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corcerning the unexpected behaviour of DC shunt motor

Discussion in 'Electronic Basics' started by Banting, Dec 22, 2003.

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  1. Banting

    Banting Guest

    I got confused in an experiement experiment concerning shunt motor. In
    the experiment, I was asked to kept increase the load and measure the
    final speed and armature current.

    the experimental results were compared with the theorectical ones and
    they matched as long as the loading is small. But when the load was
    increased to 0.65Nm there was a sudden increase in armature current
    and drop in speed, devated from the original tendency.

    My demostrator said that this was "superloading". I ve never come
    across the term before. What is "superloading"? And how did it affect
    the performance of the shunt motor?
  2. Never heard of it. I would have expected a nearly linear reduction of
    speed as the load increased, and a linear increase in armature current
    all the way to stall.
  3. Thinker

    Thinker Guest

    Are you sure it is true shunt motor and not a compound motor. The action
    will happen in a compound motor when the it is too heavily loaded and the
    series field
    is supplying most of the magnetising force.
  4. Banting

    Banting Guest

    I am glad to see that there are a number of responses these days, and
    i d like to give more information on my experiment.

    I ve performed the experiment on both series and shunt motors, but NOT
    a compound motor. The shunt motor showed a linear torque-speed
    relationship and current- torque relationship at the beginning.
    Attached is the experimental result for the shunt motor:

    (the columns represent
    1)Torque(Nm) 2)Speed(rpm) 3)Voltage(V) 4)Armature Current (A) 5)Field
    Current(mA) 6)Effieciency% respectively,
    where efficiency is calculated, not measured.

    0.05 1700 110 0.20 240 18.391
    0.10 1690 110 0.30 238 29.905
    0.15 1660 110 0.40 236 37.272
    0.20 1640 110 0.50 234 42.542
    0.25 1620 110 0.60 232 46.341
    0.30 1600 110 0.70 231 49.083
    0.35 1590 110 0.81 230 50.941
    0.40 1560 110 0.94 229 50.817
    0.45 1550 110 1.07 228 51.157
    0.50 1530 110 1.21 227 50.680
    0.55 1500 110 1.45 233 46.667
    0.60 1480 110 1.72 231 43.330
    0.65 1190 110 2.60 230 26.020

    If anyone bothered to plot a graph, he would find a linear
    relationnship, but the point at torque= 0.65Nm deviated.

    I expect the term superloading may somehow related to the large
    armature current.

    Hope that these infomation would help to anyone kind respondeers.
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