Calculation of the starting capacitor of the electric motor

Asynchronous electric motors are widely used in production and at home. By design, they can be made in a three-phase or single-phase version. The most efficient motor performance is achieved by connecting a three-phase supply voltage of 380 volts. With inclusion in a single-phase network for the normal start of the electric motor, it is necessary to use a starting capacitor. The calculator below will help you calculate the starting capacitor.  

The formula used in the calculations is:

C_slave=K*I/U

• K – the coefficient of inclusion of the motor windings. For windings connected according to the “star” type, the coefficient is equal to 2800, and connected by a triangle – 4800.
• I – current in amperes (A).
• U – mains voltage in volts (V).
• C_slave – capacitance of the working capacitor in microfarads (uF).

However, this formula may give an error, since it does not take into account some engine parameters. You can improve the result if you decompose the current strength into components:

I=P/√3*η*U*cosα

The formula for calculating the capacitance of a working capacitor will take the form:

C_slave=K*P/√3*η*U*cosα or C_slave=K*P/1.73*η*U*cosα

Where:

• η – efficiency;
• cosϕ – Power factor.

All the data necessary for the calculations are indicated on the plate attached to the motor.

The formula calculates the capacitance of a working capacitor that is constantly in operation. A starting capacitor is used to quickly start the engine. Since during start-up the current can exceed the rated current indicated on the motor by 2-3 times, then from here we get the approximate starting capacity:

C_start≈2,5* C_slave

The result obtained basically does not coincide with the capacitors produced in the industry. In this case, we do this:

• With a slight difference of 1–3 microfarads, we take a close value.
• If there is no close value, then we take a capacity with a small, smaller value.
• When the difference is significant, it is allowed to select the capacitance by connecting the capacitors in parallel. In this case, the containers are added.

If the plate on the electric motor is damaged and hard to read, but there is data on its power, then select the capacitor based on the average values: 100W – 6,6-7uF.