Whether a contactor is used between the inverter (VFD) and the motor depends on the specific situation. When the contactor is on and off when the motor is running, the operating overvoltage is easy to cause VFD damage. Therefore, whether to use a contactor, we must make the right choice according to the actual situation.
1. Contactors cannot be used.
The contactor between the VFD and the motor is switched on and off when the motor is running, which will generate an operating overvoltage and cause VFD damage.
Therefore, the user manual requires that in principle, a contactor should not be installed between the VFD and the motor.
2. Able to use contactors (use them with care)
It is still necessary to set up the VFD when using it in the following cases.
1) When the frequency conversion speed regulation system used for energy-saving control often operates at the rated speed, in order to complete the economic operation, the VFD needs to be blocked.
2) Participate in important technological processes and cannot stop work for a long time. In order to improve the reliability of the system, it is necessary to switch the backup control system.
3) One VFD controls multiple motors (including motors that are spare to each other).
When the VFD output side is equipped with electromagnetism, the peripheral circuit should be designed to avoid the contactor action when the VFD has an output, and prevent the power supply from being connected to the VFD output side at any time.
In order to facilitate the insulation of the test load cable and the motor, some users set an automatic air switch on the output side of the VFD to block the VFD during the test. This approach can do more harm than good.
Because the output cable (line) of the VFD needs to be shielded or piped, the probability of cable failure is very small. Usually, when measuring the insulation of motors and cables, the input and output terminals of VFDs, DC reactors and braking units can be reliably shorted with lead wire or soft copper wire, and then tested. Only when the interphase insulation of the cable needs to be measured, there is really no need to increase the investment, otherwise reliable methods should be adopted to avoid misoperation in operation.
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