Rotary Phase question

[AlanR]

If you are planning to bring in a separate 3 phase service to your shop, then yes you can run everything off of the 3 phase panel. Be mindful of the wild leg, about 180 V to neutral and ground.

Strictly speaking not so.

Usually three phase 240V is supplied from the power company in delta configuration. Transformers within the building can be used to convert to three phase 208 volt wye which is 120V from a three phase leg to neutral. We have two such transformers in this building.

Lots of motors are rated to run at 208 volts three phase. Lots of large three phase machines have internal transformers to step down from 240V to 120V for control mechanisms.

 

[John Hasler]

Strictly speaking not so.

Usually three phase 240V is supplied from the power company in delta configuration. Transformers within the building can be used to convert to three phase 208 volt wye which is 120V from a three phase leg to neutral. We have two such transformers in this building.

Lots of motors are rated to run at 208 volts three phase. Lots of large three phase machines have internal transformers to step down from 240V to 120V for control mechanisms.

If he uses an RPC he will have a wild leg. I agree that he's unlikely to get one from the utility, though, if he gets three-phase service from them. If he is going to get three-phase directy from the utility he should discuss it with their sales people. They might be able to supply 120/208 wye directly.

 

[Wireaddict]

If he uses an RPC he will have a wild leg. I agree that he's unlikely to get one from the utility, though, if he gets three-phase service from them. If he is going to get three-phase directy from the utility he should discuss it with their sales people. They might be able to supply 120/208 wye directly.

My RPC output only has 3-240V leads so I doubt if the mfgr added a center tap in one winding to create a wild leg, assuming it's wired in a delta configuration. You've got me wondering, though, so tomorrow I'll turn it on & measure the phases to ground & report back if there's anything more than stray AC noise present. If you choose to install a panel after your RPC make sure it's a 3-phase panel [has 3 (hot) main lugs & no neutral] & use 3-pole breakers to supply your 3-phase machines. To protect the RPC don't exceed 80% of its full-load output current rating. If the output is only rated in KVA divide it by 415.7 to find the amperage rating. If the combined current rating of all the 3-phase motors exceeds 80% of the RPC's output current you should install a 3-pole breaker or fused disconnect switch between the RPC & the 3-phase panel to protect it & also satisfy the code.

 

[John Hasler]

My RPC output only has 3-240V leads so I doubt if the mfgr added a center tap in one winding to create a wild leg, assuming it's wired in a delta configuration. You've got me wondering, though, so tomorrow I'll turn it on & measure the phases to ground & report back if there's anything more than stray AC noise present. If you choose to install a panel after your RPC make sure it's a 3-phase panel [has 3 (hot) main lugs & no neutral] & use 3-pole breakers to supply your 3-phase machines. To protect the RPC don't exceed 80% of its full-load output current rating. If the output is only rated in KVA divide it by 415.7 to find the amperage rating. If the combined current rating of all the 3-phase motors exceeds 80% of the RPC's output current you should install a 3-pole breaker or fused disconnect switch between the RPC & the 3-phase panel to protect it & also satisfy the code.

The 240 single phase supply comes from the power company's center-tap-grounded transformer. The RPC provides no isolation: L1 and L2 of the single phase go straight through and become L1 and L2 of the three phase. L3 is generated by the RPC. Thus you end up with what looks like a center tap grounded delta.

 

[JimDawson]

This is a typical 3 phase, 240V, w/center tap ground/neutral service connection from the power company.

 

[AlanR]

This is a typical 3 phase, 240V, w/center tap ground/neutral service connection from the power company

We don't have that here but this is a 30,000 square foot, industrial building with its own HV from the pole, (big) step down transformer out back. The building internally sees straight 240 delta hence the transformers inside to provide 120 for lighting and such.

From the transformers we get 208 three phase and 120V, so we have either 240 or 208 three phase, our choice, depending where we connect things.

 

[JimDawson]

We don't have that here but this is a 30,000 square foot, industrial building with its own HV from the pole, (big) step down transformer out back. The building internally sees straight 240 delta hence the transformers inside to provide 120 for lighting and such.

From the transformers we get 208 three phase and 120V, so we have either 240 or 208 three phase, our choice, depending where we connect things.

That would be typical of a heavy industrial power system. I should have noted that I was talking about a light industrial/commercial service < 200 amps.

 

[Wireaddict]

The 240 single phase supply comes from the power company's center-tap-grounded transformer. The RPC provides no isolation: L1 and L2 of the single phase go straight through and become L1 and L2 of the three phase. L3 is generated by the RPC. Thus you end up with what looks like a center tap grounded delta.

I stand corrected about that. Up to now I thought that RPCs did provide isolation unlike the quick & dirty 3 phase idler motors running on single phase & started via phase shifting caps, evidently RPCs don't have isolated outputs either, including mine. Sorry if I added to anyone's confusion.

 

[Cheeseking]

Wdnich I am in a somewhat similar situation in that i have 4 machines to run and am trying to decide how to best distribute from the converter. 7.5 hp rpc fed by 60A breaker. Right now I have 2 outlets daisy chained but this is sort of a "rig" for time being. I would not feel comfortable adding any more without proper distribution. As suggested the best thing will probably be a 3 phase breaker panel fed by the converter. Sorry for the crude sketch but this is how I wired mine.






Sent from my iPhone using Tapatalk

 

[Wireaddict]

Nice use of UniStrut to mount your equipment.