By John Papiewski, Updated Mar 24, 2022
Utility generators produce electricity in three distinct phases, each running a 60‑cycle alternating current. The phases overlap, creating a rotating magnetic field that drives industrial motors efficiently.
Residential wiring typically uses one or two phases because it is cheaper. A standard outlet supplies 110 V from a single phase, while a 220‑V outlet combines two phases. These single‑phase feeds power everyday appliances.
A three‑phase motor has coils that are energized sequentially. As each coil reaches its peak, the magnetic field rotates around the rotor, creating continuous torque. The phase shift is essential for smooth operation.
With single‑phase power, all coils start together, producing no rotating field. Engineers solve this by adding a starter capacitor—typically 10 µF—into a separate coil. The capacitor creates a second phase that leads the first by 90°, generating the necessary rotating field to start the motor. Once the motor reaches speed, a centrifugal switch removes the starter coil and capacitor, allowing the motor to run efficiently.
For larger motors, a dual‑capacitor system improves performance. A large capacitor provides the initial boost, while a smaller capacitor maintains optimal torque once the motor is running.
