What is Meant by Short Circuit Torque

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 fault occurs on the motor circuit. The energy stored within the motor's magnetic field causes 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 can overstress the attachment of the motor to its foundation or damage shafts and couplings 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 the circuit) all influence that magnitude. The most damaging occurrence is the socalled 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, the likelihood of such an event is extremely small. Much more apt to occur is the single-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. The worst case, although least likely, is right at the motor terminals. Second, despite the name "short-circuit torque," occasional torque peaks can also result from various switching operations on the circuit. Of particular concern is rapid reclosing of a suddenly opened circuit, as occurs during bus transfer. Relative phase angles of motor internal voltage and external circuit voltage will have an effect. Third, the theoretical SCT depends upon other circuit components besides the motor. Capacitors can be a major influence.

Fourth, SCT is a transient phenomenon. Consider it analogous to the flow of fault current through a current-limiting fuse. Although the "available" or possible peak value may be dangerously high, externally detectable torque begins to die away before 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 significantly lower because of electrical and mechanical energy absorption. Estimating torque magnitude Given all those variables, what torque magnitude can be expected? Technical papers over the past half century have offered a bewildering variety of answers. Rigorous calculation procedures are hard to come by and impossible for anyone but the motor designer to apply. For estimating purposes, peak SCT values have been quoted throughout the range from 3 to 15 times the rated full-load torque of the motor. A few authors have suggested even higher ratios. Again, this is an internally developed electrical torque. As one engineer puts it, "The peak shaft torque cannot be directly related to the peak electrical torque:' Any quoted value of SCT is probably going to be quite conservative. No test method exists to verify the value; a short-circuit can't be simulated on the test floor. Based on what's been published, a reasonable value of maximum SCT at the shaft or motor base is six times rated torque. That should not endanger the motor shaft itself, which is typically designed with a significant factor of safety applied to the stress expected at maximum torque (the breakdown or pullout value, which is usually from 200% to 250% of rated torque). Evidence suggests that shaft failures from excessive torque are not common. More likely causes are torsional oscillation, fatigue in reversing stress applications such as belt loading, and corrosion.

Like shafts, hold-down bolts are under continuous stress during normal motor operation. Such loading isn't dangerous as long as the bolts remain tight. Let's look at a simple example taken from one manufacturer's product line. The rating is 700 hp, 1,765 rpm, 4,000 volts. Figure 1 is an end view of the motor. The center-to-center hold-down bolt distance is 20", or 1.67'. Size of the four holes is 16"; good practice dictates use of the largest standard bolts such holes can accommodate, which in this example would be 7/8" diameter. Assuming an SCT of six times fullload torque, the value is: (6)(5,250)(700)/1,765=12,500 lb-ft The upward reaction force at the left side in the illustration will be 12,500/1.67, or 7,480 pounds, equally shared between two bolts at 3,740 each. That is little more than half the steady-state safe load for a 7/8" bolt. Furthermore, this assumes no initial tightness of the bolts. The reaction force will not impose any added bolt stress until it becomes great enough to cancel the initial tightening tension. Of course, if the bolts are initially loose, allowing reaction forces to bounce the motor up and down, impact damage will inevitably lead to failure. The use of bolts smaller than the motor base is designed for is another obviously unwise practice. In all of this, remember that a serious system fault or emergency bus transferunlike normal motor starting and running-is a rare occurrence for any normal motor application. Metal fatigue is therefore not a concern. The technical literature discloses no significant history of motors breaking loose from their foundations for any reason unless the mounting bolts (or the foundation structure itself) were plainly inadequate, or were not properly tightened.

Short circuit or electrical fault analyses are performed with the motor speed adjusted so that the applied electrical frequency is coincident with the torsional resonant frequency to produce the maximum dynamic torque (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. It produces a braking torque which has fundamental and second order frequency components. On the other hand, 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 phase difference between the generator and network voltages exists. It produces a braking torque which has a fundamental electrical frequency component. Proper controls can prevent the occurrence of a nonsynchronizing short circuit.