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Everyone knows that the new motor has a nameplate on it. The above details the motor power, current, capacitance and other data. If you encounter an old motor without a nameplate on a certain day, can you calculate the capacitor? The selection of the starting capacitor of the single-phase motor is too small, which is difficult to start. It is very easy to burn the motor. Therefore, it is very important to choose a suitable capacitor. The following is how to choose the capacitor for single-phase motor operation.
First, withstand voltage
The withstand voltage must be greater than the maximum peak value of the AC input voltage: 220V × 1.414 ≈ 311V, which can take 400V withstand voltage or higher withstand voltage.
Withstand voltage formula: UC ≥ 1.5U (V).
Second, the capacitor matching
Involving physical quantity and unit, constant: C is the capacitance value, the unit is μF; I is the current value, the unit is A; U is the voltage value, the unit is V; P is the power value, the unit is W; cosφ is the power factor, generally 0.5 to 0.8. Usually take 0.65.
1. Single-phase motor with only one capacitor: use a capacitor, which is both a starting capacitor and a running capacitor. It is commonly used in small-capacity motors such as electric fans and washing machines. Capacitance calculation formula: C=1950Icosφ/U=1950Pcosφ/U2 =1950×P×0.65/2202≈0.026P.
2. Use a capacitor only when starting up: disconnect during normal operation, switch with a transfer switch or a centrifugal switch. Starting capacitor capacity formula: CQ=3500Icosφ/U=3500Pcosφ/U2=3500×P×0.65/2202≈0.047P.
3. Use 2 capacitors: one is responsible for running, one is responsible for starting, and the running capacitor capacity formula is: CY=1200Icosφ/U=1200Pcosφ/U2=1200×P×0.65/2202≈0.016P.
Starting capacitor capacity formula: CQ=2.5CY (usually taken as 2~3 times of the running capacitor value, usually 2.5). Generally, if you do not need to calculate, run the capacitor 2~3μF per 100W, usually take 0.025μF/W, start The capacitance is 2 to 3 times the operating capacitance, usually 2.5 times.
The capacitor selection of the motor is strictly voltage-dependent and must 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 is usually not lower than 400V. The capacitance value has a certain broadness. The large point and small point do not have much relationship in a short time. In particular, the starting capacitor can be selected from 2 to 5 times the working capacitance. However, in order to increase the starting torque of the motor, it is often necessary to replace the capacitor with a large capacity. It is mistaken that the larger the capacity, the better. In fact, although this method can increase the starting torque, the starting current of the motor will increase at a larger rate, which is extremely disadvantageous for the motor. In general, in a single-phase capacitor-starter motor, the capacitor capacity in series in the startup winding is doubled, the starting torque can only be increased by 50%, and the starting current is increased by 200%. In a single-phase capacitor-operated motor, when the capacitance is increased by 2 times, the starting torque can be increased by nearly 2 times, but the efficiency of the motor is reduced by 50%. This will make the motor almost unable to drive the original load. If the power is continued, the motor will be overloaded for a long time and the winding will be burned out.
It can be seen that if the capacitors used are not properly selected, it will have serious consequences for the motor. When replacing the starting and running capacitors, it is best to use the same capacitor as the original configuration parameters. If the capacitor is damaged and you do not know or can't see the parameter, you can calculate it as above.
----- Editor-in-Chief: Dalan Oil Pump Motor 02-Procurement Consultant
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