Start capacitor size selection _ How will the startup capacitor be large?

Capacitive induction motors have two windings, the start winding and the running winding. The two windings are spatially different by 90 degrees. A large-capacity capacitor is connected in series on the starting winding. When the running winding and the starting winding pass through single-phase alternating current, the current in the starting winding is advanced by 90 degrees from the current of the running winding due to the action of the capacitor. Maximum value. Two identical pulsed magnetic fields are formed in time and space, so that a rotating magnetic field is generated in the air gap between the stator and the rotor. Under the action of the rotating magnetic field, an induced current is generated in the rotor of the motor, and the current interacts with the rotating magnetic field. The electromagnetic field torque causes the motor to rotate.

Start capacitor size selection

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 a single-phase motor is too small to cause difficulty in starting. If the selection is too large, it is easy to burn the motor, so it is very important to choose a suitable capacitor.

It can be calculated according to the following formula:

Starting capacitor capacity: C=350000×I/2p×f×U×cosφ

I---current;

f---frequency;

U---voltage;

2p---power factor is taken as 2, power factor is taken as 4;

Cosφ---power factor (0.4 ~ 0.8) is generally 0.8 for the motor.

Starting capacitor withstand voltage: Capacitance withstand voltage is greater than or equal to 1.42 × U, the pressure is small and easy to be broken down. Running capacitor capacity: C=120000×I/2p×f×U×cosφ In the formula:

I---current;

f---frequency;

U---voltage; 2p---take 2.4;

The cosφ---power factor (0.4 to 0.8) is generally 0.8.

Operating capacitor withstand voltage: Capacitance withstand voltage greater than or equal to 2 × U.

Note: The starting capacitor capacity of the two-value capacitor motor: C = 2 × running capacitor capacity. Starting capacitor withstand voltage: The capacitor withstand voltage is greater than or equal to 2 × U.

What happens when the startup capacitor is large?

The capacity is large, the motor speed is fast, but the heat is large, and the motor burns out due to long working hours; the capacity is small, the motor does not start or the start is difficult, the motor is weak, and the speed is decreased. The capacitor of the standard capacity should be configured.

The large capacitance is the starting capacitor, which improves the torque. The operating capacitance is relatively small, which is to improve the power factor. To improve the efficiency of single-phase AC, it is impossible to form a rotating magnetic field. Even if the two sets of windings are arranged up and down, each group is left and right. The magnetic field generated by it is also unable to rotate the rotor.

So, use a capacitor. Connect it in the start winding and use it to advance the current phase by 90 degrees. Thus, when the motor is energized, the 90-degree current that is advanced will cause the rotor to produce a 90-degree rotation, but once the rotor rotates, its own internal "winding" (actually just a group like a winding) The aluminum strip) cuts the magnetic lines of force generated by the stator to generate an induced potential. This potential causes the rotor to generate a magnetic pole, which uses the magnetic force generated by the stator to drive the rotor to continue to rotate. And because of the inertia, the rotor can rotate in a full circle. At this time, the function of the starting capacitor and the starting coil is completed.