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3 phase motor runs on single phase

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  • 3 phase motor runs on single phase

    Hi guys.I have a question.I am building a car lift powered by a 5hp 3 phase hydraulic unit.I was working on the lift and had forgot to fire up the rotary convertor.To my surprise it ran fine on single phase.I lifted my truck fine on single phase.I have saw many blown fuses working in a shop that when one was blown the motor wouldn't budge let alone run at working load.Any explanation to this? The pump should be using approx. 3 1/2 hp max.
    Last edited by j king; 03-26-2014, 09:32 AM.

  • #2
    Maybe your shop had three phase all along and you didn't know it?

    Show us a diagram how you wired it.

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    • #3
      Maybe the rotary started by itself somehow when you pushed the switch for the "second" motor?

      Your building the lift from scratch? Can we see pics?
      Andy

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      • #4
        Originally posted by 1-800miner View Post
        Maybe your shop had three phase all along and you didn't know it?

        Show us a diagram how you wired it.
        Lol! No the rotary sits right next to where the hydraulic unit is sitting.The RPC is 15 hp so I know it isn't running.

        It is wired into a 3 phase breaker panel.The rpc is plumbed into the panel to supply the 3rd leg in the panel.I don't know why this works but it does.

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        • #5
          Some of the motor-spurts here can probably answer best but I think some 3-phase motors will run on single phase but not at full power. They will also draw more current. I think that's how it works.

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          • #6
            The rotary requires me holding a momentary switch in till the rpc comes up to speed.The button pulls in a contractor to use more large caps.My rpc is a beast ! Lol. Capacitor bank is out of an electric furnace.I can start the rpc with no motor.

            Kinda got off tract..

            Lift is a mid rise scissor lift that I'm setting into the floor flush so no space taken up or trip hazards.Will be sweet.

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            • #7
              Did you have another 3 phase motor running in the building? Either that or you got lucky and the Hyd pump is not that much a load on the motor.
              Retired - Journeyman Refrigeration Pipefitter - Master Electrician

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              • #8
                The run capacitors from your rotary phase converter are also connected to your load so you wound up with a static phase converter.
                Location: Long Island, N.Y.

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                • #9
                  Some three phase motors will start up under no load on single phase. I have a 1 hp motor one here that does that every time, but they are the exception. I'd also go along with the theory that the rpc correction (not starting) caps created a phase shift for the wild leg and helped the starting process.

                  BTW... it may have worked BUT you are limited to about 50% power, so real easy to burn up your motor.
                  Last edited by lakeside53; 03-26-2014, 04:37 PM.

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                  • #10
                    Originally posted by lakeside53 View Post
                    Some three phase motors will start up under no load on single phase. I have a 1 hp motor one here that does that every time, but they are the exception. I'd also go along with the theory that the rpc correction (no starting) caps created a phase shift for the wild leg and helped the starting process.

                    BTW... it may have worked BUT you are limited to about 50% power, so real easy to burn up your motor.
                    Ok well I won't do it on purpose .Thanks everyone

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                    • #11
                      Originally posted by RichR View Post
                      The run capacitors from your rotary phase converter are also connected to your load so you wound up with a static phase converter.
                      +1. There has to be a phase shift induced in one leg somehow. I don't understand how you have things wired such that the lift motor gets power, but the RPC does not try to spin up. Regardless, make sure you don't have a safety issue with either live wires or unexpected starts.

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                      • #12
                        I have an old 3ph motor with a oil filled cap. hooked across a hot lead and the open lead . It will start when I
                        hit the start button on the mag relay

                        gt

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                        • #13
                          RichR has the answer....

                          A 3 phase motor WILL. NOT. START. on single phase unless something provides just enough phase shift to get it moving.... The most likely culprit is the run caps,. but the rotary should not have power to it, and so the balance/PF correction capacitors should not be able to have any effect.

                          There might be some other capacitor in circuit.... unloaded, it doesn't take very much to get it moving, although it may take a while to get up to speed.
                          CNC machines only go through the motions

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                          • #14
                            I have seen some start without a capacitor, they were not engaged to any machine...they were used either 1) To show they would start on single phase & 2) they were used as RPC's (Rotary Phase Converters) to start & run 3 phase machines...of course some use a single phase motor as a "Starter Motor" to get the 3 phase motor spinning before they energize it. I'm not a big brain know it all...don't claim to know more than anybody, but seen this on Youtube...

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                            • #15
                              I have one that will start unloaded. It's an older non-nema design from the 50/60's. Starts sluggish, but it accelerates quickly after a second or two. Maybe something to do with the stator construction? Not sure. I found this when testing it on single phase. Had the rope ready to pull start it but didn't need to.

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