transistors, the drive designengineer must maintain a transistor’s on-time to off time ratio for motor stability. Indoing so at higher carrier frequencies, thetransition-time from off to on and then onto back off again must be maintained as asmall percentage of the overall PWMcycle.

Figure 1 shows the switching of a Bi-polar Junction Transistor (BJT) versus anIGBT as an example of how the increasedswitching speeds effect the turn-on andturn-off times as a ratio of the overallcycle. Note that the BJTs are switching ata frequency of 2 kHz and the IGBTs areswitching at a frequency of 8 kHz, or fourtimes faster.

The high rate of change in voltageover relatively short periods of time isknow as the dv/dt of the voltage pulse.The dv/dt is quantified in units of voltsper microsecond. As the number of

pulses increase, so must the dv/dt. Notethat this voltage waveform is a functionof the drive design and is not a usersettable parameter.

The maximum design carrier frequencysets the limits on how fast a transistormust cycle on and off. If the waveformshown at the output of the drive was iden-tical at the motor terminals, this high dv/dt would not be a concern, but as thiswave propagates through the conductor,the characteristics can change and bevastly different at each end of the cable.

This article describes the relation of dv/dt in the IGBT to that of the older BJT. Animportant concept here is that when de-sign engineers use IGBTs, they must setratios of on-time to off time versus the turn-on and turn-off times to maintain stabilityin the motor. Essentially the turn-on andturn-off times (which sets the dv/dt) mustbe a small percentage of the overall pulsecycle. If a design engineer wants to estab-lish the maximum carrier frequency of thedrive to be 8kHz for example, the turn-on,turn-off, and dv/dt are set for this fre-quency in hardware. The dv/dt is not a pro-grammable function.

If a user runs the drive at a lower car-rier frequency, 2 kHz for example, the dv/dtis still set for the 8kHz running condition.Comparing IGBT to BJT from a dv/dtstandpoint must account for these oper-ating characteristics. IGBT drives couldbe set at a maximum of 2 kHz carrier fre-quency and the dv/dt would be identicalto that of an older BJT, but in applicationthis is never done. All commercially avail-able IGBT-based drives switch at these

Design Considerations for MotorsAnd Variable Speed Drives

By Thomas F. Lowery

About the Author

Member ASHRAE

Thomas F. Lowery is the product line managerfor custom/configured drives at Rockwell Auto-mation/Reliance Electric in Cleveland. He is chairof ASHRAE Technical Committee 8.11, Motors andMotor Controls.

Lead lengths should not exceedmanufacturer’s recommendations. If pos-sible, design the system by placing thedrive as close to the motor as possible.This should be the primary goal of theengineer because this method usually isthe most cost effective. However, certainapplications may not permit short leadsand other methods must be considered.Engineers now must consider these fac-tors when applying newer IGBT-basedvariable speed drives.

Switching Times and dv/dtControl algorithms for PWM drives

must accurately output a specific volt-age at a given fundamental motor fre-quency. This ratio, known as the V/Hzpattern, keeps motor current and torque

The following article was published in ASHRAE Journal, February 1999. © Copyright 1999 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paperform without permission of ASHRAE.

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higher carrier frequencies so the comparison between the twodevices is accurate.

Drive to Motor ConductorsThere are several conductor characteristics that affect the

voltage pulse wave form and specific dv/dt effects at both thedrive and motor terminals. Impedance or electrical resistance inAC circuits has an impact on the voltage pulse as it travels fromthe drive to the motor. The motor impedance and the relation-ship to the cable impedance is important when analyzing thepulse wave transmission. When the cable impedance closelymatches the motor impedance, the voltage pulse is evenly dis-tributed across the system. However, when the motor imped-ance is much larger than the cable impedance, the pulse willreflect at the motor terminals, causing standing waves.1

A filter and/or reactors can be placed between the drive andmotor. There are a number of commercially available devicesthat are designed to eliminate the standing waves. If pursuingthis method of attenuation, an engineer or user should followthe recommendations set by the manufacturer in applying thesedevices. Cost is also a factor when selecting filtering devices.MTE, TCI and EMS are companies that have published a greatdeal of information on the subject.

