AC Current draw characteristics -- Any AC gurus?

[Brian Diehl]

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!

[DrivingMissLazy]

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!

[gumpy]

How are you determining power consumption? 

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

[Brian Diehl]

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.

[DrivingMissLazy]

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.

[FloridaCliff]

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

[Brian Diehl]

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.

[gumpy]

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.

[gumpy]

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.

[TomC]

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

[DrivingMissLazy]

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

[Brian Diehl]

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.

[gumpy]

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.

[kyle4501]

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
kyle4501

[Nick Badame Refrig/ACC]

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-