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Thread: Replacing DC drive with AC drive in ungounded 480V delta system

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    Replacing DC drive with AC drive in ungounded 480V delta system

    Hi,
    We have DC drive and motor and we are going to replace it with AC drive and motor. We are going to use same location to install drive and same wires to power it up. And our system is 480 V ungrounded delta. For the existing DC drive, there are 3 wires for AC power supply(L1-L2-L3) between MCC bucket and DC drive cabinet. However, there is not a ground wire coming to the cabinet from MCC. So, do we have to pull a ground wire for the new AC drive from MCC bucket(approx. distance 200 ft) or can we pull a ground wire from the closest ground point ( For instance; from the steel structure body, approx distance 10 ft) ?

    Thanks.

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    Quote Originally Posted by ozkan View Post
    Hi,
    We have DC drive and motor and we are going to replace it with AC drive and motor. We are going to use same location to install drive and same wires to power it up. And our system is 480 V ungrounded delta. For the existing DC drive, there are 3 wires for AC power supply(L1-L2-L3) between MCC bucket and DC drive cabinet. However, there is not a ground wire coming to the cabinet from MCC. So, do we have to pull a ground wire for the new AC drive from MCC bucket(approx. distance 200 ft) or can we pull a ground wire from the closest ground point ( For instance; from the steel structure body, approx distance 10 ft) ?

    Thanks.
    Pull the ground wire from the MCC ground bus. Then also run your ground to the motor directly to the VFD ground terminal, NOT to a ground lug or bus in the VFD cabinet. There is likely a 2nd ground terminal on the VFD that you connect to that ground lug / bus. RTFM.

    Side note to a question not asked:
    Did you check with the VFD mfr as to whether or not it is suitable to be connected to a delta system? Some are not, those that are will usually have instructions for how you must modify the VFD to avoid having it blow itself up and those mods sometimes void the warranty and/or UL listing. What most VFD mfrs will tell you is to NOT feed a VFD with a delta power system, they are not really designed for that (if you post the make and model, I can tell you). They will recommend a Drive isolation Transformer ahead of the VFD that will be a 1:1 ratio, but 480 delta to 480/277 wye and you ground the wye on the secondary.

    Also, just because your MCC only has 3 bus bars and no neutral, does NOT mean it is a delta system. The only place it matters is at the secondary of the service transformer. If the transformer is delta-wye and the wye point is grounded, it's a 4 wire system even if no neutral conductor is brought out from the transformer. That is actually very common now. If that's the case, the VFD is fine with it as is.
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