Square Dial MG AC Motor Uneven Amp Draw

[Shiseiji]

Power source is a North American UL-10 Rotary Phase Converter. I had similar numbers using a static phase converter. I've checked the windings resistance, nothing unusual. The primary questions are do I need to be concerned? If yes, what options? The lathe was built in '53 as a MG, delivered to DOD prepared for long term storage. No idea when it was placed in service. Purchased as GSA surplus, last known use was at a Navy Depot. Lever wear indicates it turned a lot of screw threads. Getting delivery, a cross country move, yada yada, has drug out work over 3 years now.

T1 = 7.45a
T2 = 6.8a
T3 = 4.7a

TIA

 

[markba633csi]

You might try swapping the three legs around and see if the unbalance improves a bit. Those machines have several transformers
which presents a complex reactive load to the RPC. The generated leg has different source impedance than the two main legs.
Some additional balancing caps might be in order too

 

[Shiseiji]

Unfortunately swapping legs isn't an option because T3 can't be swapped for T1 or T2 and T1 & T2 can't be reversed because it will reverse the spin of the co-axial DC generator. Cal has started me on the balancing caps, but I'm also trying to get a response on ideas from North American. Had completely forgotten about the Monarch sub-forum though I contribute to the Logan sub-forum. So I decided it wouldn't hurt to ask.

Thanks,

 

[dfloen]

You could convert it to single phase using the steelman method. I did mine, wasn't too bad.

 

[Ulma Doctor]

You do not need to be concerned.
There isn’t anything wrong with a low leg for this situation. Don’t use the 3rd leg for control purposes and you’ll be fine

 

[Shiseiji]

You do not need to be concerned.
There isn’t anything wrong with a low leg for this situation. Don’t use the 3rd leg for control purposes and you’ll be fine

Thank you. I ended up calling North American and they were a bit surprised at the amperage question. So obviously it's not common. They told me the voltage match is the most important.

Act 2. After getting off the phone with North American, I tried to start the RPC. Best I can tell, the main contactor failed in a light show.

Called them back, am waiting on a response. 10 year warranty, we'll see.

 

[Shiseiji]

You could convert it to single phase using the steelman method. I did mine, wasn't too bad.

If I didn't have both a RPC and a SPC I'd probably tackle it. I have given some thought to getting a second MG so I won't be in danger of creating even more of a boat anchor. Did you document your conversion?
There is a conversion being documented on the Practical Machinist Monarch sub-forum as part of a complete refurbishment by Grant Gunderson. He stopped working on the Steelman conversion when things weren't going well and decided to put it off until the mechanical portion is complete. Mr. Gunderson, works as a winter sports photographer so work tends to be in the warm months.

 

[Ulma Doctor]

Thank you. I ended up calling North American and they were a bit surprised at the amperage question. So obviously it's not common. They told me the voltage match is the most important.

Act 2. After getting off the phone with North American, I tried to start the RPC. Best I can tell, the main contactor failed in a light show.

Called them back, am waiting on a response. 10 year warranty, we'll see.

most likely a capacitor or relay has failed, there is the possibility that the contactor could have a fault as well.

any pictures of the converter may help to explain what happened

 

[Shiseiji]

NA UL-10 SPC insides. All the caps tested good. The white box on the left is where the fireworks were.



Inside cover. The help desk told me it's not uncommon for the diagram to not be 100% accurate.

 

[Shiseiji]

Closure. Operator error. Swapped a cap wire during the cap testing.