First of all, let's understand the working principle of the three-phase AC asynchronous motor: There are three pairs of coils on the stator of the three-phase AC asynchronous motor. When the motor is connected to the three-phase AC power supply, a rotating magnetic field will be generated, so the rotating magnetic field will cut the metal on the rotor. Bars (or windings), and induced currents are generated on the metal bars, so the rotor will be rotated by the electromagnetic force of the rotating magnetic field.
The single-phase AC asynchronous motor only needs a pair of coils. When the pair of coils is connected to the single-phase AC, the pair of coils will only generate a pulsating magnetic field, but not a rotating magnetic field! Therefore, another pair of coils needs to be added. This pair of coils is what we call the starting coil, and the spatial angle between the starting coil and the working coil on the stator is 90 degrees different. Therefore, the single-phase AC asynchronous motor actually has two pairs of coils, namely the main coil (working coil) and the auxiliary coil (starting coil). The rotor forces are the same and opposite, so the rotor is stationary. In order to rotate the magnetic fields generated by the primary and secondary coils, it is necessary to supply alternating currents of different phase sequences to the primary and secondary coils.
How to realize alternating current with different phase sequences?
Since a single-phase AC motor can only be connected to a single-phase 220V power supply, how can we simply and economically obtain two different-phase AC powers to obtain a rotating magnetic field? At this time, it is necessary to use a capacitor to realize the phase shift, that is, a capacitor is connected in series with the secondary coil. As shown below:
In this case, the current waveform of the main coil is shown as curve a, and the current waveform of the secondary coil is shown as curve b
As shown in the figure above, the main coil current a reaches the maximum value at time 1, while the secondary coil current b is zero; then the main coil current a decreases to zero at time 2, while the secondary coil current b rises to the maximum value; then the main coil current a becomes the maximum value in the opposite direction, while the secondary coil current b is reduced to zero... The two alternating currents of the primary coil a and the secondary coil b successively reach the current maximum value, and the phase difference between them is 1/4 cycle, also That is, the difference is 90 degrees, so the magnetic fields generated by them also reach the maximum value in turn. In this way, the magnetic field of the main coil can push the rotor, the magnetic field of the secondary coil can push the rotor, and then the rotor can be rotated.
The principle of starting with a large capacitor and running with a small capacitor
For low-power single-phase AC asynchronous motors, because of its low power, light load, and low starting torque requirements (such as electric fans), it only has a small capacitor, which only plays the role of starting (the motor starts After the centrifugal switch disconnects the starting coil, only the running coil works, the rotor continuously cuts the pulsating magnetic field generated by the working coil through its own rotation, and the rotor realizes continuous rotation), or plays the role of starting and running at the same time (the motor does not disconnect after starting The starting coil, the starting capacitor, the starting coil and the running coil work together. At this time, the rotor cuts the magnetic field lines in the continuous rotating magnetic field generated by the starting coil and the working coil, and the rotor realizes continuous rotation).
However, for industrial high-power single-phase AC asynchronous motors, if only one capacitor is used to take into account both starting and running, at this time, due to the small starting torque of the motor and the heavy load carried by the motor, it is easy to cause the motor to Difficulty starting. At this time, a large capacitor needs to be connected in parallel with the running capacitor to increase the starting torque. We call this capacitor "starting capacitor".
Some friends may be curious, why not directly connect a large capacitor for startup and operation? Because when the capacitance of the connected capacitor is too large, although the torque can be increased, it will also cause serious heating of the single-phase AC asynchronous motor, and even burn the motor, so the high-power single-phase AC asynchronous motor has a centrifugal switch. The function of the centrifugal switch is to disconnect the starting capacitor after the motor speed reaches a certain level (about 70~80% of the rated speed) to prevent the winding from being burned due to excessive current and overheating. Therefore, the single-phase AC asynchronous motor adopts the principle of "starting with a large capacitor and running with a small capacitor".
There are two functions of the capacitor in the single-phase AC asynchronous motor: one is to realize the phase shift of the single-phase power supply between the two pairs of main and auxiliary coils of the single-phase motor stator to form a rotating magnetic field; the other is to start and run the motor. Provide larger excitation current.
