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Shunt-wound vs. PM DC motor?

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  • Shunt-wound vs. PM DC motor?

    I'm bidding on a Baldor DC motor for my lathe which will be powered by a KBIC-120 control. I noticed the motor is shunt-wound. Is there an advantage or disadvantage in using a shunt-wound motor?

    I looked through the KB manual and it's seems to be happy with either type other than this statement: "Note: * Performance is for the SCR rated PM motors only. Lower performance can be expected with other motor types."
    Milton

    "Accuracy is the sum total of your compensating mistakes."

    "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

  • #2
    For smaller motors, generally the disadvantages outweigh the advantages.
    Motor is larger frame size, motor can run away to destruction if field goes open with no field loss fitted, field supply required.
    Up side field weakening can be done in order to achieve higher RPM, (probably not applicable with KB) at the cost of torque.
    Most wound field motors are of the vintage that used SCR controls anyway, before the PWM became prevalent.
    Max.
    Last edited by MaxHeadRoom; 04-21-2012, 12:16 PM.

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    • #3
      Originally posted by MaxHeadRoom
      Motor is larger frame size, motor can run away to destruction if field goes open with no field loss fitted, field supply required.
      Theoretically possible, unlikely with small motors if even a belt drive is used. Friction will usually overcome the weak torque.

      What happens is that the back EMF drops due to low field, which makes the speed at which there is a balance between back EMF and input voltage increase. The motor goes faster. But the weak field (only remanent field if the field current is zero) also means the motor has very low torque.

      Because with a shunt motor, there is no automatic weakening of the field further (as with series) either the speed will limit, or the torque will drop to near-zero and the motor will not be able to overcome friction.

      A series motor can run away if the load is released. A shunt cannot run away unless the field goes away AND there is no load. Two simultaneous failures, not just one.

      If the motor has a clutch before any belts, gearbox, etc, it is more possible, since the minimum load is less.
      1601

      Keep eye on ball.
      Hashim Khan

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      • #4
        Thanks for the response gents. I owe you'uns an apology as further review revealed that the motor is actually a PM type, not shunt wound.

        The link supplied by the ebay seller was to a spec sheet that was a bit iffy and I interpreted it as having separate field connections. My bad!

        On a happier note, I just now won the auction for what I feel is a fair price. $143.53 shipped for a new Baldor 3/4 hp 56C motor. The ORAC ought'a sing with that motor twisting on it. Here's hoping it arrives safe & sound and is as represented.
        Milton

        "Accuracy is the sum total of your compensating mistakes."

        "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

        Comment


        • #5
          Field weakening is/was traditionally done on DC Lathe spindles etc, to obtain a higher rpm than possible with the applied armature supply, at a cost of torque, but field loss was always fitted, it is not a pretty sight to see one of these motors run with field failure when the field loss has either been removed or not fitted.
          Max.

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          • #6
            Originally posted by DICKEYBIRD
            On a happier note, I just now won the auction for what I feel is a fair price. $143.53 shipped for a new Baldor 3/4 hp 56C motor. The ORAC ought'a sing with that motor twisting on it. Here's hoping it arrives safe & sound and is as represented.
            I take it you passed on the R.O.C. one then?
            Max.

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            • #7
              Originally posted by MaxHeadRoom
              I take it you passed on the R.O.C. one then?
              Max.
              Yes, I did. I stumbled across the Baldor soon after we talked about the Chinese Leeson clone and I decided to wait and try to get it.

              Hopefully, I made the right choice. If it's as represented I'll be in good shape as it's a genuine, made in Arkansas motor.
              Milton

              "Accuracy is the sum total of your compensating mistakes."

              "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

              Comment

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