Problems with home built RPC

I put together a 5hp RPC using the pattern that Jim Rozen has in the first thread, kicker motor turning the 3 phase motor to get it started and then turning on the power. Got it assembled and tested it by wiring into welder plug for the single phase feeding into it and the short lead that was on the motor coming out. Started the kicker motor and then used breaker to supply power to it and it started right up and ran fine with 120 volts coming out of the regular leads and 135 volts on the wild leg. I just wire nutted one input into one set of wires hooked to the short lead that had originally powered the motor and second input into another and left the last one like it was.

Had electrician come to wire up lathe and when he was finished wiring it in went to start RPC to start lathe it just hummed and would not start turning on its own. He changed wires around and tried it again, still would not start. Unhooked lathe and changed wires around again and could not get it to start. He claims that the RPC has to have a start capacitor to work but if so why did it start and run when I first wired it up to test? Any reasons that it would not work after wiring it up second time? I am not very electric savvy so not sure why it worked one time and not another. Thanks for your help and input on this.

Are you sure about the voltages you posted? The RPC, if on a 220V "welder plug" should be generating ~220V with ~235 on the wild leg. You do not need a start capacitor - that's what the kicker motor is for. You didn't say how big the motor on the lathe is. If it's bigger than 3 hp, you might not have enough of an RPC to handle the starting current.


Tom

One question...

Are you trying to start the RPC with the lathe connected and on?

If so, that's why... can't have a load on the RPC when you fire it up...

I had nothing hooked to the RPC when I tested it and when hooked to the lathe nothing was turned on. Lathe has a 3 hp motor on it. Welder plug is 120 volts on each leg making a total on both of 240.

Ober Gun Works said:

I had nothing hooked to the RPC when I tested it and when hooked to the lathe nothing was turned on. Lathe has a 3 hp motor on it. Welder plug is 120 volts on each leg making a total on both of 240.

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I believe a 50 amp welder plug should be wired 2 hots and a ground rather than a neutral. Three phase readings should be taken from hot to hot.
Rob

Can you post a drawng of your wiring?

It reads like you have the welder plug wired to the lathe and the RPC which is technically correct but where the wild leg from the RPC is connected is not clear.

I wired the RPC to the welder plug to test and see if it worked before having the electrician come to wire the RPC and lathe. I was not sure if the motor was any good as it had been given to me several years ago by a friend. If it did not work I did not want to waste electricians time wiring it to lathe only to find out it did not work. When RPC was wired into switch box and then to lathe and did not work he changed input wires to L1 L2 and L3 in every combination and it would just humm when power was applied with each change. We took motor to neighbors commercial RPC and it ran just fine so eliminated that motor was bad. When it was wired to welder plug there was only the short wires that had been used to connect motor to line power coming out that I used to see what I had for voltages.

Assuming that your RPC and lathe are wired correctly, I still think that the RPC might undersized for the task. From what I've read, an RPC based on a 5hp motor will start a 3 hp easy-starting load. A 3 hp lathe (a hard-starting load according to the link below) would need a 7.5 hp or larger RPC.

Tom

P.S - Maybe you could test it by removing the belt from your lathe motor and see if the RPC will start the lathe motor with no load. If it starts the motor but won't start the lathe, you need a bigger RPC idler.

tmb

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Talked to electrician and he high pot checked motor used to make the RPC and it failed the test. Guess I will buy an VFD and go from there. Thank you for all your input.