How is a higher torque achieved in a single phase cap-start cap-run motor?

Question

The starting winding come capacitor circuit is there for starting the motor. When the motor reaches 80% rated speed, the centrifugal switch cuts the starting circuit out and leaves the run winding only.

Now, in a cap-start cap-run motor, the centrifugal switch cuts out only the start capacitor and leaves the run capacitor in series with the starting winding. For what purpose is this?

Also, If torque depends on the current and the starting winding has a higher resistance, wouldn't it make more sense to cut out the starting winding completely to achieve a higher torque?

I am a bit confused as to how this cap-start cap-run arrangement produces a higher torque as opposed to a normal cap start motor. Please assist.

Answer 1

Now, in a cap-start cap-run motor, the centrifugal switch cuts out only the start capacitor and leaves the run capacitor in series with the starting winding. For what purpose is this?

A permanently-connected auxiliary winding with some capacitance in series makes the motor more like a two-phase motor. Since the current in the main winding lags the voltage, and adding the capacitance in series with the auxiliary winding can make the auxiliary current lead the voltage, the phase difference between the two can approach 90 degrees. That improves the overall power factor and efficiency and reduces the torque pulsations that are inherent in single-phase motors.

Also, If torque depends on the current and the starting winding has a higher resistance, wouldn't it make more sense to cut out the starting winding completely to achieve a higher torque?

With just one winding, there are two components of the magnetic field rotating in opposite directions with the torque produced by one subtracting from the other. Adding a second winding with a phase displacement reduces that effect and increases the net torque. Look at my answer to: If single phase supply can not create rotating MMF, how does Induction motor generate torque?

Answer 2

This is not completely true. A cap motor can use 2 capacitors. One big for the starting torque with more amps and one smaller for driving and to create the 90% phase shift for the 2nd winding to turn. The switch you mentioned doesn't cut the starting winding but only the starting capacitor. To achieve a better starting torque as said you need bigger capacitance thus 2caps are connected in parallel. After the motor has achieved enough n it cuts one cap from the circuit and leaves the motor with a smaller capacitor. Having a big capacitor while the motor is running can create high currents and burn the motor. And the starting winding can't be cut because you need 2 windings with 90° current phase shift to make it turn.

Answer 3

In short: The starting torque is the same as a normal cap. start motor, but, instead of just sitting there taking up space when the motor is running, the start winding can do its bit and taking some of the load off the run winding as well as providing some rotation to the stator flux and smoothing it a bit, reducing the rotor losses slightly.