In theory, if measures are taken to make the AC current flowing in the two sets of windings of the single-phase motor have a certain phase difference, it can be started. How to make a certain phase difference appear between two magnetic potentials or magnetic fluxes that have been staggered by a certain angle in space is the starting point for solving the startup problem. According to this, single-phase AC asynchronous motors can be divided into two categories: split-phase type and shaded-pole type.
Split-phase single-phase motor
The split-phase single-phase motor uses capacitors or resistors in series with the inductive starting winding to shift the phase, so that the current phases of the starting winding and the working winding are staggered, that is, the so-called "phase splitting".
(1) Capacitor split-phase single-phase motor
Figure (a) shows the principle wiring of a capacitor split-phase single-phase motor. Since the phase-shifting effect of the capacitor is relatively obvious, as long as a capacitor with an appropriate capacity (usually about 20~50μF) is connected to the starting winding, the current phase difference between the two windings can be made close to 90°, and the synthetic rotating magnetic field at this time is close to Because of the circular rotating magnetic field, the starting torque is large and the starting current is small. This single-phase motor is widely used, and can be retained (called a capacitor-run motor) or removed (called a capacitor-start motor, performed by a centrifugal switch placed inside the motor) after starting as needed. If you need to change the direction of the motor, you only need to swap the outlet ends of any one of the windings. At this time, the current phase relationship of the two windings is opposite.
(2) Resistance split-phase single-phase motor
Compared with the running winding, this kind of motor has fewer turns and thinner wires, and has smaller reactance and larger resistance than the running winding. When using resistance split-phase starting, the starting winding current is ahead of the running winding, and the synthetic magnetic field is an elliptical rotating magnetic field with a large ovality, and the starting torque is small. The starting winding of the resistance split-phase single-phase motor is generally designed for short-time operation. After starting, it is cut off by the centrifugal switch and maintained by the working winding. Figure (b) shows the principle wiring of the resistance split-phase single-phase motor.
Shaded Pole Single Phase Motor
A part of the stator pole is embedded with a short-circuit copper ring or a short-circuit coil (group) to form a shaded-pole single-phase motor. Shaded pole single-phase motors include two types: salient pole type and recessed pole type.
When the single-phase alternating current is applied to the stator winding, most of the magnetic flux of the pulsed magnetic field generated by it is directly coupled to the rotor through the air gap, and a small part of the magnetic flux generates induced magnetic flux when passing through the copper ring of the shaded pole. After being synthesized with it, it enters the rotor magnetic circuit through the air gap. According to Lenz's law, the induced magnetic flux always hinders the change of the original magnetic flux, and the induced magnetic flux lags behind the original magnetic flux in phase. In this way, there are two magnetic fluxes that are staggered by a certain angle in space and have a certain phase difference, and the combined magnetic field is a rotating magnetic field with great ellipticity. The rotation direction of the shaded-pole motor is fixed from the unshaded pole part to the shaded-pole part. Shaded pole motors are generally used in small blower motors and fan motors.
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