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

[ozkan]

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.

 

[Jraef]

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.

 

[ozkan]

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.

Thanks for the help and reply. And, I would like to tell what we did for this project and it may be helpful for other people. The electrical system that we worked has been in use since 1950s and the susbstation transformers are ungrounded delta. The VFD that we used was Yaskawa A1000 rated for 250HP(Model CIMR-AU4A0296FAA). We installed a ground wire directly from the MCC ground busbar. And, as you mentioned above we used VFD cable and we connected the ground wire, which is between motor and VFD, to VFD terminal. It has been in use now and everything seems ok.

 

[kwired]

Was the original DC drive supplied by metal raceway? if so that was the EGC.