Synchronous converters and motor-generators

At the present time the alternating-current motor in a motor-generator set of 100-kw. capacity or more is practically always a synchronous motor; an induction motor is rarely used for this purpose. The reason for this is threefold: the lagging current taken by an induction motor makes the motor undesirable at the end of a long line; from an operating standpoint the mechanical construction of an induction motor makes it less reliable than a synchronous motor; and the cost of induction motors has heretofore been considerably higher than that of synchronous motors. The usual objections to the synchronous motor — that it has a low starting torque and that it requires external excitation — do not apply to the case of a synchronous motor used in a motor generator set; a high starting torque is unnecessary in this case, and there is always some way of exciting the motor whether it is coupled to a direct-current or to an alternating-current generator. Thus it has become standard practice to use synchronous motor-generator sets in all sizes except where the output is too small for a standard synchronous motor. This being the case I shall only consider synchronous motor generator sets in addition to synchronous converters.∗