Starting Performance of Synchronous Motors

This paper deals with the theory underlying the starting performance of the salient-pole synchronous motor equipped with damper windings. The theory, while involving some approximations, is accurate enough for practical engineering calculations. Formulas are developed for the starting torque, pull-in torque and inrush. A method is also given for calculating the speed torque curve from stand still to synchronous speed. Due to the fact that the damper winding is not continuous around the periphery and due to the presence of the single-phase field winding, the rotor circuit is not a perfect polyphase secondary but is unbalanced to some extent. In order to take care of this unbalance, it is necessary, in addition to the usual system of positively rotating vectors, to employ a second system of negatively rotating vectors as is done in unbalanced three-phase problems. The stator resistance has been disregarded in working out the general case of the unbalanced or partial polyphase secondary in order to obtain a torque formula which will be simple and at the same time accurate enough for practical calculations. Mr. Q. Graham is working on this problem and expects to present in an Institute paper, in the near future, the general solution including the stator resistance. The use of the double squirrel-cage type of damper winding in salient-pole machines has been examined both theoretically and experimentally. Other methods of obtaining unusual starting performance are suggested and the results of some actual calculations presented.