yet another neophyte RPC question
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  1. #1
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    Default yet another neophyte RPC question

    OK, I'm great with fractured bones and joint related issues, am school trained machinist (fully qualified to destroy a perfectly good chunk of steel) but I am hopelessly lost in the world of electricity and hence have refitted all of my machines with single phase motors. But the vintage Reid surface grinder that has sat in the corner for several years is begging to be brought into service and so I am being forced to build a PRC. I have 2 questions.

    First - while many, but not all, of the various authors have recommend an idler motor of at least 1.5X the machine motor, given the extreme low load on a surface grinder motor why can I not use the 1HP 3Ph motor I have on hand.

    Secondly, in the interest of $$ I would prefer to build something along the lines of Jim's RPC with a small motor to start the idler but would like (I think) to add enough electronic "stuff" to get balanced voltage. The fellow I bought the grinder from had a simple RPC running it and he used a rope to start the idler running ala old lawnmower technique. I did not pay much attention to the RPC as I thought I would change the motor - now I understand grinder motors are balanced motors and proprietary in construction hence not easily retrofitted with a single phase motor.

    While I have read several of the articles on various websites I am still lost. I think all I need is a capacitor (or 2) and resistor. I'm begging here for a bit of hand holding and steerage.

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    You forgot to mention HP rating of the grinder motor.

    And keep the ratio 1:1.5, otherwise you may not be able to start your motor.

    As for the capacitors, I'd use, at least, two: a starting one and an oil-filled run capacitor. Then instead of a pony motor or a rope use a button (any normally open momentary switch) : turn the RPS on, press the button to start the idler, release it and you're in business. No need to spend money on a relay.

    All the button does is it temporarily connects the start capacitor to the run one, sharply increasing the total capacitance. This causes the idler to start rotating. In essence, this replaces the rope or a pony motor.

    Naturally, you may choose to install a higher value oil-filled running capacitor, so that the idler will start by itself, but the balance and energy waste will be very far from ideal.

    The most expensive items are the motor and oil-filled capacitor, but if you want an RPC, you must have both, in any case.
    Last edited by MichaelP; 09-24-2009 at 07:51 AM.

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    Yes, if you mention the hp rating on the load motor you intend to run, that would
    help direct the discussion.

    Jim

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    Red face

    Oops - despite rereading the post several times to ensure I had all the details I certainly did miss one. Sorry. The grinder has a 1Hp motor. The idler I have is a Baldor Super E and the HP data reads 1TE (what is TE?) and NEMA Nom Eff 85.5.

    So if I understand MichaelP I need to get a bigger idler. OK - will start that search tomorrow. But where do these capacitors go and what values? I need this at the "poets and lovers" level please. I did take a HS electronics class (a long time ago) and so can read a schematic but all the technical AC talk in the various, well written I must say, articles the author is writing to someone that at least has some level of AC understanding.

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    My solution on this you know, it was to simply get a much larger motor than
    the load motors, to use as an idler. The voltages came in balanced within
    ten percent which was OK for me.

    You can go to a smaller idler motor but need to tune with capacitors. There's
    a link to a couple of sites in the 'sticky' post in this (the converters and vfd forum)
    that explains how to do this.

    Just a thought: if that grinder is the only three phase machine you are likely to
    own, a VFD would do the job quite nicely. You can run a one hp motor from one
    of those TECO drives that's powered from 120 volts. Because you don't need
    to change speed, it would be dead simple to set up.

    If you can get an idler motor for free, in the 3 to 5 hp range, that would work. But
    the VFD drive (department of redundancy department....) is somewhere around a
    hundred bucks or so. It's a low-impact retrofit. Nothing to start, nothing to tune.

    Jim


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