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ASD overvoltage andundervoltage trips

Drives have diagnostic codeswhich identify the cause oftrip. Generally speaking, theycan be classified as overvolt-age, undervoltage, or overload(overcurrent). Note that

mechanical starters only haveoverload trips. Theyre notconcerned with over or under-voltage. What makes drivesdifferent?

Drives turn sine wave acinto dc (converter section), andthen turn the dc back into ac(inverter section). However, theac at the output is not a sinewave. It is a special waveformknown as the pulse-widthmodulated (PWM). The PWM,from the motors point of view,is accepted as if it were a sine

wave almost (see Advancedmotor measurements below).For now, though, lets focus onthe drive internals, specificallyon whats commonly referredto as the dc link. The dc link isnothing but a capacitor bank,usually with a series linkinductor (reactor) thrown in forfiltering and protection. The dclink is carefully monitored bythe drive; overvoltage orundervoltage refers to the volt-age of the dc link.

Undervoltage can be caused

externally by voltage sags onthe drive input. The Sags andSwells function on Fluke powerquality analyzers can help toidentify line-related undervolt-age problems. Problems couldalso exist internally with the dclink capacitors and/or reactor.In many drives, there are testpoints to measure the dc linkvoltage. To check the capaci-tors, use the min/max functionof a digital multimeter, or,preferably, the trend function ofa Fluke power quality analyzeror ScopeMeter test tool. Checkif voltage regulation is withinthe manufacturers specifica-tion. To check the reactor,check the waveform on bothsides there should be nochange.

When troubleshooting asystem, the tendency is to viewthe drive or PLC as the mostsusceptible to voltage sags. Theice-cube control relay is mostoften the source of sag-related

problems. Studies have shownthat these low-cost compo-nents are the first to drop outwhen voltage sags occur. Sodont forget to look at anyexternal control circuit whileyoure troubleshooting intermit-tent system shutdowns.

Overvoltage could be a

symptom of problems in thecapacitors or reactor. Or it couldbe caused by line-related volt-age transients. At one point,utility capacitor switching tran-sients were notorious forcausing overvoltage trips indrives. Overvoltage could alsobe caused by regenerativeloads. Loads such as cranesand elevators feed back voltagewhen they decelerate. Dynamicbraking circuits are installed toshunt off this energy from thedrive, where they would other-wise show up as overvoltageon the dc link. Problems suchas improper installation canresult in overvoltage trips.

Overload problems areusually load related and will beaddressed below.

Load profiling

To troubleshoot the interactionbetween the load and themotor, you have to understandthe relationship betweentorque and current. A motor is

nothing but a device to turnelectrical energy (current) intorotational mechanical energy(torque), via the magical effectsof magnetism. What a loaddemands of a motor is torque.For all practical purposes, thistorque is directly proportionalto current used by the motor.This should make perfectsense, because we all knowthat for constant-speed motors which include all motorsstarted across the line (withelectro-mechanic starters)

voltage is, or should be, stable,and current is the variable.When a load demands moretorque and current than amotor can supply, the result isan overload condition. Over-loading will cause overheatingof the motor. Motor controllerswill shut down the motor (andthereby the load) rather thanallow permanent winding insu-lation damage to occur.

Overloading is always relative totime: a high overload will trip themotor in a short time, while alower level of overload will takelonger to trip the motor.

When we want to evaluate theimpact of a load on the motor-drive system, we have tomeasure the current it draws. Of

course, this current draw typi-cally varies over time as the loadvaries. The measurement ofcurrent over some period of timeis called load profiling. For loadprofiling, the power-record func-tion of Fluke power qualityanalyzers is ideal for capturing atrend line of current consumption(and kW too, if you want it). Acursor enables you to identify thecurrent values at different pointson the trend line, along with atime stamp for those points. It isnot necessary to measure allthree phases of the inductionmotor because the motor is abalanced load. Before loadprofiling, first make the currentimbalance measurement to makesure the motor is healthy. If yourconcern is nuisance tripping,then pick the high leg andmeasure that (an overload onone leg will trip all three legs).

