WHAT IS MEANT BY SHORT circuit torque (SCT) of an induction motor?Should it concern the motor installer or user? The simple answer is that SCT is produced within a running motor when a faultoccurs on the motor circuit. The energy stored within the motor's magnetic fieldcauses it to act briefly as an induction generator, feeding high current to the fault,accompanied by a high transient torque. If that torque is great enough, it canoverstress the attachment of the motor to its foundation or damage shafts andcouplings in the drive train.Determining the SCT magnitude, however, is far from simple. First, the type of fault, its location, and its duration (how rapidly protective devices open thecircuit) all influence that magnitude. The most damaging occurrence is the so-called three-phase bolted fault, in which all three phase conductors become solidly joined to one another through zero impedance.Although the resulting short-circuit current is the maximum possible, thelikelihood of such an event is extremely small. Much more apt to occur is thesingle-phase line-to-ground fault, with a consequently lower value of fault current(and associated torque).The farther the fault is from the motor, the less will be the resultant SCT. Theworst case, although least likely, is right at the motor terminals.Second, despite the name "short-circuit torque," occasional torque peaks can alsoresult from various switching operations on the circuit. Of particular concern israpid reclosing of a suddenly opened circuit, as occurs during bus transfer.Relative phase angles of motor internal voltage and external circuit voltage willhave an effect.Third, the theoretical SCT depends upon other circuit components besides themotor. Capacitors can be a major influence. Fourth, SCT is a transient phenomenon. Consider it analogous to the flow of faultcurrent through a current-limiting fuse. Although the "available" or possible peakvalue may be dangerously high, externally detectable torque begins to die awaybefore that theoretical peak is reached.Finally, and most important, what's produced within the motor is an "air gap" or"electrical" torque. What appears at the shaft or mounting will be significantlylower because of electrical and mechanical energy absorption.Estimating torque magnitudeGiven all those variables, what torque magnitude can be expected? Technicalpapers over the past half century have offered a bewildering variety of answers.Rigorous calculation procedures are hard to come by and impossible for anyonebut the motor designer to apply.For estimating purposes, peak SCT values have been quoted throughout the rangefrom 3 to 15 times the rated full-load torque of the motor. A few authors havesuggested even higher ratios. Again, this is an internally developed electricaltorque. As one engineer puts it, "The peak shaft torque cannot be directly relatedto the peak electrical torque:' Any quoted value of SCT is probably going to bequite conservative. No test method exists to verify the value; a short-circuit can'tbe simulated on the test floor.Based on what's been published, a reasonable value of maximum SCT at the shaftor motor base is six times rated torque. That should not endanger the motor shaftitself, which is typically designed with a significant factor of safety applied to thestress expected at maximum torque (the breakdown or pullout value, which isusually from 200% to 250% of rated torque). Evidence suggests that shaftfailures from excessive torque are not common. More likely causes are torsionaloscillation, fatigue in reversing stress applications such as belt loading, andcorrosion.
Short circuit or electrical fault analyses are performed with the motor speed adjusted so that the appliedelectrical frequency is coincident with the torsional resonant frequency to produce the maximum dynamictorque (conservative assumption). For the short circuit analysis, the motor air gap torque as a function of time is usually obtained from the motor manufacturer.A line-to-line short circuit is a short between two of the phase circuits while the motor is running. Itproduces a braking torque which has fundamental and second order frequency components. On the otherhand, a three phase short circuit produces a braking torque at the fundamental with no second order.A non-synchronous short circuit is a short that occurs when the generator is synchronized when a phasedifference between the generator and network voltages exists. It produces a braking torque which has afundamental electrical frequency component. Proper controls can prevent the occurrence of a non-synchronizing short circu
