Electrical measurements on adjustable speed drivessupport.fluke.com/educators/download/asset/2161101_a_w.pdfElectrical measurements on adjustable speed drives Ten measurements that

Electrical measurementson adjustable speed drivesTen measurements that tell you a lot

Troubleshooting philosophyThere are many different ways to go about troubleshooting an electrical circuit, and a goodtroubleshooter will always find the problem —eventually. The trick is to track down the prob-lem as quickly as possible, keeping downtime toa minimum. The most efficient procedure fortroubleshooting is to begin looking at the motor,and then systematically work back towards the electrical source, looking for the most obviousproblems first. A lot of time and money can bewasted replacing perfectly good parts when theproblem is nothing more than a loose connection.

Next, take care to make accurate measure-ments. Nobody makes inaccurate measurementson purpose of course, but it is easier to do thanyou may think, especially when working in ahigh energy, noisy environment like that of anASD.• If possible, do not use grounded test instru-

ments. They can introduce noise into a meas-urement where none existed before.

• Avoid touching instruments and probes whiletaking the reading, as electrical noise can getcoupled through your hands which may alsoaffect the reading.

• Because of the high noise environment, whenmaking current measurements use either aclamp meter designed for this environment or,if using a scope, use a current clamp that putsout 10 mV/ amp or 100 mV/amp. They pro-vide a better signal to noise ratio than 1 mV/amp clamps when making currentmeasurements less than 20 amps.

• If using a digital multimeter with a clampaccessory always use a current transformermilliamp output clamp as this connects to thelow impedance current input jacks of theDMM and is much less susceptible to thenoise in the environment.Finally, it is a good idea to document electrical

measurements at key test points in the circuitwhen the system is functioning properly. If agood drawing does not exist, make one. A simpleone-line or block diagram will do nicely. Writedown voltage and temperature measurements atkey test points. This will save a great deal oftime and head-scratching later.

Making safe measurementsBefore making any electrical measurements, besure you understand how to make them safely.No test instrument is completely safe if usedimproperly, and you should be aware that manytest instruments on the market are not appropri-ate for testing adjustable speed drives.

Safety ratings for electrical test equipmentThe International Electrotechnical Commission(IEC) is the primary independent organizationthat defines safety standards for test equipmentmanufacturers. The IEC 61010 second editionstandard for test equipment safety states twobasic parameters, a voltage rating and a meas-urement category rating. The voltage rating isthe maximum continuous working voltage the instrument is capable of measuring.

IntroductionMost experienced motor technicians are well prepared to deal with traditionalthree-phase motor failures that result from the effects of water, dust, grease,failed bearings, misaligned motor shafts, or just plain old age. However, modernelectronically controlled motors, more commonly referred to as adjustable speeddrives, present a unique set of problems that can vex the most seasoned pro.This application note describes the electrical measurements you need to makeduring the installation and commissioning of a drive, as well as when diagnosingbad components and other conditions that may lead to premature motor failurein adjustable speed drives (ASDs).

F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y

Application Note

When the voltage rating is coupled with a cate-gory rating, it can be confusing. The categoryratings depict the measurement environmentexpected for a given category. The measurementenvironment for adjustable speed drives is notalways simple and may vary from installation toinstallation. Most all three-phase ASD installationswould be considered a CAT III measurement envi-ronment. Single phase ASD installations would bea CAT II environment. If you are working in bothenvironments, play it safe and use only CAT IIIrated test instruments. What may not be readilyobvious from looking at the following table is thedifference between a 1000 V CAT II rated meterand a 600 V CAT III rated meter. At first glance,you might think the 1000 V CAT II meter is thebetter choice because it has a higher working

voltage than the 600 V CAT III meter and it canhandle the same level of high voltage transient,which is true. However, the 600 V CAT III metercan safely handle six times the power as the1000 V CAT II meter, should a transient cause afault within the meter.

Also, avoid meters that claim to be “designedto meet” EN61010 specifications or that do notcarry the test certification of an independenttesting lab such as UL, CSA, VDE, TÜV or MSHA,as they do not always meet the specifications forwhich they claim to be designed. Always lookfor independent certification of test instrumentsfor ASD measurements. Refer to the ABC’s of DMMSafety from Fluke for additional information oncategory ratings and making safe measurements.