When load profiling, we arelooking for periods of especiallyhigh current, relative to the fullload amps of the motor. Full loadamp information is available onthe nameplate of the motor. Ifthere is a service factor, therange calculation should be madeon the basis of full load ampstimes service factor.

While high current is the mainconcern, low current should alsobe avoided. A motor is most effi-cient, and has the best powerfactor, in the 60 to 80 percentrange of its full load amps. Thereis no immediate penalty forunderloading the motor willnot trip. In fact, many motors are

routinely oversized for the load,on the theory that the motor isless likely to trip from overload.However, as is most often thecase, there is no free lunch. Inthe case of underloading, theenergy company sends a higherbill.

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2 Fluke Corporation Is it the drive, the motor or the load?

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Is it the drive, the motor or the load? Fluke Corporation 3

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Two different loads:variable torque andconstant torque

Most drive systems are usedwith variable torque orconstant torque loads. Variable

torque loads include fans androtary pumps. These are by farthe majority of loads, from anenergy consumption point ofview. When ASDs are usedwith these loads, dramaticenergy savings can be realized.For example, a fan at half-speed (30 Hz) ideally uses onlyone-eighth of the power of thesame fan at 60 Hz (we sayideally, because there is alwayssome level of loss due to ineffi-ciencies in the drive-motor-load system). From a trou-

bleshooting point of view, theimportant thing to realize isthat these variable torque loadsrarely cause overload-relatedproblems for drives (assumingthe load has been sizedcorrectly). That is because theyspend a lot of their timerunning at lower speeds (lessthan 60 Hz) and drawing lesscurrent. If this were not thecase, that is, if the loaddemanded full speed (andtorque) most or all of the time,there would be no economic

justification; i.e., energysavings, to install an ASD inthe first place. Sometimes theseloads will cause a trip at start-up, but that is usually anindication that the load has notbeen sized correctly to thedrive. Another possibility is thatthe load has changed. Forexample, a bearing starting toseize up demands more initialtorque to get the load going.

Constant torque loads canbe more challenging. Frictionalor gravitational loads areconstant torque loads. The keything to understand aboutthese loads is that they requirethe same level of current (moreor less) at lower speeds. Thiscan be dangerous for themotor. Motors are usuallycooled by fans built onto therotor; when the motor slows,the fan cools less. Thereforeexcessive heating can occur.The danger is that motor over-load circuits are built to

measure heat indirectly bymeasuring current (there aremotors with heat sensorsembedded in their stators, butthese are obviously moreexpensive). Here we have asituation where normal current

draw at low speed can causeoverheating. The common solu-tion is to install externallypowered fans to cool the motor.

Before we leave the subjectof load troubleshooting, itshould be noted that there is awhole area of expertise havingto do with the mechanical link-age of the motor and load.These include vibration, shaftalignment, motor mounting,etc. These are obviously impor-tant issues, but they areoutside of the scope of this arti-

cle.

Advanced motormeasurements

In ASD motor systems, thereare a few measurements thatneed to be taken that wouldnot be taken in mechanicalstarter (across the line) motorsystems. This is because thefast switching, high-frequencyelement of the PWM outputwaveform causes special prob-lems that the sine wave

doesnt. At first glance, themotor as a current-drawingload looks like nothing but abig set of inductors or coils(stator windings), and thenature of inductors is that theyfilter out the high frequencycurrent elements. That is whythe current waveform looks likea sine wave. But unfortunately,those high-frequency elementsof the voltage waveform do notget filtered out, and are capa-ble of causing some mischief.

The first two of these meas-

urements should be made witha ScopeMeter or with the Scopefunction of a Fluke powerquality analyzer: Overvoltage reflections.

