What is the purpose of providing a DC supply for an induction motor during starting and braking?

Purpose of Direct Current Power during Startup

Limit the starting current

When an induction motor starts, if it is directly connected to an AC power supply, the rotor, being stationary, experiences a strong induced effect from the rotating magnetic field of the stator, resulting in a very large starting current.

When a DC power supply is provided, it can alter the magnetic characteristics of the motor, thereby limiting the size of the starting current. For example, in some soft start devices, a specific magnetic field is generated using a DC power supply, allowing the motor to smoothly start from a stationary state and avoiding the impact of excessive starting current on the power grid and the motor itself.

 This is because the interaction between the magnetic field produced by the DC power supply and that produced by the AC power supply changes the electromagnetic relationship inside the motor, which in turn limits the starting current.

Generating Initial Torque

When an induction motor starts, it requires a certain initial torque to overcome the static friction and inertial force of the load, so as to start rotating. A direct current power supply can establish an initial magnetic field inside the motor, and the interaction between this magnetic field and the rotor can produce an initial torque.

This initial torque helps the motor overcome the resistance of the load at the moment of starting and start smoothly. For example, in some special starting methods, the magnetic field provided by the direct current power supply will change the distribution of current in the rotor conductor, thus generating electromagnetic forces consistent with the rotation direction, and then forming the initial torque.

The purpose of direct current power during braking

Achieve rapid braking

During the braking process of an induction motor, a direct current (DC) power supply can be used to alter the direction or magnitude of the magnetic field within the motor, generating an electromagnetically induced torque opposite to the direction of the motor's rotation.

This reversed electromagnetic torque enables the motor to quickly decelerate until it comes to a stop. For instance, in energy dissipation braking, by connecting the DC power supply to the stator windings, a stationary magnetic field is formed inside the motor. As the rotor continues to rotate due to inertia, it cuts through this stationary magnetic field, inducing a current. This induced current, in turn, interacts with the stationary magnetic field to produce a braking torque, thus achieving rapid braking.

Precise Control of the Braking Process

Using a DC power supply allows for more precise control of the braking process. By adjusting parameters such as voltage and current of the DC power supply, it is possible to change the magnitude of the braking torque, thus achieving braking according to predetermined requirements. For example, in some equipment that requires precise parking positions, precise control of the parameters of the DC power supply enables the induction motor to stop accurately at the designated position, meeting the requirements of production processes or equipment operation.