J.C. Gomez, M.M. Morcos, C. Reineri, G. Campetelli

Induction Motor BehaviorUnder Short Interruptions

and Voltage Sags

Of all power quality issues, voltage sags and short interrup-tions are considered to be the main cause of more than

80% of the problems experienced by sensitive equipment. Theconsequences of a power quality problem are sensitive equip-ment dropout and possible full-process or industrial-line disrup-tion, with the obvious customer economic losses andcomplaints. This type of problem occurs frequently due to theincreasing widespread of highly sensitive control equipment,such as programmable logic controllers, adjustable speeddrives, and personal computers.

Customers normally suffer from the effect of the induc-tion-motor and supply-system interaction, and utilities can expe-rience significant loss of load [1]. The motor undervoltageprotection could trip the motor contactor if the supply voltagestays too low for a long time [2]. New power quality requirementshave an important effect on the motor system interaction, for ex-ample, the increasingly popular motor fast reconnection to thesame source or to an alternative source. The load characteristicsduring the reconnection instant are also critical for the motor be-havior, since it is possible thatthe motor would stall and notstart when the supply voltage isrestored [3].

Several reports related tothe modeling of induction mo-tor behavior under voltage sagconditions have been pub-lished, but only a few onshort-interruption behavior [2].To the authors’ knowledge, noextensive experimental studyon these two phenomena hasbeen available in the literature.This article documents an ex-perimental study that is the first

part of an extensive project with the goal of developing a simpletool for the study of induction motor behavior and system effectsunder short interruption and voltage sag conditions.

Short Interruptions and Voltage SagsA short interruption is defined as the complete loss of voltage(< 0.1 pu) on one or more phases for a time period between 0.5cycles and 3 s. This phenomenon can be due to the supply in-terruption or due to the trip and subsequent reconnection by themotor undervoltage protection. The present situation is moresevere than the normal motor start due to several reasons, suchas the motor generated voltage that is out of phase, heavilyloaded machinery, and a rigorous hot-load pickup. The indus-trial plant should have a reacceleration scheme in order to al-low its production process restart without interfering with itsown sensitive equipment and with other customers connectedto the same supply system.

Without considering the induction motor effects, voltagesags are normally represented by a square waveform [4]. Most

IEEE Power Engineering Review, February 2001 0272-1724/01/$10.00©2001 IEEE 11

J.C. Gomez, C. Reineri, and G.Campetelli are with the ElectricPower System Protection Institute,Rio Cuarto National University, RioCuarto, Cordoba, Argentina. M.M.Morcos is with the Department ofElectrical and Computer Engi-neering, Kansas State University,Manhattan, KS, United States.

Digital Vision Ltd.

of the voltage sags lasted 10 cycles or less and were 20-30% inmagnitude. Transmission faults are usually cleared in less than 6cycles, while distribution faults last between 10 and 20 cycles.

Voltage sag results in the initial reduction of the motor speed,keeping for a while a higher voltage supplied by its internal, orback, electromotive force (emf). When the voltage sag ends, themotor speed increases, demanding more energy from the supplyuntil the steady-state speed is reached, hence, extending thevoltage sag duration. The load torque in this case shows very dif-ferent characteristics as compared to normal startup conditions(presence of compressor unloading valves, counter pressure, fandampers, etc.). The motor current is now a function of two phe-nomena, mechanical and electrical, each having its own timeconstant. The presence of the induction motor causes a voltagesag distortion, smoothing and prolonging the voltage variation.The result is that some of the sensitive equipment that was ableto withstand the original voltage sag would drop out during thepost-sag period due to the induction motor effects. This indi-cates that the addition of motor loads to a system known to beoperating without harmful voltage sags can be critical to the sen-sitive equipment operation. It would be very convenient to deter-mine the motor load limit for each particular system based on itssensitive equipment.

The single line-to-ground fault is the most probable type offault and, through a delta-wye transformer, is transferred as atwo-phase voltage sag, in which case voltage sags should beconsidered as a case of unbalanced transient supply.Three-phase voltage sags (to the same level of the imbalancesags) represent the worst stability condition [3]. Therefore, onlybalanced phenomena were experimentally studied, leaving theunbalanced behavior for future investigation.

Experimental SetupThe tested induction motor is a standard three-phase, squir-rel-cage machine of the following ratings: 5.5 kW, 380 V, 50 Hz,and 1,450 rpm. The load was based on an eddy-current brake,having torque characteristics nearly proportional to the squareof speed. Voltage and current were measured and recordedthrough a digital oscilloscope and a standard power data ana-lyzer, both sampling 32 bits. The investigated voltage sags andshort interruptions were always balanced with a duration of ap-proximately 5 cycles.

