Single-phase operation of three-phase motors and calculation of capacitance - News - Global IC Trade Starts Here.

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When the three-phase motor is connected in delta, the capacitor is connected as shown in the figure; when it is in the star connection, the capacitor is connected as shown in Figure 2. In the figure, C2 is the running capacitor: it is the starting capacitor. After the switch K is closed, the power is turned on and the motor starts to run when the motor reaches! After the rated speed, c1 should be disconnected by switch K, otherwise the motor will heat up or even burn out.

The capacity of capacitor C2 can be calculated as follows: C2=1950*In/(Un*COSФ) (μF)

In the formula, 1N, UN, and cos are the rated current, rated voltage, and power factor values ​​on the original three-phase motor nameplate. If there is no power factor on the nameplate, the cosy may take about 0.85. For example, the nameplate of a three-phase asynchronous motor on the day is marked with an "A" connection, the rated voltage is 220V, and the rated current is 0. 85A, power factor is 0.8. Then change to single-phase operation when the working capacitor C2 is:

C2=1950In/(Un*COSФ)=1950*0.85/(220*0.8)=9.42(μF)

Take C2 = 10μF.

The capacity of the capacitor C1 can be selected according to the magnitude of the load when the motor is started, and is usually 1 to 4 times that of C2. For small motors with a power below 1kw, C1 can also be removed, but the C2 value should be increased appropriately. After this change, the capacity of the motor is reduced by 10% to 40% according to the power factor of the motor.

The capacitors in the above circuits should be selected as non-polar capacitors such as oil-immersed capacitors or metallized capacitors. Electrolytic capacitors cannot be used, and the withstand voltage value should be noted. Generally, if the working voltage of the motor is 220v, the withstand voltage of the capacitor should be 400v; if the working voltage of the motor is 380V, the compressive stress of the capacitor is about 600V.

For low-power three-phase asynchronous motors up to 1 kW, not only can it be operated in three phases, but also in single-phase operation.
1. Connection method of single-phase operation of the motor (1) The triangular connection of the three-phase winding is shown in Fig. 1. The capacitor is connected in parallel at either end of any phase of the three-phase winding (connected at both ends of the U phase), and then 220V mains is applied to one end of the capacitor C and the intersection of V2 and W1. In this way, the motor can be rotated. If it is necessary to change the direction of rotation of the motor, it can be connected as shown in Figure 2, and the mains can be switched from one end of the capacitor to the other.
(2) Star connection of the three-phase winding The capacitor is connected to any two terminals (such as V1 and U1 in Fig. 3), and 220V mains is applied to either terminal W1 and capacitor C. In this way, the motor can be rotated. To change the motor steering, switch one end of the mains from U1 to V1 (as shown in Figure 4).
2. Capacitor capacity When selecting a small three-phase asynchronous motor for single-phase operation, the selected capacitor capacity must be suitable. If it is too small, the rotation is weak and the startup is difficult; if it is too large, the loop current is too large, causing the motor to overheat. The general capacitance capacity value selection is shown in the attached table.
If you do not check the table, you can also obtain according to the empirical formula: when the star is connected, the required capacitance capacity C(Μf)=P(W)/17, the unit of C is μF, the unit of P is W; when used as a triangle When wiring, the selected capacitor capacity C(μF) = P(W)/10.