compressor amp test

[OAR]

To check amp draw on a compressor, We used to cut the end off an extentsion cord, splice in a push to start switch into the ground wire,  then re-wire ground to hot side of cord. this way we could manually start the compressor without a start device...can this still be done with modern P.S.C. compressors? ie. Just use a 2 way switch and wire in the run cap to the start/old ground line???? Or Is there a better / more simple way to check for amp draw on new compressors???

[Samurai Appliance Repair Man]

Sounds like you have a good system works just fine. Keep doing it that way.

[appl.tech.29501]

 dont get much easier than this. fluke T5-600

[Samurai Appliance Repair Man]

That's a damn fine meter, appl.tech-- I own and use that same one.

OAR, in re-reading your post, I didn't see any explicit mention of you actually measuring the current draw. Your rig would be very amenable to doing that with an instrument such as the one appl.tech linked you to.

[himeros]

I use an 3 in one hard start wired to a line cord, the overload will turn off any locked or shorted compressors.  With it wired direct to the ac line with no safety device, there could be a lots of sparks if the compressor is shorted or locked up. 

Himeros

[BrntToast]

hehhe funny you said that.... when i went to skool for dis 17 years ago they had us make those test cords with the start switch, i had mine for about 10 years before i plugged it into a shorted compressor and the switch blew up in my hand when i hit the button

i have the 3 in one on a cord now too

i prefer click over BOOM

[OAR]

Thats a great idea using a 3 in 1...but how do check true amp draw on the new P.S.C. compressor (ie. start +run winding) during run phase?..if my understanding is correct on a P.S.C. compressor using a P.T.C. relay both start and run windings operate during run phase.

[himeros]

I was thinking the same thing as brnttoast said happened to him, I would not want anything in my hand like a switch if that compressor was shorted out, 25 or more amps at 115 volts, just waiting to damage my body parts.  Unless I am mistaken, if you measure the current on the common wire to the compressor, would that not tell you how much current the compressor is using?  You could of course put an 10 to 15 amp circuit breaker in the common line if you want to use the switch method, when you wire it direct to the 115 line.

Himeros

[Budget Appliance Repair]

[user=8993]OAR[/user] wrote:

Thats a great idea using a 3 in 1...but how do check true amp draw on the new P.S.C. compressor (ie. start +run winding) during run phase?..if my understanding is correct on a P.S.C. compressor using a P.T.C. relay both start and run windings operate during run phase.

I don't believe it has anything to do with being a P.T.C. replay compressor, most newer compressors use a run capacitor and that is what causes both run and start windings to be utilized during run, thus letting the compressor run at a lower amperage. The start windings aren't running at full voltage once the compressor has started and is running.

You can run the compressor without the run capacitor and it will still operate ok but at a higher amp draw, thus consuming more energy.

As Himeros said, nothing special in checking the amperage, just put your amp clamp around one of the incoming lines, (Line or Neutral), and you should be able read the correct amperage.

[OAR]

Budget, your on the right path. this is what I'm trying to figure out..If you use a manual start cord or a 3 in 1 connected to a Permenent Split Capacitor compressor, you are going to read a higher than normal amp draw during run phase becouse start windings are not getting power...and you wont be able to messure amp draw on the start windings.  How can I rig a manual start cord allowing me to check amp  draw on both start and run seperatly while in run phase...The only thing I've come up with is the 2way switch and cap wired into a manual start cord described in my first post. but I'm not sure if this is the best way or simplest way to do this.  Any other ideas are greatly appreiated.

I always ohm out all terminals before dumping raw power into any motor.  And the switch I use is an auto-type push to start.(will carry lots and lots before melt down) 20amp breaker should go long before switch. I really liked the idea of an inline fuse as a safety. You can never count on someones home being wired correctly.

[himeros]

Let see if I understand now compressors work, can we forget about the start cap and overload protector for now . This is what I think, all the power to the compressor comes from the ac line, which has two wires.  All the current to power the compressor has to come from those two line.  During the start process, both the main and the start windings have full power applied to them, furnished by the start relay.  There is a common terminal, start winding terminal, run winding terminal.  At the starting of the compressor, power is applied from the common to both the start and main windings, power is applied to the start windings through a start relay. When the start relay shuts off power to the start windings, the compressor will then run with only the main run winding power up.  If you then have an run capacitor wired from the run windings to the start windings, you will have a condition when the start windings aid in powering the compressor.  I suspect the capacitor is also used to correct the compressor winding power factor which causes an inductive phase shift.  If this is right, you can measure the total amps, watts, on either of the two wires going to the compressor.  If using a 3 in 1, you could put your meter on any of the three wires going to the compressor terminals, if you wanted to see power going to each winding.  Please correct me if I am wrong.

Himeos

[OAR]

Himeros,  Im not sure without doing some more research but I dont believe start and run windings are wired in series like that, My understanding was that they are wired in paralell with the start winding being interupted by a run cap after initial start up. if this is the case should'nt you  be able to measure amp draw seperatly off of L1 and L2 (start line in and run line in) even though they share a common..?

[himeros]

The compressor start relay disconnects all power from the start winding after the motor starts.  At that time one side of the 115 is applied to the common terminal, and the other side of the line is applied to the run winding.  The start winding is open from one side of the line so no current can flow, in effect at that time, you could cut off the start terminal and nothing would change.  Now when you add a run capacitor from the run winding terminal to the start winding termimal, the capacitor passes  the alternating currrent from the run winding to the start winding.  Now you have a complete path from the common terminal ac line to the start winding that is now getting it power through the run capacitor from the other side of the line that is connected to the run winding.  The start winding with the run capacitor is now in parallel with the run winding. In other words, the start winding is now in series with a capacitor connected between L1 and L2, which is in parallel with the run winding. I am sure there is a better way to explain it, and if I could I would post a dragram on how the circuit looks.  And to answer the your question, yes, you can measure the total current going through the compressor by checking either side of the ac line, it will be the same. The run capacitor is by passed when the compressor first starts, because the start relay shunts the run capacitor, but only for a fraction of a second.

Himeros

[kwantlenappliance]

Here's a simple little motor start box you can construct to make you diagnosis of motors and compressors more accurate. We have all our students make one of these during our Appliance course.

Visit us at:   www.kwantlenappliance.ca

Motor Start Cord.doc

[kwantlenappliance]

For anybody interested in building the motor start cord I've posted it at:

http://www.kwantlenappliance.com/technology.htm

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