What precautions should be taken in the speed regulation mode of three-phase asynchronous motor

There are several speed regulation methods for three-phase asynchronous motors:

Change the number of poles of the motor. Two sets or one set of stator windings are used, and the connection mode is changed to achieve the purpose of changing the number of poles. This speed regulation is graded, not smooth. Change the frequency of the power supply. Therefore, a dedicated variable frequency power supply is required. Varying the applied voltage changes the slip. This approach has little practical value. Additional resistors are connected in series in the rotor circuit. This method is only suitable for wound-type asynchronous motors, which can achieve smooth speed regulation.

1. Frequency control:

Smooth speed regulation is achieved by supplying a continuously varying frequency and corresponding voltage to the motor stator. There should be a dedicated variable frequency power supply. This speed regulation applies to both synchronous and squirrel-cage motors.

2. Variable pole number speed regulation

The speed n is proportional to the number of poles p, if the number of poles increases, the speed can be reduced; if the number of poles decreases, the speed can be increased. Suitable for squirrel cage motors.

3. Rotor external resistance speed regulation;

When the grid voltage and frequency are constant, the starting torque of the motor can be increased by connecting a resistor in series with the rotor circuit, and the starting current can be reduced by connecting a starting resistor in series with the rotor of the wound motor. Suitable for slip ring motors.

4. Rotor applied voltage speed adjustment:

This method is also called cascade speed regulation, which is to connect the rotor circuit of the wound motor in series to adjust the additional potential, change the slip rate of the motor, and achieve the purpose of speed regulation. There are two modes: motor feedback mode and electrical feedback mode. Cascade speed regulation mostly uses thyristor cascade speed regulation. Thyristor low-synchronous cascade speed control system is a way to use high-power thyristors or rectifier diodes on the rotor side of wound asynchronous motors to change the rotor speed, and then use thyristor inverters to return the rotor current to the power supply. The control speed regulation system has high efficiency and high utilization rate, and can realize stepless smooth speed regulation, and the mechanical characteristics are relatively hard at low speeds. This method is suitable for slip ring motors. There are two ways of doing this:

1) Motor feedback mode, the slip voltage of the rotor is input to the DC separate excitation motor coaxial with the main motor through silicon rectification, and the feedback amount is adjusted through the excitation adjustment of the DC motor to obtain speed regulation.

2) Electric feedback method, the slip voltage of the rotor is input into the thyristor inverter through silicon rectification, the AC output of the inverter is connected to the power grid through the feedback transformer, and the speed is adjusted by changing the inverter angle.

5. Stator voltage regulation and rotor variable resistance speed regulation;

The torque of the asynchronous motor is proportional to the square of the stator terminal voltage, and the rotor changes the external resistance to adjust the speed. The rotor variable resistance is coarse adjustment, and the stator voltage adjustment is fine adjustment. Suitable for slip ring motors.