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  1. #1
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    Off-Cycle Compressor Motor Heat



    I'm sure some of you are familiar with an old/early method of crankcase heating which energizes the compressor run capacitor during the off cycle. Since the capacitor is in series with the start/aux winding, there is current flow and the winding generates some heat. If not familiar, here's a link explaining it:

    Off Cycle Motor Heat

    For those who are familiar: does anyone know a typical current/amperage value for the circuit?

    I've seen a lot of units with this crankcase heating method over the years but never measured the current till yesterday and found it to be 3.0 amps. For some intuitively based reason, I always expected it to be much lower, as some comments I've seen referred to it as "trickle" current...and 3 amps seems more than a "trickle" to me.

    My reason for being on the call in the first place was to investigate a "hum", which turned out to be the off cycle heating arrangement.

    The 30f capacitor "tested" OK, but I tried a new one to be safe, and the amp draw was the same 3 amps. The "running" amps on the aux are 6-7.

    It's an old GE unit, some 30 years (believe it or not) and I'm wondering if the motor winding has lost some "integrity" resulting in a higher than normal amperage.

    Thanks in advance...



  2. #2
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    Re: Off-Cycle Compressor Motor Heat

    Hi.
    After 30 yrs. Anything would start to hum!
    A trace voltage through the motor windings is as you say quite common On large comps anyway.
    However not as efficient as an actual crankcase heater (on small Comps.) I believe.
    3A is quite a lot for a crankcase heater system, usually they are 1- 1,5A.

    When do you intend to retire the unit?
    Grizzly
    Despite the High Cost of Living it still remains Popular!

  3. #3
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    Re: Off-Cycle Compressor Motor Heat

    Quote Originally Posted by Grizzly View Post
    Hi.
    After 30 yrs. Anything would start to hum!

    3A is quite a lot for a crankcase heater system, usually they are 1- 1,5A.

    When do you intend to retire the unit?
    Grizzly
    Thanks...

    I'd have been less surprised with a 1-1.5 amp range reading.

    Retirement isn't an option for me and I don't force retirement on equipment. So the old GE will run till it expires...which may not be much longer.

  4. #4
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    Re: Off-Cycle Compressor Motor Heat

    I know you were concerned about the load being drawn.
    But Hey as you say "if it isn't broke don't fix it!"
    Grizzly
    Despite the High Cost of Living it still remains Popular!

  5. #5
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    Re: Off-Cycle Compressor Motor Heat

    Quote Originally Posted by Grizzly View Post
    I know you were concerned about the load being drawn.
    I found some related info that said the power consumption is supposed to be 40w-80w...which obviously ain't much. I know volt-amps and watts aren't the same but at 3 amps, 80 VA would require 90% of the voltage dropping across the capacitor, which I didn't think to measure at the time. The weather in my part of the world is warm anyhow, so I disconnected the heater circuit until I could learn more about what's supposed to be.

    But I'm concerned the "hum" is more the result of a stall condition than magnetic flux noise...I also didn't do a ground-fault check on the motor windings. I suppose a high resistance fault could conceivably result in an abnormally high amp draw.

    Needless to say, I've got to do some more analysis...

  6. #6
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    Re: Off-Cycle Compressor Motor Heat

    Here's a good reference from a Bard manual on the particulars of the off-cycle motor heat wiring connections and some numbers useful in making an intelligent analysis.

    Bard.PNG

  7. #7
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    Re: Off-Cycle Compressor Motor Heat

    I went back today and measured 24V between C and S, so the-off cycle heat circuit is performing as it should...or at least close. The 72VA (3 X 24) would probably result in a wattage within design range.

  8. #8
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    Re: Off-Cycle Compressor Motor Heat

    Its an old way to keep the compressor warm instead of a crankcase heater.
    Amperes drawn in the US will be higher as they use half the voltage in Europe.

  9. #9
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    Re: Off-Cycle Compressor Motor Heat

    Quote Originally Posted by chemi-cool View Post
    Its an old way to keep the compressor warm instead of a crankcase heater.
    Amperes drawn in the US will be higher as they use half the voltage in Europe.
    I knew what it did...didn't know to what degree it did it.

    It operates on 240V/60Hz...

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