Second-order torque components in the Schrage motor operating at synchronous speed

The operation of a Schrage motor at synchronous speed is considered, and on the assumption that the air-gap flux at this speed contains space harmonics, an expression for the torque in terms of flux-axis position is derived. This shows that the torque consists of the main component, which is constant for constant brush separation, and variable components depending on (a) unbalance in the secondary circuits owing to errors in brush-separation angles and/or unequal secondary resistances, and (b) the assumed space harmonics of flux. The variable torque components give rise to superimposed speed oscillations and primary-current hunting, when the average speed is just above or below synchronism. An experimental method of investigating the torque is then given, together with two methods of measuring the harmonic content of the air-gap flux. The latter measurements are used to predict the amount by which the total torque varies with the air-gap flux-axis position at synchronous speed. Possible errors are discussed, and an approximate correction for non-linearity due to the brush-contact effect is derived and applied. Experimental results given for a 2-phase machine show reasonable agreement with the theory.