What Are The Starting Methods For An Electric Motor?

The starting method of a motor varies depending on the motor type (DC/AC), power rating, and load characteristics. Common starting methods can be mainly categorized as follows:

I. Starting a DC Motor

The main problem with starting a DC motor is the large starting current, thus requiring current limiting.

1. Direct Starting: Only suitable for very low-power motors.

2. Series Resistor Starting: A starting resistor is connected in series in the armature circuit. The resistor is gradually removed as the speed increases to limit the starting current.

3. Reduced Voltage Starting: An adjustable DC power supply is used. The armature voltage is reduced during startup and gradually increased as the speed increases. This method results in smooth starting and low energy consumption.

II. Starting AC Asynchronous Motors

1. Full-Voltage Starting

Method: The rated voltage is directly applied to the motor stator windings via a circuit breaker, contactor, or knife switch.

Application: Generally used for small-power motors (typically less than 20%-30% of the transformer capacity), or in situations where the power grid capacity is sufficiently large.

2. Reduced-voltage starting

To reduce starting current, when the motor power is high, a method of reducing the stator winding voltage is usually used for starting.

Star-delta (Y-Δ) reduced-voltage starting

Method: The stator windings are connected in a star configuration during startup. Once the speed increases to near the rated value, the connection is switched to delta.

Applicable to: Motors that normally operate in delta configuration, and for no-load or light-load applications where starting torque requirements are not high.

Autotransformer reduced-voltage starting

Method: An autotransformer is used to reduce the voltage applied to the motor stator windings. After the starting process is complete, the transformer is disconnected, and the motor switches to full-voltage operation.

Applicable to: Motors with slightly higher starting torque requirements and larger capacity; widely used.

Stator series reactor (or resistor) starting

Method: A reactor or resistor is connected in series in the stator circuit during startup, using the voltage divider principle to reduce the motor terminal voltage. The reactor or resistor is disconnected after the speed increases.

Applicable to: Commonly used for high-voltage motors or applications requiring smooth starting.

3. Soft Start

Method: Utilizes power electronic devices such as thyristors to smoothly and steplessly increase the motor terminal voltage from an initial value to full voltage by controlling the conduction angle.

Applications: Widely used in applications requiring high smoothness of start-up and reduced mechanical shock (e.g., water pumps, fans, belt conveyors).

4. Variable Frequency Start

Method: Achieves constant torque start-up by simultaneously changing the motor's power supply frequency and voltage using a frequency converter.

Applications: Applications requiring frequent starts, precise speed control, or heavy-load start-up (e.g., cranes, rolling mills, elevators, high-performance fans and pumps).