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  • 3 phase questions

    I posted earlier about a Rockwell drill press with a 1/2 Hp, 3-phase motor. I do not intend to use this motor to run the drill press; instead, I have a 1/2 Hp 1-phase one to do the job.

    My plan is to use the 3-phase motor as a shaft for an idler pulley. I gather that when the 3-phase is driven by the 1-phase motor the former becomes a "sort-of" generator. I have capped off (Marr connectors & electrical tape) the wire leads on the 3-phase. Is this all that is required? I ask as 3-phase is, in practical terms, an unknown for me. I do not want sparks flying - or worse - when the machine is in operation.

  • #2
    As long as the each wire end is taped separately you should have no problem. I would think you would gain some inertia with the extra motor in line.

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    • #3
      Are you looking to generate 3 phase with this or are you looking to just use this motor as an idler pulley and don’t want any colts going to places I shouldn’t?

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      • #4
        As an induction motor, it will NOT generate unless it has two conditions satisfied, at least one of which you will not have.

        1) it must have some residual magnetism in the iron, to act as a "starting" generator.

        2) It must be connected to suitably sized capacitors, which will supply the "exciting current" when the residual magnetism generates a small voltage. The proper value allows the thing to "build up" as a generator.

        You might have #1, but you will not have #2, and there is really very little to worry about. If you ever generated more than a half a volt it would surprise me. And the residual magnetism will decline with time.
        Last edited by J Tiers; 04-29-2018, 12:24 AM.
        1601

        Keep eye on ball.
        Hashim Khan

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        • #5
          strictly as an idler pulley

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          • #6
            You’d be money and time ahead to simply buy a vfd and run the dp three phase which would give you fingertip speed control. A vfd for a .5 hp motor could be run off 110v and around $100.

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            • #7
              What is the OP trying to accomplish here? Jim

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              • #8
                A Rotary Phase Converter (RPC) is nothing more than a 3 phase motor that is run on single phase power. The third leg (or phase) is actually generated from the motor spinning. Its output is sent to the third leg on the 3 phase motor that's being used for whatever application. The other two phases are simply connected in parallel. So a 3 phase motor should be able to be operated as a generator on all three legs (although I've never done it).

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                • #9
                  Guys......

                  He wants to use the motor as a handy pulley shaft because it is already equipped with bearings and mount. He specifically does NOT want it to generate any voltages etc. It's not going to, so he has little to worry about. Taping or capping the wire ends is fine.
                  1601

                  Keep eye on ball.
                  Hashim Khan

                  Comment


                  • #10
                    Don't cap the leads! If you wire them back to the single phase driving motor, you will have a perpetual motion drill press. And you'll still have one spare lead to use for something else like lighting.

                    Put the DP on a rolling platform to move from place to place since it won't require a power outlet. Heck, you can make a powerfeed for the platform by adding belts to the idler pulley. When you're done, just push the cart once to start everything up.

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                    • #11
                      Originally posted by MichaelP View Post
                      Don't cap the leads! If you wire them back to the single phase driving motor, you will have a perpetual motion drill press. And you'll still have one spare lead to use for something else like lighting.

                      Put the DP on a rolling platform to move from place to place since it won't require a power outlet. Heck, you can make a powerfeed for the platform by adding belts to the idler pulley. When you're done, just push the cart once to start everything up.
                      Plus he can add one of those new Chinese made AC batteries to store excess power and just let it run when he does not have a use for it. That spare lead is great, Its free energy and would make what's his name, you know the guy who invented the Internet very happy!
                      Retired - Journeyman Refrigeration Pipefitter - Master Electrician - Fine Line Automation CNC 4x4 Router

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                      • #12
                        Using the 3ph motor as a jack shaft will require that you fit a new pulley to it somewhere, so work is going to be involved. If you have a suitable step pulley on the motor, you should still read on.

                        I would suggest that you take out the 3ph motor and sell it. You can get a decent price for a working 3 ph motor in some areas. Decent defined as more than I'd WANT pay but not more than I HAVE paid.

                        Then you use that money to buy a couple of pillow blocks to hold the jackshaft. Turn down a thick wall pipe for an axle and bob's your uncle.

                        Dan
                        At the end of the project, there is a profound difference between spare parts and extra parts.

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                        • #13
                          Ahhh... the vagaries of physics...

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                          • #14
                            thankx danlb...

                            Yes, it seem a waste to have the weight of all that iron simply to support a pulley but it was there. I'll set up a slow-speed adapter.
                            Had not really thought about selling the 3ph; my uneducated guess was that a 1/2 Hp wouldn't be a high demand item and thus not worth much. For now, I'll just set it aside - who knows what the future holds. As for "vfd'ing", again, maybe in the future.

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                            • #15
                              Don't assume that motor is not useful.... I am using a 1/3 HP Doerr 3p motor on the Logan, one that was apparently made for overhead doors. Limited duty or not, I have had it shut down on me only once when I was really working it for an extended time. 1/2 HP would be overkill with a 1" flat belt.

                              They come in handy. And, as 3 phase, they are somewhat more "effectively powerful" for their nominal HP than a single phase motor, particularly in cases where belts can slip, as with my 1" flat belt.

                              If it happens to be a 240/480 motor, they are less common in that size, and are worth a bit more.
                              1601

                              Keep eye on ball.
                              Hashim Khan

                              Comment

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