Ethan said:
... like a fast NTC thermistor switch--but those things are pretty slow to warm up...AC might not be that patient.
If a thermister is the right resistance cold, right in that unknown margin between too low R shutting down the inverter, and too high R not starting the A/C compressor properly, things can only get better from there, as a thermister warms up and does better than a long wire with ZeroTC. Of course, this thermister would probably be the size of a hockey puck., and cost more than season tickets.
My humble advice, your next course in testing should be to see how wide that margin is, keep increasing the series resistance until the compressor fails to start. Probably best to fine tune this in August when it is 100F out, and 120F inside, where system pressures will be at the highest, for most possible head load on the compressor. I'll bet you have a wider range than you expect, A/Cs are designed to start and run on all sorts of weak power situations.
My gut feeling is that a pair of 1 ohm 50W resistors in parallel will be fine. Follow my logic through three stages of powering up the compressor:
1) Locked Rotor amps. Too much for the 4KW peak inverter, but adding a half ohm, puts it under the trip limit. This means initial surge is more than 33.33 amps. Adding a measly 0.36 ohms (plus connector resistance) in series, drops the current to less than the 33.33 amp shutdown threshold, so it can't be much more than that, lets assume 35 amps, just over the limit of what that inverter can put out.
35 A through 0.36 ohms is a 13.3 voltage drop from a start surge through the extension cord to dissipate 465 watts, for milliseconds. Or with an 0.5 resistor, a voltage drop of 17.5 (current will be less with a drop of 0.5 resistor vs 0.36 extension cord), recalculate with 0.5 ohms, is a 612 watt surge This tapers down over a few power cycles, to the "get turning and come up to speed" stage.
2) Come Up To Speed. About twice the normal running amps once it gets spinning, as the stator coils actually start to have inductance at 60hz, as the relative frequency between stator and the rotor decreases, less coupling into shorted secondary turns. This current tapers down until full speed is achieved. About 10A draw getting there. Resistors drop 5 volts, watts to dissipate is 50W.
3) Up To Speed. 600W draw, 2.5V drop, 25W dissipation. That's nothing.
If a power resistor can take 4X power for 5 seconds, I suspect it should take 6.2X power for 1/100 of that time. These would be good spec's to have.
No need to wind your own and risk smelling cat piss. Seriously, it is not all that it is cracked up to be.
I was in a farm supply place today, galvanized steel electric fence wire comes in 17 gauge, too thin for this.
You could get clever and mount these resistors inside the A/C itself, where there is some outside condenser airflow for cooling to the outside.
Gene
PS: a 2KW continuous , 4KW peak sine wave inverter would be ideal to run a small PA system off of a well-batteried truck, do you care to share the make and model #? I could use something like that, with that sort of "headroom", in the live PA business.
