Q. Split-phase or resistance-split-phasemotor?

Split-phase or resistance-split-phasemotors: Split-phasemotors have two statorwindings (amainwinding and an auxiliarywinding)with their axes displaced 90 electrical degrees in space. As represented schematically in Figure (a), the auxiliary winding in this type of motor has a higher resistance-to-reactance ratio than the main winding. The two currents are thus out of phase, as indicated in the phasor diagram of Figure (b). The motor is equivalent to an unbalanced two-phase motor. The rotating stator field produced by the unbalanced two-phase winding currents causes the motor to start. The auxiliary winding is disconnected by a centrifugal switch or relaywhen themotor comes up to about 75%of the synchronous speed.The torque-speed characteristic of the split-phase motor is of the form shown in Figure (c). A split-phase motor can develop a higher starting torque if a series resistance is inserted in the starting auxiliary winding. A similar effect can be obtained by inserting a series inductive reactance in the main winding; this additional reactance is short-circuited when the motor builds up speed.