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How to rectify ac to run a dc motor?

Discussion in 'Sensors and Actuators' started by boomerdog, May 24, 2013.

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  1. boomerdog

    boomerdog

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    May 24, 2013
    Hi all, new to the site and need help. I am not an electrician and have only a basic understanding of how things electric work, with that said, on to my problem. I have a Leeson permanent magnet gearmotor that I want to run with rectified ac power. The specs on the motor are as follows,

    volts 130
    amps .46
    h.p. .044 kw
    duty S1
    torque 23 lb in.
    RPM 139

    I bought a full wave bridge rectifier, 400v 8a, and when wired to 120ac, I measured 108vdc output. When I wired my motor to it, it ran but smoke immediately came from the motor. What do I need to make this motor run on 120ac?
     
  2. duke37

    duke37

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    Jan 9, 2011
    The figures seem to be right. The mean voltage of a rectified sine wave is Vrms*0.9.
    The volts, amps, power are consistent.

    I cannot help with the smoke.
     
  3. boomerdog

    boomerdog

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    May 24, 2013
    The motor has 3 leads, black, red, and green. I wired black to - and red to + on the rectifier, the green was left un-wired to ground, instant smoke. Then I reversed black and red, which reversed the rotation of the motor, but still instant smoke. I then removed the brushes and the end cap/bearing housing from the motor to have a look inside. The brushes looked fine to my un-trained eye, meaning they were still plenty long. The inside of the motor is wet with a light oil though, I don't know what this motor came from, so no idea of it's previous enviroment.
     
  4. john monks

    john monks

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    Mar 9, 2012
    Can you post the make and model of the motor or where it came from?
     
  5. boomerdog

    boomerdog

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    May 24, 2013
    No idea where it came from, but here's a pic of the specs-

    [​IMG]

    and with a tape measure for scale...

    [​IMG]
     
  6. DuctDuck

    DuctDuck

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    Jan 26, 2013
    Greetings, I have small passing familiarity with DC motors and have never seen such strange specs...
    :confused: 130VDC (not 13VDC huh, hmm)

    If you have DC power supply to vary the volts and current to your perm.mag motor, slowly increase current

    Else you can try to use big power resistors/rheostats to estimate the motor's resistance through its stall current (i.e- increase rheostat's resistance untill current level can't turn motor).

    Sorry, I have no real world experience to offer you.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I'm not sure the post above is actually helpful.
     
  8. eKretz

    eKretz

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    Apr 8, 2013
    If he has a DC power supply he could just steadily increase voltage and check RPM to see if there's a misprint. Most likely, the smoke is the oil burning off though. A brushed DC motor shouldn't be wet with oil anywhere near the brushes. My suggestion would be to degrease the motor insides and dry it well, then try it again.
     
  9. duke37

    duke37

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    Jan 9, 2011
    You need to determine the type of motor, If it is a permanent magnet motor then there will be some external magnetic field and there will probably be some 'cogging' where the motor goes round in slight jerks when turned by hand.
    If it is a shunt motor, there will be a field winding which must be energised before the brushes have power.
    It is not likely to be a series wound motor since it is DC.

    Since you have taken it apart, check where the wires go to with a meter. Two could go to the brushes and one to the field, the other end of the field may go to a brush.

    In any further test, put a bulb in series to limit the current, you could also try it at a lower voltage. My battery charger gets abused for such jobs.
     
  10. boomerdog

    boomerdog

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    May 24, 2013
    It runs fine, albeit very slowly when hooked up to a 12v 10a batt charger. It didn't make any unruly noises when fed 108v, just smoke. Could I put a rheostat (common light dimmer) on the ac side before the rectifier, and measure the dc voltage before trying again?
     
  11. duke37

    duke37

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    Jan 9, 2011
    That looks as if not too much damage has been done.
    Measure the voltage on the spare wire relative to each of the input wires. Is it connected to the body?

    A common light dimmer is not just a rheostat, it is a phase controlled triac. It may work but may give complications.

    Use a filament bulb to limit the current. Start with a low wattage and work up. See if the bulb lights brightly before the motor starts to turn. You may need a slow start circuit.
     
  12. boomerdog

    boomerdog

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    May 24, 2013
    OK, I'll try a few different bulbs and see what I can figure out. I assume you mean ac bulbs before the rectifier? Please bear with me, I'm not even qualified to be an amateure! :eek:

    Here's a pic of it cleaned and de-greased,

    [​IMG]
     
  13. poor mystic

    poor mystic

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    Apr 8, 2011
    Hi :)
    That'd be a case of "suck and see". (BTW a light dimmer is not a rheostat although a rheostat can be used to dim lights). You sound pretty cautious, so I'd say go ahead. However there's a chance the inductance of the motor could upset the "triac phasing" in the light dimmer, so if things seem to be going out of control another approach will be needed.

    (I see that Duke has recommended similary. Use his idea about the (tungsten filament) light bulb, they're great at saving electrical parts under test!
     
    Last edited: May 25, 2013
  14. boomerdog

    boomerdog

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    May 24, 2013
    If nothing else, this is a fun learning experiance! I tried a few bulbs, here are the values I measured at the rectifier with each as the motor was running,

    40w 9.4vac 7.2vdc
    65w 15vac 12vdc
    100w 23vac 19vdc

    The motor ran at not quite 14rpm using the 100w bulb, the biggest bulb I have on hand. I also tried the dimmer for a brief second, there seems to be no happy medium with it. Turning up the power only makes smoke as soon as the motor starts to turn, so I've abandoned that method. Where do I go from here?
     
  15. poor mystic

    poor mystic

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    Apr 8, 2011
    what voltage is the light bulb rated for please? and what voltage was being supplied to the circuit?

    this smoke does not sound good. Is it really acrid, richly nasty chemical type smoke?
     
    Last edited: May 25, 2013
  16. poor mystic

    poor mystic

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    Apr 8, 2011
    and this your circuit? (see attachment)
     

    Attached Files:

  17. duke37

    duke37

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    769
    Jan 9, 2011
    What are the bits attached to the brushes? Normaly they slide in a metal sleeve and have a spring at the back to keep them at the right pressure against the armature.

    It looks like a permanent magnet motor with red and black for inputs as you have used. Does the green wire go to the case?

    Edit. If you put a bulb across the motor, this may provide a load sufficiently resistive to make the dimmer behave.
     
    Last edited: May 25, 2013
  18. boomerdog

    boomerdog

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    May 24, 2013
    Bulbs are 120v, voltage supplied was 119-120, I'm in the USA. Yes, acrid is a good description of the odor, also the smoke was white, if that means anything. I'm on my phone now, so i can't make out that diagram. I won't be back to a computer for a few days.
     
  19. eKretz

    eKretz

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    Apr 8, 2013
    This is very unusual. It's still smoking after you degreased it and tried again? Could it be a bad rectifier? The motor runs fine on the battery but not so much on rectified DC. Out of curiosity, if I had the motor, I'd probably spin it at the rated RPM and see what voltage it generated. If that checks out it would seem to me that you definitely have an issue with the rectifier. Either damage or improper connections somewhere.
     
    Last edited: May 26, 2013
  20. glewis721

    glewis721

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    Apr 30, 2013
    Are you sure that the Motor is 130v?
     
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