AC Current draw characteristics -- Any AC gurus?

Brian Diehl · June 07, 2006, 12:41:46 PM

So, if I'm starving the condensor it could be because of a restricted intake or exhaust. I know my intake has plenty of air as it is open to the baggage bay and the baggage bay door was open. However, I do have the exhaust discharge partially blocked. I'll try rerouting the exhaust air and see if that changes the behavior of the compressors.

Nick Badame Refrig/ACC · June 07, 2006, 06:29:36 PM

Brian,

It would be best to totally seperate intake and exhaust air. It's very possible to regergitate and cause increased amp draw.

Mabe it's possible to draw air through the floor and exhaust it out the baggage door somehow?

Nick-

gumpy · June 07, 2006, 08:06:29 PM

FYI, I ran a similar test on mine tonight.

Unplugged shore, shut off all other loads. Then I kicked on the first compressor. Fan came on first. Drew about 4 amps.
Soon, the first compressor came on. I saw some instantaneous loads in the 15 amp range, and it immediately fell back to 10-11 amps (flashing between the two, so high 10, reaching 11 amps). I let it run that single compressor for 10 minutes. In that time, the current draw climbed to 13 amps in the first 5 minutes, and stayed pretty much steady for the next 5 minutes. 24.5 volts on the battery bank, and about 40-44 amps coming out of the battery bank.

I then turned on the second compressor. Current draw went to 20-21 amps, and over the next 5 minutes, climbed to about 23 amps draw and steadied there. My voltage dropped to about 24.2. Current out of the batteries was around 77 amps.

What I don't understand is why my e-meter shows 77 amp draw @ 24 volts, but my trace meter shows output of 23 amps @ 120 volts. The two values don't match up.

pvcces · June 07, 2006, 11:54:42 PM

Craig, I don't know exactly what kind of setup you have, but I have an odd symptom where the values don't match up. This is regarding a 12 volt Trace, RV2012.

With the inverter we recently purchased, there is a shunt built into it, so that the inverter can separate battery and house currents. This means that there is a shunt report and an inverter/charger report. The shunt report is always right on. The inverter draw from the batteries is really close. But the charger current is understated by around 1/3.

It took me a while to figure out that even though the inverter/charge current is a single report, it must get it's information in two different ways. The shunt report is from a single source. The charger current is just not being calculated correctly.

The other odd thing I ran into was comparing volt amps and watts using a Kill-a-Watt meter while running a floor buffer of around 1 hp. With no load on the buffer, I was showing over 1600 volt amps and 450 watts. The inverter was drawing 140 amps.

Then, I stood the buffer up on it's brush and the volt amps dropped to 1500 and the watts went up to around 1000 watts. The inverter draw increased to about 170 amps.

At first, I just thought that the current lag explained the readings, but then, I realized that the inverter draw with no load was awfully high. So, I checked around to see if something was getting hot, but the buffer and inverter were staying nice and cool.

We both know that wasted power turns into heat. Do you have any idea where I might have been getting a bum reading? I figure that that is the only reasonable explanation for those readings. However, I do intend to view all the readings with caution until I am sure of them.

For what it's worth.

Tom Caffrey
Suncatcher
Ketchikan, Alaska

DrivingMissLazy · June 08, 2006, 04:46:24 AM

Unloaded AC motor power factor is generally extremely poor. Maybe around 0.4 or so. At load, the PF should be around 0.9 or so. You really need a Power Factor meter to accurately measure this. The poor power factor is the current lagging the voltage by several degrees. What it appears is that something is reading kva instead of kw and without the proper meters, you can not rally tell much. The kva (poor pf) is not using more power (watts) but I do not think I am capable of explaining more in detail. It is where Clarke needs to chime in as a true engineer. LOL
Richard

When I get to the office I will try and find the formula for converting kva to watts. Just too old to remember all those things I used many years ago.

Well I did some checking and this really gets very complicated to try and explain. At least for me. But I will give it the old college try.

