VFD’s and Load Side Disconnects

National Electric Code (NEC) requires there to be a disconnect within sight of the motor. Specifically, Article 430, Part IX details that “within sight” means visible and not more than 50 feet from each other.film Rings

If a Variable Frequency Drive (VFD) is provided to control the fan or pump motor, the VFD would usually come with a built-in disconnect and be mounted within sight of the motor. This would comply with the NEC.

But what if the VFD cannot be mounted within sight of the motor? This would require a separate disconnect to be mounted closer to the motor in order to meet code. The question then becomes, what needs to be done to protect the VFD when there is a disconnect between the drive and the motor?

If the disconnect is closed, then the VFD will operate normally, as if the disconnect were not even there. The potential problem arises when the disconnect is opened while the VFD is running. In this case, there is a strong possibility that the subsequent voltage spike could cause faults/alarms in the VFD. The drive is likely getting a speed command signal from a controller (BMS), and when the disconnect is opened, the flow is reduced. This would typically lead to the drive being told to speed up. If the disconnect is then closed and the drive is outputting 60Hz, it will trip the VFD.

Another thing to consider is that typically disconnect switches are not designed to make/break under load. This now becomes a safety issue for the technician with his hand on a switch that is not rated to handle what is being done to it (i.e. the switch could fail if operated under load).

In order to prevent this from happening, it is recommended that a set of auxiliary contacts be run from the disconnect back to the VFD. This will allow the VFD to shut down when the disconnect is open and then re-engage when the disconnect is closed again.