Many motor applications use single phase power; especially for smaller horsepower motors. Most of these applications are residential and light commercial, where three phase power is generally unavailable. Single phase AC motors differ from three phase motors. In a three phase motor, the incoming power produces a rotating magnetic current on its own. This allows the three phase motor to be self starting. Single phase motors require additional power in order to produce a rotating magnetic field. Once started, the motor has a changing magnetic field at each pole, allowing the motor to continue running.
The most common method of starting a single phase motor combines a capacitor and auxiliary winding or start circuit. A schematic view shows an auxiliary starting winding, a capacitor, and a centrifugal switch. The auxiliary winding is actually a second winding in the motor.
When current is applied to the motor, both the run winding and the start winding produce magnetic fields. Because the start winding has a lower resistance, a stronger magnetic field is created which causes the motor to begin rotation. Once the motor reaches about 80 percent of its rated speed, a centrifugal switch disconnects the start winding. From this point on, the single phase motor can maintain enough rotating magnetic field to operate on its own. The graph shows a typical torque/speed curve for auxiliary starting on single phase motors.
There are a variety of starting methods used in the different single phase motor types. These are covered in more detail in the CONTROLS drawer under "Starting." What these starting methods all have in common is the ability to produce a rotating magnetic field using the input power that is applied to the motor.
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