Motor control refers to the control of starting, accelerating, running, decelerating and stopping the motor

Motor control has different requirements and purposes according to different motor types and motor use occasions. For the motor, through motor control, the purpose of the motor's fast start, fast response, high efficiency, high torque output and high overload capacity can be achieved.

Start control

Three-phase asynchronous motor start methods include: full voltage direct start, reduced voltage start, and start by increasing the resistance of the rotor loop.

For step-down start, it mainly includes: autotransformer start, star-delta change start, and variable voltage start. When the asynchronous motor starts, the rotor is at a standstill, and its slip rate s=1. At this time, the rotor side resistance of the T-type equivalent circuit is very low, so the starting current is larger, and the starting current can be reduced by step-down starting. Since the starting torque of an asynchronous motor is proportional to the square of the voltage, it is necessary to ensure that the motor has a certain starting ability for step-down starting.

The method of increasing the rotor loop start is suitable for wound rotors, deep slot rotors and double cage rotors. This method cannot be used for squirrel cage rotors.

When the rotor resistance of the asynchronous motor is increased, the maximum torque of the motor will not be affected, but the point of occurrence of the maximum torque will move, and the motor torque-slip curve will be compressed along the slip axis. Since the motor curve has a tendency to increase first and then decrease with respect to slip, the starting torque of the motor will increase. However, its value is affected by the maximum torque of the motor.

The starting methods of single-phase asynchronous motors include: capacitor start, resistance start, PTC start, etc., shaded pole start, etc.

Since the single-phase winding of the induction motor cannot generate a rotating magnetic potential when the rotor is stationary, an asynchronous motor with only a single-phase winding cannot start automatically. In this regard, it is necessary to install an auxiliary winding 90° from the main winding on the single-phase asynchronous motor. This winding is mainly used to start the motor. When the motor is started, it can be cut off or used for the operation of the motor.

In order for the motor to generate a rotating magnetic potential, the motor winding must be able to generate a rotating magnetic potential when the rotor is stationary. To this end, two windings that are spaced at 90° to each other are required, and currents with a phase difference of 90° to each other are passed in. Since the motor windings are inductive, capacitance and resistance can be used to make the two windings 90° each other. PTC startup uses PTC resistance. When the motor runs to a certain speed, the temperature of the motor will rise. At this time, the PTC resistance reaches the temperature in the drama, and the resistance is automatically cut off.

The synchronous motor has no slip because the rotor rotates at a synchronous speed. When the speed of the rotor differs greatly from the synchronous speed, it will be out of step, so it cannot start automatically. The starting methods of synchronous motors include: variable frequency starting, asynchronous motor driven starting, and linear motor self-starting.

For variable frequency start, usually set the rate of change of the start voltage frequency, when the motor runs to 60 to 80 of the rated speed, the rated frequency is added to the motor to directly bring into synchronization. Asynchronous motor drives the start similarly. For linear motors, the rotor structure is permanent magnet + squirrel cage. The squirrel cage is used for the start-up process. When the motor runs to the synchronous speed, the squirrel cage no longer generates electromagnetic torque.

Speed control

Motor speed control methods include: series resistance speed control, frequency conversion speed control, pole-changing speed control and vector control, direct torque control, etc.

Series resistance speed regulation is mainly used for asynchronous motors. The speed adjustment range is limited by the maximum torque of the motor.

Frequency conversion speed regulation is suitable for induction motors. The purpose of speed regulation is achieved by adjusting the synchronous speed.

Pole-changing speed regulation produces 1/2, 1/3... rotation speed by changing the number of motor poles.