Starting and Speed ​​Regulation of Three-phase Induction Motor

At present, there are five main speed regulation methods for three-phase induction motors: variable frequency speed regulation, pole-changing speed regulation, cascade speed regulation, electromagnetic slip clutch speed regulation, and voltage regulation speed regulation. Although variable frequency speed regulation has good speed regulation smoothness, it brings harmonic pollution to the power grid that cannot be ignored. This pollution has become a public nuisance. At the same time, harmonics also bring adverse effects such as increased losses, reduced efficiency, and decreased power factor to the three-phase induction motor itself. Although pole-changing speed regulation is low-cost and has no harmonic pollution, its application is limited because of the small number of speed regulation levels.

Although cascade speed regulation has good performance, it is complex and expensive, and is difficult to be applied in medium and small capacity three-phase induction motors. Although the cost of electromagnetic slip clutch speed regulation is lower than that of cascade speed regulation, it has low efficiency, large low-speed static differential rate, small overload capacity, and a dual-machine combination structure with large size and high cost.

Voltage and speed regulation require additional voltage regulating devices (such as transformers, saturated reactors, etc.), which increases costs; if the voltage regulating device uses power electronic equipment, it will also cause harmonic pollution and affect the three-phase induction motor itself.

The starting and speed regulation of three-phase induction motors are relatively important aspects of motor control. The following is a brief introduction to their implementation methods.

How to start:

(1) Direct starting method: directly connect the motor to the power supply and start the motor directly. This method is simple and easy to implement, but it has a greater impact on the power grid and requires special treatment.

(2) Auto-coupling starting method: Use auto-coupling to reduce the starting current of the motor and reduce the impact on the power grid. The advantages of the auto-coupling starting method are small starting current, but long starting time and low efficiency.

(3) Star-delta starting method: Start the motor with star connection first, then switch to delta connection after the motor reaches a certain speed to achieve a smoother starting process.

(4) Transformer starting method: Use a transformer to adjust the input voltage to a lower level, reduce the starting current of the motor, and achieve smooth starting.

Speed ​​adjustment method:

(1) Variable frequency speed regulation method: Use a frequency converter to control the power frequency of the motor to achieve motor speed regulation. The advantages of the variable frequency speed regulation method are high control accuracy and continuous speed regulation. The disadvantage is that the equipment cost is relatively high.

(2) Resistance speed control method: The speed of the motor is changed by changing the resistance in the motor rotor circuit. This method is low cost but low efficiency.

(3) Excitation speed control method: The motor speed is controlled by changing the excitation current or voltage of the motor. This method is widely used, but the control accuracy is low.

(4) Mechanical speed control method: The speed of the motor is controlled by changing the load or mechanical transmission of the motor. This method is simple and easy to implement, but the control accuracy is low.

It is necessary to select appropriate starting and speed regulation methods according to specific application requirements to achieve normal operation and control of the motor.