adjustable speed drive motor protection 2014

1364 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 2, MARCH/APRIL 2014

Adjustable-Speed Drive Motor ProtectionApplications and Issues

Working Group J1 of the Rotating Machinery Protection Subcommittee, Power System Relaying Committee

Jonathan D. Gardell, Chairman, and Prem Kumar, Vice ChairmanMembers of the Working Group are: M. Bajpai, M. Basler, S. P. Conrad, T. L. Crawley, T. A. Farr (Corresponding

Author), E. C. Fennell, D. Finney, D. Fredrickson, A. Guggisberg, W. G. Hartmann, P. Kerrigan, H. J. King,F. Lopez, J. Park, S. C. Patel, M. L. Reichard, C. Ruckman, S. Thakur, J. T. Uchiyama, S. M. Usman

Abstract—The uniqueness of the electrical environment (asyn-chronous connection to power system, variable frequency oper-ation, and harmonics) on the output of adjustable-speed drives(ASDs) requires special consideration to be given to the motor pro-tection. The Working Group has investigated and addressed theseconcerns. This paper announces the report on the findings of thisactivity and the recommendations made to the motor protectionengineer. The full report can be found at www.pes-psrc.org underPublished Reports.

Index Terms—Adjustable-speed drives (ASDs), ASD motors,drives, motor protection.

I. INTRODUCTION

THE FULL report provides the background and guidancewith regard to protecting motors controlled by adjustable-

speed drives (ASDs). The report does not cover the specificsof all drive topologies for motor applications due to the largenumber of types and technologies. The intent is to provideguidance for the protection application engineer to implementadequate motor system protection given the special electricalconditions found on the output of a drive.

ASDs have been widely used in industry for many years.The Rotating Machinery Protection Subcommittee of the IEEEPower System Relaying Committee believed that, prior to re-vising the IEEE Guide for AC Motor Protection, this equipmentand its impact on motor protection applied external to the ASDshould be investigated. Manufacturers of ASDs have providedmotor and drive protection functions in their system controls,and some have suggested that additional external protection

Manuscript received January 30, 2013; accepted July 7, 2013. Date ofcurrent version March 17, 2014. Paper 2012-PSPC-778, presented at the 2013IEEE/IAS Industrial and Commercial Power Systems Technical Conference,Stone Mountain, GA, USA, April 30–May 3, and approved for publicationin the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the PowerSystems Protection Committee of the IEEE Industry Applications Society.

T. A. Farr is with the Eaton Ampgard, Arden, NC 28704 USA (e-mail:[email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIA.2013.2274632

for the motor and system is not required. The Subcommitteewanted to better understand this technology and have a clearerposition on appropriate protection.

II. OVERVIEW

This Working Group assignment is to provide a recommen-dation to the Rotating Machinery Protection Subcommitteeof the IEEE Power System Relaying Committee regardingprotection requirements of motors applied with ASDs. Therecommendation will be incorporated into the next revision ofC37.96–IEEE Guide for AC Motor Protection [1]. The reportpresents the results of the Working Group J1 investigation basedon industry knowledge, experience, literature, current drivesprojects, and knowledgeable guest speakers.

III. HISTORY OF APPLICATION AND PURPOSE OF ASDs

Various drive technologies have been used to control thespeed of motors for many years. DC motors have been appliedfor process control beginning in the late 19th century. To varythe speed of a dc motor, one simply adjusts the applied dcvoltage or field strength. DC motors offer high torque andprecision control but are burdened with the high maintenanceof brush assemblies. AC drives have been used since the early20th century on a limited basis. The principles applied forvarying speed have been well understood by motor designersand application engineers for a long time, but the advent ofpower electronics in the late 20th century is what fosteredthe rapid growth and widespread use of high-power ac drivetechnology.

Two key benefits of using ASDs are improved process con-trol and efficiency. Due to the mechanical affinity laws in acentrifugal pump or fan application, the process flow is directlyproportional to speed, and mechanical horsepower varies asthe cube of speed. Therefore, for a centrifugal load driven at50% speed (flow), only 12.5% of the mechanical horsepoweris required, providing substantial energy savings when com-pared to line-connected operation. Traditional process controltechnologies, such as dampers and flow control valves, andgearboxes require maintenance and are not as energy efficientcompared with varying the process speed with an ASD.