2 Fluke Education Partnership Program Electrical measurements on adjustable speed drives

Table 1. Measurement environment examples

CAT IV • Refers to the “origin of installation”, i.e. where low-voltage connectionis made to utility power.

• Electricity meters, primary overcurrent protection equipment.• Outside the building and service entrance, service drop from the pole to

building, run between the meter and panel.• Overhead line to detached building, underground line to well pump.

CAT III • Equipment in fixed installations, such as switchgear and three phase motors.• Bus and feeder in industrial plants.• Feeders and short branch circuits, distribution panel devices.• Lighting systems in larger buildings.• Appliance outlets with short connections to service entrance.

CAT II • Appliance, portable tools, and other household and similar loads.• Receptacle outlets and long branch circuits.• Outlets at more than 10 meters (30 feet) from CAT III source.• Outlets at more that 20 meters (60 feet) from CAT IV source.

CAT I • Protected electronic equipment.• Equipment connected to source circuits in which measures are taken to

limit transient voltages to an appropriately low level.• Any high-voltage, low-energy source derived from a high-winding

resistance transformer, such as the high-voltage section of a copier.

OvervoltageCategory Examples

Table 2. Transient test values for overvoltage installation categories

Overvoltage Working Voltage Peak Impulse Transient Test SourceCategory (dc or ac-rms to ground) (20 repetitions) (Ohm = V/A)

CAT I 600V 2500V 30 ohm source

CAT I 1000V 4000V 30 ohm source

CAT II 600V 4000V 12 ohm source

CAT II 1000V 6000V 12 ohm source

CAT III 600V 6000V 2 ohm source

CAT III 1000V 8000V 2 ohm source


Low voltageThis troubleshooting step should always be donebefore you attempt any other measurement.Periodic tightening of connections is oftenrequired to maintain a low resistance connectionbetween conductors. Visually inspect all connec-tion points for looseness, corrosion, or conductivepaths to ground. Even if the visual inspectionlooks okay, you should use at least one, or somecombination of the following methods for check-ing the connections.

Voltage dropsCheck for voltage drops across the various con-nections. Compare with the other two phases.Any significant variation between phases, ormore than two or three percent (depending on motor current and supply voltage) at eachconnection, should be suspect.

Temperature measurementsInfrared thermometers are a fast and easy way to check for bad connections. Any significantincrease in temperature at the connection terminal will indicate a bad connection or con-tact resistance due to infrared heat loss. If thetemperature of the terminal was not previouslyrecorded onto your system diagram, comparewith the other two phases.

Voltage measurementsAs the voltage applied to the motor terminals bythe ASD is non-sinusoidal, the voltage readingsdisplayed by an analog meter, an averageresponding digital multimeter (DMM) and a true-rms DMM will all be different.

Motors — Measurement 1Low voltage

Analog metersMany troubleshooters prefer using an analogmeter because the coil in the meter movementresponds in the same way as the motor to thelow frequency component of the waveform andnot the high frequency switching component.The analog meter should correspond closely tothe voltage displayed on the ASD housing if one exists.

Analog meters read the average voltage of themodulation frequency of the PWM drive. Whileit’s true the analog meter displays a voltagereading close to what the PWM drive is display-ing and the motor is responding, safety is a bigconcern with the analog meters as they gener-ally don’t have any EN61010 safety rating.

Digital multimetersMany DMMs will respond to the high frequencycomponent of the motor drive waveform and willtherefore give a higher reading. A true-rms DMMwill give an accurate reading of the heatingeffect of the non-sinusoidal voltage applied tothe motor, but will not agree with the motorcontroller’s output voltage reading. However, itshould be noted that even though the motor isnot responding to the higher frequencies interms of torque or work being done, high fre-quency currents might be flowing outside of the windings due to various capacitances inother parts of the motor. The issue is bandwidth.Process meters,, power quality analzyers, oscilloscopes, some clamp meters, and DMMswith low-pass filters will all provide voltagereadings similar to that of the analog meter andASD display.

