Influence of Delta impulse current on frequency converter

In the process of maintenance of frequency converter, it is found that due to the back electromotive force in the stator winding, the electromotive force in the motor under the condition of synchronous generator and the impulse current in the self-induced electromotive force in the self-loop rotor, the current impact will certainly affect the motor, frequency Drive (Variable-frequency Drive) and the power grid.

1. When the VFD works under normal load, the current flowing through the power equipment in the VFD flows through the winding of the motor, and its energy and voltage are mainly consumed in the winding of the motor, which will not affect its unlucky. Once the VFD suddenly shuts off the load, the current in the power equipment loses the loop, and a huge d/df occurs, resulting in a sharp rise in the terminal voltage of the power equipment by 70%, which will cause the power equipment to withstand too much current impact and damage.

2. The surge current will also damage the relaying diode, filter capacitor and insulation of the VFD, greatly shortening the service life of the VFD.

3. Different loads on the motor shaft have different effects.

Different capacities of motors have different effects. If the motor with blower and exhaust fan switches to the power supply after a delay of 13s because of the low back pressure formed by air, it will not be impacted by large current, and the motor can fully withstand the impact current;

If the motor is loaded with a pump, the "water hammer" effect, coupled with the back electromotive force and high water pressure when switching, will make the motor more than 20 times the rated current impact and huge torque impact, the formation of motor damage. If the motor is an old type, because the power and power factor of the motor is low, the copper loss and iron loss is large, the impact current of the switch is mostly consumed in the loss of the motor, the motor can withstand the impact current; If the motor is a new type, because of its high power, high power factor, low power consumption, small volume, light weight, switching impact will become most of the torque impact, the formation of great harm to the motor.

Impact on the power grid. If the switching opportunity is good, the impact current will not have much impact on the power grid; If the switching time is not selected properly, the air switch will trip; If the switching time is not selected properly, the power grid will be disturbed and fluctuated, and the safety and product quality of the power supply system will be adversely affected, especially the large capacity motor.