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3Vibrations Vol 17 No.4 December 2001

Process engineers consider variable-speed drives, especiallyAC variable-speed drives, a boon. Some would like to replace

all cylinder and constant-speed drives with variable frequencydrives (VFDs, or AC servos). Simply tie an AC induction motorto a VFD for in nitely variable speed. Add a process computer,and the process is under control. It is a fact that variable-speed drives optimize processingparameters, thereby saving money. It is better to operate a motorat one-half speed than at full- speed with constant starts andstops. Operation at half speed allows a smooth ow of materi -al. In addition, the number of times a motor can be started andstopped is nite, and it is cheaper and more ef cient to operatea motor continuously at a reduced speed. Variables-speed drives have unique problems, however,that present challenges to maintenance personnel. The objective

of this article is to describe some of the problems and waysto monitor and minimize them. It is important to have someknowledge of basic electrical theory, linear and nonlinear cir-cuits, harmonics in power conversion, harmonic amplitudes andlimits, design parameters, and current analysis.

Basic Electrical TheoryThree forces must be overcome if current is to ow in an ACcircuit: resistance, capacitance, and inductance. The amplitudeof the AC voltage in the circuit with respect to time at a givenfrequency is a sine wave. The amplitude of current ow withrespect to time as a function of resistance is also a sine wave.This sine wave is in phase with the voltage sine wave. Theamplitude of the current ow with respect to time as a functionof capacitance is a sine wave that leads the voltage by 90. Theamplitude of current ow with respect to time as a functionof inductance is a sine wave that lags the voltage by 90. Therelationships are shown in Figure 1.

Linear and Nonlinear Circuits and LoadsThe two classes of electrical circuits and loads are linear andnonlinear. In a linear circuit the current varies in proportion tothe voltage to maintain a sinusoidal waveform. This is not thecase with a nonlinear circuit in which the three forces due toresistance, capacitance, and inductance can vary independently

Harlow C. HallGM Powertrain Division

Saginaw, Michigan

Variable Frequency Drives:Are They Heroes or Villains?

Summary. This article describes a study of harmonicgenerators in an industrial environment. The fre-quencies and amplitudes of the harmonics generatedby variable frequency drives are presented. Examplesare given.

of each other. As a result the current waveform is not sinusoidaland harmonics form. The combination of resistance, capaci-tance, and inductance is called impedance. Thus, nonlinear loads cause harmonics in an electricalcircuit. In addition, the impedance of the circuit controls theamplitude of the harmonics at any given frequency. Nonlinearloads can be caused by anything that contains recti ers anddiodes, including AC and DC variable-speed drives, powerrecti ers and inverters, arc furnaces, discharge lighting, com -puters, and X-ray machines. No current ows initially when a voltage is applied to a circuitcontaining a diode or recti er. As voltage increases, current beginsto ow in pulses. When voltage decreases, current stops owingsuddenly. Each diode conducts or pulses once each AC period.The nonlinear conduction pattern is shown in Figure 2.

Figure 1. Relationships of Current Waveforms.

F e a t u

r e

A r t i

c l e

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The pattern in Figure 2 is for two diodes. It is for a full-waverecti ed single-phase circuit. For one diode one pulse wouldbe for a half-wave recti ed single-phase circuit; three diodeswould have three pulses in a single AC period and would indi-cate a half-wave recti ed three-phase circuit. Six pulses would

indicate a six-recti er bridge circuit for full-wave recti cationof three-phase power. Occasionally half-wave recti ed single-phase occurs inindustry. More typical is full-wave recti ed single- and three-phase power. Twelve-pulse circuits are created by using twosix-recti er bridge circuits with a 30 offset in phase betweenthe bridges, between which one recti er res every 30. Sucha design has very low harmonics and thus also low mechanicalstress and noise. It is for this reason the US Navy developed a36-pulse system in their submarines to have a very quiet mechan-ical system.