Measure phase-to-phase atthe motor terminals. Theleading edges of the PWMpulses can have peak valuesmuch higher (up to 200percent in theory) thannormal. These overvoltagereflections can causedamage to motor windings.

These overvoltages areclearly visible on the scopewaveform. Solutions fall intothree categories: shorten thedrive-motor cable length;use a motor with highergrade insulation, so-called

inverter duty motors; usefilters.

Motor shaft voltages andbearing currents. Measurevoltage rotor-to-frame(ground), using strandedwire or a carbon brush.Motors run by sine waveshave a "normal" shaft/bear-ing-to-frame voltage of 1 to2 V. The PWM waveform cancause breakdown voltages of8 to 15 V to occur betweenthe shaft (more specifically,the bearing) and the frame.This damages bearings,causing pitting and scarring.Many solutions have beenproposed, but the mostcommon is the shaft ground-ing device.

Leakage current. Measurewith a current clamp aroundall three phase conductors.High frequencies causeincreased leakage betweenstator windings and theframe. This ground or leak-age current can interfere

with control and communi-cation signals. Commonsolutions are the use of EMIsuppression cables or acommon mode choke.

What about harmonics at theoutput of the drive? Wouldntthe PWM-turned-into-sine-wave current-waveformcontain a lot of harmonics?Absolutely. But we dont haveto measure these. First of all,they dont get into the rest ofthe power distribution system;they only affect the motor.Specifically, they cause addi-tional heating in the motor.However, motor and drivemanufacturers have addressedthis problem by supplyinghigher grades of motor insula-tion. In those cases where anolder motor is retrofit with anASD, the recommendation isthat the motor full load amp bederated.

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Summary

To determine whether its theload, the motor, or the drivethat is causing problems, ithelps to proceed systematically.Start with the basic motormeasurements (imbalance) tocheck the health of the motoritself. Then do some simpledrive measurements to checkfor causes of over or undervolt-age trips. Profile the load tofind the cause for intermittentoverload trips. Finally, forpremature motor bearing orwinding failures, as well ascontrol signal interference, usea Fluke ScopeMeter test tool tomake additional motor meas-urements. This by no meansexhausts all the possible

sources of problems, but it willcertainly identify a lot of themost common ones.

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Work safely

The high voltage and currentspresent in electrical powersystems can cause seriousinjury or death by electrocutionand burns. Consequently, onlytrained, experienced electri-cians who have knowledge ofelectrical systems in generaland the equipment under testshould perform testing andmodification of electricalsystems.

Fluke cannot anticipate allpossible precautions that youmust take when performing themeasurements described here.At a minimum, however, youshould: Use appropriate safety

equipment such as safety

glasses, insulated gloves,insulating mats, etc.

Be sure that all power hasbeen turned off, locked out,and tagged in any situationwhere you will be in directcontact with circuit compo-nents. Be certain that thepower cant be turned on byanyone but you.

Read and understand all ofthe applicable manualsbefore applying the informa-tion in this application note.Take special note of allsafety precautions andwarnings in the instructionmanuals.

Do not use instruments onapplications for which theyare not intended, andalways be aware that if theequipment is used in amanner not specified by themanufacturer, the protectionprovided by the equipmentmay be impaired.

Fluke.Keeping your worldup and running.

Fluke CorporationPO Box 9090, Everett, WA USA 98206

Fluke Europe B.V.PO Box 1186, 5602 BDEindhoven, The Netherlands

For more information call:In the U.S.A. (800) 443-5853 orFax (425) 446-5116In Europe/M-East/Africa (31 40) 2 675 200 orFax (31 40) 2 675 222In Canada (800)-36-FLUKE orFax (905) 890-6866From other countries +1 (425) 446-5500 orFax +1 (425) 446-5116Web access: http://www.fluke.com/

2004 Fluke Corporation. All rights reserved.Printed in U.S.A. 10/2004 1989052 A-US-N Rev B