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Shaded pole motor no-load current?

Discussion in 'Electronic Design' started by Joerg, May 9, 2013.

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  1. Joerg

    Joerg Guest

    Folks,

    Got a replacement motor for an evaporative cooler. They failed to mark
    the direction of rotation so I hooked it up. The core became hot
    quickly. So I measured the no-load current and at 120V it was right at
    the rated 4A (yes, four amps!).

    Now I know that shaded pole motors are inefficient, but are they really
    this bad?
     
  2. Tauno Voipio

    Tauno Voipio Guest

    Are you sure that you're not being misled by reactive current?
     
  3. Joerg

    Joerg Guest

    Yes, there will be a large reactive part. But the motor is dissipating
    tons of power without load. After less than one minute you cannot touch
    the laminated core packet any longer without pain, too hot.
     
  4. P E Schoen

    P E Schoen Guest

    "Joerg" wrote in message
    Try measuring the DC resistance of the winding. That will tell you the
    wasted power dissipation at 4 amps. Does the motor actually run? Can you
    lower the voltage to see where it stops running, and also to see if the
    current draw is proportional to voltage or if it shoots up a lot at 120V. I
    would expect only a couple hundred mA for a small fan motor (1/100 to 1/20
    HP or 8 to 40W). Most of these motors are "impedance protected", so that the
    current draw is low enough even at locked rotor to stay under a safe
    temperature. It sounds like a bad motor or a mismatch of voltage and/or
    frequency.

    Paul
     
  5. P E Schoen

    P E Schoen Guest

    I see that the rated current is 4 amps, so that's a hefty shaded pole motor.
    The evaporative cooler motors I found at Grainger were all split phase or
    capacitor start, and about 1/2 HP. The biggest shaded pole motor I found was
    1/20 HP with full load current 2A:
    http://www.grainger.com/Grainger/DAYTON-HVAC-Motor-5K004?Pid=search.

    The in-phase current should be about 750/20/120 = 312 mA. So it might draw
    close to 2A whether loaded or not.

    Paul
     
  6. Joerg

    Joerg Guest

    It's this guy:

    http://store.essickair.com/search/?query=110441-C&x=0&y=0&records=6

    DC resistance on the high-speed tap is 4.28 ohms and on the low speed
    tap 6.31 ohms.
     
  7. Guest

    Some motors have an "air over" rating (it might say "air over" or "AO"
    on the rating plate), which means they expect to be in some kind of
    airflow, usually from the fan they are driving. So the case temperature
    might not be as hideous when the thing is actually installed. On the
    other hand, if there is a squirrel cage wheel on each shaft, I'm not
    sure how much airflow the motor will get.

    I agree that getting about 93 watts (1/8 hp) of spin for about 480 watts
    of electricity would be a pretty bad deal. Have you tried powering it
    through a Kill-a-watt (or equal portable kWh meter) that can read both
    watts and VA?

    Matt Roberds
     
  8. Jamie

    Jamie Guest

    Yup, they get warm regardless of load. They get a little warmer when
    stalled.

    Does your motor have terminations for the shaded pole configuration
    so you can set the direction? Not sure if you have a motor of that type
    but it's important that you only use one set of shaded poles when
    picking a direction.

    In the case where you don't have this option, you are kind out to pasture.

    On a second note, they can be speed varied simply by voltage control.
    Lots a slip is acceptable with these and for some apps, it's a perfect
    fit.

    Jamie
     
  9. Joerg

    Joerg Guest

    Well, I installed it, running right now. We'll see :)

    This one is an el cheapo version that they likely sell at a fat profit
    margin, only three wires and ground.


    On evap coolers you have to run them full bore most of the time anyhow.
     
  10. P E Schoen

    P E Schoen Guest

    "Joerg" wrote in message
    So at 4 amps on the high speed tap there will be about 65 watts of heat, and
    on the low speed tap it *should* draw less than the 4 amps. There are
    several motors shown on the following page and it seems that yours may be
    the 10441-C which is 1/17 HP or 44 watts. But the 10441-2 is a 1/3 HP (250W)
    2 speed motor, where 65 watts no load would be acceptable with 75%
    efficiency.

    Paul
     
  11. Joerg

    Joerg Guest


    Yes, on low it "only" draws about 2.5A. Which I consider a lot.

    Mine is a 1/8HP. Not sure whether the calculations about horses and all
    that are right because this cooler is rated at 2800cfm of air at high,
    while many larger coolers have big 3/4HP or even 1HP and rate 5000cfm.
    Somehow all that doesn't really compute. I think I have to get myself a
    real wattmeter so see what it truly pulls under load.

    What became hot was not the winding. I could touch it through the cage
    after turning off and disconnecting. What got hot was the laminated
    section which is openly accessible with this motor.

    I just installed it and so far it works. Knocking on wood.
     
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