The single-phase AC power that comes into your home poses a problem for electric motors. For technical reasons, a single-phase AC motor has difficulty starting on its own. When engineers added start and run capacitors to the motors, product designers could use them in all sorts of appliances, from vacuum cleaners and blenders to air-conditioning units. A single-phase AC motor has good efficiency, but needs a capacitor or two to run correctly.
Every electric motor has to overcome its own inertia to start spinning. Many motors operate compressors or some other heavy load, making start-up even harder. Engineers call the force needed to begin turning a motor its start-up torque. Single-phase AC by itself doesn’t give a motor any start-up torque, but adding a start capacitor creates an additional phase that solves this problem. Capacitors cost little, have excellent durability and keep the motor design simple.
Some motors have a run capacitor in addition to the start capacitor because the two states have different requirements, especially for larger electric motors. A start capacitor loads the motor slightly after it comes up to speed. This reduces the motor’s peak efficiency because the capacitor uses some of the electric power. Some motors have two capacitors, one to start and a smaller one to run. The smaller value of the run capacitor lets the motor run more efficiently. A switch inside the motor turns the start capacitor off and the run capacitor on when the motor reaches its operating speed.
Start and run capacitors allow motors to run on single-phase AC. Motors that run on two or more phases, called polyphase motors, don’t need capacitors but require special wiring. For most home appliances, the convenience of operating from a single AC outlet gives single-phase motors a big advantage. While polyphase designs work well for large industrial motors, single-phase motors dominate for most other applications.