Induction Motor Start Run Capacitor Function

 

In an alternating current (AC) electric motors, the starting capacitor functions to increase the initial torque of the motor in one particular direction of rotation. So that the rotor of the motor can rotate in the specified direction. Without a starting capacitor, the rotor just vibrates and makes a buzzing sound. Because it is supplied by alternating current not direct current, so it is uncertain which direction the rotor should turn. Rotor is the rotating part of the motor.

Whereas in direct current (DC) motors, the direction of rotor rotation is determined by the polarity of supplied current, by determining the positive and negative wire connections to the motor.

Some AC motor designs use a switch that is activated by centrifugal force, to cut off the start capacitor when the rotor has reached a certain speed. Furthermore, the motor works with a running capacitor.

Starting capacitor is designed to stay connected in the circuit up to a predetermined speed, which is usually around 75% of full speed. Then the starting capacitor is disconnected from the circuit, usually by a centrifugal switch which is activated at 75% of motor full speed.

In a small single-phase AC motor, the starting capacitor is always connected to the circuit. So that it also functions as a running capacitor.

If the capacitor value does not match the design parameters such as: power, voltage, power factor, etc., it can result in: noise, decreased power, wasteful power consumption, temperature increases so that the motor overheats.

Induction type AC motors are very popular because of their simple design and do not use permanent magnets, so they are cheap. The working magnetic field is only the electromagnetic field in the stator, there is no field from the permanent magnet. While the magnetic field in the rotor is the result of the induction of the magnetic field from the stator. So it does not require a carbon brush to conduct electricity to the rotor. These motors are usually applied to: washing machine, fan, blower, air conditioner condenser and evaporator, refrigerators, exhaust fan, water pump, compressor, and others.

Failure that often occurs in AC motor is a weak or broken capacitor. In washing machines, it often happens that the coil wire breaks, because it is exposed to water. Replacing capacitor is actually quite easy, you can do it yourself on Sunday or holiday. The difficulty is usually in access to damaged components, having to open many other components, such as in the air conditioner (ac).

The following YouTube video shows the characteristics of a faulty capacitor in a fan motor.

If the capacitor is disconnected, and motor is connected to the electric grid, the motor will hum, the rotor is not rotating. If rotor is rotated by fingers, the motor can rotate to the right (clockwise), or to the left (counterclockwise). Furthermore, the rotor will continue to rotate according to the direction of rotation of the finger, but at a lower speed and power than it should.

The circuit schematic for a single-phase induction motor is as follows:

 Schematic description:

ACV = alternating current supply

L1 = main winding

L2 = auxiliary winding

R = squirrel cage rotor

C = capacitor start/run

If the ACV connection is changed from L1, so that ACV is supplying to L2, the rotor will rotate in the opposite direction.

The size of the capacitor can be calculated by the formula:
C = (P . n . 1000) / (V . V . F)

Where:
C = capacity micro Farad (mF)
P = power Watts (W)
n = efficiency in percent (%)
V = alternating supply voltage (VAC)
F = supply voltage frequency Hertz (Hz)

For example, a single-phase induction motor with 100 Watts power, 80% efficiency, supply voltage 220 VAC, frequency supply voltage 50 Hz.

Please note that the efficiency calculation is not written as 80% or 0.8, but it is calculated as 80.

C = (100 . 80 . 1000) / (220 . 220 . 50) = 3.31 mF