Motors — Measurement 2Voltage and current unbalance

Voltage unbalanceNext measure the phase-to-phase voltagebetween the three motor terminals for voltageunbalance. Voltage unbalances of as little astwo percent can cause excessive heating due tounbalanced currents in the stator windings andloss of motor torque. However, some motorinstallations are more forgiving towards unbal-ances so be sure to check out the entire motorsystem for other causes should an unbalanceexist. As the relative difference between phasevoltages is what is being measured, not absolutevoltages, a DMM will give more accurate read-ings with better resolution than an analog meter.Use the following procedure to calculate voltageunbalance.

For example, voltages of 449, 470 and 462give an average of 460. The maximum deviationfrom the average voltage is 11, and percentunbalance would be:

11____ x 100 = 2.39 %460

Possible causes of voltage unbalance are: oneof the phase drive circuits is only partially con-ducting, or there is a voltage drop between theASD’s output and the motor terminal on one ofthe phases due to a poor connection.

There are other concerns about the motor terminal voltages with regard to distortion, butthey must be measured and viewed using anoscilloscope and will be discussed later in thisapplication note.

Current unbalanceMotor current should be measured to ensure thatthe continuous load rating on the motor’s name-plate is not exceeded and that all three-phasecurrents are balanced. If the measured load current exceeds the nameplate rating, or the cur-rent is unbalanced, the life of the motor will bereduced by the resulting high operating temper-ature. If the voltage unbalance is within accept-able limits, then any excessive current unbalancedetected could indicate shorted motor windingsor one or the phases shorted to ground.Generally, current unbalance for three-phasemotors should not exceed 10 percent.

As the current measurement will be made in ahigh energy, electrically noisy environment, besure the proper current clamp is used as well asgood measuring technique as discussed earlier in this application note. To calculate currentimbalance, use the same formula as stated forvoltage but substitute current in amps. For exam-ple, currents of 30, 35 and 30 amps would givean average current of 31.7 amps. The maximumdeviation from the average current would be 3.3 amps with a current unbalance of 10.4 %.

% (V or I) unbalance = Maximum deviation from the average voltages x 100Average voltage

Motors — Measurement 2Voltage and current unbalance


The trend with PWM drives has been to makethe rise time of the pulses as fast as possible toreduce switching losses and increase the effi-ciency of the drive. However, fast rise times coupled with long cable lengths produce animpedance mismatch between the cable and themotor causing reflected waves, or “ringing” asshown in Figure 3A.

If the rise times are slow enough, or the cableshort enough, the reflected waves will not occur.The main problem with this condition is thatordinary motor winding insulation can breakdown quickly. Additionally, higher than normalshaft voltages can develop causing prematurefailure of bearings and excessive common modenoise (leakage currents) can interfere with lowvoltage control signals and cause GFI circuits totrip.

The relationship between cable length, risetime and the resultant increase in peak voltage isillustrated in Figure 3B. The peak voltage at themotor terminals will increase above the dc busvoltage of the ASD as cable length increases andthe rise time of the ASD output pulse gets faster.

Overvoltage reflections — troubleshootingAs mentioned earlier, fast rise times on the ASDoutput pulses and long cable runs between theASD and the motor will cause overvoltage reflec-tions approaching double the dc bus voltage andeven higher. An oscilloscope is required to dis-cover the full extent of this problem, as seen infigure 3C.

Figure 3A shows the ASD L-L voltage meas-urement at the motor terminals with six feet ofcable, while Figure 3B shows the ASD L-L volt-age with 100 feet of cable. Notice the differencein peak voltage measurements — about 210 volts.Also notice that there is only 5 V rms differencebetween the two waveforms (small digits on thedisplay). This means your voltmeter will not findthis problem.

Very few scopes will trigger as nicely andeasily as this oscilloscope did for the measure-ments in Figures 3C and 3D. For other scopes usethe following procedure to measure the extent ofthe overvoltages.

PWM Drives — Measurement 3Overvoltage reflections at the motor terminals

Figure 3C. Normal PWM waveform. Figure 3D. PWM waveform with reflectedvoltages.


Figure 3A. Reflected voltages (ringing).Figure 3B. Impact of rise time and cable length on magnitude ofreflected voltages.

