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Need help understanding / wiring vintage RPC

Dwayne D

Plastic
Joined
Feb 2, 2024
I recently purchased a lathe with a 2 speed 3 phase motor, 5hp. It was sold with a cheap VFD that they ran it with, and the motor will work that way, just none of the other controls on the lathe work as it's all bypassed. I bought what was sold as a used rotary phase converter that had been running 3 phase motors on a farm. The guy I bought it from couldn't tell me how to hook it up, and I've been having no luck figuring it out from web research. I had an electrician buddy come over to help look at it, but he wasn't sure what to do with it either.

I'm well versed in single phase residential wiring, but I'm not a commercial electrician. It has a 25hp motor, and I believe it has a generator head on the motor. I have it hooked to a 100 amp breaker.The top contactor has L1,L2, and L3 hookups on the top. L3 passed through the contactor and travels straight down to the motor connections at the bottom of the cabinet. L1 and L2 connect tp the other contactors below and go over and do something with the transformers. I dont understand exactly what its all doing. I've tried hooking to L1 and L2 and leaving L3 unconnected on the contactor. I've put a jumper wire between L2 and L3 so they both have voltage. It will pull both of tue top contactors either way with this. I've tried putting the neutral on the L1. It won't pull the first contactor like this. I do know the motor and generator head spin freely. When you try to start it, the motor jumps back and forth. If you leave it trying long enough the middle contactor releases and the lower contactor engages, but nothing changes. Eventually it will pop the breaker. Hopefully someone has more knowledge than me and can tell me what I'm doing wrong.
 

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Need more pics of the motor, typically its just a 3ph motor, it could possibly be a motor/generator, but we need to see it, and you may need to explain why you think its both. As for the panel, it looks very odd, you will probably need to draw out a schematic for anyone to understand what it does.
 
If it's a 25hp motor, that "generator" may be a pony motor to spin the big motor up. Once the motor is up to speed, line voltage is applied and it starts producing 3ph.

Some details like make, model no, etc might help. It's kind of hard to tell what you have from those pics.
 
Hopefully this helps.
 

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This is my attempt to diagram how it's wired. I don't have the small control wiring drawn out, if I need that it will take me a while because I'll have to trace them all. Hopefully what you are asking is answered with the plates I posted above.
 

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Well, it surely is a motor-generator setup.

There is just ONE big problem.... the generator is 400 Hz. Not any use to you as an RPC.

What it seems to be is a test set made to produce 400 Hz presumably for testing military equipment, possibly for civilian aircraft equipment. It is all one piece, obviously made as a unit for the purpose. Back at a prior employer, we had a setup like it for 50 Hz, since we had a good-sized export product line.
 
I completely missed where it says 400 cycles. I don't know where the guy bought the RPC, but he was/is a farmer. He said at his prior farm that he rented this was used to run all his silo / grain motors. I wouldn't think any of those would be running 400hz. I'm not sure how to test to find out if the plate is correct, but I'm sure it would jeed to be running to figure that out. Currently I'm still where the motor just jumps back and forth and won't start. After doing more reading I'm wondering if maybe I still don't have enough amperage going to it? It's on a 100 amp breaker. Reading somewhere online it said to multiply the amps on the motor plate by th sq rt of 3 which was 97.68. Elsewhere I was reading about uprising that so it would be 125amp breaker.
 
Perhaps the 16HP motor was being used as an RPC, and the generator was just freewheeling?

I'm not sure whether the 'synduction' motor would result in any unusual characteristics from an RPC point of view.

The transformer is probably an autotransformer for reduced voltage starting, which fits with the time delay you mentioned. I don't see any starting capacitors so you likely need to add those.
 
If it was being used for 60 Hz, it must have been wired up as an idler. it's a lot of extra "stuff" for a "simple" RPC, though.

There is a wire coming from the generator, and it comes up to what appear to be a switch. That and the lack of visible start capacitors makes it a bit unclear as to what it really was doing.

A motor-generator set makes a great RPC. You can have a single phase drive motor, and the generator makes very good 3 phase. But of course you need a mains frequency generator.

A schematic of the innards would show what is really going on.
 
If it was being used for 60 Hz, it must have been wired up as an idler. it's a lot of extra "stuff" for a "simple" RPC, though.

There is a wire coming from the generator, and it comes up to what appear to be a switch. That and the lack of visible start capacitors makes it a bit unclear as to what it really was doing.

A motor-generator set makes a great RPC. You can have a single phase drive motor, and the generator makes very good 3 phase. But of course you need a mains frequency generator.

A schematic of the innards would show what is really going on.
What exactly do you mean by a schematic? I have had no luck finding anything online about this piece of equipment, so I don't know how to get a schematic. I could possibly draw up a diagram of where all the control wires go, but I really don't know for sure what all the pieces inside even are.
 
What exactly do you mean by a schematic? I have had no luck finding anything online about this piece of equipment, so I don't know how to get a schematic. I could possibly draw up a diagram of where all the control wires go, but I really don't know for sure what all the pieces inside even are.
At this point I think you are SOL on using that thing for anything more than parts to build an rpc. Scrap the 400hz generator, hopefully you can use the motor, cabinet, and contactors.
 
........................... Currently I'm still where the motor just jumps back and forth and won't start. After doing more reading I'm wondering if maybe I still don't have enough amperage going to it? It's on a 100 amp breaker...................
You apparently do not have enough current on the third wire going to it. Maybe no connection at all?

The start capacitors are connected to that wire during the start, so that there is a "sort-of" third input at the right electrical phase to get the three-phase motor started. The capacitors need to be the correct size so that they provide enough current, and provide it at the correct electrical phase vs the other two wires.

Make sure your start capacitors are being connected when you try to start the thing. And check that they are disconnected after it does start.

That is commonly done by a relay that is normally closed, and is opened when the generated voltage on the third wire gets high enough.
 
What exactly do you mean by a schematic? I have had no luck finding anything online about this piece of equipment, so I don't know how to get a schematic. I could possibly draw up a diagram of where all the control wires go, but I really don't know for sure what all the pieces inside even are.
Drawing up a diagram is helpful, even if all the parts are just black boxes with at best terminal numbers.
 








 
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