Speed regulation of three-phase asynchronous motor

To meet the needs of production, it is necessary to change the speed of the motor under the same load to meet different production requirements. The relevant factors affecting the speed of the electric motor are:

According to the formula for motor slip rate:

n=(1-s)n₁=(1-s)60f₁/p

To adjust the speed, it can be achieved by changing physical quantities such as the power frequency f1, the number of magnetic poles p of the stator winding, or the slip rate s.

Since the speed n of an asynchronous motor is directly proportional to the power frequency f1, changing the power supply frequency of the motor can achieve speed regulation. Generally, variable frequency speed regulation requires the main magnetic flux of the motor to remain constant, so that the torque T can be kept constant within the speed regulation range. According to the formula             

        U₁≈E1=4.44f₁N₁KΦ

可知Φ=U1/4.44f₁N₁K

In order to maintain the main magnetic flux Φ constant, while changing the power frequency f1, it is also necessary to change the power voltage U1 and keep the U1/f1 ratio constant. This is the characteristic of variable frequency speed regulation.

This method has good speed regulation performance, a large speed regulation range, smooth speed regulation, and relatively hard mechanical characteristics, but it requires the use of a specialized frequency modulation power supply. Due to the large investment in equipment, there are not many practical applications. With the development of electronic technology, it is now more convenient to obtain variable frequency power supplies. The application of thyristor frequency converters has opened up broad prospects for frequency conversion speed regulation.

When the connection method of a stator winding is changed, the number of magnetic poles of the rotating magnetic field generated by it changes. This method of variable pole speed regulation is only applicable to three-phase cage asynchronous motors and not suitable for wound rotor asynchronous motors. Because the number of magnetic poles in the rotor of a cage type motor can change with the number of stator poles, while the number of magnetic poles in the rotor winding of a wound rotor motor is already determined when the rotor is embedded, it is generally difficult to change the number of magnetic poles.

The advantage of variable pole speed regulation is that it requires simple equipment, but its disadvantage is that the motor winding has more leads and fewer speed regulation poles. In order to avoid the difficulty of rotor winding pole changing, wound rotor asynchronous motors do not use pole changing speed regulation, that is, pole changing speed regulation is only used in cage asynchronous motors.