Is a Contactor Required between VFD and Motor?
1. When a VFD controls a motor and does not need to switch between the power supply, an output contactor is not needed to be connected between the variable frequency drive and motor. The main reason is that: if an output contactor is connected, the motor may be started directly when the output frequency of VFD is high. As a result, a large starting current would be generated, which may lead to trip of VFD.
2. The output contactor has to be connected in following two cases.
Similar to output contactor, when a VFD controls only one motor and doesn’t need to switch with a power frequency, there’s no need to connect the thermal relay as the VFD has thermal protection function itself. However, when a VFD connects with several motors, the capacity of each motor is much smaller than that of VFD, VFD can’t provide thermal protection for each motor. On this basis, each motor can only be protected by its own thermal relay. When the motor needs to be switched between variable frequency and power frequency, as the VFD can’t provide thermal protection for motor when running at power frequency, the thermal relay is necessary.
2. The output contactor has to be connected in following two cases.
- When a VFD controls several motors, each motor shall have an independent controlling contactor, as shown in figure a.
- Switch between variable frequency and power frequency. When the motor is connected to the power frequency power supply, it must disconnect from the VFD. So the contactor is necessary between the motor and VFD, as shown in figure b.
Similar to output contactor, when a VFD controls only one motor and doesn’t need to switch with a power frequency, there’s no need to connect the thermal relay as the VFD has thermal protection function itself. However, when a VFD connects with several motors, the capacity of each motor is much smaller than that of VFD, VFD can’t provide thermal protection for each motor. On this basis, each motor can only be protected by its own thermal relay. When the motor needs to be switched between variable frequency and power frequency, as the VFD can’t provide thermal protection for motor when running at power frequency, the thermal relay is necessary.
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