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Author Topic: AC Current draw characteristics -- Any AC gurus?  (Read 3647 times)
Brian Diehl
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« on: June 07, 2006, 06:59:09 AM »

So, I did some wiring changes of my basement air conditioner so that I could run them off the inverter while going down the road.  I have the coleman mach II basement air with dual compressors and the heat pump function.  Anyway, I wanted to test how the inverter would work so I told the inverter to drop the 110v connection and pull power from the batteries.  I let the system run for a while and noticed some odd behavior that I'm wondering if anyone can explain.  When I first started the system on one compressor it was drawing about 1500 watts and then slowly over about 5 minutes it worked up to 2000 watts.  Then I turned on the second compressor the current draw only went up to 2500 watts and while inverting the current draw was stabalized out at about 2600 watts total.  The outside temperature was about 80 degrees. 

I'm curious why when using the first compressor the current draw kept increasing.  Turning on the second compressor had the same behaviour of lower current draw at first increasing as the system worked towards steady state.  I had always thought the greatest current draw would be during the initial system startup and not later on.  Can anyone point me to a "white paper" on how the physics of the AC system works that would explain the increasing current draw as the system reaches steady state?

BTW, just before I shut down my battery monitor showed the current draw right at 121 amps at 22.9v!
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DrivingMissLazy
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« Reply #1 on: June 07, 2006, 07:10:35 AM »

Brian, I seriously doubt if you would see the initial inrush current at startup. This only lasts for a few seconds at most.

It sounds like there is some kind of automatic compressor loading that is slowly increasing the load, which would be great if that is what it is. It is also possible that there is some kind of soft start system on the compressor.
Richard


So, I did some wiring changes of my basement air conditioner so that I could run them off the inverter while going down the road.  I have the coleman mach II basement air with dual compressors and the heat pump function.  Anyway, I wanted to test how the inverter would work so I told the inverter to drop the 110v connection and pull power from the batteries.  I let the system run for a while and noticed some odd behavior that I'm wondering if anyone can explain.  When I first started the system on one compressor it was drawing about 1500 watts and then slowly over about 5 minutes it worked up to 2000 watts.  Then I turned on the second compressor the current draw only went up to 2500 watts and while inverting the current draw was stabalized out at about 2600 watts total.  The outside temperature was about 80 degrees. 

I'm curious why when using the first compressor the current draw kept increasing.  Turning on the second compressor had the same behaviour of lower current draw at first increasing as the system worked towards steady state.  I had always thought the greatest current draw would be during the initial system startup and not later on.  Can anyone point me to a "white paper" on how the physics of the AC system works that would explain the increasing current draw as the system reaches steady state?

BTW, just before I shut down my battery monitor showed the current draw right at 121 amps at 22.9v!
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« Reply #2 on: June 07, 2006, 07:24:30 AM »

How are you determining power consumption? 

Does your inverter show output watts or are you calculating it?


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Craig Shepard
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« Reply #3 on: June 07, 2006, 07:50:02 AM »

Craig,
When on shore power the inverter shows power consumption in watts in 500w increments.  While inverting the inverter shows watts in kw down to the tenth.  That is where the 2.6kw figure came from.

Richard, you are correct, I couldn't see the initial inrush current on the battery monitor as it is just not fast enough to show it.  However, the inverter handled it just fine.   Do you think a soft start/ramp up would take over 5 minutes?  I suspet there is some other physics involved here that I'm not aware of.
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« Reply #4 on: June 07, 2006, 08:04:08 AM »

Does it do this when on shore cord power directly?
Richard

Craig,
When on shore power the inverter shows power consumption in watts in 500w increments.  While inverting the inverter shows watts in kw down to the tenth.  That is where the 2.6kw figure came from.

Richard, you are correct, I couldn't see the initial inrush current on the battery monitor as it is just not fast enough to show it.  However, the inverter handled it just fine.   Do you think a soft start/ramp up would take over 5 minutes?  I suspet there is some other physics involved here that I'm not aware of.

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« Reply #5 on: June 07, 2006, 08:04:26 AM »

Brian,

Just a thought.

I couldn't tell from your post if the units were already running on the 110v shore before you switched to inverter or cold started on the inverter.

Could this power usage curve be caused by the forces the compressor motor must over come as everthing heats up?

Cliff
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« Reply #6 on: June 07, 2006, 08:10:51 AM »

I initially ran the 1st compressor on shore power.  I watched the power usage while on shore power.  Then I wanted to run the second compressor, but don't have enough power on shower power and therefore switched over to just the inverter for the testing process.
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« Reply #7 on: June 07, 2006, 08:20:35 AM »

My thoughts are that as the pressure comes up in the system, the compressors will have to work harder against the system pressure until it stabilizes. But I'm certainly no expert on how A/C compressors work. Nick or others on here will know more about that.

Also, you can probably run the fans separate without the compressors to get an idea of their consumption. Subtract that to determine how much is actually compressor, and then you can compare the two compressors.

I'd also be checking both input volts/amps from your E-Meter and output volts/amps from your inverter to compare to it's power display. They all should be similar, with maybe 5% difference for inverter inefficiency.

