Re: DC pwr for AC
I run my Marine Air 5k btu off a honda 2000 generator. It runs fine off the genny since it draws only about 5 amps (don't forget the water pump load) but it requires close to 20 amps to start up. My genny can't do that on eco mode. I wanted to find some kind of capacitor or something to help with the start load and found this document. I haven't tried anything yet. Xantrex's site has a few faq about running AC off an inverter.
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Technical Note
PROwatt
512-0037-01-01 Rev 1
Operating Air Conditioners or
Refrigerator Compressors
Overview
We do not have specific model test data regarding compressors. However, it appears there is considerable
variation in start surge from model to model of air conditioner or fridge compressors. In our experience the
newer small rooftop-style air conditioners (5000–7000 BTU) used on RVs in some cases actually draw only 8
amps running (with moderate start surge), and can usually be operated by a PW1500. The older models may not
be as efficient, and may draw a much higher surge, causing even a PW1500 to trip into overload. The PW2500
inverter in most cases can operate air conditioning units up to 11 A/10,000 BTU. The DC wiring and battery
capacity must be optimal to provide the large start surge without starving the inverter of power during a surge.
A three-foot-high, bar-style fridge, which draws up to 2.5 A, in most cases can be run by a PROwatt 800. Full
height fridge rated from 2.5–5 A can usually be operated by the PW1500. The PW2500 would be more suitable
to operate compressors up to 5–8 A.
The compressor amp/watt rating does not tell the whole story. The compressor spec plate rating is usually the
max running draw. The start surge can range far higher; the rule of thumb is 5–10 times the spec plate rating. If
the compressor demands more surge than the inverter can provide, the inverter would go into overload and shut
down the AC output until the overload is removed, and the inverter reset (switch off, then on again). In a
borderline situation (i.e. inverter surge capacity is just short of compressor surge demand) the addition of an
inductor, or filter choke of 5 mHenry/10–30 amp rating may be sufficient to buffer the surge spikes and get the
compressor started. The inductor would be wired in series in the hot lead between inverter AC output and
load/compressor (see diagram below). The start surge typically lasts less than two seconds until the motor’s
magnetic field and motor impedance rises, which lowers the current demand from the inverter to a manageable
level for continuous running.
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1998 2700 SCR
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