[Acmanz28]

I have a question with regards to LRA of a compressor or motor. What would be a normal range difference from what LRA/RLA that is printed on name plate? Example if I had a LRA of 40, but measured 38 amps on start up then drops to 7 amps and the RLA is said to be 10, would this be acceptable? And what breaker would be acceptable if the name plate says a 15amp max breaker? It seems that a 15 dual pole breaker is to small, if on start up its pulling 38 amps...

:det:

 

[beenthere]

Yes.

 

[54regcab]

A compressors amp draw if heavily influenced by the high side pressure. Most of the time is will pull less than rated load amps. My own compressor is rated at 13.5A, but typically pulls about 1/2 that amount. If the name plate says 15A breaker, that's what you use. Breaker won't trip for a quick surge 3X it's rating. Direct short will trip breaker immediately.

 

[beenthere]

High side pressure per say isn't what influences amp draw/wattage.

 

[54regcab]

It looks that way from the chart, is there something that I'm missing?
If not head pressure, what is the biggest influence on amp draw?

 

[Acmanz28]

So would that mean the same amps on C-R-S??

 

[SoFlaDave]

It's more to do with the compression ratio, the difference between low and high side that effects amp draw. Typically in air conditioning applications the ratio is 3:1 for example 70 PSIG suction and 210 head pressure for R22. If the ratio increases to 4:1 or 70/280 the compressor is working much harder because of the difference.

 

[beenthere]

It's more to do with the compression ratio, the difference between low and high side that effects amp draw. Typically in air conditioning applications the ratio is 3:1 for example 70 PSIG suction and 210 head pressure for R22. If the ratio increases to 4:1 or 70/280 the compressor is working much harder because of the difference.

Yep, compression ratio.

70 to 239 is 3:1, and 60 to 224 is 3:1 compression ratio.

From the posted chart. 155 to 575 is 3.47:1, amps 11.05, and 118 to 575 is 4.44:1, amps 11.25 Same head pressure, but lower vapor pressure.

So would that mean the same amps on C-R-S??

No. But your question is becoming more in-depth then what is answered in the AOP forum.

 

[54regcab]

double post

 

[54regcab]

Yep, compression ratio..

From the posted chart. 155 to 575 is 3.47:1, amps 11.05, and 118 to 575 is 4.44:1, amps 11.25 Same head pressure, but lower vapor pressure.

A 28% increase in compression ratio is less than 2% increase in amps? Compression ratio makes more sense, but why does the chart show very little difference in amps across a wide range of suction pressures?

Using the same 155 suction pressure: Dropping the head pressure from 575 to 475 (18% decrease) drops the amps from 11.25 to 9.05 (19% decrease). Compression ratio goes from 3.47 to 3.06 (12% decrease).

 

[beenthere]

Instead of looking at it from decreasing the compression ratio. recalc it on the increase. 457 to 575 is a 21% increase, and gives you a 24% increase in amp draw, and a compression ratio increase of 13%.

Increase in amp draw will from system to system varying on coil size, and scroll size and RPM.

 

[54regcab]

Instead of looking at it from decreasing the compression ratio. recalc it on the increase. 457 to 575 is a 21% increase, and gives you a 24% increase in amp draw, and a compression ratio increase of 13%.

OK, I think I got it now:
A 28% increase in compression ratio by lowering suction pressure results in a 2% increase in amp draw.
A 21% increase in compression ratio by increasing head pressure results in a 24% increase in amp draw.

- - - Updated - - -

Increase in amp draw will from system to system varying on coil size, and scroll size and RPM.

Changing Scroll Size/RPM would mean a different compressor chart.

How would changing coil size affect the numbers on the chart? I can see using a different spots on the chart based on the different suction/pressures caused by different coil sizes, but not a change in amps for a specific head/suction pressure.

 

[beenthere]

Increase the size of either coil, and the amp draw will drop. As the compression ratio will drop.

 

[54regcab]

Check out the 100f/316psi line. Going from 36 to 155 on the suction side only makes a 0.1amp difference even though compression ratio changes from 8.78 to 2.04. Considering the compression ratio is over 4 times as much, I would have expected more than a measly 0.1amp difference. Is this particular compressor an oddball? Do piston compressors behave differently?

 

[54regcab]

It's more to do with the compression ratio, the difference between low and high side that effects amp draw. Typically in air conditioning applications the ratio is 3:1 for example 70 PSIG suction and 210 head pressure for R22. If the ratio increases to 4:1 or 70/280 the compressor is working much harder because of the difference.

What happens if you change the suction from 70 to 52 instead of changing the head from 210 to 280 (still a 4:1 ratio)?

 

[beenthere]

What happens if you change the suction from 70 to 52 instead of changing the head from 210 to 280 (still a 4:1 ratio)?

3.66:1

 

[54regcab]

Where is 3.66 coming from?
280/70 = 4?
210/52 = 4?

 

[garyed]

Where is 3.66 coming from?
280/70 = 4?
210/52 = 4?

Don't forget that you have to add 14.7 to your pressures to get psia.
280/70 becomes 294.7/84.7 = 3.48 for your compressor ratio

 

[54regcab]

Don't forget that you have to add 14.7 to your pressures to get psia.
280/70 becomes 294.7/84.7 = 3.48 for your compressor ratio

Makes sense since even at 14.7psi (Zero on gauges) still has refrigerant capable of being compressed.

Why is it that compression ratio increases by reducing suction pressures have very little effect on amp draw, while head pressure changes make a HUGE change in amp draw?