Test Cases and ResultsIn order to get a clear and step-by-step idea about the inductionmotor behavior, the following tests were carried out.

No-Load and 85% Motor Rated Load Direct StartsSteady-State Voltage: This test shows a small distortion (< 3%),and very small voltage imbalance (< 1%) and a phase-to-phaseopen circuit rms voltage of 390 V (slightly higher than rated).

Steady-State Current: This test shows distortion without lowfrequency swings or oscillations, current imbalance higher thanvoltage imbalance (approximately 3%), and steady-state no-loadand 85% load currents of 5.56 A and 9.38 A, respectively.

From the steady-state voltage and current oscillograms, noimportant constructive asymmetries were detected.

Start Voltage: The voltage waveforms for both conditionsshow a smooth increase caused by a normal start phenomenonplus a slight oscillation. These slight voltage variations are dueto voltage drops caused by power oscillations.

Start Current: Figure 1 shows the characteristic shape (startcurrent approximately 8 times the rating value) and an important

oscillation with initial amplitude of 12%. The observedoscillation seems to have constant frequency and exponentialamplitude-attenuation. The maximum values and oscillationamplitudes are similar for both no-load and 85% rated loadstarts, with start durations of approximately 0.18 s and 0.24 s,respectively. The effect of the phase angle, at the instant of con-necting the motor to the supply, on the maximum current peaksis noticeable - ranging from 125 A to 148 A. It should be notedthat the voltage recovery is slower in the 85% rated load startthan in the no-load start.

No-Load and 85% Motor Rated Load DecelerationsUntil Zero Speed due to Supply InterruptionThe voltage drop follows a double-exponential variation due tothe speed reduction in addition to magnetic decays. The initialvalue is the back emf that differs from the supply voltage by13% for no-load and 17% for 85% rated load, respectively. Be-sides, the speed reduction is governed by the mechanical timeconstant, which is proportional to the kinetic energy at the shaftpower. The magnetic decay is governed by the rotor-circuit timeconstant. As the rotor resistance is a function of speed, the timeconstant will change with the speed. Since the circuit is open,the current goes immediately to zero.

The voltage measured values are easily fitted by the applica-tion of two time constants, 0.3 s for the magnetic phenomenonand 27 s for no-load (or 0.44 s for 85% rated load) for the speedchange, which is very small for the time period considered in theno-load case. Analytical and experimental values show an ex-cellent agreement up to at least 0.2 s after the disconnection hastaken place, Figure 2.

12 IEEE Power Engineering Review, February 2001

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Figure 2. Back emf decay for no-load motor and analytical exponential line

No-Load and 85% Motor Rated Load Decelerationsdue to Supply Interruption During 5 Cyclesand then Reconnection to SupplyThe voltage drop follows the previous double-exponentialvariation. The same principle was applied to determine thevoltage difference (magnitude and phase angle) for thereconnection instant (off time 96.7 ms and 93.5 ms for no-loadand 85% load, respectively), with a very good agreement be-tween experimental values (0.375 pu for no-load and 1.35 pufor 85% rated load) and analytical values (0.378 pu for no-loadand 1.46 pu for 85% rated load). The voltage difference mea-sured for the loaded case is very close to the reclosing maxi-mum allowed value of 1.33 pu. At the reconnection instant,three transient phenomena take place that are due to three dif-ferent processes:

● Magnetic inrush current● Mechanical inrush current● Power oscillation.The magnetic inrush is caused by the discrepancy between

the supply-established magnetic field and air-gap residual flux(in spatial position and value). The mechanical inrush is due tothe difference between the actual and steady-state speeds. Thepower oscillation is caused by the induction motor response tothe applied power step, generating several power interchangeswith negative and positive torque until passive loads smooth thephenomenon down.

For the no-load case, the speed drop is very small, then thereconnection takes place with small magnetic field and speed dif-ferences, showing overcurrent values not higher than 2.5 times

the rated current. There are also slight power oscillations with atotal transient duration shorter than the direct start phenomenon.

The situation is noticeably different for the loaded case,where the reconnection is completely out of phase, showing alarge voltage shift [5]. The measured intensity values werehigher than the direct start currents, but their time duration wasshorter, showing 60% amplitude oscillations (Figure 3). Thevoltage oscillations due to voltage drops on the circuit imped-ances are, therefore, more noticeable in the out-of-phasereconnection than in the no-load case. The thermal effect pro-duced by the reconnection current is only 53% of that corre-sponding to the 85% load start current.

