measurement of adjustable speed drives with fluke...

Measurement of Adjustable Speed Drives with Fluke Meters Fluke Corporation 1

Measurement ofAdjustable Speed Drives

with Fluke Meters

2 Fluke Corporation Measurement of Adjustable Speed Drives with Fluke Meters

Table of ContentsIntroduction ..................................................................................................................................... 3

Troubleshooting Philosophy ...................................................................................................... 3

Making Safe Measurements (Sidebar) ..................................................................................... 4Safety Ratings for Electrical Test Equipment .............................................................................. 4

Adjustable Speed Drive Theory ................................................................................................. 6

Simple Things First ...................................................................................................................... 8Resistance Measurements ............................................................................................................. 8Voltage Drops .................................................................................................................................. 8Temperature Measurements .......................................................................................................... 8

Motor Measurements .................................................................................................................... 9Temperature Measurements .......................................................................................................... 9Overloading ..................................................................................................................................... 9Voltage Measurements ................................................................................................................... 9Current Imbalance Measurements .............................................................................................. 10Single Phasing .............................................................................................................................. 11Overvoltage Reflections - Theory (Sidebar) .............................................................................. 12Overvoltage Reflections - Troubleshooting ............................................................................... 14Bearing Currents ........................................................................................................................... 17Leakage Currents .......................................................................................................................... 17

Measurements at the Inverter .................................................................................................. 18Control Circuit Noise ..................................................................................................................... 18Volts/Hertz Ratio ........................................................................................................................... 18Inverter Drive Circuits .................................................................................................................. 18

Voltage Source Inverters .......................................................................................................... 18PWM Inverters ........................................................................................................................... 19

The DC Bus .................................................................................................................................... 20DC Voltage Too High ................................................................................................................. 20DC Voltage Too Low ................................................................................................................. 21

AC Line Input ................................................................................................................................. 22Diode Bridge .............................................................................................................................. 22Voltage Notching ....................................................................................................................... 23Voltage Unbalance .................................................................................................................... 23

Harmonics and IEEE-519 Compliance ................................................................................... 24

ASD Measurement Guide for Fluke Test Instruments ......................................................... 26


IntroductionMost experienced motortechnicians are well preparedto deal with traditional threephase motor failures causedby water, dust, grease, failedbearings, misaligned motor shafts,or even just old age. However,modern electronically controlledmotors, more commonly referredto as adjustable speed drives,present a unique set of problems.This application note will focuson electrical measurements thatcan be used to diagnose badcomponents and other conditionsthat may lead to prematuremotor failure in adjustable speeddrives (ASDs).

There are many different waysto go about troubleshooting anelectrical circuit, and a goodtroubleshooter will always findthe problem - eventually. Theprocedure set forth in thisapplication note will beginlooking at the motor and workback towards the electricalsource. Additionally, it willemphasize checking the simpleand easy things first. A lot oftime and money can be wastedreplacing perfectly good partswhen the only thing wrong isa loose connection.

Make accurate measurements.Of course nobody makes inaccu-rate measurements on purpose,but this is easier to do than youthink when working in a highenergy, noisy environment likeASDs. Don’t use grounded testinstruments if it can be avoided.They can introduce noise intoa measurement where noneexisted before. Avoid touchinginstruments and probes ifpossible while taking thereading, as electrical noisecan get coupled through yourhands which may also affectthe reading. Use current clampswhich are well shielded andterminated with BNC connectors.

Troubleshooting Philosophy

Current clamps that put out10 mV/amp or 100 mV/amp willhave better signal to noise ratiothan 1 mV/amp clamps whenmaking current measurementsless than 20 amps.

Finally, document electricalmeasurements at key test pointsin the circuit when the systemis functioning properly. If a gooddrawing doesn’t exist - makeone. A simple one-line, or evenblock diagram will do. Writedown voltage and temperaturemeasurements at key testpoints. This will save lots oftime and head-scratching later.


Before any electrical measure-ments are made, be sure youunderstand completely howto make them safely. No testinstrument is completely safeif used improperly, but youshould also be aware that manytest instruments on the marketare not appropriate for testingadjustable speed drives. Thefollowing information isexplained in more detail in theFluke application note, ABCs ofMultimeter Safety and the videowith the same name.1 The mainpoints are summarized below.If you install or maintainmodern adjustable speed drives(ASDs), then please don’t skipthis section as there could behazardous voltages that exist inyour measurement applicationthat you are not aware of.

Safety ratings for electricaltest equipmentThe International Electro-technical Commission (IEC)is the primary independentorganization that defines safetystandards for test equipmentmanufacturers. There is muchconfusion about what thesestandards mean and how theyshould be used to determinethe right instrument for theright application. The followingsection will help clarify thisselection process.

The IEC 1010 standard fortest equipment safety states twobasic parameters; a voltage rat-ing and an overvoltage categoryrating. The voltage rating is themaximum continuous workingvoltage that can be measured.This is fairly straightforwardand simple, although I’m suremany of you have noticed thatyour DMM or scope will oftengive a reading higher than itsmaximum voltage rating. Yourtest instrument should neverbe operated above its voltagerating. Most test instrumentsare designed to have a 10%overvoltage safety margin, butif for some reason you see a

Making Safe Measurements

voltage displayed above therating, or see the overload (OL)being displayed, disconnectthe measurement deviceimmediately.

Probably the most confusingissue is when the voltage ratingis coupled with a categoryrating. The category ratingsbelow in Figure 1 show themeasurement environmentexpected for a given category.The main criteria for thedifferent categories is thetransient voltage that the testinstrument can withstand, andmaybe even more importantly,how much energy (volt/amps)is available to feed any shortcircuit that may occur asa result of the transient.Transients may originate fromsources outside the building,i.e., lighting strikes, utilityswitching and power line faultclearing activity, or they mayoriginate from inside the build-ing for example, as a result ofload switching. The measure-ment is considered to be saferthe further the test instrument isaway from the transient source,

as the transient voltage getsdampened considerably as itmakes its way into the building,and through the internaldistribution system.

The measurement environmentfor an adjustable speed driveis not always straight-forwardand may vary from installationto installation. For example, a100 HP drive instal-led 25 feetfrom the switchgear would beconsidered a CAT III measurementenvironment, but if that same drivewas installed 70 feet from theswitchgear and had an isolationtransformer or line reactorspreceding it, it could be arguedthat it is more like a CAT IIenvironment. If you are workingin both environments, be safeand use only CAT III rated testinstruments.

What may not be readilyobvious from the looking at Table2 is the difference between a1000V CAT II rated meter and a600V CAT III rated meter. At firstglance, you might think the 1000VCAT II meter is the better choicebecause it has a higher workingvoltage than the 600V CAT III

Figure 1. IEC-1010 category ratings diagram for various measurement locations

1 ABCs of Multimeter Safety Application Note,Literature code BO317UEN Rev. A. The ABCs ofDigital Multimeter Safety Video, P/N 609104.


meter and it can handle thesame level of high voltagetransient, which is true.However, the 600V CAT IIImeter can safely handle sixtimes the power as the 1000VCAT II meter, should a transientcause a fault within the meter.

The bottom line recommen-dation for test instruments usedon ASDs is they should be 600Vor 1000V CAT III rated. Somemanufacturers have dual rated600V CAT III/1000V CAT II testinstruments and probes whichare the next best thing to a1000V CAT III test instrument.CAT II instruments shouldbe avoided altogether unlessyou are certain to be makingmeasurements only onfractional horsepower drivesthat are plugged into a wallreceptacle. As the designguidelines for CAT IV haveyet to be defined and adopted,there are no official CAT IVtest instruments available.

Also, avoid meters thatclaim to be “designed to meet”IEC-1010 specifications or thatdo not carry the test certificationof an independent testing labsuch as UL , CSA, VDE, TUVor MSHA, as they do not alwaysmeet the specifications for whichthey claim to be designed.Always look for independentcertification of test instrumentsfor ASD measurements.See Table 7 at the end of thisapplication note for recommendedFluke instruments.

Once you’ve selected a testinstrument with an adequatesafety rating, then it’s up toyou to follow the measurementsafety practices as outlinedbelow, as even a meterdesigned with safety in mindcan be unsafe if misapplied.• Work on de-energized

circuits whenever possibleusing proper lockout/tag-outprocedures.

• Use protective gear whenworking on live circuits(insulated tools, safetyglasses, insulated mat,

flame resistant clothing andremove jewelry).

• Do not measure voltagesabove the working voltagerating of the test instrumentor in high energy measure-ment environments for whichit’s not rated.

• Use the three-point testmethod:1. Test a known live circuit.2. Test the target circuit.3. Test the live circuit againto be sure the meter is stillfunctioning properly.

• Hook on the ground lead first,then connect the hot lead.Then reverse the procedureby taking off the hot leadfirst, then the ground second.

• Hang or rest the meter ifpossible to avoid holding itin your hands.

• Use the old electrician’s trickof keeping one hand in thepocket to lessen the chanceof a closed circuit across thechest and through the heart.

WARNING: To avoid electricshock or other injury, whenmaking measurements on CATIII circuits do not use dividerprobes (e.g. 10:1, 100:1,1000:1, etc.) that are only ratedIEC 1010 CAT I or CAT II. Makesure the scope probe you useis rated for the measurementcategory environment in whichyou work.

