How single-phase AC motors work

Update:19 Oct 2019
Summary:

A single-phase AC motor has only one winding and the ro […]

A single-phase AC motor has only one winding and the rotor is squirrel-cage. When a single-phase sinusoidal current passes through the stator winding, the motor generates an alternating magnetic field. The strength and direction of the magnetic field change sinusoidally with time, but it is fixed in spatial orientation, so the magnetic field is also called alternating. Pulsating magnetic field. The alternating pulsating magnetic field can be decomposed into two rotating magnetic fields that are opposite to each other at the same rotational speed and rotation direction. When the rotor is stationary, the two rotating magnetic fields generate two equal-sized and opposite-direction 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 The cutting magnetic line motion becomes larger. This balance is broken, the total electromagnetic torque generated by the rotor will no longer be zero, and the rotor will rotate in the direction of the push. In order to make the single-phase motor rotate automatically, we can add a starting winding to the stator. The starting winding is 90 degrees out of phase with the main winding.

The starting winding must be connected in series with a suitable capacitor so that the current with the main winding is The phase is approximately 90 degrees out of phase, the so-called phase separation principle. 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 start. After starting, when the speed rises to a certain value, the starting winding is disconnected by means of a centrifugal switch or other automatic control device mounted on the rotor, and only the main winding operates during normal operation. Therefore, the starting winding can be made in a short-time mode of operation. But there are many times when the starting winding is not broken. We call this kind of motor a capacitive single-phase motor. To change the steering of this motor, it can be realized by changing the position where the capacitors are connected in series. In a single-phase motor, another method of generating a rotating magnetic field is called a shaded pole method, also known as a single-phase shaded pole motor.

The stator of the motor is made of a salient pole type, and has two poles and four poles. Each pole has a small slot at the 1/3--1/4 full pole face, the magnetic pole is divided into two parts, and a short-circuited copper ring is placed on the small part, as if the magnetic poles are covered. So called the cover pole motor. The single-phase winding is set on the entire magnetic pole, and the coils of each pole are connected in series, and the polarity generated by the poles must be arranged in order of N, S, N, and S. When the stator winding is energized, a main magnetic flux is generated in the magnetic pole. According to Lenz's law, the main magnetic flux passing through the short-circuited copper ring generates an induced current that is delayed by 90 degrees in phase in the copper ring, and the magnetic current generated by the current is generated. The pass also lags the main flux in phase, and its function is equivalent to the start winding of the capacitive motor, thereby generating a rotating magnetic field to rotate the motor. Second, 220V AC single-phase motor starting mode is divided into several types: The first type, split-phase starting type, as shown in Figure 1, is assisted by the auxiliary starting winding to start, its starting torque is not large. The operating rate remains approximately constant. Mainly used in electric fans, air conditioning fan motors, washing machines and other motors. Secondly, when the motor is stationary, the centrifugal switch is turned on. After the power is supplied, the starting capacitor participates in the starting work. When the rotor speed reaches 70% to 80% of the rated value, the centrifugal switch will automatically jump off, and the starting capacitor completes the task. Was disconnected.

The starting winding does not participate in the running operation, and the motor continues to operate with the winding coil running, as shown in Figure 2. Third, when the motor is stationary, the centrifugal switch is turned on. After the power is supplied, the starting capacitor participates in the starting work. When the rotor speed reaches 70% to 80% of the rated value, the centrifugal switch will automatically jump off, and the starting capacitor completes the task. Was disconnected. The running capacitor is connected to the start winding to participate in the running work. This connection is generally used in places where air compressors, cutting machines, woodworking machines, etc. are heavily loaded and unstable. As shown in Figure 3. A motor with a centrifugal switch, if the motor cannot be started successfully in a short time, the winding coil will burn out quickly. Capacitance value: Double-valued capacitor motor, large capacity of starting capacitor, small capacity of running capacitor, and withstand voltage is generally greater than 400V.

 

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