Explain the principle of the old fan not moving the blade (the principle of single-phase asynchronous motor)

In the hot summer days, electric fans are a must-have for ordinary people to cool down their homes. However, I believe most people have encountered this situation. The electric fan is connected to the power supply. When the switch is pressed, the blades do not move. It won't work for a few files! But just dial the fan blade and it will turn right away. What is going on here?

The general household electric fan is a single-phase AC 220V power supply, and the motor used is a single-phase asynchronous motor. This motor has a series of functions such as simple structure, low cost, and operation, so it is widely used in electric fans, washing machines, and electric power. In home appliances such as refrigerators and air conditioners. So what is the operating principle of a single-phase motor?

When the stator winding of a single-phase motor passes a sinusoidal current, the motor generates an alternating magnetic field. The strength and direction of the magnetic field change sinusoidally with time, but the orientation is fixed. This magnetic field is an alternating pulsating magnetic field. The alternating pulsating magnetic field can be decomposed into two rotating magnetic fields with opposite rotation speeds at the same rotation speed. When the rotor is stationary, the two rotating magnetic fields generate two equal and opposite torques in the rotor, so that the synthesis The torque is zero, so the motor cannot rotate. When we use an external force to rotate the motor in a certain direction (such as clockwise rotation), the movement of the cutting magnetic field between the rotating magnetic field of the rotor and the clockwise rotation direction becomes smaller; between the rotating magnetic field of the rotor and the counterclockwise rotation direction As the cutting magnetic field moves, the total electromagnetic torque generated by the rotor will no longer be zero and the rotor will rotate in the direction of the pushing.

Figure 1 The pulsating magnetic field is divided into two rotating magnetic fields that turn opposite.

This seems to understand, but why when the electric fan just bought it, you can turn it without dialing? I can't always let me dial the fan every time I turn it on. That's too much trouble, and it's dangerous! Don't worry, let's analyze.

Figure 2 Schematic diagram of the wiring of the capacitive split-phase asynchronous motor

In order to make the single-phase motor rotate automatically, we add a starting winding to the stator. The starting winding is 90 degrees out of phase with the main winding. The starting winding is connected in series with a suitable capacitor so that the current with the main winding is in phase. Approximate 90 degrees difference is the phase separation. Such two currents that differ by 90 degrees in time into two windings that are spatially different by 90 degrees will spatially generate a (two-phase) rotating magnetic field, under which the rotor can automatically initiate rotation. stand up.

Figure 3 two-phase rotating magnetic field

Figure 4 two-phase current waveform

Can you know now? The electric fan can't start up, but when it is turned, most of the reason is that the starting capacitor is aging, and the capacity is reduced. It is not enough to provide the rotating magnetic field needed for the motor to start. In this case, decisively replace the new starting capacitor, the electric fan. It will definitely run as fast as "activation"!