0093-9994 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

GARDELL AND KUMAR: ADJUSTABLE-SPEED DRIVE MOTOR PROTECTION APPLICATIONS AND ISSUES 1365

Fig. 1. Basic blocks of an ASD system.

Other benefits that ASDs offer are as follows:

1) soft-start capability;2) short circuit reduction;3) reduced maintenance over other process control

technologies.

These benefits are important from an operational and powersystem design point of view. For example, an ASD controllinga large horsepower motor will not burden the connected buswith the negative impact of high inrush currents that come withacross-the-line starting and will limit the motor short circuitcontribution on that bus. ASDs provide the lowest possiblemotor inrush of any starting method. This ultimate soft-start ca-pability also reduces wear and tear on the attached mechanicalsystems. Unlike reduced voltage starters, the ASD-controlledmotor has full torque at any speed. In some distribution systems,it is possible to add more motors or horsepower capacitywithout exceeding the voltage drop limits for motor starting andshort circuit ratings of the associated breakers.

ASDs provide maximum energy savings on variable torqueloads where torque increases with speed, such as fans, blowers,centrifugal pumps, and most kinds of compressors. Constant-torque loads require the same torque regardless of speed suchas reciprocating compressors, positive-displacement pumps,conveyors, center winders, and drilling/milling machines. ASDmust be carefully sized to ensure adequate starting torque forthose applications. Loads in which torque decreases with speedusually involve very high inertia loads such as vehicular (drivesor flywheel) loaded applications. Custom-engineered solutionsare often required for those applications.

IV. ASD AND MOTOR SYSTEM DESCRIPTION

AC drive systems convert ac line power to dc, and the dcis then inverted to a simulated ac and applied to the motor tocontrol the speed of the rotating machine. The ASD system canbe categorized by various criteria. The following section givesan overview of different criteria. In order to define the properprotection scheme, it is essential to categorize the ASD system.

A. Categorization by Load Type

The following basic load types can be distinguished.

1) Variable torque load: Torque increases by the square ofspeed. Typical applications are fans, pumps, and centrifu-gal compressors.

2) Constant torque load: Torque is constant over the speedrange. Typical applications are conveyors, reciprocatingcompressors, paper mills, and rolling mills.

B. Categorization by the ASD Topology—SystemConfiguration

The following basic types can be distinguished:1) stand-alone ASD;2) ASD with manual bypass (motor can be operated in an

across-the-line start configuration);3) ASD with synchronized bypass (ASD can synchronize

the motor to the line and back to the drive);4) ASD with more than one motor connected to one single

drive;5) multimotor ASD system (multidrive); a common dc link

is used for several inverters to drive several motors.

C. Categorization by Operation Area

The following basic types can be distinguished.1) 1q, 2q, or 4q operation: 1q operation means that the motor

runs only in one direction and only in one mode (eitheras motor or as generator), 2q operation means that themotor runs in both direction but only in one mode (eitheras motor or generator), and 4q operation means that themotor runs in both directions and in both modes. Thisrequires either an active front end or dynamic breaking.

2) The motor is operated in field-weakened range.3) The motor is operated continuously in low-speed opera-

tion (0 Hz to approximately 5 Hz).4) The motor is operated at high speeds (in excess of ap-

proximately 100 Hz).

D. Categorization by ASD Topology

The basic blocks of an ASD system are shown in Fig. 1.All the blocks shown are not necessarily needed for an ASDsystem. Depending on the topology, some blocks are required,and some are not. A short explanation of each basic block isgiven hereinafter. Table I provides example ASD topologieswith characteristics.

Input Section: The input section provides a means to dis-connect the ASD from the main supply. It also provides thelast level of protection. This can be a circuit breaker, fusedcontactor, or load break switch.


TABLE IEXAMPLE ASD TOPOLOGY CHARACTERISTICS

Input Filter: The input filter is used to limit the harmoniccurrents induced to the network and improve the displacementpower factor. A harmonic filter is usually used in conjunctionwith an active front end or a current source inverter (CSI).

Input Transformer: The input transformer is used to isolatethe motor from the power system and can also be used tostep up or down the applied system voltage. The isolationtransformer will provide harmonic mitigation when suppliedwith multiple phase-shifted secondary windings. These multi-winding secondary transformers are used with a rectifier with apulse number greater than six, for example, a 12-, 18-, 24-, or36-pulse rectifier.

The transformer also provides the galvanic separation of theASD system from the network. Specifically, the zero-sequenceor common-mode voltage can be isolated between the networkand the motor.

