Abridgment of short-circuit torque in synchronous machines without Damper windings

The torque produced by a short circuit is first discussed in a general way, showing that it is pulsating in nature. The average value of torque is determined by the resistance and other energy, losses but the instantaneous value rises jar above the average value. The major part of this pulsating torque is produced by the change in stored magnetic energy. General equations are derived for the torque in a machine having negligible resistance and constant self-inductance of each winding and which is referred to as an “Ideal Machine.” These equations are useful in comparing single-phase and various polyphase short circuits and for determining general tendencies. However, the numerical value of the torque is usually of greatest interest in salient-pole machines and here the self-inductance of the armature winding is not constant. The calculation of torque from the actual inductance of the windings of a salient-pole machine is a very complicated problem. A relatively simple step-by-step calculation, called the “semi-graphical method,” is developed for calculating the torque from the actual currents as given by an oscillogram. The effect of resistance can be included in this calculation. The method holds for any variation in self inductance with rotor position provided that saturation is not a large factor in determining the change in stored magnetic energy, which seems to be true in cases thus far investigated. Short-circuit torque can be measured by an instrument described in a previous paper.
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The torque as measured is compared with the torque as calculated by various methods. This shows that calculations based on constant self-inductance of each winding may be seriously in error, particularly if equations derived on this basis are applied to the actual currents of salient-pole machines. The “semi-graphical” method agrees reasonably well with measured values for cases tested so far.