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Author Topic: Air Conditioner amperage draw at low voltages  (Read 3867 times)

Mike Sokol

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Re: Air Conditioner amperage draw at low voltages
« Reply #40 on: December 22, 2018, 07:02:29 pm »

To sum in up, it's about watts Mike.  Watts in, less the heat dissipation or power loss (motor efficiency) = horsepower out.  To compensate for voltage drop, the current will increase in general terms - to maintain power output of the motor.  They are designed to run within an optimal range - too far out with high/low voltage or frequency and things go china syndrome.

Dometic has sent me one of their 15,000 BTU air conditioners for the experiment, so as soon as it gets a little warmer out I'm going to hook it up to a bit 30-amp variac and change the voltage while monitoring the amperage. But yes, my basic research suggests that a 10% drop in voltage will result in a 10% increase in amperage, and a 20% voltage drop will cause a 20% current increase, etc...
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Mike Sokol
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Steven A. White

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Re: Air Conditioner amperage draw at low voltages
« Reply #41 on: December 24, 2018, 12:27:46 am »

Going from memory here for my shop AC.  It's a window unit, seems like the plate said it will run on 208V or 240V and rating was 28,000 BTU on 208V & 28,500 BTU on 240V.  It's jammed through the wall and sealed around it with foam crammed into the voids so I can't read the spec label on the side or I would check it again.

Interesting stuff for sure.  When I wired my shop I did overkill on everything from the number of 20A 125V circuits for the plugs to the welder, AC, electric heat & compressor runs in terms of wire gauge.  Low volts is way bad for air compressors.
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Stephen Swaffer

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Re: Air Conditioner amperage draw at low voltages
« Reply #42 on: December 25, 2018, 05:42:07 pm »

The reason it is "way bad" for air compressors is the same reason it is bad for AC units- the main power draw of which is a compressor- just a different gas that is being compressed.

Motors typically draw 2-3 times their full load on start up- " locked rotor" is several times more than that.  With low voltage, the motor takes longer to start, drawing closer to " locked rotor" current for a longer time. The more current draw, the greater the voltage drop in the wiring- so a nasty snowball effect- especially if the low voltage is caused by undersize supply wiring to begin with.
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Steve Swaffer

Steven A. White

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Re: Air Conditioner amperage draw at low voltages
« Reply #43 on: December 26, 2018, 10:53:05 pm »

The reason it is "way bad" for air compressors is the same reason it is bad for AC units- the main power draw of which is a compressor- just a different gas that is being compressed.

Motors typically draw 2-3 times their full load on start up- " locked rotor" is several times more than that.  With low voltage, the motor takes longer to start, drawing closer to " locked rotor" current for a longer time. The more current draw, the greater the voltage drop in the wiring- so a nasty snowball effect- especially if the low voltage is caused by undersize supply wiring to begin with.
Bingo.
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