One final word on safetyis covered in the section onOver-voltage Reflections —Troubleshooting as it willbe much better understoodand appreciated after thepossible condition is explainedin more detail.

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


The term adjustable speeddrive (ASD) includes a widevariety of mechanical,pneumatic and electronicallycontrolled motors whose speedis adjustable. Another term,variable frequency drive (VFD)is used for electronicallycontrolled ac induction motorsas they vary the frequencyof the voltage to the motor tochange its speed. The term ASDwill be used from here on outas this is the conventional termas used in the IEEE standards.

DC motors and other methodsof adjusting motor speed andtorque will not be discussedin this application note.

Figure 2 shows a simplifiedblock diagram of a typical ASD.There are four main sections.The three sections that arein-line between the ac lineinputs and the motor are theInput Converter, dc Link andthe Output Inverter. The fourthsection, the Control Circuitry,is the “brains” of the drive. Each

Adjustable Speed Drive Theory

DC LinkDC Link

DC Voltage

Line VoltageTo MotorCurrent

ProcessFeedback/

Control Signal

Input Converter Output Inverter

Control Circuits

DisplayOperatorControls

Figure 2. Block diagram of a typical ASD

of these sections has a job toperform, and for most ASDs thetask is the same:• The input converter

transforms ac sinusoidalcurrent to dc, as dc iswhat is required for theelectronically controlledpower transistors in theoutput inverter.

• The dc link is the source ofpower for the output inverter.The larger horsepower driveswill have a large bank ofcapacitors to store voltagefrom the sine wave input.This section may alsocontain a series inductorto help regulate the voltageand current.

• The output inverterprovides a variable acvoltage and frequency.Voltage and frequency (V/Hz)are either varied together toprovide a constant torquewith varying speed, or theV/Hz is varied at differentrates to provide variabletorque at different speeds.

• The control circuitryof the ASD performs a varietyof tasks. It times the switchingof the input and outputdevices. It carries outcommands from the operatorcontrol panel. It also monitorsfor and reports malfunctionsand improper operatingconditions to the operatoror troubleshooter. Controlcircuits shut down the ASD,if necessary, to protect themotor or output inverterand prevent unsafe operatingconditions.

Torque =Horsepower

Speed

(V x I x Eff)746

120 x Hzno. of poles

=

You can see from the formulaabove that varying the ratio ofhorsepower to speed will alterthe torque. Therefore, changingthe volts/hertz (V/Hz) ratio willchange the motor torque. Forconstant torque applications,the V/Hz ratio is held constantby varying the voltage andfrequency together; or forvariable torque applications,the voltage and frequency arevaried at different rates toaccomplish variable torque.

The speed and torque of themotor are usually controlled by0-5 Vdc, 0-10 Vdc, 0-20 mA,or 4-20 mA control signalsapplied to the control inputblock of the ASD. These controlsignals may come from feed-back sensors and encoders orfrom a control panel.

The following paragraphsdescribe in more detail the twomost common variations ofASDs, the voltage sourceinverter, and the pulse widthmodulated (PWM) inverter.

x 5250


Figure 3 shows a simplifieddiagram of a voltage sourceinverter. The input section usesthyristors to rectify all, or part ofthe incoming line-to-line phasevoltages to produce varyinglevels of dc voltage at the dclink capacitor. The dc voltagedetermines the peak voltage ofthe drive’s output to the motor.The frequency of the drive’soutput voltage is determined bythe control signals applied tothe SCRs in the output section.To summarize, the voltage partof the V/Hz ratio is controlled byfull or partial rectification of theac to dc in the input section,while the frequency part of theV/Hz ratio is controlled by thefrequency of the control signalsapplied to the inputs of theSCRs (or transistors) in theoutput section.

Figure 3. Simplified diagram of a voltage source inverter with output current and voltage waveforms

Figure 4 shows a simplifieddiagram of a PWM drive. Noticethe input section is using diodeswhich provide full rectificationof the ac line input, therebycharging the dc link capacitor to1.414 times the input RMSphase-phase voltage (unloadedbus). Unlike the voltage sourceinverter, the dc voltage remainsat a constant level.

However, to provide varyinglevels of RMS voltage, thevoltage waveform is a series ofconstant magnitude pulses thatvary in width. To the motor, onegroup of positive pulses lookslike the positive half of a sinewave, while the negative pulseslook like the negative half.The wider the pulses, thehigher the RMS voltage.Therefore if narrow pulses areat the beginning of the positivecycle, and wide pulses aretowards the middle, it willproduce a nearly sinusoidalRMS voltage. Although thevoltage waveform looks very

3φ AC Input

M

Controlled Rectifier Six-Step Inverter

Induction Motor

Figure 4. Simplified diagram of a pulse width modulated inverter with current and voltage waveforms taken at the motor terminals

distorted (non-sinusoidal),the large motor inductancewill “smooth out” the currentwaveform so to the motor,it looks more like the dashedlines superimposed on thevoltage waveform in Figure 4.The nearly sinusoidal currentwaveform in the bottom ofFigure 4 shows that the motoris in fact only responding tothe RMS voltage of the pulses.

To summarize, the voltagepart of the V/Hz ratio is controlledby varying the pulse width ofthe pulses, thereby varyingthe RMS voltage, even with aconstant peak-to-peak voltage.The frequency part of theV/Hz ratio is controlled by themodulation frequency whichis the time period comprising onepositive set of pulses and onenegative set of pulses. It is themodulation frequency that themotor is responding to and onlya very small amount of the highfrequency pulses can be seen inthe motor current waveform.

indicates VRMS


Be sure all connections are good.Seems obvious doesn’t it?However, honest troubleshooterswill admit to having overlookedthis condition at least once intheir career—right?! Thistroubleshooting step shouldalways be done before anythingelse. Periodic tightening ofconnections is often required

Resistance measurementsThis is probably the leastpreferred of the three methods,but will still detect completelyopen circuits or “leakage” pathsto ground that are resistive.Keep in mind however, thatmost ohmmeters use smallamounts of current in theirmeasurements and may reada good connection, when infact the connection may opencircuit when a large amountof current attempts to conductthrough it. This is also knownas contact resistance.

Voltage dropsCheck for voltage dropsacross the various connections.Compare with the other twophases. Any significant variationbetween phases, or more than2-3% (depending on motorcurrent and supply voltage)at each connection, shouldbe suspect.

TemperaturemeasurementsAn inexpensive infraredtemperature probe usedwith a handheld DMM is afast and easy way to checkfor bad connections. Anysignificant increase intemperature at the connectionterminal will indicate a badconnection or contact resistancedue to I2R heat loss. If thetemperature of the terminalwas not previously recordedonto your system diagram,compare with the other twophases. More on how to usetemperature measurementsto diagnose motor and driveproblems is discussed in thefollowing sections.

Simple Things First

Figure 5. The 1 mV/degree output of the Fluke 80T-IR can be used with any DMMcapable of reading millivolts. The Fluke ScopeMeter instruments can read outdirectly in Fahrenheit or Celsius.

to maintain a low resistanceconnection between conductors.Visually inspect all connectionpoints for looseness, corrosion,or conductive paths to ground.Even if the visual inspectionlooks okay, you should use atleast one, or some combinationof the following three methodsfor checking the connections.


TemperaturemeasurementsTemperature is a key indicatorof a failing, or overloaded motor.Use an infrared temperatureprobe such as the 80T-IR fromFluke to measure motor tem-peratures at key points includ-ing; bearing locations and motorterminal block connections (ifone exists), as well as the motorhousing itself. Are the readingsclose to what was measuredwhen the motor was operatingnormally? If these readingsweren’t taken before, is themotor operating temperaturewithin the NEMA classificationfor that motor? If the answersto these questions are yes,then you may want to move tothe ASD controller for the nextmeasurements, or you can goahead and make the followingmeasurements just to be sure.

OverloadingIf the motor is trying to delivermore torque/power to themechanical load than for whichit is rated, an overload conditionwill exist, i.e., the motor drawscurrent above its nameplaterating. Measure the motorcurrent and check it againstthe nameplate rating. Be sure tomultiply the service factor (SF),if one is shown on the name-plate, times the rated current.

If the motor is at or above itsrated capacity and the overloadis intermittently tripping thedrive, the solution is not toadjust the overload on the drivehigher to prevent the tripping!Find out if the motor is ratedproperly for the application.Sometimes a cheaper, lowerrated motor is substituted; orperhaps a miscalculation of theload requirement accounts forthe mismatch. It’s also possiblethat the mechanical load beingdriven by the motor has beenincreased by an operatoror process supervisor whodoesn’t understand that doingso will put the motor into anoverload condition.

Voltage measurementsAs the voltage applied to themotor terminals by the ASD isnon-sinusoidal, the voltagereadings given by an analogmeter, an average respondingdigital multimeter (DMM) and atrue RMS DMM will all bedifferent. Many troubleshootersprefer using an analog meterbecause the coil in the metermovement responds in the sameway as the motor, i.e., tothe low frequency componentof the waveform, not the highfrequency switching component.The analog meter will alsocorrespond to the ASD’sprogrammed V/Hz ratio andthe voltage displayed on theASD housing if one exists.