Harmonics in Power ConversionThe diodes or recti ers in the power conversion circuit pro -duce a characteristic harmonic pattern, regardless of how thepower is used; e.g., computers, a variable frequency drive,radio transmitter. A change in the pattern indicates a problem.The Table contains the characteristic harmonic patterns for

ve power conversion circuits. They range from a half-waverecti ed single-phase circuit with one diode to a full-wave rec -ti ed three-phase circuit with three six-diode bridges, a total of18 diodes and recti ers. Three types of harmonics are generatedduring power conversion: negative sequence, zero sequence, andpositive sequence.

Negative sequence harmonics. Examples of negativesequence harmonics include second, fth, eighth, eleventh,fourteenth, and seventeenth. In an AC induction motor theseharmonics oppose normal motor oppose normal motor rotationby creating a magnetic force in the rotor that opposes this normalrotation. As a result the motor works harder, draws more current,

and creates more heat. The mechanical impacts caused by thereverse torque damages bearings, drive couplings, rotors, andgears. Torsional resonance problems can also arise in driveshafts and rotors. In addition, motor starter contacts can chatterand fail prematurely, and solenoid operated valves can operateerratically. Other problems can also occur. Zero sequence harmonics. Zero sequence harmonics in-clude the third, sixth, ninth, twelfth, fteenth, and eighteenthand are termed triplen harmonics because they are divisible bythree. They add current to the neutral conductor. Triplens areadditive. They can burn the coils out of solenoid-operatedvalves; add heat to motors; trip circuit breakers; disrupt sensorsignals; and cause process computers to lock up, problemswith software, and overheating of transformers. The neutralconductor is not protected from excessive current, and thecables can burn and cause res. This is not a complete l ist ofproblems. Positive sequence harmonics. Positive sequence harmon-ics (fourth, seventh, tenth, thirteenth, sixteenth, nineteenth)create a magnetic eld that rotates in the same direction as

Table. Harmonics Generated in Power Conversion.

Figure 3. Motor Operating at 1,800 RPM.


Figure 2. Voltage and Current for a Computer.

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ambient conditions. The data in Figure 5 were taken on a warm

and dry day; those in Figure 4 were taken on a cold and snowyday. Because the system is not in a controlled environment,such factors affect standing wave formation. Cable length also affects the amplitude of harmonics byaffecting impedance. In Figures 3, 4, and 5 the distance betweenthe variable frequency drive and the motor is about 80 feet. InFigure 6 the distance between the drive and the motor is sixfeet. Both are full-wave recti ed three-phase 480-volt circuits.The spectrum in Figure 6 contains only the fundamental powerfrequency. If the amplitude axis is allowed to oat to negativeamplitude values, a characteristic harmonic pattern for a six-pulse circuit is present, indicating that the harmonics are presentbut are not ampli ed.

Standing WavesThe ampli cation of voltage or current at a harmonic frequencyresults from standing waves, also termed re ected waves ortransmission line effects. A pulse of energy traveling along atransmission line encounters a standing wave that re ects partof the energy in the pulse at a given frequency back toward thesource. As the re ected energy tries to return to the source, itencounters the next pulse, combines with it, and returns to thestanding wave. The standing wave again re ects a portion of theenergy but more energy than before. The process of re ectingand combining continues until the amplitude of the current orvoltage reaches a limiting value. The amplitude then stabiliz-es for that set of conditions and is dependent on impedance.

The cable between the drive and motor represents substantialimpedance and is proportional to length. If the cable surgeimpedance does not match the motor surge impedance, voltagere ection occurs. Pulse width modulation (PWM) frequency, also termedthe switching or pulse frequency, is typically in the range from2 kHz to 10 kHz. PWM for the VFD also plays a role in theformation of re ected waves. Most VFDs are rated full loadwith the PWM at about 2 kHz. As the PWM increases, the drivemust be derated to maintain the heat buildup within acceptablelimits. Thus, adjusting the PWM to optimize the circuit may not be an option.