BTW, was the fridge and all other loads turned off?

I've never tried this kind of test on mine. I suppose I could. My inverter shows line voltage and current draw. I have an E-Meter, too, for input volts/current.


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Craig Shepard
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« Reply #8 on: June 07, 2006, 08:22:38 AM »

I'm also suspicious that some of what you're seeing is the result of drawing the batteries down to lower voltage, but I can't figure out how to relate the two. I'll have to think about it for awhile.

Again, this may be all wet, too.
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Craig Shepard
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« Reply #9 on: June 07, 2006, 08:27:05 AM »

Can you read the amp draw on shore power and did it have the same characteristics as when being driven by the inverter? A/C's have a momentary surge to get them going that is less than a second then is in running mode.  What does happen is the head pressure on the A/C will build up, in direct relation to the outside temp and pull more amps as it has to pump more.  Another- are you able to read the inverter output voltage? If the voltage output on the inverter starts to lower during the operation, this can cause the amperage to go up-since volts times amps=watts.  If it starts at 120 then drops to 110, that can cause the amps to raise by about 1.5 amps.  Another-do you have sufficient air flow to the basement air?  If the exhaust is being sucked in by the intake, then you'd be super heating the A/C-another way of raising amperage as it runs.  Just thoughts.  Good Luck, TomC
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« Reply #10 on: June 07, 2006, 08:57:42 AM »

Another far out thought. Is it possible that you are not disconnected completely from the shore power?

 I know that some inverters are set up so that only the excess required above the capacity of the shore cord supply is drawn from the batteries.

If you ever find out the answer, please let us know.
Richard
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« Reply #11 on: June 07, 2006, 09:17:37 AM »

The Inverter was consistently putting out 120volts for the entire time it was on and while on shore power I didn't watch the voltage. 

Yes, all other AC loads were off.

Yes, I can test the air circulation fan only, but not the external fan as there are no controls for that.

My inverter does not have an AC amp display on shore power, only AC watts.  However the measured watt consumption could not be exactly compared because while on shore power the wattage is measured in increments of 500 on the display while in Inverter mode the wattage is measured down to the 100 watt. 

I was not worried about amp draw in the original question as I understand the relationship between amps, watts, and volts.  That is why I was asking about total power consumption in watts.  While running the test it was fairly windy outside, so I don't think the superheated air problem is a big issue for the test, but definitely something I need to do a better job of eliminating for future use. 

Craig, remember I watched this increase in power consumption while on shore power with the 1st compressor.  This eliminates the battery voltage issue. 

Richard, the Outback inverter allows for complete disconnect from shore power and unlike the trace units does not have the power "fill in" capabilities to deal with higher loads than allowed by the shore line.  I do wish I had this option on the inverter though.  I need to ping Outback again and ask them if they've added this feature yet.

I think the increasing head pressures are the reason for the increase in current draw, but I don't understand the physics of the closed system that cause the pressures to increase during operation.  This is probably one of those things that I'd need a text book to fully understand.  I'm hoping Nick B. can chime in on this thread with more ideas.
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« Reply #12 on: June 07, 2006, 09:40:36 AM »

Craig, remember I watched this increase in power consumption while on shore power with the 1st compressor.  This eliminates the battery voltage issue. 

I misunderstood. I thought you had disconnected completely before starting up the first compressor.

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« Reply #13 on: June 07, 2006, 10:20:06 AM »

If I remember correctly from High School Auto Mechanics class;

- the compressor sucks freon from the evaporator & raises the pressure (this work adds heat),
- the freon then goes to the condenser to dump the added heat to the air passing over it (this removal of heat causes the freon to condense in to a liquid form),
- the freon passes thru a small hole (expansion valve) where it vaporizes to a gas (this change from liquid to gas takes energy, so it gets cold)
- the cold freon gas passes thru an evaporator where it removes heat from the air passing over it.
- the freon then goes thru an accumulator / dryer where moisture is removed & excess freon is stored before getting sucked back into the compressor.

The expansion valve on some systems opens & closes depending on the evaporator temp. If closed, the compressor would dead head & power required would increrase. On my car, there is a pressure switch that turns the compressor clutch off or on depending on the system pressure.

OK Nick, How far off am I on this? What did I leave out?

Thanks
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« Reply #14 on: June 07, 2006, 11:56:19 AM »

Brian,

Tom C hit it on the head!

Your head preasure off the compressor is increasing due to improper discharge of the condencer air and or restriction of condencer intake air!
You stated that it was 80deg's outside while testing. That tells me that your A/C's were not overloading due to high ambiant outside temps, so
that leads me to belive that you are starving the condencer. One other thought, your battery bank could be dropping in voltage and resulting in
inverter overloading??

Hope this helps-
Nick-
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« Reply #15 on: 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.
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« Reply #16 on: 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-
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« Reply #17 on: 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.



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Craig Shepard
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« Reply #18 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
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« Reply #19 on: 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


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
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Ketchikan, Alaska
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« Reply #20 on: 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.

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Craig Shepard
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« Reply #21 on: 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?
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« Reply #22 on: 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.

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« Reply #23 on: 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
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Ketchikan, Alaska
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