In most cases, single-phase asynchronous motor has not only one phase or one set of windings, but the power supply is single-phase, which is widely used in daily life and low power drive in various industries.


Compared with three-phase asynchronous motor, it is required to work under the condition of single-phase power supply. The stator has a winding structure suitable for single-phase power supply, and the rotor is a cage short-circuit rotor with simple structure and reliability.

Single winding asynchronous motor.

When the single-phase sinusoidal current passes through the stator winding, the single winding on the stator produces the pulse magnetic potential, and the decomposition will result in a rotating magnetic potential with the same amplitude of positive and inverse sequence.


The motor is a static short-circuit rotor (blocking state) which has the same reaction effect as the positive and reverse sequence magnetic potential, and will produce the same magnetic field (magnetic density) after the stator is synthesized, so the positive and reverse sequence torque produced by the motor at the speed n ≥ 0 is also equal, that is to say, the starting torque Tst= Tf-Tb=0 of the Tf=Tb motor means that the single-winding single-phase asynchronous motor has no starting torque.


After the rotor starts, when the rotor rotates at a certain speed or rotates by hand, the reaction of the rotor to the positive sequence magnetic potential of the forward rotation is very different from the reaction to the reverse magnetic potential. The slip rate between the rotor and the positive sequence magnetic potential is small, the rotor is close to open circuit, and the positive sequence current of the rotor is small. The positive sequence rotating magnetic field synthesized by the rotor reaction is still large, the magnetic density and back potential are large, and the positive sequence torque value Tf is also larger. However, the rotor is short-circuited to the reverse sequence magnetic potential, showing a strong demagnetization damping effect. At this time, the inverse sequence impedance value is small, and the reverse sequence current component of the rotor is larger. Because of the demagnetization of the rotor, the amplitude of the synthesized reverse order magnetic field will be very small, the magnetic density, the back EMF and so on will also be small, so the reverse order torque Tb will be smaller. The difference between positive and reverse torque is the actual torque produced by the motor, that is, the steering torque T=Tb-Tf > 0 is positive after starting.


Unique features of single winding asynchronous motor The above analysis shows that the single winding asynchronous motor does not play a torque. But if the rotor can be driven to turn, the motor can produce a driving torque in the direction of rotation, making it turn continuously. Dual-winding single-phase asynchronous motor In order to realize the single-phase motor can rotate automatically, the single-phase asynchronous motor is usually a double winding motor.


Resistance or Capacitance Start Single Phase Asynchronous Motor These two kinds of motors are collectively referred to as phase split starting asynchronous motors. In addition to the main winding that determines the performance of the operation, there are spare auxiliary secondary windings (abbreviated as secondary windings). When the starting motor is closed, the main and secondary windings are electrified at the same time, which produces the starting torque and the motor turns up. After the starting process is finished, the motor is operated only by the main winding when the secondary circuit is switched off by the relay switch (cutting off the starting winding or the secondary winding). The secondary winding of this kind of motor is usually placed in 90 electric angles with the main winding in space, and also has any angle θ non-orthogonal discharge. The starting winding should be connected with a suitable capacitance in series, so that the current of the main winding is approximately 90 degrees different in phase.


In order to generate rotating magnetic field, in addition to the different spatial position of the two-phase winding, there is also a phase difference in the current time phase. This requires that the proportional relationship between the resistance and reactance of the auxiliary phase winding circuit is different from that of the main phase parameters. By increasing the resistance of the auxiliary winding to realize the phase displacement between the main and the secondary phase, the asynchronous motor is called single-phase resistance to start the asynchronous motor, and the capacitor starts the asynchronous motor by starting the asynchronous motor by the capacitance on the branch of the auxiliary winding in series, so that the current phase of the two windings is different.


In single-phase motor, another method of producing rotating magnetic field is called cover pole method, also known as single-phase cover pole motor. The stator of this kind of motor is made of salient pole, there are two kinds of poles and four poles. Each pole has a small slot at the full pole surface of 1 / 3 / 1 / 4. The magnetic pole is divided into two parts and a short-circuit copper ring is mounted on the small part, as if it were covered with this part of the pole, so it is called a cover-pole motor. The single-phase winding is arranged on the whole pole, and the coils of each pole are connected in series. The polarity produced by the single-phase winding must be arranged according to N, S, N and S in turn. When the stator winding is electrified, the main flux is generated in the magnetic pole. According to Lenzundefineds law, the main flux passing through the short-circuit copper ring produces an induced current with a phase lag of 90 degrees in the copper ring, and the flux generated by this current also lags behind the main flux in the phase, and its function is similar to that of the starting winding of the capacitive motor. As a result, a rotating magnetic field is produced to make the motor turn.


Single-phase capacitor running asynchronous motor single-phase capacitor running asynchronous motor referred to as capacitance motor. When this kind of motor is running, the auxiliary windings are also connected to the power grid at the same time, and the motor can run in a circular rotating magnetic field to the established load by proper design. The two-phase load is average, the efficiency is high, the noise is low, and the operation performance is good. Therefore, capacitor motor is the most widely used, and its connection mode is also many. There are L-type series or parallel wires with 90 °electric angle on the axis of two-phase winding, a non-orthogonal connection with arbitrary electric angle theta on the axis of two-phase winding, and a tap type or T-type connection with 90 °electric angle on the axis of two-phase winding. Sometimes, due to the need for a large starting torque, you can start with another capacitor, and automatically disconnect the capacitor after starting. This can not only meet the requirements of starting performance, but also maintain good operation performance, which is called single-phase capacitor starting and running asynchronous motor.


How does the single-phase asynchronous motor turn?