Most digital multimeters(DMMs) will respond to thehigh frequency componentof the motor drive waveformand will therefore give ahigher reading. Even thougha true-RMS DMM will give anaccurate reading of the heatingeffect of the non-sinusoidalvoltage applied to the motor,quite often the analog meterreading is preferred sinceit gives a (lower) voltagereading similar to the motor’sresponse to the voltage applied.

Motor Measurements

If mechanical load requirements and behavior seem normal, then it’spossible some of the motor windings have become shorted and are thesource for increased current, heat and loss of torque. Things to checkfor now are current imbalance, single phasing, voltage imbalance andsomething unique to PWM drives, overvoltages caused by reflectedvoltage waves. High motor shaft voltages, a problem also associatedwith PWM drives, could explain excessive bearing temperatures andpremature bearing failure. How to test for these conditions is describedin the following paragraphs.

What next?

However, it should be noted thateven though the motoris not responding to the higherfrequencies in terms of torqueor work being done, highfrequency currents may beflowing outside of the windingsdue to various capacitancesin other parts of the motor. Thiswill be discussed in more detaillater on.

The reason for making voltagemeasurements at the motorterminals is to verify that thevoltage is not too high, too low,or is unbalanced. A DMM/Scopewith a low pass filter probe orFluke 41B, can be used to verifythat the voltage at the motorterminals corresponds with thecalculated voltage readout at theASD’s display. Using a scopeor 41B has the advantage inthat a simultaneous frequencymeasurement can be made whichis much more meaningful. This isdiscussed further in the Volts/Hzratio section. If the voltage atthe motor is too low, then badconnections are a likely cause,or the dc bus voltage at the ASDis too low. If the motor terminalvoltages are too high, then thedc bus voltage may be too high,which in turn could be due to theASD input voltage being too high.


Next measure the phase-to-phase voltage betweenthe three motor terminals forvoltage imbalance. Voltageimbalances of as little as 3%can cause excessive heatingdue to unbalanced currents inthe stator windings and loss ofmotor torque. However, somemotor installations are moreforgiving towards imbalancesso be sure to check out theentire motor system for othercauses should an imbalanceexist. As the relative differencebetween phase voltages iswhat is being measured, notabsolute voltages, the DMMwill give more accuratereadings with better resolutionthan an analog meter. Use thefollowing procedure to calculatevoltage imbalance.

Current imbalancemeasurementsMotor current should bemeasured to ensure that thecontinuous load rating onthe motor’s nameplate is notexceeded and that all threephase currents are balanced.If the measured load currentexceeds the nameplate rating,or the current is unbalanced,the life of the motor will bereduced by the resulting highoperating temperature. If thevoltage imbalance is withinacceptable limits, then anyexcessive current imbalancedetected could indicate shortedmotor windings. Generally,current imbalance for three-phase motors should notexceed 10%.

As the current measurementwill be made in a high energy,electrically noisy environment,be sure the proper currentclamp is used as well as goodmeasuring technique. An aconly current transformer (CT)style current clamp will usuallywork best as it is a low imped-ance device and less likely to“capture” electrical noise acrossit. Most ac/dc current clampsare the hall effect style whichhave a high impedance andare therefore more susceptibleto noise. Either style clampcan have problems with noiseif not properly shielded andterminated. Clamps usingcoaxial cable terminated withBNC connectors work best.

Clamps with higher select-able output ranges for currentmeasurements less than 10or 20 amps will help get thesignal-to-noise ratio of theclamp to an acceptable level.

The integrity of themeasurement can sometimesbe enhanced by not touchingthe clamp or the meter whiletaking the reading.

To calculate current imbal-ance, use the same formula asabove for voltage but substitutecurrent in amps. For example,currents of 30, 35 and 30 ampswould give an average currentof 31.7 amps. The maximumdeviation from the averagecurrent would be 3.3 amps witha current imbalance of 10.4%.

Percent VoltageImbalance =

For example, voltages of 449, 470 and 462 gives an averageof 460. The maximum deviation from the average voltage is11 and percent unbalance would be:

x 100Maximum deviation from the average voltagesAverage voltage

x 100 = 2.39%11

460

Possible causes of voltageimbalance are: one of thephase drive circuits is onlypartially conducting, or thereis a voltage drop between theASD’s output and the motorterminal on one of the phasesdue to a poor connection.

There are other concerns aboutthe motor terminal voltages withregard to distortion, but theymust be measured and viewedusing an oscilloscope and willbe discussed in a later section.


If the current in the motor isunbalanced (Figure 6), you candetermine if the unbalance iscaused by the motor or the ASDby rotating the phase connec-tions on the motor terminals.But first, measure the current inall three phases with the motorunder load. Next, rotate thephase conductors from the ASDto the motor terminals.

If the phases A∅, B∅ and C∅are connected to motor termi-nals T1, T2 and T3 respectively,then change the connections sothat phases A∅, B∅ and C∅ arenow connected to terminals T2,T3 and T1 (Figure 7). Nowmeasure the phase currents withthe motor under load again.

Let’s say the first measurementfound the current imbalancewas on the B∅/T2 connection.If the imbalance moved to theB∅/T3 connection in the secondmeasurement, then the imbal-ance is coming from the ASD. Ifthe current imbalance in thesecond measurement was onthe A∅/T2 connection then theimbalance is due to the motor.

Single phasingSingle phasing results from thetotal loss of one of the phasevoltages applied to a threephase ac induction motor andcan be a tricky problem todetect. In an ASD application,this would usually be caused byan open connection at eitherend of the cabling between themotor and drive, or an open inone of the conductors in thecable itself. It’s also possibleone of the insulated gate bipolartransistors (IGBT) which arethe devices that drive eachphase of the motor, could havebecome open circuit, althoughsome ASDs are able to detectthis condition.

Figure 6. Measure the motor phase currents.

Figure 7. Rotate phase connections to determine whether problem is in themotor or the drive.

T1 30A

T2 30A

T3 35A

T1 30A

T2 35A

T3 30A

Rolled PhasesUnbalanced causedby supply

Unbalanced causedby motor

C∅

A∅

B∅

C∅

A∅

B∅

A∅

T2B∅

T3

C∅ T1

Single phasing is a fairlycommon cause of failure forthree phase induction motorsas the other two phase wind-ings must conduct more current,and therefore produce moreheat, which eventually leadsto premature failure of the motor.What makes single phasing atricky problem to detect is thatthe motor will continue to runnormally, although there will bean increase in heat, and possiblyloss in torque—subtle conditionsthat may go unnoticed. Anotherclue to a possible single phasingcondition is if the motor isstopped and restarted, it mayrun backwards.

The measurement sideof detecting this problemis also a little tricky. Whenthe voltage measurement ismade at the motor terminals,the voltages will read closeto normal as motor action isinducing voltage into the openwinding. The best way toreliably detect this condition isto make current measurementson all phases until the openphase is detected through anabsence of current flow.

T1 A∅ 30A

T2 B∅ 35A

T3 C∅ 30A

B∅C∅

T2T3

T1A∅

FLUKE 87

80i-400 A COMmA A

1000V MAX

400mA MAXFUSED

10A MAXFUSED

COMmA A

mAA


Overvoltage Reflections — Theory

For those not interested in thetheory of how this phenomenonoccurs, skip ahead to the nextsection on troubleshooting theproblem.

The trend with PWM driveshas been to make the rise timeof the pulses as fast as possibleto reduce switching losses andincrease the efficiency of thedrive. However, fast rise times,coupled with long cable lengthsproduce an impedancemismatch between the cableand the motor causing reflectedwaves, or “ringing” as shownin Figure 8. If the rise timesare slow enough, or the cableshort enough, the reflectedwaves will not occur. The mainproblem with this conditionis that ordinary motor windinginsulation can break downquickly. Additionally, higherthan normal shaft voltagescan develop causing prematurefailure of bearings andexcessive common mode noise(leakage currents) can interferewith low voltage control signalsand cause GFI circuits to trip.

The relationship betweencable length, rise time andthe resultant increase in peakvoltage is illustrated in Figure 9.Basically, the peak voltageat the motor terminals willincrease above the dc bus voltageof the ASD as cable lengthincreases and the rise time ofthe ASD output pulse gets faster.

Figure 8. Reflected voltages (ringing) on themodulated pulse

Figure 9. Impact of rise time and cable length on magnitude of reflected voltages


For example, if we have a480 Vac L-L voltage with a648 Vdc bus, 50 feet of cable,and RL = 0.9 we can see theeffect of the peak voltagecalculation using two differentrise times.