Figure 6. Effect of Cable Length on the Amplitude ofHarmonics.

normal rotation, thereby slightly increasing torque. However,the frequency is higher than the fundamental so that unwantedheat results. All harmonics produce unwanted heat somewhere in thecircuit. The quantity of heat produced is expressed as the squareof the harmonic number times the square of the current at thatfrequency. When multiple triplen harmonics are present, smallamplitudes, which are additive, produce large quantities of heat.Many production stoppages are caused by harmonics, but thecauses are not sought until the problem is very serious.

Harmonic AmplitudeThe amplitude of each harmonic is a function of the circuitimpedance. As resistance, capacitance, or inductance across acircuit changes, the amplitude of each harmonic is affected ina different way and is usually considered a function of circuitdesign. Changes are continuously made, however, as equipmentis removed or added to a process and power is removed from acircuit. Resistance can increase as contacts age. Aging of knifeblades in a disconnect can change resistance. Such changesaffect vibration levels and circuit impedance. Amplitudes of theharmonics in a variable frequency drive circuit are a function

of operating speed of the motor as a percentage of full speed;see Figure 3 and Figure 4. The motor represented in Figure 3 is operating at about1,800 RPM with fth and seventh harmonics. In Figure 4 themotor is operating at about 3,060 RPM with fth, seventh, elev -enth, and thirteenth harmonics. Late in 2000 the amplitude of the

fth harmonic was about 1% of the amplitude of the fundamentalpower frequency. Earlier, when the motor was operating at 3,240RPM (Figure 5) the amplitude of the fth harmonic was 7.1% ofthe fundamental power frequency. The only harmonic observedwas the fth. Later the other three harmonics were seen (seeFigure 4). The IEEE speci cation for harmonics on a circuit for a

computer (IEEE Standard 519-1992) states that no harmonic willhave an amplitude greater than 3% of the fundamental powerfrequency. The circuit represented by the data in Figure 4 wouldhave passed; the earlier data shown in Figure 5 would not havepassed. Data from the same motor, same variable frequencydrive, and almost the same power level differed only by day and


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Harmonic Limits and Design ParametersIEEE Standard 519-1992 recommends for circuits that supplypower to computers that total harmonic distortion (THD), thetotal of all harmonics, not exceed 5% of the amplitude of thefundamental frequency; that is, the main power frequency. Inaddition, no single harmonic amplitude is to exceed 3% of theamplitude of the fundamental frequency. In industry, however,most electrical circuits supply power to some kind of computer.Very few circuits with power conversion devices consistentlymeet this speci cation. The probable result is computer glitches. Design parameters have been established for power conver-sion units. Full-wave recti cation with multiple bridge circuitselectrically phase shifted to minimize harmonic distortion areused. For three-phase power, two six-recti er bridge circuits aregood; three are better. The VFD output is rated for the upperend of the PWM, not the lower end, to allow greater exibilityfor tuning the VFD to an individual circuit. The length of trans-mission lines is kept to a minimum. Placing the VFD close tothe motor minimizes impedance matching, re ected waves, andthe resulting harmonics. Using isolated power sources for thecomputer minimizes glitches. It is best to minimize or eliminatethe formation of harmonics during the design phase.

Current AnalysisCurrent spectra should be collected in a harmonic-rich envi-ronment on critical circuits. Routes can be established and datacollected as with mechanical data except that harmonic patternsand amplitude changes are sought. Amplitudes are a percentageof the amplitude of the fundamental frequency. Trends can beestablished, and data can be collected in dB amps (the mostsensitive scale), RMS amps, or peak amps. If dB amps are used,the amplitude conversion for percentage of the fundamentalfrequency involves logarithms. It can be dif cult to determine the root cause of problemscaused by harmonics in a power distribution system. Problemsinclude failures of electronically-operated valves, conductorinsulation, bearings, drive couplings, and motor windings;tripping of circuit breakers; pitting of contacts; and fatigue andbreakage of drive shafts.

ConclusionThe most effective way to eliminate harmonics is at the designstage. If harmonics do exist, a good predictive maintenanceprogram that includes collection of current spectra and thermalimaging should minimize problems.

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