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# DINGFENG CAPACITOR - - How to choose single-phase motor running capacitor and starting capacitor

Issue Time:2020-07-03

1. Selection formula of single-phase dual-value motor running capacitor:

C = 1100×I/U×cos φ where 1100 is a coefficient of the empirical formula;

I is the rated current of the motor, U is the power supply voltage;

cos φ is the power factor of the motor (between 0.7 and 0.8, generally 0.75 is appropriate)

2. The calculation formula for single-phase motor current is: P=IUcosφ. P: is the single-phase motor power; I: is the motor current, generally required; U: is the motor voltage, generally 220V; cos φ: is the motor power factor, generally 0.75, if there are specific data according to actual conditions.

For example: a single-phase motor with a power of 1.5KW, calculate its working current; P=l×Uxcosφ

=1500/165=9.1A

Its capacitance value is

C ＝1100×I/U×COS φ ＝1100×9.1/220×0.75≈34(μF)

3. The starting capacitor can be selected according to 3.75 times the operation of the motor; 34×3.75=127.5uF.

According to the formula, the rated current of the 750 motor is calculated as I=P/U×cosφ=750/220×0.75=750/165=4.54A;

Running capacitance C=1100×I/U×cosφ=1100×4.54/220×0.75=11000×0.0155=17uf; starting capacitance is 17×3.75=63uf;

According to the operating capacitor of a 0.75kw single-phase motor in the physical picture of a single-phase motor, the actual capacitance is 16uf/450vAC, and the capacitance of the starting capacitor is 60uf/450vAC. See the figure below.

For capacitors, because they work in an AC 220V power supply, their withstand voltage is generally calculated as 1.5 to 2 times the withstand voltage, so the capacitor's withstand voltage is 450V.

For a single-phase motor with a running capacitor and a starting capacitor, it becomes a split-phase start single-phase asynchronous motor, also known as a dual-value capacitor motor.

The two-value capacitor asynchronous motor of this structure has a starting capacitor and a running capacitor.

When the motor starts, a starting capacitor with a larger capacitance is disconnected from the circuit under the action of the centrifugal switch to complete its task, leaving the running capacitor still connected in series in the starting winding of the motor to work.

The single-phase motor of this structure has better starting performance and running performance, higher power factor and efficiency, and is suitable for load starting and loads requiring low noise, such as household appliances, pumps, small machine tools, etc. If you want to change the rotation direction of the single-phase capacitor start-up and running asynchronous motor, you only need to swap the two outlet ends of one of the two winding coils.