Figure 2 shows the surge impedance of both the motor andthe cable for different horsepowers. Note that relatively smallmotors, less than 2 hp, have very high impedance with respectto typical cable. Larger motors, greater than 100 hp, closelymatch cable impedance values.

Damaging reflected waves are more likely to occur in smallermotors because of the mismatch in surge impedance values.2,3

If multiple small motors are run from a single drive, the potentialfor reflected waves is high. Special considerations must be givento designing such a system.

Riding the Reflected WaveReflected waves damage motors because transmitted pulses

and reflected pulses added together can cause very high volt-age levels. Since these voltage pulses are transmitted throughthe conductor over specific distances, the cable length is a keyvariable when examining the potential for damaging voltages.Figure 3 shows the relationship between cable distance, switch-ing times, and voltage levels of pulses at motor terminals.4

The voltage level at which motor damage begins is deter-mined by the materials used in the insulation system.5 When

specifying motors for operation on variable speed PWM drives,engineers should specify the voltage withstand level based onthe dv/dt of the drive and the known cable distance.

There are no simple calculations to show how peak voltagerelates to lead lengths. The problem here is that there are manyvariables that go into these calculations. The best thing to dowould be to work with your drive and motor supplier to deter-mine appropriate lead lengths for each application. Standingwave and reflected wave mathematical equations are very com-plex, and even by making several assumptions by fixing someof the variables this analysis is difficult. Manufacturers shoulddetermine the allowable lead length with their drive and state itto the application engineers. This is less of a concern with 208/230 volt applications, since the motor insulation material isalways higher rated than low voltage drives can generate.

Because motor manufacturers do not use lower rated materi-als specifically for these lower voltages, the insulation systemsare identical to that of a 460 or higher rated motor. Theoretically,voltage peaks generated by drives on a 208/230 volt systemcan never reach these rated values and therefore there is noconcern for damage due to over-voltage at any lead length atany carrier frequency.

Figure 1: BiPolar vs. IGBT PWM switching.

Figure 2: Motor/drive relative impedance.

Figure 3: Switching times, cable distance and pulse peakvoltages.

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To describe the reflected wave phenomena, oscilloscopemeasurements were taken at each end of the drive to motorconductor. These traces, shown in Figure 4, demonstrate theeffect of transmitted and reflected pulses adding together form-ing damaging voltage potentials. The induction motor must bedesigned to withstand these voltage levels or insulation break-down will occur.6

Motor Ratings and NEMA StandardsA 460-volt induction motor is constructed to withstand volt-

age levels higher than the nameplate might suggest.7 The spe-cific maximum voltage withstand value should be obtained fromthe manufacturer, but typical values for 208V and 460V motorsrange from 1,000 volts up to 1,800 volts. Higher voltages suchas motors fed from 575V power systems may be rated up to2,000V peak.

This rating is determined by the design and materials usedto insulate the induction motor. NEMA, the National ElectricalManufacturers Association, has established a standard to as-sist motor specifying engineers as part of the MG1 standard.NEMA MG1-1993, Part 31.40.4.28 states the established PWMdrive fed motor limits and is shown in Figure 5.

This standard establishes a V peak of 1,600 volts and a mini-mum rise time (dt) of 0.1 microseconds for motors rated lessthan 600 Volts. Specifying motors applied on drives that meet or

exceed these limits assures the design engineer that insulationbreakdown will not occur in the motor from excessive voltagepeaks or fast IGBT switching times.

Motor Insulation Break DownDamage to the motor can occur only if the peak voltage or

minimum rise time is exceeded. If reflected waves generate volt-age levels higher than the allowable peak, insulation begins tobreak down. This phenomenon is known as Partial Discharge(PD) or corona. Figure 6 shows how intense electrical fieldsaround motor conductors can ionize surrounding gases caus-ing damaging corona effects.

When two phases or two turns in the motor pass next toeach other high voltage peaks can cause a spark plug effect,damaging the insulation. The voltage at which this begins isreferred to as the Corona Inception Voltage or CIV rating of themotor6 (Figure 7). Again, NEMA specifies this level at 1,600Volts in MG1.