Rectifier: The rectifier for a voltage source inverter (VSI)typically uses diodes. For low voltage (LV) ASD systems,a six-pulse rectifier or an active front end is very common.For medium voltage (MV) ASD systems, a 12-pulse rectifiermeets, in most cases, the network harmonic limit requirementsdefined in IEEE 519. However, for higher power ASD systems(typically above 5000 hp), pulse numbers greater than 12 arevery common. The rectifier for a CSI is usually an SCR rectifier.


Typical pulse numbers are between 6 and 24 pulses. In mostcases, an input filter is required to meet the harmonic limitsdefined in IEEE 519; 12-pulse configurations are very common.When SCRs are used in the rectifier, the power factor is verypoor at certain operation points. The power factor is basicallyequal to the firing angle of the SCR. Therefore, the harmonicfilter is also used for power factor compensation.

DC Link: The dc link is used to decouple the line-sideconverter (rectifier) from the motor-side converter (inverter). Inthe ideal case, the intermediate dc link provides infinite energystorage to filter all harmonics, which can couple effects fromeach side to the other. A VSI converter uses a capacitor bank todecouple both sides, where a CSI uses an inductor to decoupleboth sides. Both elements provide energy storage and act asfilters.

Inverter: The inverter is used to convert dc to variable ac.There are many different inverter topologies. The most commontopology is a two-level inverter. This topology is widely usedfor LV ASD systems. CSI converters are also based on a two-level inverter. In particular, for MV ASD systems, higher levelsof inverter configurations are built.

1) Two-level inverter. The output of this inverter can beswitched to two different + or − levels: 0 and fulldc bus.

2) Three-level inverter. The output of this inverter can beswitched to three different + or − levels: 0, half bus dc,and full bus dc.

3) Five-level inverter. The output of this inverter can beswitched to five different + or − levels: 0, 1/4 bus dc,1/2 bus dc, 3/4 bus dc, and full dc bus.

Output Filter: An output filter is used in order to smooth theoutput waveform. For VSI topologies, an output filter is used tolimit the voltage rise time or even eliminate all major load-sideharmonics. The cable distance between the drive and motor isthe determining factor in many cases. For CSI drives, the filteris used to smooth current harmonics which can create torquepulsations if not properly mitigated. Output filters can also beused to mitigate the effects of common mode.

Motor: The motor is always used in order to convert theelectrical energy into mechanical energy.

V. CHANGES TO MOTOR OPERATING CHARACTERISTICS

AND DYNAMICS THAT CAN IMPACT PROTECTION

When motors are controlled with ASDs, certain operatingcharacteristics of the motor are modified. The operating fre-quency impacts how the motor behaves during both the startingand the running operation as well as during abnormal operationand fault conditions. The areas that will be discussed arepertinent to the protection of the motor and drive system.

The following characteristics are pertinent to the protectionof the motor.

1) Motor ground fault conditions:a) ASD with input transformer: The input transformer

provides galvanic isolation between the motor/ASDand the feeder bus. A ground fault on the motor/ASDwill not influence the ground fault protection of thefeeder bus.

b) ASD without input transformer: There is no galvanicisolation between the motor/ASD and the network inthis configuration. A ground fault on the motor maytrigger the ground fault protection on the feeder bus. Itis recommended to check the ground fault protectionscheme of the ASD with the manufacturer to assurethe selectivity of the ground fault protection scheme(ASD ground fault protection should trip faster thanthe feeder bus ground fault protection).

2) Motor fault contributions: When a drive is applied toa motor, it provides a current-limiting feature such thatit will limit the contribution to the system short circuitlevel. In some cases, the contribution to short circuit canbe eliminated by switching the power electronics in thedrive such that any short circuit current contribution fromthe motor will not flow back to the point of the faultin the system. This is a significant benefit with regardto a large motor with a long short circuit time constantwhen considering limits on the system breakers for faultduties.

3) Reduced frequency operation effects: The frequency ofthe source to the motor dictates the operating speed. Atlower speed operation, the motor is not cooled as effi-ciently as it is at rated speed. Therefore, this must be takeninto consideration with regard to motor thermal overloadprotection. For constant-torque applications, auxiliarymotor cooling may be required. Actual motor full loadcurrent (FLA) is a function of the frequency, as lowerFLA is drawn at lower frequency. The actual FLA mustbe used in the overload protection. This is particularlyimportant for a sustained motor operation at off-nominalfrequency. Note that motor manufacturers typically statethe rated FLA at nominal frequency. Also, it is importantfor the protective device to accurately measure motorcurrent at off-nominal frequencies (by frequency trackingor other means) to provide effective overload protectionat all frequencies.

