How to turn a single-phase asynchronous motor?

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

 

 

 

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

 

Single-winding asynchronous motor

 

When the single-phase sinusoidal current passes through the stator winding, the pulsating magnetic potential generated by the single-phase winding on the stator will be decomposed into the rotating magnetic potential with the same amplitude of the positive and negative sequence.

 

 

 

 

The motor is stationary.

 

The short-circuit rotor (blocking state) has the same effect on the positive and negative sequence magnetic potential, and the magnetic field (magnetic density) which is equal to the positive and negative sequence will be produced when the stator is synthesized. Therefore, the positive and negative sequence torques produced by the motor at the speed n=0 are equal, that is, the positive and negative sequence torques produced by the motor are equal.

 

Tf=Tb

 

Starting Torque of Motor

 

Tst= Tf-Tb=0

 

That is to say, single-winding single-phase asynchronous motor has no starting torque.

 

 

 

 

After starting the rotor

 

When the rotor rotates at a certain speed or is turned up by hand, the reaction of the rotor to the positive magnetic potential rotating in its direction is quite different from that to the reverse magnetic potential.

 

The slip ratio between the rotor and the positive sequence magnetic potential is small, the rotor is close to open circuit, the positive sequence current of the rotor is small, the positive sequence rotating magnetic field synthesized by the reaction of the rotor is still large, the magnetic density and the reverse potential are large, and the positive sequence torque value Tf is also large.

 

However, the rotor is short-circuited for the inverse magnetic potential, showing a strong demagnetization damping effect. At this time, the reverse sequence impedance value is smaller, and the reverse sequence current component of the rotor is larger. Because of the demagnetization of the rotor, the amplitude of the synthesized reverse magnetic field will be very small, and the magnetic density and back EMF will be smaller, so the reverse torque Tb will be smaller.

 

The difference between positive and negative sequence torque is the actual torque generated by the motor, that is, the forward-steering driving torque T = Tb-Tf > 0 is positive after the rotor starts.

 

 

 

 

Unique characteristics of single-winding asynchronous motor

 

The above analysis shows that single-winding asynchronous motor has no torque. But if the rotor can be driven to turn, the motor can generate driving torque in the direction of rotation, making it rotate continuously.

 

Double-winding single-phase asynchronous motor

 

In order to realize the automatic rotation of single-phase motor, single-phase asynchronous motor is usually a double-winding motor.

 

 

 

Resistance or capacitance starting single-phase asynchronous motor

 

These two kinds of motors are collectively called Separated-phase starting asynchronous motors. In addition to the main windings which determine the operation performance, there are also auxiliary auxiliary windings (referred to as secondary windings).

 

When the starter motor closes, the main and secondary windings are electrified at the same time, which generates the starting torque and the motor rotates. After the starting process is completed, the secondary circuit is disconnected by the relay switch (the starting winding or the secondary winding is cut off), and the motor only works by the main winding. The secondary windings of this kind of motor are mostly positioned at 90 electric angles in space with the main winding, and also at arbitrary angles with theta non-orthogonal. The starting winding should be connected with a suitable capacitor in series, so that the current of the main winding is approximately 90 degrees different from that of the secondary winding.

 

 

 

In order to generate a rotating magnetic field, the phase difference of current and time phase is required besides the difference of space position between two-phase windings. This requires that the proportional relationship between the secondary winding circuit resistance and reactance is different from that of the main phase parameters. By increasing the secondary winding resistance to achieve the phase shift between the main and secondary phases, it is called single-phase resistance starting asynchronous motor; by connecting capacitors in series on the branch of the secondary winding, the capacitor starting asynchronous motor can make the phase of the current of the two windings different.

 

 

 

 

The shaded pole

 

In the single-phase motor, another method to produce a rotating magnetic field is called shaded-pole method, also called the shaded pole motor. The motor stator made of salient pole type, there are two kinds of poles and quadrupole.

 

 

 

Each pole in the 1/3-1/4 side at a small groove is arranged, the magnetic pole is divided into two parts, in the small part is sheathed on a short-circuit copper ring, if this part of the pole cover, so called shaded pole motor. In the whole set of single phase winding magnetic poles, each pole coils are connected in series, polarity connection must be followed by the N, S, N, S arrangement.

 

 

 

When the stator winding is energized, the main magnetic flux in the magnetic pole, according to Lenz's law, in which the main flux through the circuit copper ring in copper ring generates a lag induced current 90 degrees in phase, the current flux generated in the phase lag in Lord flux, starting with equivalent winding capacitance it's the type of motor. In order to produce a rotating magnetic field of the motor to rotate.

 

 

 

 

The single-phase capacitor running asynchronous motor

 

Referred to as the single-phase capacitor running asynchronous motor capacitor motor. This type of motor in operation, the secondary winding connected to the power grid can work at the same time, and the appropriate design can make the motor load is established on circular rotating field operation.

 

 

 

Phase average burden load, high efficiency, low noise and good running performance. So the capacitor motor is used most widely, also the connection. L Type 90 electrical degrees axis two-phase winding in series or parallel connection, two phase winding axis of arbitrary electric angle line, non orthogonal phase winding axis electric angle of 90 degrees with the tap type or T type connection.

 

 

 

Sometimes due to the larger starting torque, starting in another and a starting capacitor, after automatically disconnect the capacitor. It can meet the requirements of starting performance, and can maintain good performance, called single-phase capacitor start and run asynchronous motor.