The machine crashes when switching between frequency conversion and power frequency? Here's how to fix it

When switching from variable frequency to industrial frequency, the frequency converter explodes and the circuit breaker trips. Various explanations have emerged, making the frequency converter-industrial frequency switching a difficult threshold to overcome. For example, some people say "it is necessary to ensure that the phase sequence of the frequency converter output is consistent with the phase sequence of the industrial frequency, so that it is possible to switch in" and so on. If the phase sequence of the frequency converter output is really consistent with the phase sequence of the industrial frequency, the frequency converter will still explode and the circuit breaker will still trip when switching from variable frequency to industrial frequency. Obviously, the reason is definitely not because of the phase sequence, phase, etc.

There is a simple method. Use a voltmeter to measure the voltage between the inverter output terminal and the power frequency phase line. No matter how the inverter output phase sequence, phase or other factors are adjusted, the measurement result is the power frequency 380V line voltage. The voltage between the inverter output terminal and the power frequency phase line is the power frequency 380V line voltage. Can the variable frequency-power frequency switch be performed directly? Can the direct switch not explode or trip the machine? Therefore, the technical secret of the variable frequency-power frequency switch is that the output terminal of the inverter cannot be short-circuited with the power frequency. As long as the output terminal of the inverter is not short-circuited with the power frequency, this method will definitely ensure the successful switching. How to ensure that the output terminal of the inverter is not short-circuited with the power frequency?

The method is also simple. Use a contactor 1 to disconnect the inverter output from the motor, and then use a contactor 2 to connect the power frequency to the motor. Use the normally closed contact of contactor 1 to connect the electromagnetic coil of contactor 2, that is, contactor 1 and contactor 2 must be interlocked. This ensures that the output end of the inverter and the power frequency cannot be short-circuited, and such switching will never cause the machine to explode or trip.

Operation Notes:

1. To switch the power frequency motor, set the parking mode to free parking, and avoid soft parking;

2. Cut off the contactor of the motor from the inverter output end. The control stop button and the inverter stop button are the same composite button. When the stop button is pressed, the inverter stops and the contactor coil is powered off to cut off the connection between the motor and the inverter.

3. Cut off the contactor of the motor from the output end of the inverter, and interlock the start button of the inverter with the start button of the inverter, that is, the inverter can only be started after the start contactor connects to the motor;

4. The motor is connected to the power frequency contactor, and its coil control circuit is controlled by the normally closed contact of the contactor that cuts off the motor at the inverter output end, ensuring that the power frequency is connected after the inverter output end cuts off the motor;

5. If the switching process is fast and accurate, that is, the shorter the time the motor is out of the power supply inertia, the less the speed drops, and there is less "shock", that is, the motor switches at the rated current;

6. Here we must pay attention to the phase sequence of the motor connected to the industrial frequency to ensure that the direction of the motor is consistent after switching!

"When switching a 400KW motor, the high voltage side tripped"

1. It seems that everyone has doubts about switching the power frequency of high-power motors;

2. There is no need to worry about the motor generating electricity during its inertial motion, but what causes the tripping?

3. There are two issues worth considering. One is that after the large motor is disconnected from the power supply, the winding still has electrostatic voltage due to the distributed capacitance, and an operational overvoltage occurs during switching;

4. Another reason is that the motor has not completely separated from the inverter (for example, the arc has not been extinguished), and the power frequency is switched too early, resulting in a power frequency short circuit;

5. The solution is to first let the frequency converter stop freely, then disconnect the motor from the frequency converter, and then switch to the power frequency, which can eliminate the switching trip caused by the above reasons;

6. Be sure to control the time difference! ! !

When switching between variable frequency and industrial frequency, the secret of switching is to have a 0.1 second delay from the free stop of the variable frequency to the motor cut-off, and a 0.2-0.4 second delay from the motor cut-off from the variable frequency to the industrial frequency connection. The whole process can be completed in 0.5 seconds at most.

1. When the motor is switched from grid power supply to inverter power supply, if the output frequency of the inverter is equal to the speed of the rotating magnetic field generated in the motor, that is, the synchronous speed is equal to the motor rotor speed, the switching current of the inverter is zero and there is no impact;

2. When the motor is switched from grid power supply to inverter power supply, if the output frequency of the inverter is lower than the speed of the rotating magnetic field generated in the motor, that is, the synchronous speed is lower than the motor rotor speed, the motor will enter the braking state after the inverter is switched, causing the DC voltage to rise. If the braking resistor is not started, overvoltage protection will occur;

3. When the motor is switched from grid power supply to inverter power supply, if the output frequency of the inverter is greater than the speed of the rotating magnetic field generated in the motor, that is, the synchronous speed is greater than the motor rotor speed, the motor will enter the electric running state with a slip rate of S after the inverter is switched. The switching current is related to the slip rate S. The larger the S, the greater the switching current;

4. Therefore, when the motor is switched from grid power supply to inverter power supply, the inverter will detect the motor speed and automatically adjust the output frequency to achieve smooth switching:

5. It is a wrong explanation to say that "the output voltage of the inverter is 180° out of phase with the back EMF of the motor" because at this time there is only a weak residual magnetic potential in the motor, and after switching, it is immediately balanced by the variable frequency voltage;