What is a Split Phase Induction Motor?
Author: steve Published Under: Home
The split phase induction motor is one rather common type of fractional-horsepower electrical motor. It uses two sets of windings, with the main winding, or run, and the start winding, or auxiliary. The run winding is used as the main power source of the motor, while the auxiliary is used only when starting the motor.
If the auxiliary winding fails, the motor will not be able to start, because it would have no initial torque. The auxiliary is responsible for getting the motor up to about 75% of capacity, which is fast enough for the run winding to kick in. A centrifugal switch is responsible for disabling the start winding once the main winding turns on.
Starting a Single Phase Motor
When a single phase AC Motor is started,the field does not rotate, but instead pulsates. As a result, there is no initial rotation of the rotor as would be found in a two or three phase motor. However, there are actually two rotating fields occurring, but since they are moving in opposite directions, the rotor does not turn.
Instead, it requires a small push to begin spinning, which can occur in either direction. A single phase motor can be made to be self starting, provided that it can be given this initial push.
Starting a Split Phase Motor
A split phase motor contains both an auxiliary and a main winding. The auxiliary winding is smaller than the main winding, but has a higher resistance and less reactance. The two windings are connected in parallel, with the main winding in the lower half and the auxiliary winding in the top half.
The fact that there are two windings is what gives a split phase motor its name, specifically in regard to the starting time. When the motor is started, the two windings work together, bringing the rotor up to speed. After the rotor reaches 75% of its synchronous speed, a centrifugal switch is engaged, which disconnects the start winding.
Advantages of a Split Phase Motor
A split phase motor can maintain a constant speed, as well as allowing for adjustable torque, which is similar to that of a shunt motor. The initial torque is actually between 150% and 200% of the full load torque, with the starting current also being much higher than the running current.
By interchanging the start winding lead, it is possible to reverse the motors direction. However, the engine must be stopped or at least slowed down to reverse, because at top speed, the start winding is not engaged. These types of motors can be found in washers, ventilating fans, and oil burners.