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Thread: Current issue
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09-05-2013, 07:01 AM #1
Current issue
Hi everybody. Please, look at the readings on the table below:
Table.JPG
I was testing a new type of condenser so I took readings to see the behavior in various temperatures.
The strange thing is that the current is reducing as the ambient temperature is rising.
I would expect the opposite.
The refrigerator is a commercial cabinet with 2 doors, the compressor is Tecumseh CAJ4461Y, charged with R134a.
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09-05-2013, 12:22 PM #2
Re: Current issue
Doesn't anybody thinks this is awkward?
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09-05-2013, 05:49 PM #3
Re: Current issue
As there is a general reduction in discharge pressure then the load has come off of the compressor.
Brian - Newton Abbot, Devon, UK
Retired March 2015
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10-05-2013, 06:16 AM #4
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10-05-2013, 06:51 AM #5
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10-05-2013, 07:19 AM #6
Re: Current issue
Got it. Thank you very much!!!!!!
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10-05-2013, 07:39 AM #7
Re: Current issue
My understanding of refrigeration is that the discharge pressure should raise in conjuntion with the ambient temperature, since there is a pressure temperature relationship. The condenser's ability to dissippate heat relies on the condenser temperature being above the ambient for heat to transfer to a "colder" area.
Typically on small units like that superheat and subcooling measuring will be unusual if the system is a self contained. Weigh in charge i found to be the easiest.
I don't have a pt chart on hand but it you convert your discharge pressure to degrees you should be able to confirm this.
I don't know the exact specs of the compressor you are referring to. Little pot compressor are dependent on suction gas cooling and as the suction pressure increases there is more refrigerant cooling capabilities. Not to mention upon start up every compressor out there will pull locked rotor amps for a few seconds to get going then slowly reduce roughly 1/6th of LRA (just a rough guessimate number) Checked the tecumseh compressor guide for exact specifications.
Checking over your chart the compression ratios are about 1:6 and 1:7 starting from 12psi to 30psi, that could something to do with it as well.
That's all I got.
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10-05-2013, 08:09 AM #8
Re: Current issue
Must have been asleep at college when that piece of info was delivered Nike.
With a dedicated single piston compressor, low suction and low discharge, without excessive sub cooling would have the compressor motor drawing low amps, as compared to a higher suction and discharge. At the same ambient, there will be higher pressures involved requiring more work from compressor motor. How does the ratio become involved should product load increase, along with current draw? ( Not using original posters findings in this scenario.)To the optimist, the glass is half full. To the pessimist, the glass is half empty.
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10-05-2013, 09:15 PM #9
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11-05-2013, 08:49 AM #10
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11-05-2013, 11:01 AM #11
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Re: Current issue
As we can see from compressor characteristic on this picture, below some point (here in example at about -22°C evaporation) current consumption is opposite than above that point! Therefore, his readings are possible.
Compressor current.jpg
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11-05-2013, 06:53 PM #12
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11-05-2013, 11:13 PM #13
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Re: Current issue
I don't see how change in evaporation temperature (user defined or standard conditions) in software can affect performance curve (current).
I checked, and, in my case (as I expect), curves are unchanged no matter what value for evaporation temperature I enter.
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12-05-2013, 06:01 AM #14
Re: Current issue
hi, nike,this is user defined.you can change the evaporator/condensing/refrigerant.........have other optionsalso.
thanks
moideen
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12-05-2013, 07:49 AM #15
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I don't see the point.
My post was possible explanation of why current readings of OP are as he measured. Don't you agree that lover compression ratio mean less compressor work? Is it possible that reduction of current because of lower compression ratio and rise because of denser refrigerant at entrance of compressor at higher evaporation temperature, as resultant can be negative number and result is general reduction in current even with rise of evaporation and condensation temperature?
That is how I interpret these performance curves and readings of OP.
Unfortunately, I don't have performance curves of compressor in question of OP.Last edited by nike123; 12-05-2013 at 07:52 AM.
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12-05-2013, 01:33 PM #16
Re: Current issue
Ok, consider the compression ratio, but in our example, it is also opposite. Compression ratio has increased at lower amps condition if I define the above examples. SP=12, DP=160 RATIO IS 6.5, and consider the last reading (low amps) SP: 20, DP=240 RATIO IS 7.3.(high compression ratio=lower amps!)
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12-05-2013, 03:47 PM #17
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Re: Current issue
Your calculation is OK if these pressures are relative to the atmosphere.
I was calculating them as absolute.
