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Rotary phase converter not large enough

Mike Cop

Plastic
Joined
Apr 25, 2020
I have a Mori Seiki MS 1250 lathe. It has a 7.5 hp motor. Have a rotary phase converter that’s 7.5 hp and it operates the Bridgeport mill fine. It will start the lathe without any belts and run motor. But won’t start it under load. So we are going to either have to get a 15 hp motor for phase, someone said a VFD would work but will it on a motor that’s most likely not VFD compatible,? And or go to a single phase 240 5 hp motor on the lathe. The lathe is mine, but it’s at my neighbors house he’s a retired machinist, I got he lathe 3 years ago from my late uncle just before he passed. This might be used once a month or so. It’s not a full time shop. It’s helping local farmer here or there. Anyone got used equip for it? And advice appreciated!!
 
General rule of thumb on rotary phase converters (RPCs) is 2x horsepower of largest motor driven, or in hard start applications 3x. You can try running mill when starting lathe, or reduce lathe motor to 5hp (may/may not work), or get a larger rpc. Single phase motor may work, but may cause vibrations and poor surface finish, and if you try to run lathe at top rpm while taking too much of a cut it can slow down motor enough to cause centrifugal switch to disengage so motor is running on start windings.
 
So we are going to either have to get a 15 hp motor for phase, someone said a VFD would work but will it on a motor that’s most likely not VFD compatible,?
The RPC internal ratings might be exceeded by just changing out the idler for a 2x upgrade.
 
I have a Mori Seiki MS 1250 lathe. It has a 7.5 hp motor. Have a rotary phase converter that’s 7.5 hp and it operates the Bridgeport mill fine. It will start the lathe without any belts and run motor. But won’t start it under load. So we are going to either have to get a 15 hp motor for phase, someone said a VFD would work but will it on a motor that’s most likely not VFD compatible,? And or go to a single phase 240 5 hp motor on the lathe. The lathe is mine, but it’s at my neighbors house he’s a retired machinist, I got he lathe 3 years ago from my late uncle just before he passed. This might be used once a month or so. It’s not a full time shop. It’s helping local farmer here or there. Anyone got used equip for it? And advice appreciated!!
bridgeport are either 2hp or 3hp, so the 7.5 hp to start it is fine.
starting a lathe with a 7.5 might be a problem, find a decent used motor for it, and get a larger RPC control box and wiring upgrade. this is just a $ problem to solve.
 
General rule of thumb on rotary phase converters (RPCs) is 2x horsepower of largest motor driven, or in hard start applications 3x. You can try running mill when starting lathe, or reduce lathe motor to 5hp (may/may not work), or get a larger rpc. Single phase motor may work, but may cause vibrations and poor surface finish, and if you try to run lathe at top rpm while taking too much of a cut it can slow down motor enough to cause centrifugal switch to disengage so motor is running on start windings.
Thanks!
 
You can use more than 1 RPC at the same time. Use one when doing small loads like the BP mill and turn on the second when using the lathe. It may be easier to source a second 5-10 hp unit than a larger one.
 
Some motors (WEG brand, for instance) seem to be absolute pigs to start.

Usually, a 1.5x sized RPC is good, but yes, nasty load motors could require up to 3x.

It's not really the HP, it's actually the ability to supply current from the generated leg of the RPC. The HP suggestion is because bigger motors are also higher current, and tend to be able to supply more current when used as idlers.
 








 
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