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Thread: Rotary phase converter motor

  1. #1

    Default Rotary phase converter motor

    Looking through the American Rotary website, i saw the option to buy just the panel and add a motor. I got to looking online at surplus 3ph motors and plenty are available.
    I took from a quickie look at some DIY phase converter pages that the ideal motor needs to be a TEFC (totally enclosed fan-controlled) and a decent brand like Baldor is a plus.
    I figure a 25hp 230v TEFC motor is needed, cast cooling fins and a stand that readily accepts rubber isolators is a plus.

    I don't know what to look for in: RPM, Amps, HZ, etc.

    Also, what do you look for in quick inspections? Smooth bearings when you spin the armature, old paint still on screws indicating someone probably hasn't been messing with it, a particular OHM resistance in the accessible lugs?

    What brands besides Baldor are worth checking out?

    One of my wife's relatives works at an alternator/starter rebuilding shop. Is it worth asking him to look over a motor to replace brushes, bearings and check windings or do I need to spend the bucks on an actual electric motor shop to go through it?

    This could knock a grand or so off the price of getting 3ph. Seams too easy, like everyone would do it if there wasn't a major down-side.

  2. #2
    Join Date
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    My friend has a really nice 20 hp phase converter for sale. I can't remember what he wanted but it sounded cheap at the time. He is out of town for the holidays. I can check on it if interested.

  3. #3
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    There is no NEED to use a TEFC motor. It just does not let in any dust. Nothing about such a motor is better electrically for an idler

    I would wager that more idlers are ordinary open motors than any other kind. Mine is a commercial RPC, and it does not use a TEFC motor, it uses a modified standard motor with through-ventilation.

    The motor amps are just a function of the HP. If you need 25 HP, the amps will be the same for most any such motor, within a small variation. A lower RPM motor is better from a noise point of view.

    The motors you want will not have any brushes, they are standard AC induction motors.

    As for checking motors..... what I use is three things. 1) spin it. Bearings should be good and spin freely 2) look at it... dirty is one thing, but damaged is another. Look in the ends if you can and see if the windings look OOk and not dark in spots as if something overheated. 3) Use your nose. If it smells burnt, like burnt varnish, move on.

    That assumes you have already checked the data plate for it being what you want, of the right voltage, etc. It may list the "NEMA design". A design B, C, or maybe D is good, having lower starting current than a design A.
    1601

    Keep eye on ball.
    Hashim Khan

  4. #4
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    If you're going to power your tools with a RPC, I suggest getting a nice high quality motor. I used to have a RPC that powered my Bridgeport and Clausing lathe but you could hear it run all the time and it was loud. I prefer a VFD per machine, but I recently looked at a machine that had a really nice RPC that was so quiet that I didn't even know it was running until I got really close to it. The cooling fans for the machine it was powering was louder than the RPC itself.

    Work hard play hard

  5. #5
    Join Date
    Nov 2012
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    I'm a real rookie compared to you guys. But I built one 10 years ago.
    Went to local machine shop, asked it they had a xxhp motor that had a
    bad shaft or bearings. Had my pic. The one I picked had no fan on end
    bell. No problem, small fan. Ran for years. $50.00.
    olf20 / Bob

  6. #6
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    I believe most prefer a 1725 (1800) RPM motor but I don't know why unless its just potentially quieter. There is also a preferred winding type, but I can't remember whether its Y or Delta.

    In spite of these preferred specs, either variation will work.

  7. #7
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    Noise level, mostly.

    Delta is preferred, presumably because the winding per phase is a single one. Both work.
    1601

    Keep eye on ball.
    Hashim Khan

  8. #8
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    Quote Originally Posted by chipmaker4130 View Post
    I believe most prefer a 1725 (1800) RPM motor but I don't know why unless its just potentially quieter. There is also a preferred winding type, but I can't remember whether its Y or Delta.

    In spite of these preferred specs, either variation will work.
    I thought I had read just the opposite, a 3450rpm idler was better, which is what I have. I'll have to check over on the Phase Converter sub-forum.

    Here is one such discussion: https://www.practicalmachinist.com/v...verter-173823/

    I chose a 3450rpm idler, it was very cheap because no one wanted a c-face washdown duty motor. Nobody noticed this brand new motor also had a mounting base.
    Last edited by reggie_obe; 12-21-2018 at 02:37 PM.

  9. #9
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    The faster motor stores more energy, and theoretically might help with inrush current and voltage holdup. But that energy is dissipated fast, and the net result is likely not very significant.

    The issue is that when the motor slows, as it does when a load is put on, the generated voltage falls. The generated leg voltage starts out at a bit under the line volts, and if it falls lower, can begin to be an imbalance that can affect tthe load motors. A flywheel would tend to hold up the speed.

    The degree to which that will help is a bit questionable, it depends on how much energy is in the flywheel, the rate of slowdown, the ability of the existing back EMF to drive current through the winding impedance of the generated leg, etc. More current will cause a larger voltage drop in the windings, and the output voltage would have to go up to hold the net output voltage the same. But there is no mechanism to increase that voltage from what it was at the previous speed..
    1601

    Keep eye on ball.
    Hashim Khan

  10. #10
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    Quote Originally Posted by reggie_obe View Post
    I thought I had read just the opposite . . .
    You may well be correct. Its been many years since I researched the issue. My first RPC was homebuilt with a 3450RPM idler and it was annoyingly noisy. Since I bought one from American Rotary almost 10 years ago, its out-of-sight, out-of-mind. It is smooth and quiet and has performed flawlessly under almost daily use and abuse (I often do a hundred-plus instant-reverses per day of use).

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