That explain my perspective and your perspective.Last edited by nike123; 13-05-2013 at 09:48 AM. Reason: Refering to wrong set of data
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13-05-2013, 05:42 AM #18
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13-05-2013, 05:59 AM #19
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Re: Current issue
Be sure that you are measuring current of main winding!
Also, can you post your voltage and frequency?
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13-05-2013, 06:09 AM #20
Re: Current issue
I will check this, thanks
230V 50Hz
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13-05-2013, 09:45 AM #21
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Re: Current issue
Here are performance curves for that compressor.
It looks like your readings are possible.
Untitled picture.png
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13-05-2013, 09:57 AM #22
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13-05-2013, 10:05 AM #23
Re: Current issue
Absolute is disregarding atmospheric pressure, so: Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
To the optimist, the glass is half full. To the pessimist, the glass is half empty.
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13-05-2013, 10:09 AM #24
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Re: Current issue
I mean following:
You correctly assumed that OP pressures are relative to atmosphere and you added atmospheric pressure (cca 15 PSI) in your calculation.
On the other side, I have calculated OP pressures as they are really absolute (without adding atmospheric pressure in calculation since there is no PSIG writen in table) and I got wrong compression ratios.Last edited by nike123; 13-05-2013 at 10:13 AM.
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13-05-2013, 10:12 AM #25
Re: Current issue
Add your atmospheric pressure, at sea level, it's 14.7 PSI or 101.3 Pascals.
To the optimist, the glass is half full. To the pessimist, the glass is half empty.
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13-05-2013, 10:14 AM #26
Re: Current issue
Ah..Sorry Nike. Overlap of our comments.
To the optimist, the glass is half full. To the pessimist, the glass is half empty.
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13-05-2013, 10:24 AM #27
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Re: Current issue
No problem! Nothing to sorry about!
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13-05-2013, 10:26 AM #28
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14-05-2013, 07:10 AM #29
Re: Current issue
I would suggest you ask Tecumseh what's wrong with their program or compressor
By the way, you condenser looks small. Big condenser will work better.
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14-05-2013, 07:16 AM #30
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Re: Current issue
Are you suggesting that both Danfoss and Tecumseh software are faulty.
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14-05-2013, 07:35 AM #31
Re: Current issue
I would agree with their program first, and find out why your reading is different
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14-05-2013, 09:27 AM #32
Re: Current issue
Hi to everybody again. The readings have been rechecked once more and, they are correct.
The overall amps of the system though are normally slightly increasing as the ambient temperature rises.
It is inexplicable, I will contact the manufacturer to get some idea why.
The size of the condenser might have something to do with it?
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14-05-2013, 09:42 AM #33
Re: Current issue
Without over analysing could it be to reduced mass flow of refrigerant through compressor? Higher differential between suction & discharge & less dense refrigerant, then compressor does less work.
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14-05-2013, 12:01 PM #34
Re: Current issue
Look at the program, and look at the suction inlet temperature.
Then imagine the volumetric efficiency (as these are recips), then power factor of the motor, which is likely to be poor a certain points.
This should give you the reason for the strange readings
Lets re word it.
You measure the pressures, but did you measure the suction inlet temperatures.
I suspect that as you increase the discharge pressure, your actual suction temp increased.
Also meaning that your discharge temp increases.
Because you compressor is not 100% volumetric efficient, part of the down stroke, is driven by the pressure and not by the motor, further reducing the actual mass flow through the system, increasing further the suction inlet temp.
Re measure but include actual suction temp.
Nike, use your program and do not use a steady state suction temp at the different pressures.Last edited by mad fridgie; 14-05-2013 at 12:11 PM.
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15-05-2013, 12:29 AM #35
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15-05-2013, 04:14 AM #36
Re: Current issue
Hi,mad, the lower volumetric efficiency, mass flow reduction related to system efficiency, not the compressor load. The running or operating amperage of a refrigeration or air conditioning compressor is directly related to the load put on the compressor motor. One method of motor loading would be the mechanical load. the relatively small amount of friction created by the matched components of the compressor, such as the crankshaft to bearings, connecting rods to the crankshaft, pistons to cylinders, etc. Another method of motor loading would be the pressures entering and exiting the compression chamber, basically the suction and discharge pressures.I have another thought. As per the ohms law, Current and resistance is inversely proportional. When increased the discharge temperature will cause to increase the temperature of conductor (winding) and it might be increased the winding resistance. It may affect the reduction of the current .
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15-05-2013, 05:50 AM #37