With a rise time of 0.5 µs, thetransit time (0.1 µs) is less than1/3 the rise time (0.16 µs) so weuse the first Vpeak formula thattakes into account the effectof rise time and the calculationbecomes:

x 0.9 +1* 648 Vdc = 998 Vpeak

With a rise time of 0.1 µs, thetransit time (0.1 µs) is greaterthan 1/3 the rise time (0.033 µs)so we use the second Vpeakformula which is:0.9 + 1 * 648 Vdc = 1,231Vpeak

(See the Acknowledgments atthe end of this application notefor additional information andreferences)

There are two different for-mulas for determining the peakvoltage that can be expected atthe motor terminals, dependingon whether the transit time ofthe pulse (traveling at half thespeed of light) is greater thanor equal to 1/3 the rise time(dv/dt) or less than 1/3 the risetime. First let’s look at how todetermine the transit time (tt)with regard to cable length (lc).

tt = = 2x10-9 * lc

If the transit time of the cableis less than 1/3 the rise timethen use the following Vpeakformula:

Vpeak = * L +1 * VDCB

When the transit time isgreater than or equal to 1/3the rise time, then the rise timeportion of the first Vpeak formulais not used and the calculationbecomes:Vpeak = L +1 * VDCB

RM - RC

RM + RClc (ft)

500 (ft/µsec)

3tt

tr

.3

.5

The magnitude of the voltagethat is reflected at the motorterminals back to the ASD andadded to the peak voltage, thendriven back to the motor, isdetermined by something calledthe reflection coefficient, whichis a function of the motor resis-tance, the cable inductance andcable capacitance as shown inthe formula below:

L =

where L is the reflection coeffi-cient of the load, RM is themotor resistance and RC is thecharacteristic cable resistanceexpressed by the formula:

RC = √L/C(see cable manufacturer’s.specifications for inductance (L)and capacitance (C) per foot)

Most motors less than 10 HPhave an L between 0.9 - 1.0.Some larger horsepower motorsmay have an L as low as 0.8,occasionally less. The curves inFigure 9 use an L of 0.9 in thecalculation.


Figure 12. Leading edge of PWM pulse withno reflected voltage

Figure 13. Leading edge of PWM pulse withreflected voltage (ringing)

The signals in Figures 12 and13 were captured by triggeringon a single pulse using singleshot mode with cursors enabledto make the peak voltagemeasurement along with risetime. While this measurementrequires more button pressingand scope “know how,”the automated rise timemeasurement may be worth thetrouble. Manually resetting thesingle shot trigger periodicallywill give you a sampling ofvarious peak voltages for thedifferent pulses. Also, slowlyraising the trigger voltagewill give you an idea of themaximum peak when thescope stops triggering.

Overvoltage reflections —troubleshootingAs mentioned earlier, fast risetimes on the ASD output pulsesand long cable runs betweenthe ASD and the motor willcause overvoltage reflectionsapproaching double the DCbus voltage and even higher.An oscilloscope is required todiscover the full extent of thisproblem, as seen in Figures 10and 11 below.

Figure 10. Normal PWM signal

Figure 11. Overvoltages due to reflected waves

Figure 10 shows the ASDL-L voltage measurement at themotor terminals with six feetof cable, while Figure 11 showsthe ASD L-L voltage with100 feet of cable. Notice thedifference in peak voltagemeasurements — about 210volts. Also notice that there isonly 5 Vrms difference betweenthe two waveforms (small digitson the display). This meansyour voltmeter will not findthis problem.

Very few scopes will triggeras nicely and easily as the FlukeScopeMeter® 123 Test Tool didfor the measurements in Figures10 and 11. For other scopesuse the procedure outlinedbelow to measure the extentof the overvoltages.


Assuming you have identifieda true overvoltage, or ringingproblem, then something mustbe done about it. The simplestsolution is to shorten the cable.Table 3 shows the maximumlength of the cable before thepeak voltage goes beyond 1.15times the dc bus, (highest “safe”motor voltage) for various risetimes and reflection coefficients( L ) of 0.9 and 0.8 (see previoussection for more details).

The table only shows wherethe peak overvoltages startfor a given rise time and lengthof cable. The peak overvoltageswill continue to increase toalmost double the dc busvoltage as the cable lengthensor rise time gets faster. Thepeak voltages can even exceedvoltage doubling if the reflectedvoltage occurs on top of existingringing due to the distributedleakage inductance andcoupling capacitance.

Length where Vpeak Length where Vpeak>1.15 x DC bus voltage >1.15 x DC bus voltage

PWM Pulse Rise Time and L = 0.9 and L = 0.8

0.1 µs 2.8 feet 3.1 feet

0.2 µs 5.5 feet 6.3 feet

0.5 µs 14 feet 16 feet


1 µs 28 feet 31 feet











Table 3. Maximum cable lengths for various rise times and reflection coefficients before peakvoltages begin to exceed 1.15 times the DC bus voltage

The real danger of thisovervoltage condition is thedamage it can do to the motorwindings over a period of time,which may not show up as aproblem when the PWM drive isfirst installed. Many PWM drivesare installed without taking intoconsideration the overvoltageeffects of long cabling betweenthe PWM output and the motor.And while improved efficiencyof the newest and latest PWMdrives are achieved by makingthe rise times faster on theoutput pulses, this can makethe overvoltage problem evenworse, and the need for shortercabling even greater.

If your motor has alreadyfailed and has to be rebuilt,better insulated wire such asThermaleze Qs, or TZ Qs (byPhelps-Dodge), should be usedto rewind the motor. The mainadvantage is that it providessignificantly more protectionagainst overvoltages withoutadding insulation thicknessand the same stator can beused without modification.If the motor has been damagedbeyond repair then a motordesigned to meet NEMA MG-31specifications (sustained Vpeak≤1600 volts and rise time0.1 µs) should be used as areplacement motor for PWMapplications where sustainedovervoltages may be occurring.


If the cabling in your PWMapplication cannot be shortenedthen use one of the three waysto fix the problem as shownin Figure 14:1. An external “add-on” low

pass filter can be installedbetween the PWM outputterminals and the cable tothe motor, as one way toslow the rise time.

2. Another approach is to installan R-C impedance matchingfilter at the motor terminalsto minimize the overvoltages,or ringing effect.

Figure 14

3. In some applications, suchas submersible pumps ordrilling machines, it is notpossible to access the motorterminals and other methodsof minimizing overvoltagesare required. One way isto apply series reactorsbetween the PWM outputterminals and the cable tothe motor. While this is afairly simple solution, thereactors may be fairly large,bulky and expensive for largehorsepower applications.

The trade-offs betweenthese three alternatives forminimizing overvoltages canbe summarized in Table 4.

Motor Terminal Filter Inverter Output Filter Series Reactor

Parallel connected at the Series connected to the Series connected to themotor terminals. PWM output terminals. PWM output terminals.

Designed to match the Designed to slow rise Acts as current limitercharacteristic cable time (dv/dt) below a and also slows rise time.impedance. critical value.

Not cable length Dependent on cable Dependent on size ofdependent. length. system.

Losses are more or less Losses dependent on Losses dependent onfixed. motor kVA motor kVA

Size/cost more or less Size/cost dependent on Size/cost dependent onfixed. motor kVA. motor kVA.

Table 4.

All the solutions suggestedabove should be designed foryour specific application by aqualified engineer.

Safety Note: Reflective voltagephenomenon can mean peakvoltages 2-3 times the DC busvoltage. For 480V line voltagethis means a DC bus voltageof 648V and possible peakovervoltages of 1300V-2000Vand possibly higher given+10% line voltage variance.Therefore it is recommendedthat the measurement at themotor terminals be madewith the highest rated probeavailable and for the shortesttime possible where reflectedvoltages are likely to be present.

Filter Remedy 1: Typicaleffect of a low pass filterat the inverter output asmeasured at the motorterminals

Filter Remedy 2:Typical effect of theseries reactor measuredat the motor terminals

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


Bearing currentsWhen motor shaft voltagesexceed the insulating capabilityof the bearing grease, flashovercurrents to the outer bearingwill occur, thereby causingpitting and grooving to thebearing races. The first signsof this problem will be noiseand overheating as the bearingsbegin to lose their originalshape and metal fragments mixwith the grease and increasebearing friction. This can lead tobearing destruction within afew months of ASD operationand is thus expensive in motorrepair and downtime.

There is a normal, unavoidableshaft voltage created from thestator winding to the rotor shaftdue to small dissymmetries ofthe magnetic field in the air gapthat are inherent in the designof the motor. Most inductionmotors are designed to have amaximum shaft voltage to frameground of < 1 Vrms.

Another source of motorshaft voltages are from internalelectrostatic coupled sourcesincluding; belt driven couplings,ionized air passing over rotorfan blades, or high velocity airpassing over rotor fan bladessuch as in steam turbines.

Under 60 Hz sine waveoperation, the bearing break-down voltage is approximately0.4-0.7 volts. However, withthe fast edges of the transientvoltages found with PWMdrives, the breakdown of theinsulating capacity of the greaseactually occurs at a highervoltage — about 8-15 volts.This higher breakdown voltagecreates higher bearing flashovercurrents, which causes increaseddamage to the bearings in ashorter amount of time.

Research in this area hasshown that shaft voltagesbelow 0.3 volts are safe andwould not be high enough fordestructive bearing currents tooccur. However, voltages from0.5-1.0 volts may cause harmfulbearing currents (>3A) andshaft voltages > 2 volts maydestroy the bearing.

