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internal conversion of old motor from star to delta

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  • internal conversion of old motor from star to delta

    THis is probably aimed at Sir john how difficult is it to reconnect INTERNALLY a 3 phase motor from star to delta so I can use a 415V motor on 240V?
    I've done it myself on a 1/8 HP motor where the connections were obvious but can anyone suggest where to start looking inside a 2HP motor. I could buy a new motor but finding one with a 19mm shaft is a problem. Would prefer to keep original motor and not bore out the pulley set. (on Elliot omni mill)

  • #2
    I did this once on a 1/2HP motor, and it went fairly well. I was somewhat dissatisfied with my ability to replace the laces I had to remove to unwrap the feed wires, but it should be OK. Anyway, this motor was on the shelf.

    I decided last week to remove my mill motor, a 3/4HP motor, a Hoover, possibly original equipment on my Centec, that I was running with an inverter, and so losing half its power. I pulled this apart and the job looked impossible. The resin that covered everything was seriously brittle, and nowhere could I see any clue to where the star node might be. To do the job on this motor i would have had to dip it in something to soften the resin at least.

    I swapped the motor with the 1/2HP motor, and hey presto ! I've got much more power.

    So what I'm saying is some motors look like the job isn't on. If John replies with a 'how-to' for a very brittle resined motor, all the better.
    Richard

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    • #3
      Hi Derek

      I had a similar problem recently !
      I found that this website gave me a better understanding of the process.
      http://www.nfphampden.com/3phmotor.pdf

      Good luck

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      • #4
        That description is for a motor turning at 1725rpm (1425 in Britain at 50 Hz) and called a twelve pole motor. I am more used to seeing that called a 6 pole motor. He describes it as a nine wire motor.

        I can see how his third diagram corressponds to what I understand as the star configuration. I cannot see how his second diagram corresponds to what I understand as the delta configuration. It must be there somewhere, I suppose.
        Richard

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        • #5
          It's very easy, I just give it to the motor rewinders to fix

          Seriously it's either an easy fix, star point very obvious or very hard. Although I do loads of motors all my work is mechanical and I'm not joking when I say I get the rewinders to do it and even they struggle at times depending on the age or design.

          Very hard to get a 2HP motor with 19mm shaft, 2HP will be 24mm but no reason why you cannot shorten it and turn it down to 19mm.
          The old keyway will still show but you can put a new 6mm key opposite the original 8mm keyway if pushed.

          I always weld the old keyway up so it doesn't show and put a new keyway in opposite, just looks better.
          .

          Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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          • #6
            Unless I'm missing something, the delta would be the higher-voltage connection. So there is no star node. You would need to create one.

            What that website calls a 12-pole motor, I would call a four.
            Last edited by Alan Douglas; 11-20-2012, 07:13 PM.

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            • #7
              The way it's usually portrayed the delta has each phase flowing through one coil - in parallel with the other two coils in series, it's true - but the star has each phase going through two coils in series. So the star results in the less amperage than delta at the same voltage.

              I've seen it quoted that at the same voltage a delta configuration will result in three times the power. So when you're converting a motor from 400-440 V you expect to find a star configuration. You want to reconfigure it to delta, to allow it to take the same current when you provide it with the smaller 230 V that your inverter puts out.
              Richard

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              • #8
                Not offering any specifics, but with 3 coils there should be six wires. If the motor has all six available to connect as you choose to, then it can be wired as either delta or star. Delta has all the coils wired in series, giving you three points to feed voltage to, while star has one lead from each coil wired to a common point. That gives four points to connect to, but you may or may not need a power connection to the common point.

                If you don't find three pairs of wires to arrange, then it's going to be more difficult if not impossible to change the configuration.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                • #9
                  the delta has each phase flowing through one coil - in parallel with the other two coils in series
                  It's three-phase, so unless you think in terms of a vector diagram you'll be hard put to make sense of it.

                  If the motor currently has two wires/windings connected to each input terminal, it's a delta.

                  I'm assuming a single-voltage motor.

                  Edit: I think I had it backward. If it's now a star, you can delta-connect it and run at a lower voltage. If it's now a delta, you can make it a star but that would require more voltage, not less. The voltage across each winding stays the same.
                  Last edited by Alan Douglas; 11-20-2012, 09:38 PM.

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                  • #10
                    somewhat dissatisfied with my ability to replace the laces I had to remove to unwrap the feed wires
                    It's a pain. I have found that cable ties / 'zap straps' are a very convenient alternative.
                    "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

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                    • #11
                      if the stator is old you will be opening a can of worms with snapped of wires ect
                      when you have done it you will need to varnish it and re stove it also


                      I did armature and stator winding during my apprenticeship and it was a job we hated with passion , in truth the pry bar slipped damaged the winding " hey gaffa this one is duff needs a complete rewind ;-) "

                      that the way we fixed it with no come back for a bad job


                      Stuart

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                      • #12
                        John,

                        what sort of price to get a rewinder out fit to do it?
                        and wold you recommend any outfit (Burton Area or Down south)
                        Last edited by derekm; 11-21-2012, 10:08 AM.

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                        • #13
                          I spent three summers during college working in a motor shop, and probably rewound a couple hundred motors. I didn't mind the work; it sure beat crawling through restaurant attics removing or installing vent fans.

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                          • #14
                            Don't forget that even when you find the ends of each winding you must get the right polarity when reconnecting them. For a Y connection either the three starts must be connected together at the Y point and the three finishes must be used as the line connection or vice versa and for a delta connection it's start to finish for each phase and the three connections are the three line connections. For delta connection the rated voltage is then the rated voltage of one winding and for a Y connection the rated voltage is the square root of 3 times this. The whole thing is very simple once you get the three starts and three finishes isolated and identified. It's getting the wires isolated and identified that is the problem.

                            Ken

                            BTW--the rated speed and rated HP will not change with Y-delta or delta Y conversion but the rated current will change by a square root of 3 factor; higher current for delta and lower current for Y connection.
                            Last edited by kf1002002; 11-21-2012, 04:28 PM.

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                            • #15
                              Unless there's room in the end of the motor to set up a six terminal plate, like a new dual voltage will have. Just get the six wires out and finish the job in comfort. Getting the polarity correct is quite straightforward. When you take three coils, two of them at least must match in polarity, however you pick them up.

                              If the motor runs fine first off, you have all three with the same polarity. If it doesn't, exactly one coil has a polarity that is reversed compared to the other two. Just switch the polarity of each coil in turn until the motor runs well.
                              Richard

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