In the process of repairing the inverter, we can find that due to the back electromotive force in the stator winding, the electromotive force in the synchronous generator state and the self-induced electromotive force in the self-induced electromotive force in the self-loop rotor, the current impact will inevitably affect the motor, the variable-frequency drive (VFD) 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 on the winding of the motor, which will not affect it. 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 increase of 70% in the terminal voltage of the power equipment, which will cause the power equipment to receive excessive current impact and be damaged.
2. This inrush current can also damage the VFD's freewheeling diode, filter capacitor and insulation, greatly shortening the service life of the VFD.
3. Different loads on the motor shaft have different effects.
Motors of different capacities have different effects. If the motor with blower and exhaust fan is switched to the power supply after a delay of 13s due to the low back pressure caused by air, it will not be subjected to large current impact, and the impulse current motor is completely acceptable;
If the motor is loaded with a pump, the "water hammer" effect, coupled with the back electromotive force and high water pressure during switching, will cause the motor to have a current shock greater than 20 times the rated current and a huge torque impact, resulting in motor damage. If the motor is an old type, because the efficiency and power factor of the motor are low, the copper loss and iron loss are large, and most of the impulse current generated by the switch is consumed in the loss of the motor, and the motor can accept the impulse current; If the motor is a new type, because of its high efficiency, high power factor, low power consumption, small size, light weight, most of the impact that occurs when switching will become torque impact, which will cause great damage to the motor.
Impact on the power grid. If the timing of the handover is good, the inrush 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, it will cause disturbance and fluctuation to the power grid, and have a negative impact on the safety and product quality of the power supply system, especially for large-capacity motors.
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