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3 phase motor guru's

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  • 3 phase motor guru's

    In repairing my Baldor VM3559T, I sleeved and re-trued the shaft, installed new bearings, milled a new key way for which I had to removed and re-weld the internal fan, reassembled the whole affair and it does not like to run at all.

    In starting on my mill VFD, which is in excellent shape, the motor sort of bucks & snorts at low Hz, smooths out, bucks , snorts & shakes again up around 25Hz then rips up to top.

    Then it faults the VFD with E7 which is "DC bus voltage exceeds a threshold, due to regenerative energy from motor."

    Say what???

    The thing ran like a champ when pulled from service and, no insult, these motors aren't a complicated mechanical assembly.

    VFD is fine, runs the mill perfectly, the motor was wired for testing correctly...any ideas?
    What's regenerating in the motor?

  • #2
    Could there be cracked conductors in the rotor? Sounds as if you did some work on it, so it got handled and clamped, etc.

    Regen occurs when the motor is running faster than the VFD is commanding it, essentially a braking situation. It's not entirely clear how this can occur in your case.

    Best I can figure is that cracked rotor conductors make the rotor appear to act as a 4 pole rotor, probably intermittently, even though the motor is 2 pole. That could happen.

    The rotor is no different for 2 or 4 poles, but the way the stator "writes poles onto it" changes the number of poles. But if there is a cracked conductor, the rotor could act as if it had more poles than it does, if there is a "dead zone" on the rotor.

    Slip will change the orientation of the poles on the rotor, so the thing might buck and kick at different speeds. and if a rotor bar opens (or maybe the break closes up) up due to heat, etc, that could change the number of effective poles and cause effects similar to an over-speed situation.

    Bottom line is that if there is nothing mechanically wrong, there may be an electrical reason why it is bad.

    Why was it being repaired?
    CNC machines only go through the motions


    • #3
      There was shaft run out plus the shaft had been turned down to accommodate a pulley smaller than I wanted to use.
      It was an inexpensive CL find, ran great on my mill for some time then I mothballed it.

      Would welding the fan back on have harmed it?
      I made just small tacks as was there originally and I made certain to ground on the same end as the weld.


      • #4
        Sounds to me like you have an open / intermittent phase. Is it wye or delta wired?


        • #5
          Welding is not usual for that, what did you weld? There is usually an internal circulator fan, and also an outside blower, on a TEFC motor such as that.

          The overvolt threshold may be adjustable, if the motor can be made to run with the threshold higher, then you can try varying speeds with the VFD, and see if the behavior is repeated at any speed consistently.

          I am assuming this is being tested by itself, not attached to anything else?

          This is the motor?

          if a motor has problems at certain speeds, it can also be balance issues. Especially when run just out on the table. Might run better with a stable mounting.
          CNC machines only go through the motions


          • #6
            I had to remove the fan to get clearance in the mill to cut the key way so I turned off the welds holding the fan on then tacked it back on
            when done.

            Yes, not attached to anything.
            Yup, that's the motor.

            I think I'm just happy it didn't cost much and I'll move on to a single phase motor.
            Thanks much for your help.