2 phase to 3 phase conversion

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Johnmcca

Johnmcca

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Mid-Hudson Valley, NY
My boss is buying a used machine that is currently wound and wired 2 phase. The dealer tells him they will re-wind the motors for 3 phase. The question is how will this impact the hp rating of the motor, if at all? It may affect the use of the machine if it has a significant difference.

Thanks
 
Unless it is an antique where it is important to keep the motor iron the same, why would one want it rewound, versus buying 3-phase motors?

It should be possible to replace the old motors (Are they really 2-phase, or are they single phase?) with the same frame size of a new motor. If they are small and cost is a consideration, you can probably buy used 3-phase motors at that famous auction site. There is little market for used 3-phase motors.

I expect that the cost of a replacment motor will be less than the cost of rewinding.

Generally, a 3-phase motor will be smaller and weigh less than a single phase motor of the same HP and speed. If the frame size of the 3-phase is a little smaller than the single phase, then it should be easy to install an adapter for mounting. The shaft size will usually be the same for the same HP and speed because shaft size requirement depends on torque.
 
Two phase power is an extremely archaic system first used in the early years of AC power distribution and railway electrification.

If this machine has such an old motor, then I'd first consider replacing the motor. Modern motors are smaller and more efficient; in the same place you would expect to see an significant improvement in horsepower.

If this machine has a _modern_ motor that was rewound to work with a two phase power system, then I expect that expect a rewind back to three phase power would again improve the capabilities of the motor, however I would again consider replacement rather than rewind.

If this machine has a two phase motor which cannot be replaced (for reasons of size, for example) then I would expect a rewind to improve the capabilities of the motor; present day insulation systems can tolerate higher temperatures and take up less space. However there may be other factors (slot count, for example) that are not suitable for rewind.

Are you sure that these are _two_ phase motors, and not single phase motors?

-Jon
 
It has been my experience that rewinding large horsepower motors can be cost effective. Depends on the age of the motor and the horsepower.
 
Thanks for the answers. Yes they are 2 phase motors, came out of Philadelphia. The motor frame itself in an integral cast part of the machine, or I would just change it out. The stator is slid in and held in place with a couple of socket head set screws. Not a lot of wiggle room. We have a similar machine here and the motor is a GE with a frame #3541 according to the tag riveted to the machine frame.

Very strange I'll grant you, but that is the way the machine was built in the 50's. It is a 30" wide wood planer, made by Oliver Machinery Company.
 
Two single phase transformers in a "Scott" connection will convert 3 phase to 2 phase. The problem is that the transformers are probably no longer a standard product. The main transformer needs a center tap on the input, and the teaser transformer needs an input tap at 86.6% of the three phase voltage.

Three single phase transformers can be connected in a "Taylor" connection but again you need to have the 50% and 86.6% taps however this time they will be on the secondary.

If you are serious about keeping the motor, you should contact a transformer manufacturer.
 
You can also consider using a "Scott Tee" transformer setup which will provide you with a 2 phase output from a 3 phase input. That way you will not need to touch the old motor.
 
Jim thanks for the suggestion. but my boss told the machinery dealer we needed the 3 phase and they said they could do it. To me it doesn't seem too cost effective but if they are willing to do it...............
My real question is will rewinding the motors have any effect on the rated hp of the motors. For the feed works a slight loss would be no problem but for the cutter head it could be a problem. These machines are slightly underpowered to begin with.
 
Johnmcca said:
Jim thanks for the suggestion. but my boss told the machinery dealer we needed the 3 phase and they said they could do it. To me it doesn't seem too cost effective but if they are willing to do it...............
My real question is will rewinding the motors have any effect on the rated hp of the motors. For the feed works a slight loss would be no problem but for the cutter head it could be a problem. These machines are slightly underpowered to begin with.
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John, I'd say that, if anything, you might actually gain a bit of torque. You might also find the motor speed will be a bit different. As for the overall horsepower, with a consistent efficiency, Kw in should transfer to HP out, in theory, anyway.
 
If the stator has a reasonable number of slots for a three phase winding, then I would expect the motor to improve with a three phase rewind.

However the number of stator slots is related to the number of poles and the number of phases, and it is possible that a stator designed for two phase use would not allow for a symmetric three phase winding. If this is the case than the winding could only be approximately symmetric, and would likely result in a loss of efficiency.

From your description of the motors, it sounds like you have easy access to the stators. If you can tell us the number of stator slots, and the pole count (or rated speed and frequency) of the motors, then we could tell you if a reasonably symmetric winding is possible.

Another issue would be frequency matching; since two phase power is such a strange beastie, I would expect the frequency to be different from 60 Hz. This will also be a consideration in rewinding/reusing these motors.

-Jon
 
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