Thursday, Jan 09, 2014 at 19:13
The problem is a factor called the locked rotor current.
This is the maximum starting current that the motor can draw under load conditions and is normally around 6 times the listed running current shown on the name plate.
So, at 5.4 amps running, (some of that will be the fan/s, which will normally start before the compressor on cooling, and after it on heating) you can expect around 33 amps starting current, and this is on the 240v side, a little less for an inverter type AC.
So your instantaneous power demand at startup on the output of the inverter is about 8 kVa (close enough to 6500 watts at 0.8 power factor).
On the input side, this equates to somewhere around 550 amps.
Now, whilst this is only for a short time on initial startup, and could possibly be handled by a 2000w continuous rating inverter with a large connected input capacity, or a decent generator, once the AC has been running and the compressor turned off on thermostat, the next start will be under load for longer due to compressor head pressures, and so is likely to trip the overloads in the inverter or generator.
Due to the high current draw, severe voltage depression on the 12v side is very likely with a small battery capacity such as yours, and this is likely to cause similar undervoltage on the output side, which will cause the motor to draw its maximum amps and yet still be underpowered for startup.
There is a very real chance of over-driving the inverter electronics with subsequent failure.
Even without the above constraints, your 5.4 amps on the 240v side equates to around 110 amps on the 12v side.
Allowing for Mr Peukert, this will give you around 75 minutes running time till your batteries are at 50% Soc.
You would also be running your 1500w inverter almost flat out, which, although rated continuous, they do not like to do.
Locked rotor current is normally listed on the motor name plate as either a figure in amps or kVa, or a letter corresponding to a fixed list of values.
Your inverter would likely handle the hot water or the 3 way fridge more readily as there are no transient events.
You then have to recharge those batteries, and if doing so with solar, the catch is that the hot day (when you are most likely to use the air conditioner) is less conducive to good solar input than a cooler day.
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