No-Load and 85% Motor Rated Load Decelerationsdue to Supply Short-Circuiting During 5 Cyclesand then Reconnection to the SupplyAs soon as the motor terminals are short circuited, the voltagefalls sharply to zero. The three-phase currents follow the classi-cal two-component short-circuit time variation. The dc compo-nent is attenuated by the stator time constant, and the accomponent is also attenuated due to the emf decay. Besides, theenergy dissipation process produces a new speed-variation timeconstant, where the power value should now represent theno-load losses, shaft load, and energy dissipation.

The analytical and experimental results have similarwaveshapes and values, clearly showing that the variation israther complex. For the motor under study, the current reacheszero in approximately 80 to 100 ms, with half of the maximumvalue in nearly one-third of the time. The Joule’s heat (or Joule’s

IEEE Power Engineering Review, February 2001 13

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Figure 3. Reconnection current for 85% rated load motor

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Figure 5. Voltage recovery after a 5.5-cycle short circuit with no-load motor

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Figure 4. Short-circuit and reconnection currents for no-load motor

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Figure 6. Voltage recovery after a 5.5-cycle short circuit with 85% rated loadedmotor

integral) of short-circuit currents is much smaller than that cor-responding to the loaded-motor start current. The experimentalmagnetic-decay time constant was 0.07 s, and the speed-reduc-tion time constant was 0.09 to 0.1 s. This means that the genera-tion process does not affect the short interruption waveform. Inthis case, the first part of the short interruption (or voltage sag)shows a stepwise waveshape.

The comparison between short-circuit currents with andwithout mechanical load shows small differences. The no-loadmotor short-circuit current is approximately 10-15% higherthan the loaded current case, and the attenuation of the loadedcase is slightly higher than the no-load situation.

At reconnection, the motor emf is practically zero, since themagnetically stored energy has been dissipated in the rotor andstator resistances. The voltage difference is virtually the supplyvoltage, thus the current will follow the variation explained inthe previous case. The main difference is that the voltage recov-ery is rather slow now, lasting approximately 0.15 s and 0.65 sfor the no-load and 85 % rated load, respectively, and producingsmaller maximum current values, as shown in Figure 4. Itshould be pointed out that the transient that lasted nearly 0.65 swas caused by a short circuit present in the circuit for less than 6cycles. The slow voltage recovery is due to the test circuithot-load pickup, which represents a typical industrial system. Inthe 85% rated load case, the situation is drastically different be-cause of the long reacceleration with a high Joule’s heat (currentis kept approximately constant at 60 A), which can cause ther-mal problems to the induction motor and a possible protection

reaction. Figures 5 and 6 show the recovery voltages for no-loadand loaded cases, respectively. It can be seen that, in the no-loadcase, the steady state is reached without great difficulties. How-ever, in the loaded situation, the voltage is nearly stabilized at65% of the presag value (torque is about 42% of the ratedtorque), showing the difficulties that the induction motor is ex-periencing for the load reacceleration. The Joule’s heat for theloaded reconnection at 0.3 s is already 2.3 times the total amountfor the no-load case.

No-Load and 85% Motor Rated LoadVoltage Sags to 45% During 5 CyclesThe voltage sag is an intermediate situation between theopen-circuit and short-circuit cases described previously. Theno-load and loaded voltage waveforms present slight differ-ences during the low-voltage period. Besides, it can be seen thatthe variation from 100% to 45% and back are not stepwisechanges. After the reconnection to 100% voltage, the slow volt-age recovery for the loaded motor is noticeable, taking nearly0.12 s to reach the steady-state value as opposed to only 60 msfor the no-load case, as shown in Figure 7. During the on-sag pe-riod, the motor slows down, and a higher and more reactive cur-rent can be detected. At the moment of voltage recovery, a largeinrush current is present, which slows down the recovery pro-cess. The current magnitudes and durations were higher in theloaded case than in the no-load case, and the oscillations weremore noticeable in the first case, as shown in Figure 8. From thecomparison with the direct start, it can be concluded that the

14 IEEE Power Engineering Review, February 2001

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Figure 7. Voltage sag to 43% during 5.5 cycles with 85% motor rated load

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Figure 8. On-sag and post-sag currents for voltage sag to 43% during 5.5 cy-cles and 85% loaded motor

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Figure 9. Voltage sag to 30% during 5.4 cycles

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Figure 10. On-sag and post-sag currents for voltage sag to 30% for 85%loaded motor

whole process was faster and that the reconnection maximumcurrent was nearly 50% of the direct-start maximum current.

