What is the purpose of a run capacitor in capacitor-start, capacitor-run motors?

The purpose of a run capacitor in capacitor-start, capacitor-run motors is indeed to provide extra running torque. In these types of motors, the run capacitor enhances the performance by improving the phase relationship between the current in the start winding and the current in the run winding, thus generating a stronger magnetic field. This increased magnetic field allows the motor to operate more efficiently, especially under load conditions, providing the necessary torque for optimal performance during operation. The run capacitor remains in the circuit even when the motor has started, which continually helps to sustain the motor's efficiency and performance throughout its running time.

While the starting capacitor is there to provide the necessary torque to get the motor moving, the run capacitor takes over once the motor has started, ensuring that the motor can maintain its speed and torque effectively in operation. This distinguishes its role from other aspects like reducing wear or limiting heat production, which may also be beneficial outcomes but are secondary to the primary function of ensuring adequate running torque through improved electrical performance.