The signals in figures 3E and 3F were cap-tured by triggering on a single pulse using singleshot mode with cursors enabled to make thepeak voltage measurement along with rise time.While this measurement requires more buttonpressing and scope “know how,” the automatedrise time measurement may be worth the trou-ble. Manually resetting the single shot triggerperiodically will give you a sampling of variouspeak voltages for the different pulses. Also,slowly raising the trigger voltage will give youan idea of the maximum peak when the scopestops triggering.

Assuming you have identified a true overvolt-age, or ringing problem, then something must bedone about it. The simplest solution is to shortenthe cable. The peak overvoltages will continue toincrease to almost double the dc bus voltage asthe cable lengthens or rise time gets faster. Thepeak voltages can even exceed voltage doublingif the reflected voltage occurs on top of standingwaves due to the distributed inductance andcoupling capacitance of the cable.

The real danger of this overvoltage conditionis the damage it can do to the motor windingsover a period of time, which may not show up asa problem when the PWM drive is first installed.Many PWM drives are installed without takinginto consideration the overvoltage effects of longcabling between the PWM output and the motor.And while improved efficiency of the newest andlatest PWM drives are achieved by making therise times faster on the output pulses, this canmake the overvoltage problem even worse, andthe need for shorter cabling even greater.

If your motor has already failed and has to berebuilt, better insulated wire such as ThermalezeQs, or TZ Qs (by Phelps-Dodge), should be usedto rewind the motor. The main advantage is thatit provides significantly more protection againstovervoltages without adding insulation thicknessand the same stator can be used without modifi-cation. If the motor has been damaged beyondrepair then a motor designed to meet NEMA MG-31 specifications (sustained VPeak ≤ 1600 Vand rise time 0.1 µs) should be used as areplacement motor for PWM applications wheresustained overvoltages may be occurring.

If the cabling in your PWM application cannotbe shortened then use one of these three solu-tions to fix the problem.1. An external “add-on” low pass filter can be

installed between the PWM output terminalsand the cable to the motor to slow the risetime.

2. Install an R-C impedance matching filter atthe motor terminals to minimize the overvolt-ages, or ringing effect.

3. In some applications, such as submersiblepumps or drilling machines, it isn’t possible to access the motor terminals and other meth-ods of minimizing overvoltages are required.One method is to apply series reactorsbetween the PWM output terminals and thecable to the motor. While this is a fairly simplesolution, the reactors may be fairly large,bulky and expensive for large horsepowerapplications.A qualified engineer should design all

the solutions suggested above for your specificapplication.

Figure 3E. Leading edge of normal PWMpulse.

Figure 3F. Leading edge of PWM pulse withreflected voltage (ringing).


Inverter Output Filter Series Reactor Motor Terminal FilterRemedy 1 Remedy 2 Remedy 3Series connected to the Series connected to the Parallel connected at thePWM output terminals. PWM output terminals. motor terminals.Designed to slow rise Acts as a current limiter Designed to match thetime (dv/dt) below a and also slows rise time. characteristic cablecritical value. impedance.Dependent on cable Dependent on size of Not cable lengthlength. system. dependent.Losses dependent on Losses dependent on Losses are more or lessmotor kVA. motor kVA. fixed.Size/cost dependent on Size/cost dependent on Size/cost more or lessmotor kVA. motor kVA. fixed.

Filter remedy 1: Typicaleffect of a low pass filter atthe inverter output as meas-ured at the motor terminals.

Filter remedy 2: Typicaleffect of the series reactormeasured at the motor ter-minals.

Filter remedy 3: Typicaleffect of an R-C impedancematching filter measured atthe motor terminals.

Safety noteReflective voltage phenomenon can mean peak voltages 2-3 timesthe dc bus voltage. For 480 V line voltage, this means a dc busvoltage of 648 V and possible peak overvoltages of 1300 V-2000 Vand possibly higher given +10 % line voltage variance. Therefore itis recommended that the measurement at the motor terminals bemade with the highest rated probe available and for the shortesttime possible where reflected voltages are likely to be present.


Leakage currentsLeakage currents (common mode noise) capaci-tively coupled between the stator winding andframe ground will increase with PWM drives asthe capacitive reactance of the winding insula-tion is reduced with the high frequency output ofthe drive. Another leakage current path mayexist in the capacitance created when the motorcables are placed in a grounded metal conduit.Therefore, faster rise times and higher switchingfrequencies will only make the problem worse.

