Synopsis.-The torque produced by a short circuit is first called the "semi-graphical method," is developed for calcldating the discussed in a general way, showing that it is pulsating in nature. torque from the actutal currents as given by an oscillogram. TheThe average value of torque is determined by the resistance and other effect of resistance can be included in this calculation. The mtethod energy losses but the instantaneous value rises far above the average holds for any variation in self inductance with rotor position provided value. The major part of this pulsating torque is produced by the that saturation is not a large factor in determining the change in change in stored magnetic energy. General equations are derived stored magnetic energy, which seems to be true in cases thus far for the torque in a machine having negligible resistance and constant investigated. sel.f-inductance of each winding which is referred to as an "Ideal Short-circuit torque can be measured by an instrument described Machine." in a previous paper.4 The torque as measured is compared with These equations are useful in comparing single-phase and various the torque as calculated by various methods. This shows that calcupolyphase short circuits and for determining general tendencies. lations based on constant self-inductance of each winding may be However, the numerical value of the torque is usually of most interest seriously in error, particularly if equations derived on this basis are in salient-pole machines and here the self inductance of the armature applied to the actual currents of salient-pole machines. The "semiwindings is not constant. The calculation of torque from the actual graphical" method agrees reasonably well with measutred values inductance of the windings of a salient-pole machine is a very com-for cases thus far tested. plicated problem. A relatively simple step by step calculation, C ONFLICTING opinions are often expressed con-may produce forces which are even more severe. This cerning the torque produced by a short circuit. paper is confined to the subject of short-circuit torque One idea frequently expressed is that since the although the methods proposed can be used, with some resistance of the circuit is small compared to the re-modifications, for determining the torque during synactance, the current flowing has practically zero power chronizing and other transient conditions. factor and therefore the resultant torque produced One difficulty in understanding the torque produced should be very slight. However, the fact that the by a short circuit is the fact that the assumptions power factor is nearly zero indicates onjy that the generally used for calculating the torque during normal average torque during one complete cycle is small. operations do not represent the conditions during a It gives no information concerning what takes place short circuit. In calculating the torque during normal during parts of a cycle. In designing a machine to operation, it is usually assumed that the stored energy is withsta...