Suffice to say that when dealing with AC power, volts times amps equals volt-amps or va. To determine actual watts, you must multiply the va by the power factor. The answer is watts, and that is what generates heat. As I indicated earlier it takes a special meter to measure power factor.

For example with a motor running at 0.8 pf with 100 volts and 10 amps you would have 1000 va. Multiply the 1000 va by the 0.8 pf and you have only 800 watts of true power consumed. The rest of the current is actually circulating and doing no work so it is not generating any heat. Clear as mud?
Richard

Quote from: pvcces on June 07, 2006, 11:54:42 PM
Craig, I don't know exactly what kind of setup you have, but I have an odd symptom where the values don't match up. This is regarding a 12 volt Trace, RV2012.

With the inverter we recently purchased, there is a shunt built into it, so that the inverter can separate battery and house currents. This means that there is a shunt report and an inverter/charger report. The shunt report is always right on. The inverter draw from the batteries is really close. But the charger current is understated by around 1/3.

It took me a while to figure out that even though the inverter/charge current is a single report, it must get it's information in two different ways. The shunt report is from a single source. The charger current is just not being calculated correctly.

The other odd thing I ran into was comparing volt amps and watts using a Kill-a-Watt meter while running a floor buffer of around 1 hp. With no load on the buffer, I was showing over 1600 volt amps and 450 watts. The inverter was drawing 140 amps.

Then, I stood the buffer up on it's brush and the volt amps dropped to 1500 and the watts went up to around 1000 watts. The inverter draw increased to about 170 amps.

At first, I just thought that the current lag explained the readings, but then, I realized that the inverter draw with no load was awfully high. So, I checked around to see if something was getting hot, but the buffer and inverter were staying nice and cool.

We both know that wasted power turns into heat. Do you have any idea where I might have been getting a bum reading? I figure that that is the only reasonable explanation for those readings. However, I do intend to view all the readings with caution until I am sure of them.

For what it's worth.

Tom Caffrey
Suncatcher
Ketchikan, Alaska

gumpy · June 08, 2006, 05:53:05 AM

My setup is not real difficult. I have a 24v battery bank consisting of 8 Trojan T105s, a Trace SW4024 inverter, and a Bogart TriMetric monitor with a 500 mA shunt on the battery negative cable.

The Trace provides load amps and output voltage, plus battery voltage. The TriMetric provides battery voltage, and current through the shunt (@ 24 volts).

I just did a calculation, and it looks like my trimetric is actually thinking it's on a 36 volt battery (36 V * 77 A / 120 V = 23.1 A). I'll have to check the programming to see if it somehow got set to the wrong value, since I think that's one of the programming options.

Brian Diehl · June 08, 2006, 07:06:59 AM

Craig, the interesting thing out of your test is that your current draw experience matches mine. I know we have similar basement airs such that we are seeing the same behavior relatively. So, I guess I need to work on separating my exhaust such that there is no restriction and it is not influencing the intake air. Then I can rerun the test and see if the behavior repeats. Maybe no matter what we do this is how these coleman machII ACs work?

gumpy · June 08, 2006, 07:41:05 AM

Yeah, I was going to mention that my installation is not optimum as far as intake size, and intake vs. outlet proximity goes, as you are probably well aware, having seen my setup firsthand. I've been told I don't have enough inlet area to allow proper functioning (fortunately, the Coleman didn't hear that comment). I know it could probably be more efficient, but frankly, what little I've used it, it's done pretty well for me. Some of that may change as I finish my ductwork inside, but only time will tell.

I personally don't think you would see much difference in the performance if you ran it sitting on a bench in the garage, but I could be wrong. This is not an area in which I have any expertise.

pvcces · June 08, 2006, 10:30:15 AM

Richard, thanks for the reply. Your description of power factor agrees with mine.

We are using a special meter, the Kill-A-Watt. It measures power factor to within one percent. Unless it is lying.

My concern centered around where all the DC power was going with no load. It has to turn into heat, somewhere. I couldn't find where the heat was being produced.

That's why I said that I would be viewing the readings with caution.

Tom Caffrey
Suncatcher
Ketchikan, Alaska