It should also be noted the potential increase inleakage currents should warrant close attentionto established and safe grounding practices forthe motor frame. The increase in leakage cur-rents can also cause nuisance tripping of groundfault protection relays, override 4 to 20 mA control signals, and interfere with PLC communi-cations lines.

Measure common mode noise by placing thecurrent clamp around all three motor conductors.The resultant signal will be the leakage current.

Bearing currentsWhen motor shaft voltages exceed the insulatingcapability of the bearing grease, flashover cur-rents to the outer bearing will occur, therebycausing pitting and grooving to the bearing races.The first signs of this problem will be noise andoverheating as the bearings begin to lose theiroriginal shape and metal fragments mix with thegrease and increase bearing friction. This canlead to bearing destruction within a few monthsof ASD operation — an expensive problem both interms of motor repair and downtime.

There is a normal, unavoidable shaft voltagecreated from the stator winding to the rotor shaftdue to small dissymmetries of the magnetic fieldin the air gap. This is inherent in the design ofthe motor. Most induction motors are designed tohave a maximum shaft voltage to frame ground of< 1 Vrms.

Another source of motor shaft voltages arefrom internal electrostatic coupled sources includ-ing: belt driven couplings, ionized air passingover rotor fan blades, or high velocity air passingover rotor fan blades such as in steam turbines.

Under 60 Hz sine wave operation, the bearingbreakdown voltage is approximately 0.4 to 0.7volts. However, with the fast edges of the tran-sient voltages found with PWM drives, the break-down of the insulating capacity of the greaseactually occurs at a higher voltage — about 8 to15 volts. This higher breakdown voltage createshigher bearing flashover currents, which causesincreased damage to the bearings in a shorteramount of time.

PWM Drives — Measurement 4Motor shaft voltages

Research in this area has shown that shaftvoltages below 0.3 volts are safe and would notbe high enough for destructive bearing currentsto occur. However, voltages from 0.5 to 1.0 voltsmay cause harmful bearing currents (> 3 A) andshaft voltages (> 2 V) may destroy the bearing.

Care must be taken when making this meas-urement. While the common is connected to themotor frame ground, connect the probe tip to apiece of twisted strand wire or a carbon brushwhich in turn makes contact with the motorshaft. As the shaft voltages are caused by fastrise times of the PWM drive pulses, the voltageswill appear as narrow peaks. This measurementis best made with an oscilloscope, not a DMM.Even if the DMM has peak detect, there isenough variation between peaks to render thereading unreliable. Another measurement tip isto make the shaft-to-frame ground voltage meas-urement after the motor has warmed to its nor-mal operating temperature, as shaft voltages maynot even be present when the motor is cold.

The simplest solution to this problem is toreduce the carrier (pulse) frequency to less than10 kHz, or ideally around 4 kHz if possible. If thecarrier frequency is already in this range thanalternative solutions can be employed such asshaft grounding devices or filtering between theASD and the motor.

PWM Drives — Measurement 5Leakage currents (common mode noise)


being applied to the motor, then use the follow-ing procedure to isolate which IGBT(s) is failingin the output section.1. Check positive conducting IGBTs by connect-

ing the scope common lead to the dc+ busand measuring each of the three phases at theinverter’s motor output terminals. Check fornice, clean-edged square waves without anyvisible noise inside the pulses, and that allthree phases have the same appearance.

2. Check negative conducting IGBTs by connect-ing the common lead to the dcbus and per-forming the same measurements as in stepone above on each of the three phases at theinverter’s motor output terminals.

PWM invertersMany fractional horsepower PWM drives areintegrated to the point where the input diodeblock and IGBTs are “potted” into a single throw-away module that is bolted to the heat sink. Thecost of these units rarely justifies the time torepair them and sometimes replacement partsare not available. However, larger horsepowerdrives starting in the 5 to 25 horsepower range,have components that are accessible and thecost of such drives makes repair an economicallyviable alternative to replacement.

