Method Of Soft Start Of high Voltage Motor

- Nov 19, 2020-

The brushless automatic control motor overcomes the shortcomings of the squirrel cage asynchronous motor with large starting current and low starting torque; the wound asynchronous motor is equipped with carbon brushes, slip rings and complicated starting devices; while retaining the squirrel cage motor structure, small maintenance workload; motor with advantages of low starting current and large starting torque. The brushless automatic control motor can automatically control the size of the resistance in the motor rotor according to the motor speed, so as to increase the starting torque of the motor, reduce the starting current of the motor and realize the purpose of winding asynchronous electric brushless operation. It can not only reduce the starting current of the asynchronous motor from 5-7Ie to 0.4-1.7Ie, the motor's starting torque (0.4-1.6Me) and starting time remain unchanged, but also make the motor with the maximum torque (1.6 --3.1Me) start. It can replace the squirrel cage motor, the starting device of the slip ring, carbon brush and its starting device of the winding motor. It can be obtained by using an ordinary wound asynchronous motor, removing the slip ring, carbon brush and starting device, and replacing it with a brushless automatic motor starter.

 

The harm caused by too large starting current and too small starting torque.

 

The starting current of ordinary asynchronous motors reaches 5--7 times more than the rated current, while the starting torque is only 0.4-1.6 times. It can be started directly when the grid conditions (the grid voltage drop when the motor starts is less than 10%) and the process conditions (the starting torque is satisfied) allow. However, too large starting current, too small starting torque and too long starting time have caused great harm to the motor and the power grid.

 

When the motor starting current reaches 6-7 times of the rated current, the coil heat is 36-49 times than that of the motor in normal operation, and the electromagnetic force generated also reaches 36-49 times. Excessive temperature, excessive heating speed, excessive temperature gradient and electromagnetic force have produced great destructive force, shortening the stating coil and rotor copper bar (especially the rotor often uses the skin effect phenomenon to reduce the starting current , When the rotor copper bar is started, the surface temperature reaches 350or more). For example, the 29m2 internal main fan motor of the sintering plant is two 1000KW squirrel cage motors, which are directly started at full pressure. In 1997, the motor was burned six times in one year. After the step-down start was adopted in 1998, until 2002, it has been running for 4 years, and there has not been a motor burnt accident. For this reason, relevant statistics show that the direct start time of the motor is less than 0.1% of the motor running time, but the failure rate accounts for more than 30% of the total failure rate of the motor.

 

The power supply device of the motor is generally selected according to the rated current of the motor. Excessive starting current often causes the contact hair of the power supply device to become hot, and the insulation around the contacts is aging, which is also a major cause of damage to the power supply device. For example, the propane plant of Oil Refinery has a power transformer capacity of 800kvA, and the maximum motor capacity is only 130kw, which is fully equipped for the direct start of the motor, but the excessive starting current caused the low-voltage switch gear to short-circuit three times in 2001 and the production plant three times The suspension of production for maintenance caused great economic losses to the plant.