Care must be taken whenmaking this measurement.Connect the probe tip to a pieceof twisted stranded wire, ora carbon brush which in turnmakes contact with the motorshaft, while the common isconnected to the motor frameground. As the shaft voltagesare caused by fast rise timesof the PWM drive pulses,the voltages will appear asinconsistent peaks and should bemeasured using an oscilloscopeand not a DMM. Even if theDMM has peak detect, there isenough variation between peaksto render the reading unreliable.Another measurement tip is tomake the shaft to frame groundvoltage measurement after themotor has warmed to its normaloperating temperature as shaftvoltages may not even bepresent when the motor is cold.

The most simple solution tothis problem is to reduce thecarrier (pulse) frequency to lessthan 10 kHz, or ideally around4 kHz if possible. If the carrierfrequency is already in thisrange than alternative solutionscan be employed such as: shaftgrounding devices, bearinginsulation, faraday shield inthe motor, conductive grease,the use of ceramic bearings orfiltering between the ASD andthe motor.

Leakage currentsLeakage currents (commonmode noise) capacitively coupledbetween the stator windingand frame ground will increasewith PWM drives as the capacitivereactance of the windinginsulation is reduced with thehigh frequency output of thedrive. Therefore, faster rise timesand higher switching frequencieswill only make the problemworse. It should also be notedthe potential increase in leakagecurrents should warrant closeattention to established andsafe grounding practices for themotor frame. The increase inleakage currents can also causenuisance tripping of ground faultprotection relays, override 4-20mA control signals, and interferewith PLC communications lines.

Measure common mode noiseby placing the current clamparound all three motor conduc-tors. The resultant signal will bethe leakage current.

A common mode choke alongwith a damping resistor can beused to reduce leakage currents(Figure 15). Also, special EMIsuppression cables can be usedbetween the drive output andthe motor terminals. The copperconductors of the cable arecovered with ferrite granuleswhich absorb the RF energyand convert it to heat. Isolationtransformers on the ac inputs willalso reduce common mode noise.

Figure 15. Common mode choke with dampening resistor to reduce leakage currents

3φ AC Input

M

Diode Rectifier PWM Inverter

Induction Motor

Common ModeChoke


Control circuit noiseInduced electrical noise cansignificantly affect sensitivecontrol circuits such as speed,torque and control logic,position feedback sensors,as well as outputs to displayindicators and system controlcomputers. As many of thecontrol inputs are scaled 0 to5 or 10 Vdc maximum, withtypical resolutions of one partin 1,000, only a few millivoltscan cause improper operation.Significant amounts of noisecan actually damage the driveand/or motor.

A common source ofelectrical noise is from thecoils of relay and contactorcoils. Transients caused bythe opening of the coil circuitscan generate spikes of severalhundred volts which in turncan induce several volts of noisein adjacent wiring. Follow goodinstallation practices by usingtwisted pair, shielded wiringfor sensitive control circuitsand separate them from relayand contactor coil circuit wiring.

Additionally, addingsnubber circuits to the relayand con-tactor coils willeliminate arcing and reducenoise induced in adjacentwiring. For most ac coils, a33 kΩ resistor connected inseries with .047 µF capacitorcan be connected across eachac relay and contactor coil.For dc coils, use a reversebiased diode across the coil toachieve similar results as theRC snubber for ac coils.

Noise on the line inputscaused by SCR controlled dcdrives, current source inverters,six-step drives, and other noisyloads in the building can alsoinduce unwanted noise inadjacent control wiring. Thehigh energy, fast switchingPWM signals on the motorcabling will also contribute tothis problem if it is unshieldedand within close proximity ofcontrol wiring. The best wayto minimize this problem isto be sure line input wires andmotor cabling are containedin separate grounded, rigidmetal conduit.

Verifying whether noiseproblems exist in control circuitwiring will require the use of ascope. Special care should betaken when using a scope tomake low voltage measurementsso that noise is not coupled intothe scope and is then mistakenas noise on the control signalwiring. Using 10x probes withshort ground leads will minimizenoise introduced by the scopeprobes into the measurement.

Volts/hertz ratioAs discussed earlier, theratio of voltage to frequencydetermines the amount of motortorque produced by a givenac induction motor. If the motorexperiences a loss in torque,then this measurement maygive some clues as to whatis happening.

While the V/Hz ratio is notsomething that normally needsto be adjusted after installation,this measurement can be quiteuseful for diagnostic purposes.The Fluke 39 or 41B powermeters work well for thismeasurement as they have abuilt-in low frequency responsewhich gives a voltage readingcomparable to what the motorresponds to. Additionally, thefrequency can be viewed onthe same screen as the voltage.Using an oscilloscope with abuilt-in, or an external low pass

filter such as the PM 8918/301low pass filter probe from Fluke,can give similar results.

If the frequency of the readingis stable, but the voltage is low,high or unstable, it could indicatea problem with the dc bus circuit.If the frequency is unstable,but the voltage is okay, thensomething might be wrong withthe IGBT control circuit. If theV/Hz are fluctuating together,or the speed of the motor is off,but the V/Hz ratio is correct,then one of the speed inputs tothe control board may be bad.Vector drives that employ torquecontrol by regulating currentin a nonlinear fashion overthe entire speed range are anexception to this however,as the V/Hz ratio will varyconsiderably, be hard to predict,and should therefore not beused for diagnostic purposes.

Inverter drive circuitsVoltage source invertersWhile PWM drives are becomingmore and more popular, and arecommonly being used to replacethe voltage source inverters(more commonly called thesix-step drive because of thestepped shape of its output)there are still many of thesesix-step drives in operation thatrequire maintenance. While allthe other checks outlined in thissection—voltage and currentimbalance, single phasingand overheating—certainly applyto six-step drives, there areproblems unique to this kindof drive as well.

A “shorted” transistor onsome six-step drives can bedetected by measuring acrossthe transistor with a scope.A good transistor will havea nicely formed square wavewith sharp edges, while a badtransistor will be rounded atthe peak of the leading edge.

Measurements at the Inverter

Figure 16.ACand DC coil noise suppression

AC Coil DC Coil0.47 µF

Diode33 KΩ

+

_

RC snubber


If the shorted transistor iscausing the drive’s protectioncircuit to trip, then the convertersection that rectifies the ac intodc can be disconnected and theinverter circuit can be run withthe 10 volts or so of leakagevoltage that is present on the dcbus. The input driver circuit willstill turn on the transistors butat a much lower voltage leveland the bad transistor is easilydetected.

Likewise, the inverter sectioncan be disabled while trouble-shooting the ac to dc convertercircuit. The speed control canbe varied while monitoring thedc bus voltage to see if it varieswith the speed control.Important note: The voltagefeedback resistors must remainconnected to the dc bus toinsure the converter sectionis still controlled with the speedpotentiometer. Be sure todisconnect the ac inverter sectionafter the voltage feedbackresistors. If this procedure isnot followed, the dc converterwill turn full on immediatelyupon starting the drive.

If the converter section is notfunctioning properly SCRs canbe checked individually, out ofcircuit, using the followingprocedure (Figure 17).1. Put the DMM or ScopeMeter

in Diode Test.2. Place the red lead on the

Anode and the black lead onthe Cathode. This puts about3.5 Vdc across the device.

3. Solder some alligator leads toa 1 KΩ resister and connectone end to the anode and theother to the gate.

V

400 1 2 3 54 6 7 8 9 0

87 TRUE RMS MULTIMETER

MIN MAX RANGE HOLD H

REL Hz

mAA

A

mV

V

V

OFF

!

A COM VmA A

1000V MAX

400mA MAXFUSED

10A MAXFUSED

PEAK MIN MAX

P

N

P

N

Gate

Anode

SCR

Cathode

4. This should turn on the SCRand a voltage drop of about1.0 volt should be measuredacross it. If the SCR is notconducting, you will continueto see OL on the DMM’sdisplay. Note: some SCRs mayrequire a higher voltage toturn on. If this is the case,then connect a test lamp inseries with the SCR andconnect a 9 or 12 Vdc batteryacross the test lamp and SCR.Connect the resistor betweenthe gate and the anode of theSCR. The SCR is conducting ifthe lamp turns on.

PWM invertersMany of the newer fractionalhorsepower PWM drives areintegrated to the point wherethe input diode block and IGBTsare “potted” into a single throw-away module that is bolted tothe heat sink. The cost of theseunits rarely justify the time torepair, if replacement partsare even available. However,the larger horsepower drivesstarting in the 5-25 horsepowerrange, have components thatare accessible and becomeeconomically feasible to repair.

If it has been determined thatthe drive inverter is the sourceof an improper voltage beingapplied to the motor, then usethe following procedure toisolate which IGBT(s) is failingin the output section.

1. To check the positiveconducting IGBTs, connectthe scope common lead tothe DC+ bus and measureeach of the three phases atthe inverter’s motor outputterminals. Check for nice,clean-edged square waveswithout any visible noiseinside the pulses, and thatall three phases have thesame appearance.

2. To check the negativeconducting IGBTs, connectthe common lead to theDC- bus and performthe same measurementsas in step one above, oneach of the three phasesat the inverter’s motoroutput terminals.

