1. Cause voltage fluctuations in the power grid, affecting the operation of other equipment in the power grid
When the AC motor is directly started at full voltage, the starting current will reach 4 to 7 times the rated current. When the capacity of the motor is relatively large, the starting current will cause a sharp drop in the grid voltage, affecting the normal operation of other equipment in the grid.
During soft starting, the starting current is generally 2-3 times of the rated current, and the voltage fluctuation of the grid is generally less than 10%, which has very little impact on other equipment.
⒉ Impact on the power grid
The impact on the power grid is mainly manifested in two aspects:
① The impact of the large current directly started by the very large motor on the power grid is almost similar to the impact of the three-phase short circuit on the power grid, which often causes power oscillation and makes the power grid lose stability.
② The starting current contains a large number of high order harmonics, which will cause high frequency resonance with the grid circuit parameters, resulting in relay protection misoperation, automatic control failure and other faults.
During soft starting, the starting current is greatly reduced, and the above effects can be completely eliminated.
Damage motor insulation, reduce motor life
① The Joule heat generated by the large current repeatedly acts on the outer insulation of the wire, which accelerates the aging of the insulation and reduces the life.
② The mechanical force generated by the large current causes the wires to rub against each other and reduces the insulation life.
③ The jitter phenomenon of the contact when the high voltage switch is closed will produce an operating overvoltage on the stator winding of the motor, sometimes reaching more than 5 times the applied voltage, and such a high overvoltage will cause great harm to the motor insulation.
When soft starting, the maximum current is reduced by about half, the instant heat is only about 1/4 of the straight start, and the insulation life will be greatly extended; When the motor end voltage can be adjusted from zero, the overvoltage damage can be completely eliminated.
The damage of electric power to the motor
The large current will produce great impact force on the stator coil and the rotating squirrel cage, which will cause the clamping loosening, coil deformation, squirrel cage breakage and other faults.
In soft starting, the impact force is greatly reduced because the maximum current is small.
5. Damage to mechanical equipment
The starting torque of full voltage direct starting is about 2 times the rated torque, and such a large torque is suddenly added to the stationary mechanical equipment, which will accelerate gear wear or even tooth beating, accelerate belt wear or even pull off the belt, accelerate blade fatigue or even break the wind blade, and so on.
Using the motor soft starter to control the starting of the motor can effectively solve the above problems caused by direct starting.
Post time: Jul-24-2023