Current issue

[nikos_abel]

Hi everybody. Please, look at the readings on the table below:

10338

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.

[nikos_abel]

Doesn't anybody thinks this is awkward?

[Brian_UK]

As there is a general reduction in discharge pressure then the load has come off of the compressor.

[nikos_abel]

As there is a general reduction in discharge pressure then the load has come off of the compressor.

And where has the load gone?

As you see, the high side pressure is rising as the ambient temperature rises too.

Sorry if you meant something else, my english isn't that good, I might misinterpreted you post

[nike123]

The strange thing is that the current is reducing as the ambient temperature is rising.
I would expect the opposite.

.

Compressor current is dependant on compression ratio of compressor. Calculate pressure ratios and you will see that with lowering of ratio means lowering in current consumption.

[nikos_abel]

Got it. Thank you very much!!!!!!

[jess luquin]

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.

[mikeref]

Compressor current is dependant on compression ratio of compressor. Calculate pressure ratios and you will see that with lowering of ratio means lowering in current consumption.
Must have been asleep at college when that piece of info was delivered Nike. :o

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.)

[nike123]

How does the ratio become involved should product load increase, along with current draw? ( Not using original posters findings in this scenario.)

I did not said that is only compression ratio which influence current draw.;)

[moideen]

Hi everybody. Please, look at the readings on the table below:

10338

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.



Hi nicos, fridge is indoor application, then how reached the ambient temperature is 43c.your readings are totally opposite of the logic. Maybe your clamp meter battery is weak………………!

[nike123]

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.
10346

[moideen]

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.
10346


Hi , nike, please recheck the software again .model 7.5g compressor which you example, I changed it in different evaporator temperature ,comp.amps increasing when eva.temp going high and vice versa. If I am wrong please correct me.

[nike123]

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.

[moideen]

hi, nike,this is user defined.you can change the evaporator/condensing/refrigerant.........have other optionsalso.

thanks
moideen

[nike123]

hi, nike,this is user defined.you can change the evaporator/condensing/refrigerant.........have other optionsalso.

thanks
moideen

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.

[moideen]

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.

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!)

[nike123]

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.

[nikos_abel]

Hi nicos, fridge is indoor application, then how reached the ambient temperature is 43c.your readings are totally opposite of the logic. Maybe your clamp meter battery is weak………………!

Hi

The temperature is controlled by me for testing a new condenser to the same circuit.
Me too was expecting the opposite results, therefore the post!!!!!!

I have changed the clamp battery and today I will repeat the measurements.

Thanks everybody

[nike123]

Be sure that you are measuring current of main winding!
Also, can you post your voltage and frequency?

[nikos_abel]

I will check this, thanks

230V 50Hz

[nike123]

Here are performance curves for that compressor.
It looks like your readings are possible.
10351

[moideen]

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.


Hi,nike, What you please mean absolute? Atmosphere pressure plus gauge pressure is absolute. For calculate compression ratio, absolute pressure to be used. You said that you were calculating as absolute……………………….:cool:

[mikeref]

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.

[nike123]

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.

[mikeref]

Add your atmospheric pressure, at sea level, it's 14.7 PSI or 101.3 Pascals.

[mikeref]

Ah..Sorry Nike. Overlap of our comments.

[nike123]

;)No problem! Nothing to sorry about!

[nikos_abel]

Here are performance curves for that compressor.
It looks like your readings are possible.
10351

Measurements are legit. Test repeated and the amps are the same after changing the clamps battery.

Now, how can I explain this to others :o

[HVACRAU]

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.:)

[nike123]

Are you suggesting that both Danfoss and Tecumseh software are faulty.

[HVACRAU]

I would agree with their program first, and find out why your reading is different

[nikos_abel]

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.:)

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?

[RANGER1]

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.

[mad fridgie]

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.

[HVACRAU]

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?


The condenser size is another topic. Just forget it.

[moideen]

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.

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………………………….

[mad fridgie]

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………………………….



Yes and no.