Rotary phase converter with PM-1440GT: motor doesn't spin at full speed

[gnachman]

Background
I have a Phase-A-Matic R-3 rotary phase converter which I have wired into my Precision Matthews PM-1440GT, which has a 2 horsepower 3-phase motor.

This phase converter has only three wires in its electrical box. The two hot legs from the wall are wired into the lathe and the rotary phase converter in parallel. The rotary phase converter produces a third "generated" leg.

I ensured the generated leg is on the contact labeled "T" (the other inputs, R and S, are used for low-voltage electronics). The ground connection is fine and I measured about 240 volts between R and S and 280 between T and the other legs while idle, which is expected. According to the specs this phase converter is large enough for the 2 hp motor in the lathe since it is 50% oversized.

Problem
The motor turns very slowly, maybe a tenth of the expected speed. I ran it at the slowest speed (50 RPM) to minimize load on the motor and got about 5 RPM. The speed is also inconsistent: it speeds up and slows down. After a short time (estimating 20 seconds) the motor breaker in the lathe trips.

What I tried
I tried switching the inputs around to every permutation and the one I started with is the only one that works at all.

Questions
1. Am I overlooking something? Are there steps to figure this out that I should take?
2. Do I need a voltage stabilizer? I hate to throw good money after bad. Based on a conversation with Phase-a-matic it doesn't sound like my lathe should need it, but who knows.
2. If I return this and get a solid state phase converter from Phase Perfect should I expect the same problems?

 

[markba633csi]

I would measure the leg voltages after the lathe is switched on- measure leg to leg and not leg to ground
Something is clearly not right- should be fairly obvious. Very little to go wrong with an RPC- I suspect the lathe
Are you SURE it has a 3-phase motor? No capacitors mounted to the outside of it?
Is there a transformer in the lathe control box? I would visually check the tap connections for proper voltage

Did you just take delivery of the lathe? Infant mortality is always a possibility with new equipment
Post a picture of the motor terminals if you can- may be a wire off there. Also you would want to measure
the voltage at the motor itself- carefully though

 

[gnachman]

Before the lathe is on:

red-black: 244
red-white: 292
black-white: 297

With spindle turning:

red-black: 238
red-white: 129
black-white: 125

That does seem bad, but I'm not sure what to make of it.

 

[gnachman]

As to your other questions, I've attached a photo of the motor's nameplate and the terminals.

 

[markba633csi]

I would say you might have a problem with the rotary phase converter but...I would test the lathe motor first.
Disconnect the motor wires and measure the resistance of the three windings (all power shut off)
-they should all be the same low ohm reading. Check for readings to case ground also- should be infinity or close to.

Another test: you could run 3 wires temporarily from the RPC directly to the motor, bypassing the control box completely.
If the motor then runs correctly and the voltages seem correct then there is a problem with the control box

I suspect the motor has a defective/shorted winding and is loading down the generated leg from the RPC