3. Check for “leaky” IGBTs bymeasuring the voltage fromearth ground to the inverter’smotor output terminals withthe drive powered on, butthe speed set to zero (motorstopped). Some drives mayhave a normal earth groundto motor terminal voltageof about 60 volts, with areading of over 200 voltsindicating a leaky IGBT.Perform this measurementon a known good drive todetermine what is normalfor that drive.

Figure 17. Setup for testing SCRs


The dc busDC voltage too highTransients (less than .5 cycle)and swells (.5-180 cycles) onthe AC line inputs and motorregeneration are the two mostcommon causes of “nuisance”tripping of the overvoltagefault circuit on ASD inverters.Transients and swells can becaused by events happeningoutside the building likelightning or utilities switchingKVAR capacitors or transformertaps, as well as other loadsinside the building beingswitched on (capacitive) or off(inductive). To test for this, usean oscilloscope or power linemonitor with at least 10 µsec/div. resolution, and capable oftime-stamping the event.

The Fluke ScopeMeter® 123Test Tool is a good choice forthis measurement as it hasplenty of single shot resolution,and most importantly can time-stamp the event so it can betime correlated to whateversource—lightning, utility orelectrical equipment—is causingthe problem. Additionally, it hasthe IEC 1010-1 600V CAT IIIsafety rating which should bean important considerationwhen purposely measuring highmagnitude impulses in a highenergy environment.

If the drive is installed in apart of the country that is proneto lightning activity, be surethe building has proper surgeprotection that is functioning

properly. Additionally, thebuilding’s grounding systemmust be properly installedand functioning to help dissipatelightning strikes safely to earth,rather than through somepath in the building’s powerdistribution system. Steps canand should be taken to minimizetheir effects on your electricaland electronic equipment, sincea building that is susceptibleto transients, sags and swells,is usually a building that isdeficient in proper wiringand grounding.

If a transient voltage isexpected, then an oscilloscopelike the Fluke ScopeMeter®

can be used to measure, andmore importantly, time stampthe transient so it can timecorrelated to whatever eventcaused the ASD fault. A “freeware”software package is availablewhich was especially designedfor the ScopeMeter “B” series to

Figure 18. Single shot transient time stamped with ScopeMeter freeware

Figure 19. Transient time and date stamp retrieved using FlukeView software

Trigger (T=0.00) on 2/26/97 at 13:45:08

150

100

50

2/26/97 13:45:08

0

-50

-100

-150

-200

-250

-0.0

4

Volt

s

-0.0

3

-0.0

2

-0.0

1

0.00

0.01

0.02

0.03

0.05

0.0

6

0.07

0.0

8

0.0

9

0.10

0.11

0.12

0.13

0.14

0.15

0.16

Time

capture, time-stamp and resetsingle shot measurements.2It is especially well suited forlogging and time-stampingelusive transient events. SeeFigure 18.

The ScopeMeter 123 canalso time stamp a single shotevent which can later beretrieved using FlukeView® forScopeMeter® software version2.0 (this feature only availablewith ScopeMeter 123). Thiseliminates the need to have aPC connected to ScopeMeter123 while it’s waiting to triggeron the transient (Figure 19).

If the tripping is caused bya transient, then an isolationtransformer or series line reactorscan be placed in series withthe front end of the ASD. Analternate solution would be toplace a surge protection device(SPD) at the motor controlcenter, or the primary sideof the distribution transformerfeeding the ASD. However, ifthe source of the transient iscoming from another load onthe same secondary feed as theASD, then a separate isolationtransformer or series line reactormay need to be used directly infront of the ASD, or better yet,put the ASD on it’s own feed.

Voltage swells >30 cyclescan be monitored using theScopeMeter TrendPlot™ mode(see Figure 20) or using someother type of line monitor. Oneway to mitigate the swell is to

2 ScopeMeter freeware available at:www.fluke.com/scopemeter/


install a temporary dropoutrelay for as many cycles asthe swell, but that can stillbe tolerated by the drive. Theviability of this solution will bedetermined by the amount of“ride-through” the ASD’s inputcircuit can handle before thedc bus voltage drops to anundervoltage condition.Another possible solution is touse a voltage regulation devicelike an uninterruptable powersupply (UPS), but it shouldbe noted that most UPSs aredesigned to handle voltage sagsand momentary interruptionsand may not handle voltageswell conditions unlessspecifically designed to doso. Carefully check the UPSmanufacturer specifications.

Overvoltages or long termvoltage drift can be caused byvery large loads being turnedoff within the building or a slowresponse of the utility’s voltageregulation system to largereductions in demand on thepower grid. This condition iseasily discovered using theScopeMeter TrendPlot™ feature.The best way to deal with thisproblem is to employ localvoltage regulation with a devicelike a UPS that is designed tohandle overvoltage as well assags and dropouts.

Another common sourcefor overvoltage on the dc busis motor regeneration. Thisoccurs when the motor load is

“coasting” and begins to spinthe motor shaft rather thangetting spun by the motor,which causes the motor tochange into a voltage generatorand returns energy to the dcbus. Excessive regeneration canbe measured by checking for achange in the direction of thedc current back into the dc buswhile simultaneously checkingthe dc bus voltage for anincrease above the trip point.If regeneration is causing theovervoltage tripping, somethingcalled “dynamic braking” can beemployed which limits how fastthe regenerative current isallowed to feed back into the dcbus capacitors.

If the dynamic braking hasalready been employed and isnot functioning properly, thenit can be tested according to themanufacturer’s specifications.If the brake is the resistor type,it can be visually inspected forsigns of overheating; discolora-tion, cracking, or even smellfor that distinctive aroma ofan overheated component.The resistance value can alsobe measured against themanufacturer’s specifications.If the dynamic brake uses thetransistor type, then the siliconjunctions can be tested usingdiode test as described earlier.Also, the braking current can bemeasured and the currentwaveform compared with thatof a known good system.

DC voltage too lowThere are several possibilitiesfor “nuisance” tripping of thelow voltage fault circuit onASD inverters. Voltage sags(.5-180 cycles) and undervoltages(>180 cycles) on the lineinput to the drive are commonconditions associated with thisproblem. Sags are quite oftencaused by another load withinthe building’s distributionsystem being turned on, orperhaps from a neighboringbuilding starting a largeelectrical load.

Make the measurement withan instrument that can timestamp the sag or where theundervoltage causes the ASDlow voltage fault to trip. Youmay want to start making thismeasurement at the serviceentrance. This way you canquickly isolate whether the sagis being caused from within thebuilding or outside. Be sure tomonitor the voltage and currentsimultaneously so you cantell whether the problem isdownstream from the serviceentrance as indicated in Figure21 where the surge in currentis coincident with the voltagesag. An upstream (outside thebuilding) problem would showthe voltage sag without acorresponding surge in current.If the problem is within thebuilding, continue making themeasurement at different loadcenters until you have isolatedthe load with the correspondingvoltage sag and current surge.

Another possibility is a motorthat is drawing enough currentto cause the dc bus voltageto drop below the undervoltagefault setting, but not enoughto trip the current overload.You will need to check themotor current for overloading(compare with motor name-plate) as well as verify whetherthe program settings of thedrive are correct for the motornameplate ratings, including theapplication for which the motorand drive were intended.Figure 21. Voltage sag caused by a “down-

stream” loadFigure 20. Trend of voltage swell


Look at the line inputvoltage waveform to the ASD.The wave-shape should bea nicely shaped sine wave.Severe “flat-topping” of thewaveform (see Figure 22) canprevent the dc bus capacitorsfrom fully charging to the peakvalue, which lowers the dc busvoltage as well as the amountof current available to the ASDoutput circuit.

AC line inputMany of the problemsassociated with the ac lineinput that have been discussedin the previous section, have adirect and immediate effect onthe dc bus. However, thereare a few issues unique to theinput circuit that still need tobe discussed.

Diode bridgeThe diode bridges used in PWMdrives are pretty straightforwardto troubleshoot. The normalfailure mode for a diode isfrom transient overvoltages orover-current conditions. If theshorted diode trips the breakerbefore it has a chance to burninto an open circuit, thena DMM with diode test willuncover this problem quiteeasily. Use diode test as theohms mode may not put outa high enough voltage to getthe diode to conduct.

Once power has beencompletely disconnected fromthe ASD line inputs, use diodetest to check from both the -DCand the +DC bus to each of theline input connections. Startingwith the positive lead on the+DC bus, probe each of theline inputs with the negativelead. Each reading shouldindicate OL (overload), or areverse bias condition. Takethe readings again, only nowwith the negative lead on the+DC bus and measure eachline input with the positivelead. This will forward biasthe diodes and cause them toconduct with about 0.5-0.7volts dropped across them.Use this same procedure for theother three diodes connected tothe -DC bus. The only differenceis that the positive lead on the-DC bus will cause the diodes toconduct, and the negative leadon the -DC bus will reverse biasthe diodes and cause an OLreading on the DMM. Shorteddiodes will read as 0 volts whileopen diodes will read OL whenthey should be conducting.

Most modern ASDs willalso employ some kind of apre-charge circuit to the dcbus capacitors to reduce theinrush current. This will preventtripping of protection circuits,not to mention significant

Figure 23. Checking for open and shorted diodes in-circuit

reduction in wiring andtransformer size that wouldnormally be required to handlethe inrush. Some drives willemploy a pre-charge resistorthat limits the inrush currentuntil the dc bus charges toabout 60%, then a relay cuts into remove the resistor from thecircuit. When troubleshootingthe input circuit, be sure thepre-charge resistor and relayare not overlooked as possiblecauses first.

