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Current reading on common wire of compressor

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Posted: Thu Aug 26th, 2010 10:10 pm

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Josephine R

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I recently read a technical bulletin from Trane recommending technicians use an ammeter to measure the current through the start and run circuits. They explained how the current readings could help in troubleshooting and verifying that the problem was an external component rather than the compressor. I had never made these measurements before so I checked my own AC and got these readings- Start circuit 4.8 amps, Run circuit 9.7 amps, Compressor common 10.8 amps. My question concerns the reading on the common wire. I thought the current in the common would be the total of the Run and Start currents (approx 14 or 15 amps) since all the current flows through the common wire. Will someone please explain this. Thank you.

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Posted: Fri Aug 27th, 2010 01:17 am

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RegUS_PatOff
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depending on exactly where and when you're reading the current.

On the Common wire right at the Compressor, nothing else running through that (neutral ?) circuit ?

The Start current would only be during Starting... shouldn't be able to measure it after it's running..

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Posted: Sat Aug 28th, 2010 02:50 am

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dkpd1581

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In a single phase motor current only runs through the start winding for a short period of time - generally until the motor is at 80% of full speed.

The counter EMF (magnetic field generated by the current through the run winding) opposes the current in the start winding after start up. This opposition essentially cuts off the start winding - not completely but enough to render it ineffective and null-void.

Therefore, the common current will not be the sum total of the run and start currents when the motor is at speed.

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Posted: Sat Aug 28th, 2010 06:24 am

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JJDH

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You only need to worry about the amp draw through common. Make sure it isnt above the rla.

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Posted: Sun Aug 29th, 2010 08:07 pm

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Bobice

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Josephine R wrote: I recently read a technical bulletin from Trane recommending technicians use an ammeter to measure the current through the start and run circuits. They explained how the current readings could help in troubleshooting and verifying that the problem was an external component rather than the compressor. I had never made these measurements before so I checked my own AC and got these readings- Start circuit 4.8 amps, Run circuit 9.7 amps, Compressor common 10.8 amps. My question concerns the reading on the common wire. I thought the current in the common would be the total of the Run and Start currents (approx 14 or 15 amps) since all the current flows through the common wire. Will someone please explain this. Thank you.

You are confusing compressor ohm readings (Start to Run should toatal Common to run plus Common to Start).

Amperage RLA is suggested Rated Load Amperage measured when the Compressor is running.

Amperage LRA is Locked Rotor Amperage measured when the Compressor first starts.

Most "Electrical" problems initally started out as "Mechanical problems".

Last edited on Sun Aug 29th, 2010 08:08 pm by Bobice

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Posted: Mon Aug 30th, 2010 03:08 am

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applianceman18007260692
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When the compressor starts up all the copper (run winding and start winding) is energized to get it going. When it gets to full speed the start winding drops out of the circuit via the capacitor. What you would then read is the run winding amp draw only. Inside the start capacitor is 2 thin pieces of metal and a bunch of goop. With the capacitor sittin there minding its own business all happy happy the contactor slams down sending energy to the compressor and fan. The starting cap sends maximum power to the fan and compressor. When both get up to full running speed the 2 thin pieces of metal floating in all the goop seperate. That drops the power out of the start circuits and leaves the ac going full blast using half of the power it took to get er going. So what goes wrong? The cap can leak and all the goop come out. Then the the next time she calls for power BOOM the thang can swell up and give up the ghost. Or worse. Short out and zap the circuit breaker. LOL

Attachment: swollen capacitor.jpg (Downloaded 32 times)

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Posted: Mon Aug 30th, 2010 05:21 am

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Bobice

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Sorry JW, but Air Conditioner Compressors are Permanent Split Capacitor motors(PSC). This means that the start winding stays in the circuit through the capacitor . Shifting the windings thus making this run like a poly phase motor.

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Posted: Wed Sep 1st, 2010 06:02 pm

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8^th Post

applianceman18007260692
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You the man. The way I described it is what I was told many years ago.

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Posted: Wed Sep 1st, 2010 09:36 pm

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Bobice

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Thanks JW. Just trying to help best I can.

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