Eventually, air gaps inside the varnish material ionize due tothe high voltage gradients causing phase-to-phase or turn-to-turn short circuits. These circuits are microscopic insulationbreakdowns and are usually detected by the drive current sen-sors resulting in over-current trips. Under this short circuit con-dition, a motor may operate properly when run across the line orin bypass mode but consistently trip when run from drive power.

Figure 4: Reflected wave voltage levels at drive and motor. Figure 5: Motor voltage peak and dv/dt limits. NEMA MG1-1993, Part 30, Figure 30-5.

Figure 6: The ionization of a gas or partial discharge fromexcessive voltage peaks and high dv/dt in motor conductors.

Figure 7: Damaging reflected waves above motor CIV levels.

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Motor factory testing may be required toconfirm this failure mode. Figure 8 showshow air gaps in the varnish material ion-ize, causing these microscopic short cir-cuits across two conductors.

Special motor insulation systems canbe added to accommodate even the high-est of generated peak voltages. Thesesystems usually use special high voltagedie-electric withstand material that doesnot break down inside the motor. Usuallythis cost is prohibitive in most cases, butit is still a possible solution to a difficultproblem.

ConclusionsVariable speed drive technology

changes with advancements in power semi-conductor designs. As IGBT designs al-low drive design engineers to increase theswitching frequency utilized in PWMdrives, quieter motor operation is achievedby removing objectionable audible motorlamination vibrations in the human hear-ing spectrum. This drive advancementhelps design engineers use drives in criti-cal noise applications that previously re-lied on mechanical VAV systems. Thehigher switching frequency also forcessystem design engineers to consider newapplication criteria such as drive-to-motorlead length and motor insulation systemswhen selecting the equipment.

NEMA has adopted a standard formotors applied on variable speed drivesthat should be used by the engineer toensure a proper system. Drive and motormanufacturers now publish informationon dv/dt, lead lengths, and motor CIV lev-els to assist in the design process. Thisinformation must be used to determinethe application variables of the variablespeed drive system to insure reliable op-eration. By considering all these drive andmotor variables and using published data,VAV system designers can take advan-tage of all the energy saving and systembenefits from drives without sacrificingany of the reliability of the system.

References1. Kerkman, R., D. Leggate and G. Skibinski.1997. “Cable characteristics and their influ-ence on motor over-voltages” IEEE AppliedElectronic Conference (APEC).2. Evon, S., et al. 1996. “IGBT drive technol-ogy demands new motor and cable consider-

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ations.” IEEE Petroleum & Chemical Indus-try Conf., Sept.

3. Kerkman, R., D. Leggate, G. Skibinski. 1996.“Interaction of drive modulation & cable pa-rameters on AC motor transients.” IEEE In-dustry Application Society Conference.

4. Takahashi, T., et al. 1995. “Motor lead lengthissues for IGBT PWM drives.” IEEE Pulpand Paper Conf. Record.

5. Saunders, L ., et al. 1996 “Modern driveapplication issues and solutions.” IEEE PCICConf. Tutorial on Reflected Wave, Motor Fail-ure, CM Electrical Noise and Motor BearingCurrent.

6. Sung, J. and S. Bell. 1996. “Will your mo-tor insulation survive a new adjustable fre-quency drive?” IEEE Petroleum & ChemicalIndustry Conf.

7. NEMA Recommended Motor-GeneratorStandards, MG-1-1993 Revision 1, Part 30,

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Table 1: Drive carrier frequency engineering application considerations.

Figure 8: Ionization of insulation air gaps.

Section IV, “Standard MG 1 motors,” Para-graph 30.02.2.9, Voltage Stress.

8. NEMA Recommended Motor-GeneratorStandards, MG-1-1993 Revision 1, Part 31,Section IV, “Definite purpose inverter fed mo-tors,” Paragraph 31.40.42, Voltage Spikes.

9. Malfait A., R. Reekmans and R. Belmans.1994. “Audible noise and losses in variablespeed induction motor drives with IGBT in-verters-influence of the squirrel cage designand the switching frequency” IEEE 1994 Pro-ceedings of Industry Application Society 29th

Annual Meeting.