Split Phase Motors

Simply constructed split phase motors are among the least expensive. They're widely used on easy starting loads of 1/3 horsepower or less. Washing machines, tool grinders and small fans and blowers are among the applications that use these motors.

Split phase start motors are equipped with a special set of stator windings or starting purposes. They are called start windings or start pulls. These start windings are made of smaller wire than the run windings. Because these wires are smaller, they offer less resistance and provide higher current flow. Accordingly, the start pulls are first to become magnetized when the power is applied. The current flow through the start winding begins after power is applied to the motor by 20 degrees or so.

The current in the run windings begins even later by about 50 degrees. This timing creates a phase differential which creates the necessary rotating magnetic field. When the run pulls become strongly magnetized, the start pulls begin to fade out. As a result the magnetic field shifts to a new position creating a rotating magnetic field.

Current is induced in the rotor as the run pulls establish a stronger magnetic field. The interaction of the induced current in the rotor and the magnetic field causes the rotor to turn one quarter turn. The current induced in the rotor perpetuates its motion as speed increases and the start pulls are no longer needed. At about 75% of operating speed the centrifugal switch opens disconnecting current to the start winding.

Split phase motors have moderate to low starting torque. 100 to 125 percent of full load and high starting current. Sizes range from 1/20 to 3/4 horsepower.

Split phase motors draw 6 to 8 times normal current when starting. They usually operate on single voltages. Split phase motors have lower starting torque and are less expensive because they use no capacitors in the start winding circuit.