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Old 08-17-2010, 12:58 PM   #1
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Bad Contactors


Can a bad contactor cause a electrical feedback even if the contactor is not engaged.The compressor tries to start and the condensor fan runs very slowly with out the contacts engaged.I did replace the dual run capcitor 45UF/5UF/440volt.I did not replace the contactor.The points do look worn and pitted.I do not understand why the condensor fan runs very slowly and the compressor hums when the contactor is not engaged.It is a single pole contactor

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Old 08-17-2010, 02:52 PM   #2
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Can a bad contactor cause a electrical feedback even if the contactor is not engaged.The compressor tries to start and the condensor fan runs very slowly with out the contacts engaged.I did replace the dual run capcitor 45UF/5UF/440volt.I did not replace the contactor.The points do look worn and pitted.I do not understand why the condensor fan runs very slowly and the compressor hums when the contactor is not engaged.It is a single pole contactor
Either the contactor is making marginal contact while not engaged or the downstream leg of the contactor has a path to ground and you're getting 120V to the OFM and Compressor vs 240V. Did you double check your wiring while replacing the cap?

If your contactor is pitted then replace it.


Last edited by hennyh; 08-17-2010 at 05:28 PM.
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Old 08-17-2010, 04:26 PM   #3
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Not good. It is likely that somewhere downstream of the contactor, the switched leg is shorted to ground. This means that 120V is flowing through the unswitched leg, through the fan/compressor, and back to ground. If I remember correctly, it is frequently an internal short in the compressor, or a mixed-up wire somewhere.

Use extreme caution while looking for the short. You must operate under the assumption that all metal parts of the unit are live at 120V. Disconnect all power, double check your wiring, and check the resistance between each contactor load terminal and ground.
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Old 08-17-2010, 05:42 PM   #4
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This situation can be DEADLY. You may have a short inside the compressor. One of the windings can be shorted to the shell of the compressor and finding its way to ground. If you touch any metal parts you become the path to ground and a grave!!

You need to disconnect the power, then the wires to the compressor terminals and check for continuity from each terminal to the copper line from the compressor. Should have none.
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Old 08-18-2010, 07:29 AM   #5
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Bad Contactors


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This situation can be DEADLY. You may have a short inside the compressor. One of the windings can be shorted to the shell of the compressor and finding its way to ground. If you touch any metal parts you become the path to ground and a grave!!

You need to disconnect the power, then the wires to the compressor terminals and check for continuity from each terminal to the copper line from the compressor. Should have none.
Thank you for your electrical advice. The compressor was grounded on all three terminals.
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Old 08-18-2010, 09:26 AM   #6
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OK, so first the compressor shorted and then the contacts got baked?
But you have another problem - why didn't a fuse or breaker open?
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Old 08-18-2010, 11:03 AM   #7
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OK, so first the compressor shorted and then the contacts got baked?
But you have another problem - why didn't a fuse or breaker open?
The breaker did trip but not right away.The compressor grounded on all three terminals C,S.&R. Thank you for your input.
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Old 08-19-2010, 12:10 AM   #8
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The breaker did trip but not right away.The compressor grounded on all three terminals C,S.&R. Thank you for your input.
There is other issue with it when you replace the compressor you have to check the manufacter plate to make sure you have proper OCPD size so by reading the nameplate it will say " XX Max Amps " and Max OCPD size which it will list either breaker or fuse rating.

A nice gotcha is if they stated only fuse then you must have to install A/C fused disconnect switch that is the only way it can meet the manufacter requirement and the electrical codes.

Merci.
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Old 08-19-2010, 03:33 AM   #9
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OK, so first the compressor shorted and then the contacts got baked?
But you have another problem - why didn't a fuse or breaker open?
A short to ground doesn't mean 0 ohms to ground.
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Old 08-19-2010, 09:30 PM   #10
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A short to ground doesn't mean 0 ohms to ground.
Pass/fail specs are always welcome.

If a 240 V compressor takes 75 A locked rotor for one second this is ~3 ohms impedance and even less resistance.
How much resistance should a valid short to ground measure, in this case and in general?


I guess either the compressor or the contacts or the voltage supply could have started this chain of cascading failures.
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Old 08-20-2010, 04:14 AM   #11
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Pass/fail specs are always welcome.

If a 240 V compressor takes 75 A locked rotor for one second this is ~3 ohms impedance and even less resistance.


How much resistance should a valid short to ground measure, in this case and in general?


I guess either the compressor or the contacts or the voltage supply could have started this chain of cascading failures.
With a standard multimeter. Infinite resistance.

Chain of failures that caused this electrical problem. May not have anything to do with electric itself.

Electric motors are induction loads. OHM's law doesn't apply.

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