Help needed with Air Conditioning Compressor

Old 07-26-04, 08:03 PM
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Help needed with Air Conditioning Compressor

This is a 20 year old Carrier 3-ton central air conditioning unit. I had trouble starting the compressor about one month ago. However, it did start after I shook the old run capacitor.

To improve the cooling efficiency I successfully changed the refrigerant to the new 22A envirosafe. After changing the unit worked fine for about a week (little improvement in cooling efficiency was seen in this time). I don't suspect a leak in the tubing.

After a week of using the unit, the compressor only hums; it does not start. The fan works fine and I dont think the problem lies within the thermostat because it seems to trigger the fan correctly.

I don't suspect the compressor to be faulty because it did compress the gas on the narrow (high pressure) side tubing when I did use it for that week.

The air conditioning unit trips the outside fuse after a few seconds of being on.

The compressor is not grounded and the three wires leading to it have little resistance.

I changed the run dual 440V AC 40/5 MFD capacitor and I still have the same results.

I did some research that a hard start kick is needed.

How can I add on or replace the capacitor in the series or parallel to fix the problem?
I cant seem to find the problem or a solution.
Much help is appreciated.

Old 07-27-04, 08:54 AM
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This thread needs to be moved to the A/C forum but...

Isn't a main difference in the new freon and the old freon the lubricant? I've been told that you can't put the new freon in an old compressor because of the change in lubricant. We need an expert. Is there a moderator on line that can move the thread?

Doug M.
Old 07-27-04, 08:58 AM
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The compressor-motor resembles a 2-phase induction motor connected to a singe-phase power-supply. This requires "splitting" the single-phase source into what resembles 2 motor-windings with the currents in the 2 windings having a phase-displacement of 90 electrical time-degrees. The current in the "Start" winding "leads" the current in the "Run" winding by the 90-degree time-element. The result is a "rotating" magnetic field" in the motor-windings.

Connecting a capacitor is series with the "Start" winding causes the "leading" current in this winding, with the "Run" winding switched directly across the line-voltage.One lead of both windings connect to a "Common", or "C" terminal.This explains the 3-wire external connection--- "R", "S", and "C".

If the motor fails to start with a capacitor of the correct value in series with the "Start" winding, there may be either a defective "Start" or "Run" winding, or other problems I'm not familar with which may mechanical ones.

There are HVAC experts in other sections of this "Forum" who may be of of assistance.

Good Luck & Enjoy the Experience!!!!!!!!!!!!!!1

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