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  1. #1
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    I want to get this lathe fired up to see how it cuts etc before I spend too much on the electrics. So... I wired a rectifier up and put 220 or so to the brushes another rectifier with a rheostat 115 to the fields.
    Seems to run ok, no load, but appears to be going about 1/2 speed ie 350 rpm to 1250. Now I don't expect this to be anything close to what it should be but was wondering why only 1/2 speed and am I going to cook something in the motor.
    Thanks
    Bob

  2. #2
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    I think I'd want to put the series winding in line with the armature winding, and maybe run both with a varistat (variable transformer) if you can find one that'll give 8 amps at 250 volts. At the low end be sure to have full field voltage so the motor will start (i.e. don't try to start with the field weakened).

  3. #3
    J Tiers Guest

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    Actually, isn't that about right for full field?
    If you cut down the field and leave the armature up, it SHOULD speed up some as you do it.

  4. #4
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    It does speed up as I move the rheostat but
    can't get it upto full rated speed. Does the polarity of the connections make a difference? I thought with no voltage to fields you are supposed to get a runanway my connection won't do that either.

  5. #5
    J Tiers Guest

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    No, the only motors that really run away are series.
    With them, the back EMF reduces armature current, which is the same as field current. This lets the unloaded motor get to an arbitrarily high speed based on friction.

    There is a feedback effect that keeps enough field to do the speed increase, so it retains torque. If it slows, current and torque increase.

    A shunt motor with no field has only the residual field working. The force on a wire is proportional to the current and the field strength.
    Residual field isn't enough in general to get torque enough to even really move the armature, so it slows and draws a lot of current.

    If your armature supply voltage droops too much, you won't get the full speed increase with field weakening.

    At least yours will speed up! See R Kepler's other posting.


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