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Is a 220VAC replacement motor acceptable?

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chucksterock

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Dec 27, 2016
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#1
I am looking for a lathe and am finding some where they have replaced the 3 phase motor with a 220VAC motor. Assuming they are the same horsepower and such, is that acceptable? Any pros or cons? I already have a rotary phase converter so that investment is already made.
 

Dave Paine

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May 10, 2014
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#2
If the replacement is the same or larger HP and has the same speed it will work the same.

The advantage of single phase is that we all have single phase. Some folks want to avoid the expense of 3 phase such as RPC like you have, static phase converter or VFD.

I like 3 phase motors. Quiet and easier to achieve speed control e.g., with VFD.

Since you already have an RPC, single phase motor has no advantage, assuming your RPC is large enough to run the motor for the lathe.
 

Keith Foor

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#3
There are issues with going from a three phase motor to a single phase motor. First and foremost is that 3 phase motors with a similar HP rating will have greater starting torque than single phase motors of the same HP rating. Second, you indicated that you are finding used lathes that have a new motor that is single phase in place of a 3 phase motor. The big question here is who swapped the motor and is it actually the correct motor and not just something they had at the shop. I personally have crossed paths with a a number of machines that had single phase motors installed that weren't half the HP of what they were originally. I have seen this to the point that 13 to 17 inch lathes that would not start the spindle in full speed and would only operate at the very lowest spindle RPM and then would stall when taking any sort of cut that should be expected of that size machine. Put a 1 HP motor in place of a 5 or a 7.5 and it tends to act this way. This situation even occurs in wood working machinery that has single phase motors to begin with and had the wrong motor installed for what ever reason at some point in the past. It's very common for a farm or fan motor of 1/2 to 3/4 HP end up in a machine that had a 2 or even 3 HP motor originally. Of course the machine doesn't work right if at all and the motor will typically overheat and fail quickly when this is done.

If you are looking at re-powered equipment and can't find documentation of what HP the original motor was, look at other similar machines of that size and see what was / is used on them. A typical 16 inch lathe is going to have a 7.5 to 10 HP motor. If you find one that has a single phase 5 HP motor (or less) KNOW that you will be putting a new motor on it to get it to run correctly.

Now of course, it may be wise to create a cheat sheet and have some links to different lathes of the size you are looking at in hand when looking at one to buy. Having some knowledge of what to expect with motor size will go a long way to getting a good working machine, or a good deal on a machine that is giving the owner fits because it isn't put together right.
 
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