85% Motor Rated Load Voltage Sags to30%, 56%, 71%, and 85% During 5 CyclesThe observed behaviors were similar to the previous case. Thementioned effects are very noticeable with the 30% voltage sagbeing attenuated while the voltage sag increases from 30% to85%. The current phase shift is evident in both the sag start andend instants (Figures 9 and 10). The transient durations of volt-age decrease and increase are reduced from 0.1-0.13 s to0.03-0.025 s, as voltage sags changefrom 30% to 85%. The on-sag andreconnection peak-current to load-cur-rent relationships move from 2.9-5 to1.2-1.9, while the voltage sags changefrom 30% to 85%.

ConclusionsFrom the experimental study related toshort interruptions and balanced volt-age sags, the following conclusions canbe drawn.

● The induction motor greatly influ-ences the voltage sag waveformand duration.

● There are situations where the sys-tem recovery can be seriously af-fected by the induction motorpresence.

● The motor-load characteristicsshould be considered in voltage sagstudies.

● The on-sag and post-sag currentscan reach levels higher than the di-rect start values and the post-sagovercurrent duration can last morethan twice the normal start timeperiod.

● The circuit hot-load pickup togetherwith the motor load can drasticallyextend or delay the reaccelerationprocess and, in particular cases, pre-vent the start completely.

● The worst case is related with themotor size, system hot-load pickup,and shaft load characteristics.

● Knowledge of the circuit hot-loadpickup characteristics is decisive inorder to get a reasonable accuratecircuit representation for short-in-terruption and voltage-sag studies.

References[1] J.W. Shaffer, “Air conditioner re-

sponse to transmission faults,” IEEETrans. Power Syst., vol. 12, 1997, pp.614-621.

[2] M.H.J. Bollen, P.M.E. Dirix,“Simple model for post-fault motor be-havior for reliability/power quality as-sessment of industrial power systems,”

IEE Proc. Generation, Transmission and Distribution, vol. 143,1996, pp. 56-60.

[3] J.C. Das, “Effects of momentary voltage dips on the oper-ation of induction and synchronous motors,” IEEE Trans. Ind.Applicat., vol. 26, 1990, pp. 711-718.

[4] M.H.J. Bollen, “The influence of motor reacceleration onvoltage sags,” IEEE Trans. Ind. Applicat., vol. 31, 1995, pp.667-674.

[5] T.S. Key, “Predicting behavior of induction motors dur-ing service faults and interruptions,” IEEE Ind. Applicat. Mag.,January/February 1995, pp. 6-11.

IEEE Power Engineering Review, February 2001 15

2002 IEEE Fellow NominationsDeadline: 15 March 2001

Recognizing the achievements of its members is an important part of the mission ofthe IEEE. The IEEE grade of Fellow is conferred upon a person of “outstanding and ex-traordinary qualifications and experience in IEEE designated fields, and who has madeimportant individual contributions to one or more of these fields.” The total number ofFellows selected each year does not exceed 0.1% of the total IEEE membership.

The number of IEEE Fellow nominations for PES members has declined in recent years.As a result, fewer of our colleagues were considered for the recognition that they deservethrough their contributions to power engineering. Many of our PES colleagues made sig-nificant contributions to the profession through their work in engineering, technical lead-ership, and education. As a professional community, we need to be more proactive innominating our colleagues for this significant award.

Any person, including a nonmember, is eligible to serve as a nominator with the follow-ing exceptions: members of the IEEE Board of Directors, members of the IEEE FellowCommittee, IEEE Technical Society/Council Fellow Evaluating Committee Chairs, mem-bers of IEEE Technical Society/Council Evaluating Committees reviewing the nomination,or IEEE staff. The deadline for nominations is 15 March 2001.

The candidate must be an IEEE Senior Member at the time the nomination is submitted,and he/she must have completed 5 years of service in any grade of IEEE membership.

All the necessary material to assist you in the nomination process is available on the IEEEWeb site: http://www.ieee.org/about/awards/fellows/fellows.htm. If you prefer a hard copy,please send an e-mail to fellow-kit@ieee.org. Include your name, street address, city,state/province, postal code, country, and telephone/fax numbers. For more information,contact Chen-Ching Liu, PES Fellows Committee chair, liu@ee.washington.edu.