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Old 08-20-2010, 07:48 AM   #21
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Quote:
Originally Posted by HowieE View Post
Before he tries this please explain to me how swapping the leads on a 120 AC motor will cause it to run in the other direction.

If the unit starts for a FEW MINUTES and then fails while drawing 25 plus amps I would suspect one of the windings in the one of the motors has shorted and thus the high current draw. It seams you have a current meter. If you can get into the AC read the current on the fan motor and then disconnect the fan motor and read the running current on the compressor. Running the compressor for a minute without the fan won't be a problem.

Post those current readings.
You could also come close to this test without disconnecting things by running fan-only and checking the current, then observe the change in current when you turn it to cool. Adding vs. subtracting, but you don't have to disconnect the fan motor if that's inconvenient.
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Old 08-20-2010, 08:18 AM   #22
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My reason for separating them is to insure they are each running with max voltage available. If the test is run while they are connected have a voltmeter plugged in to make sure there is not a significant voltage drop and thus a current rise.
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Old 08-20-2010, 09:59 AM   #23
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not really "few minutes"... really about 5-10 seconds, just long enough to heat up and trip the breaker. Power Supply OK, No other loads in trailer are the issue, before i spend to much money (new shroud is 230+160 shipping ) what are replacement options?
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Old 08-20-2010, 10:03 AM   #24
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Probably a shorted winding and time for a new compressor. There's life after compressor failure, you don't need to scrap the whole A/C. Continuing to fiddle with it runs the risk of a more catastrophic failure and a shower of debris through the refrigerant lines which would complicate replacement, so don't do it.

Quote:
Originally Posted by HowieE View Post
Before he tries this please explain to me how swapping the leads on a 120 AC motor will cause it to run in the other direction.
The locked rotor current should be well in excess of 30 amps so I doubt if the reversing trick will help. However, for posterity, and the edification of all present, I offer herewith an explanation of why it does what it does.

A rigorous explanation requires calculus. In a capacitive circuit, current flow is proportional to the derivative (over time) of the voltage. Since the applied voltage is sinusoidal, the current flow through a capacitively coupled winding will lag by pi/2. Details become murky and require more calculus to illustrate rigorously because the circuit isn't purely capacitive, and in an inductor the current is proportional to the integral of the voltage, but that's the simple version.

Anyway, the delay, produced by the capacitive nature of the circuit, is what produces the twist in the magnetic field produced by the stator. Reversing the start and run windings delivers the delayed current to the run rather than the start so the twist goes the other way.
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Old 08-20-2010, 11:20 AM   #25
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The reversing trick often does work and there have been many homeowners that have paid the price for a "new" compressor when in fact they only got the reversing trick plus a can of spray paint to make their old compressor look new.

I learned about this trick while talking with an ethical ac guy about unethical repairmen in the HVAC business, and I've used it successfully in the past to delay the replacement of my own compressor.
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