Autoformer test a "back yard" report
Hi all, The equipment: Sola Basic 4000 autoformer PF 0.98 Magnum hybrid 3012 inverter PF corrected 1200 watt water heater 325 watt absorption fridge 13500 BTU Dometic ducted roof air...
Forum Discussion
Inrush current (LRA) for RV AC units is approx. 5- 6 times running current (FLA) as can be seen from this Coleman power saver model data for example which shows 58.4 LRA and 11.2 FLA. These figures will be at the nameplate voltage of 115. The figures show running watts of 1190 @ 115V and 1440 @ 103.5V. Power factor is 0.92 @ 115V but they don't give you the FLA @ 103.5V to be able to calc. the PF.
Power in minus autoformer losses = power out per ohms law. If voltage goes down on the output, current has to go up on the input and then the increased current on the input creates further voltage drop. AC units also draw more current as voltage goes down. For a momentary inrush current of nearly 60 amps (for the Coleman Mach 3 13.5 btuh above), who knows what the autoformer is doing and what the voltage & current are like at the AC terminals? Then add the hybrid inverter and what is it doing on the AC startup? A 'scope is needed to know exactly what is happening. As noted, a hard-start capacitor *might* help for the AC unit but who knows. I wonder what an EMS would do if also added into the equation on the AC startup? I've never been able to find a graph of current versus voltage as it drops for an RV AC unit but the Coleman info. above shows the current to be 13.9 amps @ 103.5V which isn't really much higher than the FLA @ 115V and I always thought it was more.
RV AC unit manufacturers spec a min. 20 amp breaker to ensure they will start and a min. of #12 wire to reduce voltage drop on startup and running. Some may find that they can get their AC to start and run fine on a 15 amp circuit, but if voltage drops enough and often enough, over time can damage an AC and cause premature failure.
I wouldn't say 1887 watts is running a 15 amp breaker hard (if at 120 volts). A breaker can run 100 percent of it's rating continuously without tripping since breakers have an inverse time-current characteristic as can be seen in the standard NEMA graph on this web page.. At 10 percent over a 15 amp breaker rating at 120 volts (1980 watts), it could hold for at least 10 min. before the thermal element trips it. (It's the brief momentary current on a motor startup that can cause the "instantaneous" magnetic element to trip.)
Power in minus autoformer losses = power out per ohms law. If voltage goes down on the output, current has to go up on the input and then the increased current on the input creates further voltage drop. AC units also draw more current as voltage goes down. For a momentary inrush current of nearly 60 amps (for the Coleman Mach 3 13.5 btuh above), who knows what the autoformer is doing and what the voltage & current are like at the AC terminals? Then add the hybrid inverter and what is it doing on the AC startup? A 'scope is needed to know exactly what is happening. As noted, a hard-start capacitor *might* help for the AC unit but who knows. I wonder what an EMS would do if also added into the equation on the AC startup? I've never been able to find a graph of current versus voltage as it drops for an RV AC unit but the Coleman info. above shows the current to be 13.9 amps @ 103.5V which isn't really much higher than the FLA @ 115V and I always thought it was more.
RV AC unit manufacturers spec a min. 20 amp breaker to ensure they will start and a min. of #12 wire to reduce voltage drop on startup and running. Some may find that they can get their AC to start and run fine on a 15 amp circuit, but if voltage drops enough and often enough, over time can damage an AC and cause premature failure.
I wouldn't say 1887 watts is running a 15 amp breaker hard (if at 120 volts). A breaker can run 100 percent of it's rating continuously without tripping since breakers have an inverse time-current characteristic as can be seen in the standard NEMA graph on this web page.. At 10 percent over a 15 amp breaker rating at 120 volts (1980 watts), it could hold for at least 10 min. before the thermal element trips it. (It's the brief momentary current on a motor startup that can cause the "instantaneous" magnetic element to trip.)
