Ideally in a two-winding machine, there will be two windings with equal mmf magnitudes, a 90° electrical phase angle between the winding currents and a 90° (electrical) physical angle between windings.
Using knowledge of three-phase induction machines, the impedance of a winding is a function of slip. This adds additional complexity if two balanced winding currents are to be obtained from a single phase supply.
If the current in the auxiliary winding must lead the main winding current, capacitance must be added to the auxiliary winding. However, if the resistance and inductance of the windings changes with slip, the capacitance required to maintain equal mmfs and 90° phase shift must also change.
The schematic diagram above shows a main and auxiliary winding placed with a 90° separation around the rotor. There are two parallel capacitors in series with the auxiliary winding. When starting, the total capacitance is given by
Ctot = Cstart + Crun
Once the rotor reaches a predetermined speed, the switch will open, and only the run capacitance will be used. This type of switching is obtained using a mechanical spring-loaded centrifugal switch. The combined-torque speed curve for a capacitor start-capacitor run motor is shown below.
The primary disadvantage of a capacitor start - capacitor run motor is cost. There are two capacitors and a switch. The combined costs of these components and their manufacture is significant when compared to the rest of the motor. If high starting torque is required, but lower efficiency run operation is acceptable, the run capacitor can be eliminated, as shown below.
In this case, the run torque is negative at synchronous speed, due to the backwards rotating field. The backwards field will also introduce torque pulsations and vibration. The combined steady-state torque-speed curve is shown below.
If run efficiency and vibration are important, but start torque can be compromised, the capacitor can be left in the auxiliary circuit at all speeds. Sizing the capacitor to provide balance at a particular load point, the backwards field can be eliminated, improving efficiency and eliminating torque pulsations.
Eliminating the centrifugal switch can reduce the manufacturing cost significantly. The trade-off is lower starting torque, since the capacitor is not sized to provide balance at starting, but for run conditions
A split-phase motor has no capacitance in the auxiliary circuit. A phase shift with respect to the main current is achieved by using narrow conductors to achieve a high resistance to reactance ratio. Increasing the resistance means that the auxiliary winding can only be used during starting, otherwise, it would overheat.
A split-phase motor has significantly lower torque at starting than any of the capacitor motors due to the reduced phase angle between main and auxiliary winding currents.
Of the two-winding machines discussed above, an evaluation of combined start and run performance results in the following list in order of performance:
The list above also is a list of relative cost of the motors. i.e. a more expensive motor will be more expensive.