The function of the motor control circuit is to control the AC motor. A higher voltage is required to start the motor, while a voltage lower than the starting voltage is required for the motor to run. Converting the motor from a higher voltage to a lower voltage is the main function of the motor control circuit.

There are three main types of motor control circuits:

Direct-on-line starter (DOL starter)

Star-delta starter

Autotransformer starter

DOL starter:

Direct-on-line starter (DOL) is used for small motors with a power of less than 10kW. In the overcurrent protection circuit, fuses and circuit breakers are usually used. If the current in the line exceeds the rated current of the motor, fuses and circuit breakers are configured to interrupt the connection.

Star-delta starter:

In order to reduce the starting current, medium and large cage motors are started at a reduced power supply voltage. Reduced power supply voltage starting is applied to the star-delta method. This is applicable to motors designed for delta connection under normal operating conditions.

Autotransformer starter:

The assembly consists of contactors, overload blocks, control power transformers, power fuses. The initial voltage applied to the motor generates the starting current and torque. The motor can be permanently connected in delta or star, switched first to the reduced voltage from the three-phase tapped autotransformer and, when it has accelerated sufficiently, switched to the run (full voltage) position.