RPC Question

Little Bill

Little Bill

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Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
If the load on an RPC is rotating the wrong way, will swapping the two single phase legs, not the derived 3rd leg, change the rotation?
Or would swapping one single phase leg with the derived 3rd leg change the rotation?
 
Little Bill said:
I've hooked up a few of them but never had to change rotation. The machines that were going to be used said where to land the generated leg.
Click to expand...
You do not want the generated leg to power any controls, it won't necessarily be a stable voltage nor will it necessarily be same voltage as the two input legs and can vary as load changes much more rapidly than the two input legs may vary.
 
kwired said:
You do not want the generated leg to power any controls, it won't necessarily be a stable voltage nor will it necessarily be same voltage as the two input legs and can vary as load changes much more rapidly than the two input legs may vary.
Click to expand...
Yes, I know that the voltage on the generated leg is higher. Similar to a Delta high leg as the voltage measured around 208V on the generated leg from a 240V supply. That would not fare well with controls that need 120V.
 
I am working on a job of wiring a newly built office in a warehouse. They had an RPC in the space where the office was being built. Before I got there, the carpenters disconnected the RPC and moved it. They didn't bother to mark any of the conductors, just took them apart. So I will need to hook it back up without knowing the rotation, or how it was wired to get the proper rotation.

That aside, this RPC is really small and I've never saw one like this. It is just a motor with no cabinet for the capacitors and other things. There is a box on top of it that looks like a pecker head/wiring compartment, but is not where the whip comes out that connects to a JB. I didn't open it up but I can only assume the capacitors, etc. are in there.
 
Little Bill said:
Yes, I know that the voltage on the generated leg is higher. Similar to a Delta high leg as the voltage measured around 208V on the generated leg from a 240V supply. That would not fare well with controls that need 120V.
Click to expand...
The 208 to ground on a high leg delta is rather solid voltage that will remain rather constant. You can run 208-240 control circuit off of it and it won't have any issues. If you ran controls line to neutral they best be able to handle 208 volts is the only catch, but they shouldn't really give you any performance troubles if it is a 208 volt control.

The voltage on the derived third phase on rotary or static phase converters is not very stable at all, can be lower or higher than nominal 240 volts to either the other two legs or just as unstable to neutral and measured voltage often will vary as loading conditions vary. Motor currents won't normally be all that balanced either as compared to running off a true three phase source.
 
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