Other drives employ SCRson the front end that will “chop”the ac voltage that is allowedto be rectified. This slowlycharges the dc bus capacitorsuntil the dc bus comes up to apredetermined level, then theSCRs goes into full rectification.Use the following procedure tocheck for a bad SCR or one thatdoesn’t go into full conductionin the pre-charge circuit.Use measurements made on aknown good drive for referencewaveforms.1. Connect the scope common

lead to the DC+ bus andmeasure each of the threephases at the line inputsone at a time. There willbe a large voltage dropacross each input SCR at thebeginning of the pre-chargecycle which will drop downto nearly zero volts once the

V

400 1 2 3 54 6 7 8 9 0



REL Hz

mAA

A

mV

V

V

OFF

!

A COM VmA A

1000V MAX

400mA MAXFUSED

10A MAXFUSED

PEAK MIN MAX

AC Input

+DC

–DCFigure 22. Voltage flat-topping caused byharmonic currents


dc bus capacitor is chargedand the SCR is fully conducting.The time it takes to goto nearly zero volts can bemeasured and comparedagainst the manufacturer’sspecification or a knowngood drive.

2. Repeat the same procedurefor the other three SCRs byplacing the scope commonlead on the DC- bus andmake the same measure-ments as explained in stepone above.

Voltage notchingThe older six-step drives usuallyuse SCRs instead of diodes asin the PWM drives to rectify theinput line voltage and convertit to dc. The reason for this isthe SCRs can be made to rectifyonly a portion of the incomingsine wave, thereby reducing orincreasing the peak voltageseen by the dc bus capacitor.Remember, by raising orlowering the dc bus voltagewe are raising or lowering thepeak voltage of the outputcircuit signal applied to themotor and therefore the RMSvoltage as well.

As you can see in Figure 24,the voltage contains notchescaused by the firing of the SCRsfrom the control circuit, which

can be a real problem if thisdistorted voltage makes its wayinto the distribution system andis applied to other sensitiveelectronic loads.

The most common way todeal with voltage notching fromsix step drives is to use linereactors in series with the lineinput to the drive, or usean isolation transformer.The advantage to an isolationtransformer is it will alsoreduce common mode noise.

This should be done withcaution however, as the additionof line reactors and isolationtransformers will increase thesource impedance which cancause problems for other loadssharing the same branch circuit.The reason for this is the ASDwill be drawing nonlinear(harmonic) currents in additionto chopping the voltage, and theharmonic currents may produceadditional voltage distortion dueto the increased line impedance.Additionally, there may beloads such as KVAR correctioncapacitors that become resonantand draw excessive current ata certain harmonic frequency,depending on the inductanceof the distribution wiring andother “upstream” devices thatmay exist like line reactors andisolation transformers (see FlukeHarmonics application note andvideo3 for more information).

Voltage unbalanceWhile voltage unbalance atthe motor terminals (discussedearlier) can adversely affectmotor operation, it can alsocause problems at the line sideof the drive. ANSI C84.1-1989recommends 3% maximumvoltage unbalance at the pointof common coupling (PCC)with the utility under no loadconditions, while the IECrecommends 2% (see previoussection to determine % voltageunbalance). However, as littleas 0.3% voltage unbalance onthe input to a PWM invertercan cause voltage notchingand excessive current toflow in one or more phaseswhich can cause tripping ofthe ASD’s current overloadfault protection.

While the 0.3% voltageunbalance is an extreme casecompounded by a lightly loadedmotor, an oscilloscope will berequired to view the notchingproblem. An accurate DMM willbe needed to make the voltagemeasurements required forthe % voltage unbalancecalculation. The Fluke 87, 863and 867B are good choices forthis measurement because oftheir accuracy and resolution.

A common cause of thisproblem is single phase loadsdropping in or out on the samefeed as the three phase ASD.The trend mode of the FlukeScopeMeter Test Tool can beused to help isolate whichsingle phase load is causingthe most problem. Additionally,providing a stiffer source byincreasing the kVA rating ofthe transformer, or by providinga separate feed for the ASDwill minimize or eliminatethis problem.

3 Understanding and Managing HarmonicsVideo, P/N 609096. In Tune With PowerHarmonics Application Note Literaturecode BO221C.

Figure 24. Notching of the line voltage causedby “chopping” of the line input SCRs


There are two basic issuesconcerning harmonics andASDs:1) the ASD operating in aharmonic environment createdby other loads, and 2) creatinga harmonic environment forloads both within and outsidethe building. That is, how theASD is itself affected and howthe ASD can create problemsfor other loads. The lattercondition, creating problemsfor other loads is what theIEEE-519 recommendedpractice is all about andis discussed further in thefollowing paragraphs.

The main problem withinstalling the ASD in a harmonicsenvironment is voltage flat-topping. As discussed earlier,the conversion of ac to dcmeans that there is currentdrawn only at the peak of thevoltage waveform (see Figure25) as the ac line voltageexceeds the dc bus capacitorvoltage. This burst of currentinto the capacitor is whatcauses the voltage to drop atits peak. Flat-topping of theline voltage caused by otherelectronic loads in the buildingmeans the dc bus capacitor inthe ASD cannot charge to itsmaximum capacity. This canmake for dramatic drops in theDC bus voltage should eitherthe motor load suddenlyincrease, or if there is a suddensag on the line voltage inputs.Stiffening the source (increasingtransformer kVA and distributionconductors) or adding voltageregulation such as with a UPSshould alleviate this problem.

Harmonics and IEEE-519 Compliance

Figure 25. Nonlinear (harmonic) currentscause flat-topping of the voltage waveform

The second condition,when the installation of theASD creates harmonic problemsfor other loads, is an issueprimarily for large horsepowerdrives, or a large number ofsmall horsepower drives, anincreasingly common situation.If your ASD installation exceedsthe capability of your building’sdistribution transformer and/ordistribution wiring, then flat-topping may occur and causeproblems for other nonlinearloads in the building such aspersonal computers and otherelectronic (nonlinear) loads.

Another concern is when alarge ASD installation generatesharmonic currents that exceedguidelines set forth in the IEEE-519 document. The purposeof IEEE-519 is to provideguidelines for how muchharmonic current should beallowed to travel outside ofyour building onto the utilitiesdistribution system. The limitswill depend on the “stiffness”of the utility system (see Tables5 and 6).

Remember, harmonic currentsare only a problem if theyencounter some kind of sourceimpedance. In other words, ifthe utility transformer has toolittle V/A capacity for the levelof harmonic currents present,the source voltage beginsto distort. Since the utility isresponsible for supplyingdistortion free voltage to all itscustomers, they will probablylet you know, and possiblypenalize you if your ASDs andother nonlinear loads aregenerating more harmoniccurrents than the distributionsystem can handle.

The IEEE-519 guidelinesset limits for harmonic voltageand current distortion asmeasured at the point of commoncoupling or PCC (usually at therevenue meter), not individualloads. Some ASD manufacturerswill promote their drives asbeing “IEEE-519 compliant.”This is a rather dubious claimas there is no way to accuratelypredict how even a lowdistortion ASD load will affectthe buildings ability to meetIEEE-519 guidelines, unlessof course ASD manufacturersdiscover a way for the driveconverters to draw current ina sinusoidal (linear) fashion.

No matter which harmonicproblem you are trying tocorrect, it will be necessary touse a power harmonics analyzersuch as the Fluke 41B, or fastfourier transformation (FFT)software with the oscilloscopewaveform to analyze harmoniccontent and voltage distortion.This measurement should bemade at the PCC with the ASDoperating at full load, then withthe ASD powered down, so theimpact of the ASD installationon the buildings ability to meetthe IEEE-519 guidelines canbe known.


The short circuit current ratio(SCR), is basically the “stiffness” ofthe system to which the buildingis connected. Table 5 shows boththe total harmonic distortion limitas well as the distortion limit foreach individual harmonic for busvoltages below 69kV. Table 6shows what the maximumcurrent distortion is for differentSCRs and for the different rangesof harmonics.

Figure 26. Voltage waveform, harmonics graph and text data from the Fluke 41B Power Harmonics Analyzer

Table 5: Voltage Distortion Limits

Individual Voltage Total VoltageBus Voltage at PCC Distortion (%) Distortion (%)

Below 69 kV 3.0 5.0

Table 6: Maximum Harmonic Current Distortion in % iL (120V - 69 kV)

SCR h<11 11-16 17-23 23-34 h>34 TDD Related Assumption

<20 4.0 2.0 1.5 0.6 0.3 5.0 Dedicated system

20<50 7.0 3.5 2.5 1.0 0.5 8.0 1-2 large customers

50<100 10.0 4.5 4.0 1.5 0.7 12.0A few relatively largecustomers

100<1000 12.0 5.5 5.0 2.0 1.0 15.05-20 medium sizecustomers

>1000 15.0 7.0 6.0 2.5 1.4 20.0 Many small customers

Figure 26 below shows howthe harmonic voltage distortiondata is presented on the Fluke41B power harmonics analyzer.Two screens are also availablefor current and power. Thismeasurement was made at theinput to the drive and showsthe total harmonic distortion(THD) as above 5%. If the samemeasurement results had beenmade at the PCC, then this

building would be out ofcompliance with the IEEE-519guidelines. However, someharmonic cancellation withother loads in the distributionsystem often occurs whichwill usually mean lower THDreadings at the PCC. Refer tothe Fluke Harmonics applicationnote and video3 for moreinformation on how to solveyour harmonics problems.