If it has been determined that the driveinverter is the source of an improper voltage

PWM Drives — Measurement 6Testing the IGBT output waveshape


Diode Rectifier PWM Inverter

Induction Motor

Common ModeChoke

3φ AC Input

Conduit

Common mode (leakage) currents

M

A common mode choke can be used to reduceleakage currents (see Figure 5A). Also, specialEMI suppression cables can be used between thedrive output and the motor terminals. The copper

conductors of the cable are covered with ferritegranules, which absorb the RF energy and con-vert it to heat. Isolation transformers on the acinputs will also reduce common mode noise.

Figure 5A. Common mode choke with dampening resistor to reduce leakage currents.

Figure 6A. Square waves. Figure 6B. Check all three phases at the inverter’s motor output terminals.

The dc bus

DC voltage too highTransients (less than .5 cycle) and swells (.5 to30 cycles) on the ac line inputs and motorregeneration are the two most common causes of “nuisance” tripping of the overvoltage faultcircuit on ASD inverters. Transients and swellscan be caused by events happening outside thebuilding like lightning or utilities switching KVARcapacitors or transformer taps, as well as otherloads inside the building being switched on(capacitive) or off (inductive). To test for these situations, use an oscilloscope or power linemonitor with at least 10 µsec/ div. resolution,and time-stamping capability.

Power quality analyzers are your best choicefor these measurements. Oscilloscopes are alsogood choices for this measurement. Both toolshave plenty of single shot resolution and, mostimportantly, can timestamp the event so it canbe correlated to whatever source — lightning,utility or electrical equipment — is causing theproblem. Also ensure your tools are EN61010600 V CAT III safety rated, an important consid-eration when purposely measuring high magni-tude impulses in a high-energy environment.

PWM Drives — Measurement 9ASD “trip” problems — overvoltage


PWM Drives — Measurement 8ASD “trip” problems — overloading

If it is determined that the cause of overloadingis too much motor current, be sure that the motorload is not causing the problem. Check for exces-sive current unbalance indicating possibleshorted phase windings. Verify ASD trip points

are set correctly to the manufacturer’s specifica-tions. Finally verify that the dc bus voltage isbeing regulated properly. Leaky capacitors may cause excessive ripple and too little inrushcurrent.

Check for “leaky” IGBTs by measuring the volt-age from earth ground to the inverter’s motoroutput terminals with the drive powered on, butthe speed set to zero (motor stopped). Somedrives may have a normal earth ground to motor

PWM Drives — Measurement 7Testing the IGBT outputs for leakage

terminal voltage of about 60 volts, with a read-ing of over 200 volts indicating a leaky IGBT.Perform this measurement on a known gooddrive to determine what is normal for that drive.

If the drive is installed in a part of the countrythat is prone to lightning activity, be sure thebuilding has proper surge protection that is func-tioning properly. Additionally, the building’sgrounding system must be properly installed andfunctioning to help dissipate lightning strikessafely to earth, rather than through some path inthe building’s power distribution system. Stepscan and should be taken to minimize their effectson your electrical and electronic equipment,since a building that is susceptible to transients,sags and swells, is usually a building that is defi-cient in proper wiring and grounding.

If a transient voltage isexpected, then the power linemonitor is an excellent choiceto measure and, more impor-tantly, time stamp the transientso it can time correlated towhatever event caused the ASD fault.

If a transient causes the tripping, then an isolationtransformer or series line reac-tor can be placed in series withthe front end of the ASD. Analternate solution would be toplace a surge protection device(SPD) at the motor control Figure 9A. Overvoltage transient capture.


center, or the primary side of the distributiontransformer feeding the ASD. However, if thesource of the transient is coming from anotherload on the same secondary feed as the ASD,then a separate isolation transformer or seriesline reactor may be needed directly in front ofthe ASD. Better yet, put the ASD on its own feed.

Voltage swells > 30 cycles can be monitoredusing a test tool line monitor. One way to miti-gate the swell is to install a temporary dropoutrelay for as many cycles as the swell, but thatcan still be tolerated by the drive. The viability ofthis solution will be determined by the amountof “ride-through” the ASD’s input circuit canhandle before the dc bus voltage drops to anunder-voltage condition. Another possible solu-tion is to use a voltage regulation device like anuninterruptible power supply (UPS), but it shouldbe noted that most UPSs are designed to handlevoltage sags and momentary interruptions andmay not handle voltage swell conditions unlessspecifically designed to do so. Carefully checkthe UPS manufacturer specifications.

