AC motor

How does a Single-Phase Induction Motor (Capacitor Induction Motor) or AC Motor work?


From the content:
0:27 How a Rotating magnetic field RMF with only one Single Phase can be created
2:38 How torque of an AC Motor is created
3:55 Characteristic curve of this AC Motor
4:54 How to connect the capacitive motor and how to reverse the sense of rotation
5:21 Motor plate and calculation of a Capacitor Motor
7:34 Appliance and Summary Single-phase AC Motor


How to create a rotating magnetic field RMF

The Single-phase induction motor or Capacitor motor has two stator windings: The main winding (2/3 of the total winding) and the auxiliary winding, which is displaced by 90 degrees to the main winding. To get to a RMF the auxiliary winding must be 90-degree out-of-phase. The capacitor motor uses a capacitor connected in serial to the auxiliary winding get this phase shift.

Capacitor induction motor - function

Capacitor induction motor - function

How create a rotating magnetic field RMF with one single phase?

The Single-phase induction motor or Capacitor motor has two stator windings: The main winding (2/3 of the total winding) and the auxiliary winding, which is displaced by 90 degrees to the main winding. To get to a rotating magnetic field, the auxiliary winding must be 90-degree out-of-phase. The capacitive motor uses a capacitor connected in serial to the auxiliary winding get this phase shift.

The following 2-dimensional representation illustrates how the capacitive motor can form a rotating magnetic field. At certain times, you see a snapshot of the current direction within the stator winding and the resulting rotating magnetic field.

Single-Phase Induction motor - how RMF is built

Capacitor induction motor - how RMF is built


How the capacitor motor creates torque

How the capacitor motor creates torque

How the capacitor motor creates torque

Without slip no torque!


Torque curve of the capacitive motor

In order to achieve a higher starting torque, another bigger capacitor can be added in serial to the auxiliary winding. Due to overheating this so-called start capacitor must be switched off at a certain speed by a centrifugal switch.

Capacitor induction motor - wiring

Capacitor induction motor - wiring

In the following, we discuss the torque characteristic curves. As you can see, a phase shift (generated by the capacitor) in the auxiliary winding is required to get a starting torque and thus a defined direction of rotation:

Capacitor motor - torque curves

Capacitor motor - torque curves


How to reverse the sense of rotation

To change the direction of rotation of the motor, the current direction within the auxiliary winding must be reversed. Thus, the sense of rotation is independent of how you plug in this AC motor to socket. If the manufacturer offers an AC motor with a 6-pin motor terminal board, is very easy to reverse the direction of rotation: Just reconnect the metal jumpers as shown:

Capacitor induction motor - how to change the sense of rotation

Capacitor induction motor - how to change the sense of rotation

Thus, the direction of rotation can not be reversed, if only the connections L1 and N are interchanged!


Single-phase Induction motor, Capacitor motor - nameplate

Single-phase Induction motor, Capacitor motor - nameplate

Determine the start and run capacitor

There exists a rule of thumb to determine the run capacitor: The capacity is between 30 μF and 50 μF per kW motor power! As you can see, this is a very rough estimate. The reason is, that the size of the capacitor depends on the motor power and the winding ratio of the working and auxiliary winding. Thus, it is very difficult to determine the capacitor. It is best to contact the manufacturer.

The start capacitor, if it exists, is about 3 times as big as the run capacitor.      CS =  3 CR      Here: CS =  3 * 25 µF = 75 µF

Work order:
Calculate the efficiency and slip for the capacitor AC motor shown above!