4) Harmonics: Harmonics in the motor current will causeadditional heating in the motors and other connectedelements. This additional heating needs to be consideredwhen sizing and protecting the equipment. At near ratedload, a typical value to accommodate the additional heat-ing can be up to a 15% increase above the fundamentalheating effects. Refer to NEMA MG-1 [9] for furtherdetails or derating factors.

5) Flux levels: State-of-the-art control algorithms used inASDs keep the motor flux constant over the entirespeed/frequency range. This results in a V/f charac-teristic shown in Fig. 2. The voltage is proportional tothe frequency (V/f = constant) in the upper frequencyrange. In the lower frequency range, the voltage is notproportional to the frequency. An extra voltage boost isapplied to compensate the voltage drop over the statorresistance of the motor. This characteristic should be con-sidered in sizing current transformers (CTs) and voltagetransformers.

6) Voltage and dielectric stress consideration should begiven for overvoltage protection due to the potential for


Fig. 2. Typical V/f curve of an induction motor.

this condition and the fact that many drives operate atfairly high semiconductor switching frequencies. Theconcern is especially important with drives applied onlong cable runs that can have high voltages developed dueto cable capacitance. Sustained overvoltage conditionscan be detected by overvoltage protection functions, butthe ringing effect must be mitigated by other voltagecontrol methods.

VI. MOTOR, SYSTEM, AND ASD PROTECTION ISSUES

The protection can be broken down into three major cate-gories: line-side protection (Zone 1), system-level protection(Zone 2), and load-side protection (Zone 3).

1) Zone 1 protection issues: The feeder breaker supplyingthe ASD typically is equipped with overload and shortcircuit protection for the input transformer and/or thedrive electronics. Typically, a phase time overcurrentelement (51) is applied for overload protection, and aninstantaneous overcurrent element (50) is applied forshort circuit protection. A 51 element that operates onthe fundamental frequency (i.e., not rms) may be set witha lower pickup, as it will not respond to the harmoniccomponents of the load current. If there is an isola-tion transformer, the 50 element is typically set with apickup of 140% of the transformer secondary through-fault current and above the transformer inrush current.In cases where the drive employs an active front end,the 50 element can be set lower as the drive normallylimits the starting current to less than two times fullload current. An instantaneous ground element (50N) canbe applied to give more sensitive protection for groundfaults. Occasionally, a differential relay has been appliedto the primary feeder to provide high-speed tripping forfaults up to the transformer high-side winding. Differ-ential protection for large isolation transformers can beconsidered only when it is practical. For large ASDapplications, the ASD isolation transformer typically hasmultiple secondary windings. In those cases, it is notpractical to have conventional differential protection. Thefeeder protection function 50 can then be relied uponto provide high-speed protection for the isolation trans-

former primary windings. Relays which mitigate dc offsetcurrents should be selected to allow for the 50 elementto be set as sensitive as possible. The feeder 51 canprovide conventional time-delayed protection. For multi-ple secondary winding configurations, the feeder 51 maynot provide protection for secondary winding faults. Thedrive integral protection would protect for those faults. Insome cases, the drive integral protection includes a powerdifferential that compares the transformer input and driveoutput power.

Where isolation transformers are used which havenot been specifically designed for harmonic load-ing, ANSI/IEEE C57.110 “Recommended Practice forEstablishing Liquid-Filled and Dry-Type Power andDistribution Transformer Capability When SupplyingNonsinusoidal Load Currents” may be used to applytransformer derating factors for each harmonic. Devicesexist that will provide thermal protection based on thisguide.

There may be additional protection applied for faultson the secondary side of the isolation transformer. Thismay include a zero-sequence voltage detection circuit ifthe transformer secondary is ungrounded or a residualor neutral overcurrent for a grounded wye secondaryconnection. Some ASD manufacturers employ fuses fortransformer through-fault protection.