3 Understanding and Managing HarmonicsVideo, P/N 609096. In Tune With PowerHarmonics Application Note, Literaturecode BO221UEN.


ASD Measurement Guide

Fluke 39/41BMeasurement Fluke 87 867 “B” GMM ScopeMeter “B” Series ScopeMeter 123 Power AnalyzerType CAT III 1000V CAT III 1000V CAT III 600V CAT III 600V CAT III 600V

Resistance X X X X

Low Voltage X X X X

Temperature w/accessory w/accessory w/accessory w/accessory

Motor Voltage Use only for X Use PM 8918/301 low pass Use PM 8918/301 low pass Xwithout voltage filter probe for “motor eqv.” filter probe for “motor eqv.”overvoltages imbalance rms voltage. Use standard rms voltage. Use standardpresent probe for waveform detail. probe for waveform detail.

Motor Voltage Use only for X DP120 CAT III 600V/CAT II DP120 CAT III 600V/CAT II Xovervoltages voltage 1000V probe recommended 1000V probe recommendedpresent imbalance

Motor Current clamps with clamps with 80i-1000s, 80i-500s, or 80i-1000s, 80i-500s, or Xcurrent output current output 80i-110s current probes 80i-110s current probes

only only recommended recommended

Volts/Hz Use PM 8918/301 low pass Use PM 8918/301 low pass Xfilter probe for “motor” rms filter probe for “motor” rms

voltage. voltage.

DC Bus X X For DC volts > 600 use For DC volts > 600 useDP120 differential probe - DP120 differential probe -CAT II measurements only CAT II measurements only

Voltage Limited X X < 2 kHz onlyDistortion

Harmonics With FlukeView software With FlukeView software X

Component X X X Xtests

Transients > 1 msec > 1 µsec > 40 ns > 40 nsand Surges

Table 7. Instrument recommendations for various ASD measurements

80i-1000s 80i-500s 80i-110s 80I-400AC current clamp 1 mV, AC current clamp AC/DC current clamp AC current clamp

Measurement Type 10 mV or 100 mV/Amp 1 mV/Amp 10 mV, 100 mV/Amp 1 mA/Amp

DC To 100 amps

AC RMS To 1,000 amps To 500 amps To 100 amps To 400 amps

Waveform Details To 100 kHz To 10 kHz To 100 kHz To 5 kHz

Table 8. Recommended Fluke current probes for ASD current measurements

The following table summarizesthe ASD measurements discussedpreviously and the recommendedFluke scopes, meters and poweranalyzers that can be used foreach measurement.


ON

OFF

ON

OL

OUTPUT100mV/A

10mV/A OFFINPUT 100Apk MAX

ZEROZERO

+ -

+ -

II

80i-400• 1-400A

• ±3% of Rdg

• 30 mm (1.18 in)

• 1 mA/Amp

Fluke 867B GMM™

• High performancemultimeter withwaveform display

• 0.025% basic dc accuracy

• 32,000 count display

• 1 MHz waveform displaybandwidth

• 2 Hz to 10 MHzfrequency measurement

• LCD backlight

• Resistance/conductance/continuity

• Amps, mA, µA

• Duty cycle/pulse width/period

• Capacitance to 10,000 µF

• Min/Max/Avg with TimeStamp

• TrendGraphTM plotting

• Built-in RS-232 interfacewith FlukeViewTM 860 PCsoftware

Fluke 87• 0.1% basic dc accuracy

• 100 mV to 1000V ac &dc

• 0.1 mA to 10A, all fused

• 33/4 digit, 4000 countdigital display

• Analog bar graph pointer

• Min/Max/Avg recording

• Frequency, duty cycle,capacitance

• Input AlertTM

• True-rms, backlit display,1 ms peak hold, 41/2 digitmode

• Touch Hold® andRelative modes

• Lifetime warranty

Model 39/41B• Three-phase readout of

kW, kVA, power factorand displacement powerfactor on a balancedthree-conductor circuitfrom one single-phasemeasurement

• Measurements to 600Vrms, 500A rms (1000Awith optional 80i-1000sprobe)

• Measures total harmonicdistortion (up to 31st)and K-factor

• Waveform, bar chart, ortext displays

• Min/Max/Avg recording

• LCD backlight

• Fluke 41B only:• Stores 8 complete

measurements sets ofdata

• RS-232 interface withFlukeViewTM

documenting software

ScopeMeter® B Series• 0.5% basic dc accuracy

• 100 mV to 600V ac & dc

• 50 to 100 MHzbandwidth

• 32/3 digit, >3000 countdisplay

• Min/Max/Trend Plotrecording

• Frequency, duty cycle,pulse width

• Direct readout in ampsand °C/°F

• True-rms, backlit display,40 nS glitch capture

• Touch Hold and Relativemodes

• Print Screen capability

• Three-year warranty

ScopeMeter® 123• 0.5% basic dc accuracy

• 5 mV to 600V ac & dc

• 20 MHz bandwidth

• 32/3 digit, >5000 countdisplay

• 2 channel Min/Max/Trend Plot recording

• Frequency, duty cycle,pulse width

• Direct readout in ampsand °C/°F

• True-rms, backlit display,40 nS glitch capture

• Touch Hold and Relativemodes

• Print Screen capability

• Three-year warranty

80i-1000s• Optional accessory. AC

current 100 mA to1000A rms continuous,1400A rms peak. BNCconnection.

• Use with 39/41B orScopeMeter

80i-500s AC• Included with both the

Fluke 39 and 41B. ACcurrent up to 500A rms,700A rms peak, withbest accuracy of 2% ofreading from 45 Hz to65 Hz. BNC connection.

80i-110s• Optional accessory.

Measures ac and dccurrent in two rangesfrom 50 mA to 100A.Best accuracy is 3% andfrequency response is dcto 100 kHz. BNCconnection.

80T-IRFor fast non-contacttemperature measure-ments.Range: -18°C to 260°C (0°F to500°F). Accuracy: ±3% ofreading or ±3°C (5°F) which-ever is greater. Internal switchselection for °C or °F. For usewith DMMs or ScopeMeter.

FlukeView® Software• Capture screen images or waveforms to

document and archive measurements

• Use data in spreadsheet programs fordetailed analysis

• Save and retrieve setups for fastpreparation of measurement routines

• Supports popular PC file formats (BMPand PCX) for image storage

100ms AVG H

k

400 1 2 3 54 6 7 8 9 0



REL Hz

mAA

A

mV

V

V

OFF

!

A COM VmA A

1000V MAX

400mA MAXFUSED

10A MAXFUSED

PEAK MIN MAX

COMPONENTTEST

SET UP

mV

V

VOFF

LOGIC HiZ

1000V MAX

COMPONENTTEST

LOGIC

10A MAX FUSED 1000V MAX 320mA MAX FUSED

CAT ALL INPUTS 1000V MAX

FREEZE

WAKE UP

A COM VmA / µA EXT TRIG

GRAPHICAL MULTIMETER867

SAVEPRINT

MINMAX RANGE

TOUCHHOLD

DISPLAYMODE

Hz

1 2 3 4 5

AmA/ Aµ


Acknowledgments[1] A. von Jouanne, P. Enjeti, V. Stefanovic,

“Adjustable Speed AC Motor Drives: Applica-tion Problems and Solutions”, Seminar notes,pp. 3.1-6.20, PowerSystems World Conf. 1996

[2] IEEE Recommended Practices and Require-ments for Harmonic Control in ElectricalPower Systems - Std 519, New York, NY: IEEEInc., 1992

[3] EC&M, “Practical Guide to Motors and MotorControllers”, pp. 9-16, Intertec ElectricalGroup, 1991

Special thanks to Dr. Annette von Jouanne forher assistance preparing this application noteand to Oregon State University for the use oftheir Motor Systems Resource Facility.

Fluke CorporationPO Box 9090, Everett, WA USA 98206

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

For more information call:U.S.A. (800) 443-5853 orFax (425) 356-5116Europe/M-East (31 40) 2 678 200 orFax (31 40) 2 678 222Canada (905) 890-7600 orFax (905) 890-6866Other countries (425) 356-5500 orFax (425) 356-5116Web access: http://www.fluke.com

©1997 Fluke Corporation. All rights reserved.Printed in U.S.A. 2/98 G0416UEN Rev B

Printed on recycled paper.

Fluke. Keeping your worldup and running.

Application NotesIn Tune with Power HarmonicsLiterature #B0221UENElectrical Power Circuit Mea-surements for ScopeMeterLiterature #B0241UENABCs of Multimeter SafetyLiterature #B0317UENElectrical Troubleshooting withFluke MultimetersLiterature #B0171UEN

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