A motor I have spent hours trying to sort out

Discussion in 'General Electronics Chat' started by steamseadog, Dec 16, 2012.

  1. steamseadog

    Thread Starter New Member

    Nov 16, 2012
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    Hello everyone
    I have a single phase, capacitor start /capacitor run 2400w/2200 r/m 240v motor. There is a run winding (res .9ohm) and start/run winding (1.8 ohm res), It was off an air compressor.
    2 capacitors - 100mfd start and 35mfd run.
    The centrifugal switch works fine and cuts out the 100mfd capacitor from circuit when increasing r/m.
    It pulls in excess of 15A when up and running on no load.
    There is no visual evidence of shorted windings nor an expensive smell.
    The bearings are fine and the rotor spins freely
    I've replaced the run capacitor with no change.
    To get it to run without blowing 15A fuses I seriesed up a 3.3ohm high wattage resistance with the motor and it runs OK but pulling abt 10A .
    The voltage measured across the resistor is 80V and across the motor is 160V. Revolutions were as per nameplate.
    Before I condemn the motor to the scrap heap has anybody come accross a similar rated motor with such low winding resistances?
    Cheers Peter
     
    Last edited by a moderator: Dec 16, 2012
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
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    Sounds like either the capacitors are wrong, or the windings are wired incorrect, i would thought the start cap was 35uF and run 100uF?
     
  3. tinkerman

    Member

    Jul 22, 2012
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    No capacitors are correct. Are you running this at the correct voltage? Perhaps someone has changed it to 120 volt operation and you're connecting to 240 volt supply. Just a thought. How about sound when you energize?
     
  4. tinkerman

    Member

    Jul 22, 2012
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    BTW, are you sure about 2200 rpm? Most 60 Hz motors operate at less than 1800 or 3600 RPM. A lot of compressors use 3450 RPM motors. (you won't find many motors that run at 1200 RPM). You might try separating all the leads, identify the coils and look for shorts between them. Quite often you will find a short between run and start windings.
     
  5. steamseadog

    Thread Starter New Member

    Nov 16, 2012
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    The thought about the supply voltage being too high makes good sense however the voltage is 240V on the nameplate.
    The motor is Australian made and 240V is our domestic supply voltage
    The motor is 50Hz.
    I meggered between the windings to earth reading was infinity (>100Meg).
    Will megger across windings-thanks for the idea
    Checking the name plate carefully this time The r/m is 2860
    I got 2200 r/m with my hand held tacho.Will check again.
    The noise when energising I would say is a rumble for about 2 secs.
    At speed it sounds happy.
    The run wiring is directly across 240V supply
    The start winding is connected to the active supply at one end and both capacitors at the other.
    Both capacitors are connected in parallel.(These are both in series with the start winding and the other leg to neutral).
    The 100mfd is connected in series with the centrifugally operated switch.
    Peter
     
  6. tinkerman

    Member

    Jul 22, 2012
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    Speed is fine. 3000 RPM theoretical for 50 cycles. They're allowing for 160 RPM slip. The start winding should have the run capacitor connected all the time. You are correct, the start cap is switched out at approx. 75% full speed.
     
  7. steamseadog

    Thread Starter New Member

    Nov 16, 2012
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    Meggered windings after disconnecting then. >100 meg so it appears there is not a short between them.
    Yes tacho speed is approx 2860 r/m.
    I put a variac as the supply 0-240v max 8A ajustable.
    At 40V ac the motor starts to roll. At 100v it is up to speed and pulling about 5A without load.
    As an exercise I disconnected the start winding whilst it was doing 2860 r/m and nothing seemed to change.
    It really has me puzzled.
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Take it to a motor repair shop. Very high running current is a sure sign of a failed winding like a shorted turn etc. It will need rewinding (or just throw it away). :)
     
  9. tinkerman

    Member

    Jul 22, 2012
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    It's not looking so good. Are you sure there is no discoloration or signs of damage to the run windings? I'd have to agree results so far are suggesting shorted turns.
     
  10. goprofishs

    New Member

    Dec 17, 2012
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    i think so,Meggered windings after disconnecting then. >100 meg so it appears there is not a short between them[​IMG]
     
  11. steamseadog

    Thread Starter New Member

    Nov 16, 2012
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    Thanks for all the advice.
    The internals look Ok and there's no burnt varnish smell.
    I'll unwind the stator and see if I can find whats going on in the slots
    I'm retired and have all the time in the world.
    My shed has become a place where people put stuff they can't get to work. Most are easy fixes but this motor has beat me.I need to suss out this problem.Then I'll scrap it

    Peter
     
  12. tinkerman

    Member

    Jul 22, 2012
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    If you had an internal growler might help pick up shorted turns. I've not had much luck with one of those things but maybe I just never learned how to run the darn thing. :(
    Kind of determination we like to see. LOL
     
  13. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Agreed, a growler or shorted turns tester would be great.

    But even if you don't have a growler, possibly the best test if it has a shorted turn is "does it draw too much current when running". ;)

    Obviously you first need to eliminate mechanical load issues like stiff bearings etc.
     
  14. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Is it at all possible that this motor was rewound at some point in it life time? At 15 amps it would make it a 5HP motor instead of the 3HP that 10 amps gives.
     
  15. tinkerman

    Member

    Jul 22, 2012
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    Rewound for a different voltage perhaps. Not possible to make a larger motor out of a smaller one. That's based on core size etc. To test HP would need a pony brake. It's possible to build such a device to fit the shaft and apply a load. It's simple but doesn't really give accurate readings. I suppose it could be improved with some experimentation on different motors.
     
  16. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    That is not always true.:) At least here in the US, and I assume all of the world that uses the NEMA standards. A 3HP and a 5HP that are the same frame size are the same physical size. If I remember correctly some frame sizes go from 1HP to 5HP in the same frame. I have several rewinding books that give formulas for the windings to change HP and/or voltage in motors of a frame size.
     
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