In addition to providing protection, there are otherZone 1 protection aspects that may need to be reviewedwhen there is an ASD on a bus. This is especially true inretrofit applications.a) The short circuit contribution from the drive to any

fault on the bus or another feeder is typically negli-gible due to the current-limiting action of the drivecontrols as mentioned previously.

b) If a large motor, particularly one associated with ahigh inertia load such as a fan, is retrofitted withASD, the residual voltage on the bus during high-speed bus transfer may be less than before the retrofit.This may result in an unsuccessful transfer or possibledamage to other motors connected to the bus. Unlessa special control action, such as the use of a regen-erative drive, is implemented, the high inertia loadwill no longer contribute to keeping the bus voltagefrequency up during transfer. In addition, the driveelectronics will most likely draw extra vars from thebus, depressing the voltage further. Consult the ASDmanufacturer.

c) If there are capacitors on the bus feeding the ASD,such as power factor correction capacitors, resonancescan occur which can be damaging to the electricalequipment on the system. Capacitors located betweenthe drive output and the motor terminals should beavoided.

2) Zone 2 protection issues: Components internal to theASDs are typically well protected by the manufacturerand do not require additional protection consideration.Zone 2 equipment protection is beyond the scope of thereport.


3) Zone 3 protection issues: ASD manufacturers typicallyintegrate most of the required Zone 3 protection withinthe ASD internal electronics (see Section VII). Supple-mental motor protection such as overcurrent protectionand flux balance/differential protection (for large motors)may be considered. If applied, however, the off-frequencycharacteristics of the individual components comprisingthe supplemental protection should be carefully scruti-nized. In particular, attention should be given to the lowfrequency saturation point of current transformers andthe low frequency response characteristics of protectiverelays placed downstream of the ASD. Because of issuessurrounding these components, many ASD manufactur-ers do not recommend supplemental motor protection andwarn of inadvertent tripping when they are used.

If supplemental motor protection is used with the ASDsystem, the following items should be considered.

A. Overcurrent Protection

In conventional motor protection, overcurrent curves are setto protect a motor based on its thermal limit curves. Timeovercurrent curves are typically set below and to the left of thesemotor limit curves and above the acceleration curve to allow themotor to successfully accelerate.

Modern microprocessor-based motor protection relays havethermal models which approximate the heating effects that var-ious system conditions have on the stator and rotor. However,these thermal models rely on motor thermal damage curvelimits which are typically reported by motor manufacturers atonly a nominal frequency of 60 Hz. Unless the motor thermallimits are known over the operating frequency range of theASD, it may be difficult to fully utilize the thermal modelavailable in many modern motor protective relays.

If the thermal model cannot be used, it may be more practicalto use simple overcurrent relaying to provide motor overloadprotection. Either way, in this application, select a pickupbased on motor FLA (corresponding to the maximum operatingfrequency which will be close to the nominal frequency). Thiswill then provide overload current protection when the motor isoperating at or near the maximum operating frequency but willprovide reduced protection at lower frequencies.

If motor thermal limits are available at various frequencies,an alternate approach might be to implement adaptive char-acteristics which would provide full overload protection at allmotor frequencies.

B. Differential Relay

For large motors, differential protection is recommendedand can be provided by flux balance CTs. The subnominalfrequency characteristics of both the CTs and the differentialrelay should be verified as adequate for the application.

C. Ground Protection

The drive side of the isolation transformer typically hasmultiple secondary windings that are ungrounded, and thus,

dedicated ground fault protection may not be practical. Thedrive manufacturer provides internal ground fault protectionto detect load-side ground faults. External motor ground faultoption is typically not required unless the motor can also bestarted or operated across line (bypassing the ASD).

D. CT/Relay Harmonics

Care should be taken to select CTs which will not saturateover the expected operating frequency range of the ASD. TheCT performance at low frequency/high harmonic should beevaluated. At reduced frequencies, the CT capacity is corre-spondingly reduced, e.g., at 10% frequency the CT capabilityis about 10%. However, the drive-side fault current is relativelysmall (because of isolation from ac system). Therefore, theCT only has to be designed for motor contribution currents(relatively small currents). The relay performance at higherharmonics should also be verified as the harmonics at lowcurrents can be considerable. The use of a higher ratio CT (fivetimes the nominal rating) and a lower nominal current relay(1-A relay instead of a 5-A relay) would be an option to enhancethe overall CT/relay performance. The settings would have tobe appropriately adjusted for those conditions.

VII. PROTECTION COMMONLY INCLUDED

IN THE DRIVE SYSTEM

Various protection elements are included in ASDs, yet thetypes of protection included vary from manufacturer to man-ufacturer. The following is the protection most commonlyincluded in ASDs.

