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# Dingfeng Capacitor -- How to choose single-phase motor running capacitor and starting capacitor?

Issue Time:2019-10-16
Generally, single-phase AC motors require one or two capacitors to operate normally, such as our common centrifugal fan, household fan, and home-set drive motor. How do we choose the capacitor that the motor matches when the motor capacitance is lost?

Commonly used starting capacitors for fans such as floor fans and ceiling fans
Starting capacitors for single-phase motors with higher power, such as washing machines, air conditioners
First choice of capacitor voltage:
Withstand voltage formula: U (capacitance) is greater than or equal to 1.5*U;
For motors such as AC220V, 400V or 450V capacitors are usually used.

Second, the choice of capacitance:
Single-phase running capacitance formula: C=1950×I/U×cosφ (using a capacitor, which is both a starting capacitor and a running capacitor, a small-capacity motor such as an electric fan or a washing machine)
Start capacitor capacity formula: C = 3500 * I / U * cos φ (use a capacitor only when the start is put, disconnected during normal operation, switch with a transfer switch or a centrifugal switch.
Two-value capacitor running capacitor capacity formula: C=1200*I/U*cosφ (with 2 capacitors, one is responsible for running, one is responsible for starting)
Double-value capacitor starting capacitor capacity formula: C=(2~3)*C(running capacitance)
C: Capacitance capacity: I: motor rated current, U: motor rated voltage, cosφ: power factor 0.7. Generally, there is no need to calculate, the operating capacitor is 2~3μF per 100W, and the starting capacitor is 2~3 times of the running capacitor.

The capacitor selection of the motor is strictly required to be equal to or greater than 1.5 times the rated voltage of the motor. For rated voltage 220V power supply, the rated voltage of the capacitor should not be lower than 400V. The capacitance value has a certain broadness. It does not matter if there is a large point. Especially the starting capacitor can be selected 2-6 times of the working capacitance.

The capacity should not be too large. Although the larger the capacity, the motor speed can be increased and the torque is increased. However, if the capacitor is too large, the motor current will be too large, and the heat will be severe and the motor will be burnt. For example, the original 4UF capacitor can be increased to 5UF, but can not be increased to 10UF, the principle is not to exceed 20% of the rated capacitance.

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