Overvoltages or long-term voltage drift can becaused by very large loads being turned offwithin the building or a slow response of theutility’s voltage regulation system to large reduc-tions in demand on the power grid. This condi-tion is most easily discovered by using anoscilloscope to trend date over time. The bestway to deal with this problem is to employ localvoltage regulation with a device like a UPS thatis designed to handle overvoltage as well assags and dropouts.

Another common source for overvoltage onthe dc bus is motor regeneration. This occurswhen the motor load is “coasting” and begins tospin the motor shaft rather than getting spun bythe motor, which causes the motor to changeinto a voltage generator and returns energy tothe dc bus. Excessive regeneration can be meas-ured by checking for a change in the direction ofthe dc current back into the dc bus while simul-taneously checking the dc bus voltage for anincrease above the trip point. When makingmeasurements on the ASD’s dc bus, choose aCAT III 1000 V instrument. Typically a DMM withmin/max recording capability. If regeneration iscausing the overvoltage tripping, somethingcalled “dynamic braking” can be employedwhich limits how fast the regenerative current isallowed to feed back into the dc bus capacitors.

If the dynamic braking has already beenemployed and is not functioning properly, then itcan be tested according to the manufacturer’sspecifications. If the brake is the resistor type, itcan be visually inspected for signs of overheat-ing; discoloration, cracking, or even smell for thatdistinctive aroma of an overheated component.The resistance value can also be measuredagainst the manufacturer’s specifications. If thedynamic brake uses the transistor type, the sili-con junctions can be tested using a diode test asdescribed earlier. Also, the braking current canbe measured and the current waveform com-pared with that of a known good system.

PWM Drives — Measurement 10ASD “trip” problems — undervoltage

DC voltage too lowThere are several possibilities for “nuisance” tripping of the low voltage fault circuit on ASDinverters. Voltage sags (.5 to 30 cycles) andunder voltages (> 30 cycles) on the line input tothe drive are common conditions associated withthis problem. Sags are quite often caused byanother load within the building’s distributionsystem being turned on, or perhaps from a neigh-boring building starting a large electrical load.

Make the measurement with an instrumentthat can time stamp the sag or where the under-voltage causes the ASD low voltage fault to trip.You may want to start making this measurementat the service entrance. This way you can quicklyisolate whether the sag is being caused fromwithin the building, or outside. Be sure to monitor

the voltage and current simultaneously. That wayyou can tell whether the problem is downstreamfrom the service entrance wherethe surge in current is coincidentwith the voltage sag. Anupstream (outside the building)problem would show the voltagesag without a correspondingsurge in current. If the problem iswithin the building, there will bea current surge coincident withthe voltage sag. Continue makingthe measurement at different loadcenters until you have isolatedthe load with the correspondingvoltage sag and current surge.

Figure 10A. Voltage sag.


SummaryIn summary, while ASDs are more complex thanstandard electrical motors, a systematic approachto measurement and problem solving and theright test tools can help significantly simplify theirinstallation, maintenance and troubleshooting.While the 10 measurements described here by nomeans cover everything you can know aboutASDs, they will provide you the information youneed for the majority of situations.

Another possibility is a motor that is drawingenough current to cause the dc bus voltage todrop below the under-voltage fault setting, butnot enough to trip the current overload. You willneed to check the motor current for overloading(compare with motor nameplate) as well as ver-ify whether the program settings of the drive arecorrect for the motor nameplate ratings, includ-ing the application for which the motor and drivewere intended.

Look at the line input voltage waveform to theASD. The wave-shape should be a nicely shapedsine wave. Severe “flat-topping” of the waveformcan prevent the dc bus capacitors from fullycharging to the peak value, which lowers the dcbus voltage as well as the amount of currentavailable to the ASD output circuit.

The top waveform in Figure 10C is taken froma three-phase circuit. The current peaks of thebottom waveform occur when the voltage wave-form is at or near its peak.

However, if the current peaks become largerthan the source can supply (i.e. source imped-ance is too high for the load), you get flat top-ping like the waveform shown here.

Figure 10B. DC bus voltage drop below under-voltage fault setting.

Figure 10C. Ideal line input voltage waveform.

Figure 10C. Flat-topped waveforms.

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