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Lesson Seven : 1-phase induction motor

7  1-phase induction motor

The 1-phase motor has the same design of the rotor as the 3-phase motor but the windings in the stator are a bit different. There are a number of different types of 1-phase motors but this chapter will focus on explaining on how the permanent split capacitor motor works.
Since the motor is now only fed 1-phase AC instead of the 3-phase AC, the motor can still create a magnetic field that switches direction in the motor windings, but since the motor have no phase shift it can’t create the rotating magnetic field in the stator as it did on the 3-phase motor. This problem is solved by dividing the motor windings into two windings, a primary winding and a secondary winding.
On the secondary winding a capacitor is connected. By connecting the capacitor on the secondary winding, the current on the primary and secondary winding doesn’t change direction at the same time. In other words it will create a phase shift close to 90 degrees between the two. This will now create a rotating magnetic field just like in the 3-phase motor but with two phases instead of three.

That means if the primary winding had the highest current value it will soon change direction and drop. Next in line will be the secondary winding that reaches its peak current and the magnetic field will move onto the secondary winding coils and then back to the primary winding coils. one can reverse the rotation of the motor by switching the capacitor to the primary windings according to Figure 17. So with only one phase the motor can still get two separate phase windings and a rotating magnetic field. 
Figure 17. 1-phase induction motor winding schematic

However the rotating magnetic field will not be as strong as the one in the 3- phase motor and thus the motor will get a lower break down torque.
Other typical 1-phase motor types are, Capacitor start induction run which means the secondary windings are only used in the start. Capacitor start and run which means it will have an extra capacitor in the start compared to permanent split capacitor in order to further increase the starting torque.
As with a 3-phase motor we can also create a motor with different amount of pole pairs. It’s possible to control the rotation speed of a 1-phase motor but it’s seldom used nowadays.
If a 1-phase motor with speed control is wanted, the standard method is to connect the 1-phase to a variable speed drive and then connected the variable speed drive to a small 3-phase motor. (LearnEngineering)
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