1) Line-side protection (Zone 1):a) Short circuit/overcurrent—some are protected with a

fuse, circuit breaker, or protective relay overcurrentfunction.

b) Overload—overcurrent protection with time delay.c) Voltage unbalance—loss of input phase.d) Ground fault overcurrent.

2) System-level protection (Zone 2):a) DC overvoltage.b) DC undervoltage—loss of control power.c) Overtemperature—this includes the rectifier and in-

verter heat sinks as well as the enclosure temperature.3) Load-side protection (Zone 3):

a) Ground fault.b) Motor overcurrent.c) Motor overload I2t.d) Motor stall.e) Motor overspeed.f) Current unbalance.g) Underload—this may indicate a process malfunction

and will protect the machinery and the process in thisfault condition.

h) External fault—an external relay input.Typically, ASDs offer a current limiter and torque

limiter function. These functions can be programmed inorder to keep the current and/or the torque at a maximumallowed limit. In case the current or torque demand from


Fig. 3. Induction motor, with transformer and no motor differential.

the process or speed controller exceeds the current/torquelimit, the actual speed is limited, and the current/torque iskept below the limits. This function can be used to limitthe current to the motor.

Although the aforementioned protection is commonlyfound in most manufacturers of ASDs, their implemen-

tation of that protection may vary. Some manufacturerssupply other protection within the ASD such as thefollowing:1) line overvoltage;2) line undervoltage;3) dc overcurrent.


Fig. 4. Synchronous motor, with transformer and no motor differential.

VIII. CONCLUSION

In summary, the Working Group has provided a basis forguidance to apply adequate protection to motors connected toASD systems.

Figs. 3 and 4 present two application examples of rec-ommended protection for consideration for an induction andsynchronous motor applied with ASDs.

Of particular importance is the inclusion of the specificprotective function, not necessarily its physical location eitherin an external protection system or internal to the drive controls.It is clear based on the group’s work that many applicationengineers lack a thorough understanding of ASD motor pro-tection. There are many unique protection scenarios that mayrequire discussion between the protection engineer and thedrive manufacturer.


REFERENCES

[1] IEEE Guide for AC Motor Protection, IEEE Std. C37.96-2000, 2000.[2] IEEE Guide for the Application of AC Adjustable-Speed Drives on 2400-

13800 V Auxiliary Systems in Electric Power Generating Stations, IEEEStd. 958-2003, 2004.

[3] Industrial Control and Systems: Adjustable-Speed Drives, NEMA Std.Publ. ICS 7-2000, 2000.

[4] Safety Standards for Construction and Guide for Selection, Installation,and Operation of Adjustable-Speed Drive Systems, NEMA Std. Publ. ICS7.1-2000, 2000.

[5] Adjustable Speed Electrical Power Drive Systems, Part 1: GeneralRequirements—Rating Specifications for Low Voltage Adjustable SpeedDC Power Drive Systems, NEMA Std. Publ. ICS 61 800-1-2002, 2002.

[6] Adjustable Speed Electrical Power Drive Systems Part 2: GeneralRequirements—Ratings Specifications for Low Voltage Adjustable Fre-quency AC Power Drive Systems, NEMA Std. Publ. ICS 61 800-2-2005,2005.

[7] Adjustable Speed Electrical Power Drive Systems Part 4: GeneralRequirements—Ratings Specifications for AC Power Drive Systems Above1000 V AC and Not Exceeding 35 kV, NEMA Std. Publ. ICS 61 800-4-2004, 2004.

[8] Application Guide for AC Adjustable Speed Drive Systems, NEMA Std.Publ., Rosslyn, VA, USA, 2007.

[9] Motors and Generators, NEMA Standards Publication MG 1-2003, 2003.[10] A. Von Jouanne, P. Enjeti, and W. Gray, “Application issues for PWM

adjustable speed ac motor drives,” IEEE Ind. Appl. Mag., vol. 2, no. 5,pp. 10–18, Sep./Oct. 1996.

[11] Recommended Practice for Establishing Transformer Capability WhenSupplying Non-sinusoidal Load Currents, ANSI/IEEE C57.110-1986,1988.

[12] A Manufacturer Reference for Determining Drive Input Current Rating.NEC430-2.

[13] Standard for Performance of Adjustable Speed AC Drives Rated 375 kWand Larger, IEEE Std. P1566/D27, 2005.

adjustable speed drive motor protection 2014

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