AC Asynchronous Motor Soft Start and Optimization Technology

1. Hazards of direct starting

Direct starting is a relatively simple way of starting, starting through the gate knife or contactor to connect the motor directly to the grid. The advantage of direct starting is that the starting equipment is simple, fast starting speed, but the harm of direct starting is very great:

(1) grid impact: excessive starting current (no-load starting current can be up to 4 ~ 7 times the rated current, with a load starting up to 8 ~ 10 times or greater), will cause the grid voltage drop, affecting the normal operation of other electrical equipment, but also under-voltage protection may be caused by the action of the equipment to cause harmful tripping. At the same time, excessive starting current will make the motor winding heat, thereby accelerating insulation aging, affecting the life of the motor;

(2) mechanical impact: excessive impact torque often cause motor rotor cage bars, end ring fracture and stator end winding insulation wear, resulting in breakdown burner, rotor shaft distortion, coupling, transmission gear damage and belt tear, etc.;

(3) impact on the production machinery: sudden pressure changes in the starting process often cause damage to the pump system piping, valves, shorten service life; affect the transmission accuracy, and even affect the normal process control.

All these are a threat to the safe and reliable operation of the equipment, but also cause excessive starting energy loss, especially when the frequent starting and stopping is more so. Therefore, there are the following limitations on direct motor starting:

(1) whether the production machinery allows the motor to drag the direct start, which is a prerequisite;

(2) the capacity of the motor should not be greater than the power supply transformer capacity of 10% to 15%;

(3) the voltage drop △ U during the starting process should be no more than 15% of the rated voltage. For the middle and high power motors are generally not allowed to start directly, but requires the use of certain starting equipment, in order to complete the normal starting work.

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