Transient Torques in Synchronous Machines

The calculation of the alternating torque developed on single-phase short circuit has been discussed by previous authors.^1,7 This paper extends the analysis of the electrical torque on short circuit to cover the transient torques due to losses, which although not so large as the alternating torques may be the most serious factor in the effects of short-circuit torques. The resulting mechanical torques in the various parts of the machine are analyzed, and some important conclusions reached: (1) That a rigid stator transmits all the electrical torque developed; while the shaft and coupling of a coupled set, or the springs in a spring-mounted stator in general need transmit only a fraction of the alternating components. The greater part of the alternating components of the electrical torque are absorbed by the inertias involved, when the natural frequency of torsional oscillation is below the rated frequency. This is the case always for spring-mounted stators, and almost always the case for coupled rotors. (2) That the sudden increase in torque on short-circuit conditions determined by losses in the negative sequence resistance (mainly the rotor losses) or in the resistance of the external circuit, may produce higher mechanical torques in shafts and couplings than the alternating components of the electrical torque. (3) That there are other transient conditions which may produce more serious mechanical torques than sudden short circuit.