Starting methods of three-phase asynchronous motors

1. Direct Starting

However, when a three-phase asynchronous motor starts directly, the current can reach 6-7 times the rated current, causing a large impact on the power grid, especially for high-power motors.

2. Frequency-sensitive Resistor Starting

Frequency-sensitive resistor starting refers to connecting a frequency-sensitive resistor in series with the main circuit when the motor starts, thereby reducing the starting current. The frequency-sensitive resistor can smoothly change the starting current, and the impact on the power grid is small, which is a more ideal starting method. However, high-power frequency-sensitive resistors are all in the form of inductance, so large electromagnetic eddy currents will be generated during use, which will reduce the power factor of the power grid.

3. Reduced-voltage Starting

Reduced-voltage starting mainly includes autotransformer reduced-voltage starting and star-delta reduced-voltage starting.

Autotransformer reduced-voltage starting refers to reducing the motor voltage during starting through an autotransformer, and at the same time reducing the starting current. It is generally reduced to about 55%-75% of the rated voltage. The advantage is that the starting voltage can be easily changed by changing the number of taps of the autotransformer. The disadvantage is that an autotransformer is required, which increases the cost.

Star-delta reduced-voltage starting refers to a method of reducing the starting current by changing the motor wiring method to change the starting voltage, which can only be used for motors with a normal wiring method of delta connection. When starting, the relay method is used to make the motor wiring method star-shaped, at which time the voltage of each phase of the motor is reduced to the original 1/√3, and the motor speed reaches about 80% of the rated speed, the control relay changes the motor wiring method to delta, and the motor starts to operate normally. The advantage is that it can save the autotransformer, reduce the cost, and the wiring method is simple and reliable. The disadvantage is that the starting voltage ratio cannot be changed, and it cannot be used in star-connected motors.