Using 208VAC to power a 3watt 120VAC synchronous motor (add a power resistor ?).

Discussion in 'General Electronics Chat' started by redwin88, Dec 18, 2007.

  1. redwin88

    Thread Starter Member

    Dec 17, 2007
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    0
    I’m in need of a cost effective way to reduce the 208 VAC (coming from 2 phases of a 3 phase line fused at 2 amps) for use with a small mechanical timer motor.

    Can I just place a 4 ohm power resistor in series with the motor ?

    It’s because we have 12 systems to run, that I’m looking for a cheaper way than buying 12 step-down transformers.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    If you grab one of the hot legs, it will measure 120 VAC to common. That should be even cheaper.
     
  3. redwin88

    Thread Starter Member

    Dec 17, 2007
    11
    0
    Well yes I could do that, but these 3 phase circuits
    don't have any commons (neutral wires)... so I would
    have to send the 25mAmps down the ground wire.
    Functional yes, but not safe if the ground wire ever becomes detached.

    Sorry I hadn’t mentioned the lack of the "fourth" wire originally, and thank you for trying to save me some cash :)
     
  4. richbrune

    Senior Member

    Oct 28, 2005
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    You probably won't save much money trying a resistor, because it would need to be a 3000 watt resistor-- I paid $40 recently for a 200 watt, 1 ohm resistor. And I'm a little surprised that you arrived at a value of 4 ohms, that's 208v/8ohms, ----26 Amps (approximately) Usually when dealing with voltage above 50, you want to get a UL approved buck boost transformer. It seems you should be able to run several motors off one of them, if the job site wiring allows. If your'e a very CAUTIOUS and competent experiment oriented worker, you might figure out how to run two of the motors in series, but the phase shift caused by the inductors in series may cause problems if the motors are anything but very small, fractional horsepower motors--- Hope that helps.
     
  5. Externet

    AAC Fanatic!

    Nov 29, 2005
    763
    57
    I would confirm first if one phase to ground (not neutral) shows or not ~120VAC.
    What about connecting pairs of the small motors in series ? Six pairs total to 208V directly ?
    Miguel
     
  6. redwin88

    Thread Starter Member

    Dec 17, 2007
    11
    0
    Whoops...thank you richbrune, you're right about the 4 ohms.
    It should have been 4,000 ohms (that way about half the power would be
    dissipated at the resistor, and it being the "same" wattage as the motor).

    Yes, the motor is a (sub)fractional type - it only drives a set of timing wheels
    similar to a car's odometer.

    The wiring for the 12 motors is all separate circuits (each using it's own
    isolation transformer - in and out = 208VAC 3 phase).
     
  7. redwin88

    Thread Starter Member

    Dec 17, 2007
    11
    0
    Thanks Externet for offering the series idea... I can't do that because each timer motor
    has it's own 3 phase source (each floats with it's own isolation transformer).

    Yes each leg does measure about 120VAC to ground (we don't have any neutral wires),
    but (intentionally, 24hrs a day) sending current into a ground wire is not favored
    because it can easily become a shock hazard.
     
  8. Externet

    AAC Fanatic!

    Nov 29, 2005
    763
    57
    ...And what is the problem to not use a 4.7K ohm / 3w resistor (or a plain 120V / 3W pilot lamp) in series with each motor into 2 - 208VAC phases ?
    Miguel
     
  9. gootee

    Senior Member

    Apr 24, 2007
    447
    50
    Hi richbrune,

    $40 for 1 Ohm 200W? Was it an Ohmite WFH230 series, or maybe an Ohmite 210-series?

    I'm not an expert with motors, and humbly seek enlightenment:

    It seems like he's saying that, for voltage-divider purposes, his motor looks like a 4800 Ohm resistance, to 60Hz 120 VAC RMS.

    If that is the case, then is there a compelling reason that he shouldn't just stick a 4160 Ohm (i.e. 4K) resistor in series with it, so that the motor and resistor act as a voltage divider for the 208 VAC, putting around 120 VAC across the motor and around 88 VAC across the 4K resistor, as long as the resistor is comfortable dissipating around 3.6 Watts?

    If that would work, and whether or not it's the best solution, how important would the choice of resistor type be, and which type might be best? e.g. Could it be wirewound, or should it be non-inductive?

    Thanks.

    - Tom Gootee
     
  10. redwin88

    Thread Starter Member

    Dec 17, 2007
    11
    0
    If I were to use a lamp in series with the timer motor, one day the
    motor would stop running (burned open filament in the lamp) and a very large
    amount of electricuty would be wasted by the timer NOT stopping that daily process, but I do thank you for that creative idea.

    Tom was right on the money, when he asked explained (better than me) what it was I was asking.

    Thanks for all that have contemplated my dilemma, and for offering ideas.

    I plan to use a series circuit of 3 resistors with the small motor.
    The only purpose of using three resistors is to spread the
    heat load out a bit (I know it's only 3 watts, but it's easy
    for me to do, the parts are available, and they will each remain
    on the cool side.

    The resistors are Ceramic Composition to avoid adding any inductance.
    Ohmite P/N OY-122KE, 1.2Kohms, 2W max.

    The 3.6Kohm resistance should drop the motor voltage to a reasonable
    value (nearish to 120VAC), and only dissipate a watt each.

    I hope to order the resistors this week, and some time in January
    update this thread with a happy success story (or 5% chance maybe
    a "Dhuoh!" - I didn't think of that issue thing to share).
     
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