Classification Of Single-phase Motors

In theory, if measures are taken to ensure that there is a certain phase difference in the AC current flowing through the two windings of a single-phase motor, it can be started. How to create a certain phase difference between two magnetic potentials or fluxes that have been offset by a certain angle in space is the starting point for solving the startup problem. Based on this, single-phase AC asynchronous motors can be divided into two categories: phase separated and shaded pole.
Phase separated single-phase motor
Phase separated single-phase motors use capacitors or resistors in series with the inductive starting winding to achieve phase shifting, causing the current phase of the starting winding and working winding to be shifted, known as "phase separation".
(1) Capacitive single-phase motor with phase separation
The schematic diagram (a) shows the principle wiring of a capacitor phase separated single-phase motor. Due to the significant phase shift effect of capacitors, as long as a capacitor of appropriate capacity (usually about 20-50 μ F) is connected in series with the starting winding, the phase difference between the currents of the two windings can be close to 90 °. At this time, the synthesized rotating magnetic field is close to a circular rotating magnetic field, resulting in a large starting torque and a small starting current. This type of single-phase motor is widely used and can be retained (called a capacitor operated motor) or removed (called a capacitor starter motor, executed by a centrifugal switch placed inside the motor) as needed after starting. If it is necessary to change the direction of the motor, simply swap the output terminals of either winding, and the current phase relationship between the two windings will be opposite.
(2) Resistance phase separated single-phase motor
This type of motor has fewer turns in the starting winding, thinner wires, and lower reactance and higher resistance compared to the running winding. When using resistance phase separation starting, the starting winding current leads the running winding, and the synthesized magnetic field is an elliptical rotating magnetic field with a larger ellipticity. The starting torque is small, and it is only used in no-load or light load situations, with few applications. The starting winding of a single-phase motor with resistance phase separation is generally designed for short-term operation. After starting, it is cut off by a centrifugal switch and maintained by the working winding. The schematic diagram (b) shows the principle wiring of a single-phase motor with resistance phase separation.
Covered pole single-phase motor
Embedding a part of the stator pole with a short-circuit copper ring or short-circuit coil (group) constitutes a covered pole single-phase motor. The covered pole single-phase motor includes two types: salient pole and hidden pole.
When a single-phase AC current is applied to the stator winding, most of the magnetic flux generated by the pulsating magnetic field is directly coupled to the rotor through the air gap, while a small portion of the magnetic flux is induced and combined with the copper ring of the cover pole to enter the rotor magnetic circuit through the air gap. According to Lenz's law, the induced magnetic flux always obstructs the change of the original magnetic flux, and the induced magnetic flux lags behind the original magnetic flux in phase. This creates two magnetic fluxes that are offset by a certain angle in space and have a certain phase difference, resulting in a composite magnetic field that is a highly elliptical rotating magnetic field. The rotation direction of the covered pole motor is fixed from the uncovered pole part to the covered pole part, which has lower power, smaller starting torque, simple structure, low price, and easy maintenance. The covered pole motor is generally used for small blower motors and fan motors.