diagnosis with amp reading

[wannafreeze]

fantastic site..
Does anyone want to tell me;
1-what current readings would I get from the common wire of a compressor that is; a)running b)humming , if there is a)bad cap b)bad pot relay c)bad compressor.In other words can I do my diagnosis by only common amp reading?
2-back emf,can you describe it?If motor is not running there is no back emf?..

[a\c don]

Say what?

In like what are you taking about?

An air conditioner compressor, ice maker, what size?

My magic 8 ball says ask again.:)

[Senior Tech]

No speaka your language...

[shaka]

No there is no magic number on amprage. you are right if there is no EMF (ElectroMotiveForce) fancy term for volts it wont be running.

[wannafreeze]

Originally posted by a\c don
Say what?

In like what are you taking about?

An air conditioner compressor, ice maker, what size?

My magic 8 ball says ask again.:)

ok.I am not claiming to be experienced but I thought experienced tech.s would understand where I am trying to get.I`ll give you one example;
since I sad common wire it`s a single ph.motor,compressor starts,first reading is LRA,it drops to RLA,than it rises considerably.Can`t you say its a bad run cap?obviously the starter is ok,the start cap is ok,run cap?..

[Milk man]

Originally posted by altan
fantastic site..
Does anyone want to tell me;
1-what current readings would I get from the common wire of a compressor that is; a)running b)humming , if there is a)bad cap b)bad pot relay c)bad compressor.In other words can I do my diagnosis by only common amp reading?
2-back emf,can you describe it?If motor is not running there is no back emf?..

Compressor running - Something below RLA on data plate.
Humming, bad caps, bad pot. relay - Something around FLA on data plate.

Back EMF - Is induced voltage on a winding not being used. Like in a motor. The start winding drops out of the circuit but is still wound close to the run winding. Voltage is induced to the start winding in much the same way the primary induces voltage to the secondary winding.

[basser]

altan, Amperage is an excellant way to diagnose a compressor. To know what the amperage should be you have to look at the compressor data sheet. Nameplate (RLA) is not operating or running amperage.

You must get the suction and discharge pressures and voltage and then compare this information to the data sheet.

If the run capacitor is bad then you will see a higher amp draw then what the data sheet calls for. If the compressor is humming you most likely will see something close to the locked rotor amperage (LRA).

To get the data sheets, that's a phone call to the supply house or to the factory. Most can also be viewed or downloaded from the factory websites.

Hope this helps!

[jackmm1]

Altron
Back EMF is when a split phase induction run motor on start up, induces a voltage into the start winding,this voltage opposses the applied voltage (causing an out of phase effect) also called back EMF or (inductive reactance). The more out of phase a motor the more starting torque. A start cap puts the motor further out of phase, increasing the starting torque. The pot relay then takes out the start cap after motor is up to 3/4 its rpm. The run cap then puts the start/run windings in phase which improves the power factor and runs more effecently(smoothly).If the run cap is N/G then the compressor will run for a short peroid of time and goe off on hi amp motor overload. Check run cap w/ohmmeter or cappy testor. As stated already this is how transformers work, the primarey side of the transformer induces a voltage into the secondary side windings. Induced voltage occurs in any type of coiled wires. Hope this helps

Note: A cap start,cap run motor uses all the MFD on start up.

[jackmm1]

Altran, sorry about first ans.spelling of your name. More on problem.
Now to try and ans.your questions. When using your amp meter on a common leg of a compressor you should get a reading of FLA or less. If the compressor is humming and running and then going off on internal overload. You could have a run cap that is N/G. and must be checked out. If your pot relay is bad and keeping the Start cap in the circuit the compressor will go off on overload drawing lock rotor amps(LRA) immediately.Compressor would not hum and run as you stated. Good Luck...

[R12rules]

The advice about using the data sheet specific for that compressor is the best advice you can take.

It will give you all the info you need to diagnose the compressor's integrity, once you have it running.

However, if it is not staying online, you have other issues. Do you have a meter that will enable you to read microfarads?
If so, test your capacitors. If they test in the range printed on their sides, then the only things left to go wrong are;

Improper voltage to the unit.
Messed up wiring.
bad overload.
Bad relay.
Mechanically bad compressor.
Bad windings in unit.


Three things ruin compressors.
Heat or cold. Too much either way wreak havoc on compressors.
Lubrication.
Last but not least is dirt or contamination.


Lets break it down how this works.

Dirty fan cooled condensor. It gets hotter and dirtier and the compressor attempts to operate like that.
In newer units, those with POE oil and a refrigerant like 404 or 134, the oil overheats and begins baking to the inside of the cap tube. This restricts the metering device and also cuts down on the amount of cooling the compressor receives back from the evaporator.

Teck comes along, (too late, usually) and cleans dirty condensor. Unit starts cooling a little bit. Tech writes up ticket and leave.
Box continues to run. Oil has been compromised in it's ability to lubricate the compressor.
So with less than adequate lubrication, the compressor struggles on.
Excessive bearing wear is just one of the nasty results.
The cap tube is plugged beyond serviceable use.
The rings have begun wearing out, scoring the cylinder walls.
The valves have taken a beating since the high head pressures while it ran before the condensor was finally cleaned.

All the while the compressor is operating with high head pressure, the suction pressure is low due to restricted cap tube. So it is low and that means increased compression ratios for the compressor to pump against.

Higher than normal amp draw means hotter than normal wiring.

Bad connections made in the field are now being pushed to their limits.

Relays are hot, start caps are hot, run capacitors are hot.


It there had been any amount of sealed system service going on prior to any of the above issues .... then you can assume there is some sort of mis amount of freon charge in the system.
In other words, every time a tech attached their gages to the system.... there is a very good chance he or she did not leave the charge the same when they got off the system.
A little psst here, a little pssssst over there ... and pretty soon a critically charged cap tube system holding 20oz. or less is now compromised.
These systems are critical to within half an ounce of refrigerant. HALF AN OUNCE!!!

HALF AN OUNCE!!!

So, unless thier gages were spotless when they tied into your little system, or thier gages happened to have some of the same gas from another system, and hopefully it was clean gas .... unless they were practicing "safe-gages" .... they may very well have not only disrupted your system's freon charge, but also introduced some small amount of air.
And air contains moisture.

And moisture makes acid when it meets heat inside a sealed system.


So .... what was your question again? "What's wrong with my compressor?"

Answer; could be any number of things.
Not the least of which is your capacitor or relay.



I hope I didnt bore you. :D

[R12rules]

Also, there are a couple of handbooks you need to carry in your truck.

One is the Copeland electrical handbook and the other is from Tecumseh.

These two books provide tremendous knowledge.

[wannafreeze]

thank you guys,more than I expected to learn.