Single-phase asynchronous motor - Solutions - Huaqiang Electronic Network

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Definition: An asynchronous motor with a single-phase AC power source is called a single-phase asynchronous motor.
Structure: stator - single phase winding, rotor - cage rotor.
Principle: When single-phase AC current is passed into the single-phase stator winding, a pulsating magnetic field whose size changes with time and the spatial position does not move with time will be generated in the stator. The analysis shows that the alternating pulsating magnetic field can be decomposed into two rotating magnetic fields with opposite steering directions. Therefore, when the motor is stationary, the two positive and negative torques are equal, that is,
The starting torque is zero and cannot be started by itself.
Applicable objects: Widely used in power tools, household appliances, medical machinery and automatic control systems.
Classification: Capacitive split phase and shaded pole type.
Capacitor phase-separated structure diagram

The rotor is a cage rotor, and the working winding A and the starting winding B are placed on the stator, and the two windings are different in space by 90°. The starting winding is connected in series with the capacitor C and connected to the working winding in parallel with the power supply. Under the same single-phase power supply, select the appropriate capacitor capacity so that the current between the working winding and the starting winding is nearly 90° out of phase, which is the phase separation.
Capacitor split-phase asynchronous motor
1. Capacitive split-phase single-phase asynchronous motor leads out: In order for the single-phase asynchronous motor to start on its own, the rotor must generate a certain starting torque at start-up.


Two-phase current

Two-phase magnetic field capacitance phase-separated single-phase asynchronous motor principle: The phase-breaking method is used to generate the starting torque.
Conclusion: (1) When two currents with a phase difference of 90° pass through a two-phase winding with a spatial position difference of 90°, the resultant magnetic field is a rotating magnetic field. The cage rotor generates electromagnetic torque and rotates under the action of a rotating magnetic field.
(2) When the rated speed is approached, the starting winding is cut off; or the starting winding is not disconnected during operation to increase the power factor and increase the torque.
(3) Changing the series position of the capacitor C can reverse the single-phase asynchronous motor.
2. Cover-type single-phase asynchronous motor structure: As shown in the left figure below, the single-phase winding is wound around the magnetic pole, and a short-circuited copper ring is placed in about 1/3 of the magnetic pole.


The structure of the cover-type single-phase asynchronous motor has the characteristics of moving magnetic field of the pole-type asynchronous motor: the structure is simple, the work is reliable, but the starting torque is small.
Adaptation object: It is often used in equipment with low starting torque requirements, such as fans and hair dryers.
Example. The following figure is a speed control circuit for a household electric fan. Try to explain its working principle.

The electric fan is driven by a capacitive split-phase asynchronous motor, and a reactor with a tap is connected in the circuit. When the speed Switch S is in different positions, the voltage drop of the reactor is different, so that the voltage of the motor terminal is changed to realize the step-by-step speed regulation. Question. Why can't a three-phase asynchronous motor start after the power supply has broken a wire? Why can I continue to rotate when I interrupt a line? Is it possible to run for a long time?
A three-phase asynchronous motor becomes a single-phase motor when the power supply breaks a wire. When starting, the line is broken. Because the starting torque is zero, it can't be started. Only the click sound can be heard. At this time, the current is very large. When the time is long, the motor will burn out.
If the line is interrupted during operation, the motor will continue to rotate. If the rated load is also driven at this time, it is bound to exceed the rated current. Over time, the motor will burn out.