autoformers for dummies (me)

pianotuna wrote:
100 volts X 12 amps = 1200 watts

Output from the autoformer

120 volts x 10 amps = 1200 watts

Your assumption here is basically correct except that the current from the pedestal has actually increased by almost 50 percent from 8.33 amps to 12 amps. and the power consumption has increased from about 833 watts to 1200 watts.

Basic Ohms law states that volts = current times resistance or current = volts divided by resistance. Ohms law has to do with DC voltage but generally remains true for AC circuits as well though there is small differences in the case of inductive and capacitive loads such as an air conditioner.

If your A/C is using 100 volts , then the amperage would be lower than if it was using 120 volts because the resistance of the load will basically stay the same. The amps used is dependent upon the volts applied to the device, even though it is an inductive load instead of a purely resistive one. So if your A/C would normally use 120 volts at 10 amps (1200 watts), the AC resistance of the unit is 12 ohms. If you only apply 100 volts to that 12 ohm device, it would be pulling 8.33 amps (100 volts divided by 12 ohms) not 10 (120 volts divided by 12 ohms).

So assuming pure efficiency and no loss across the autoformer. If you have the A/C running pulling 8.33 amps at 100 volts (100 volts divided by 12 ohms), you are also pulling 8.33 amps from the power pole (833 watts in and out of the autoformer). If your autoformer increases the output voltage to 120 volts on the output, the A/C will now pull the 10 amps it likes and use 1200 watts of power. Since the power pole can only supply 100 volts that means it has to pull 12 amps to supply the 1200 watts to the output, increasing the current from the pole by 3.67 amps.

I wouldn't call that stealing power, just trying to get what you are paying for. However, the increased current draw of the autoformer to achieve the 120 volts just exacerbates the power problem of the campground. The more current drawn across the distribution system, the lower the voltage for everyone on that system, the more the autoformer has to increase input current to achieve output voltage. Since the system can only supply so much power, if everyone was using an autoformer, it would most likely blow the main distribution breakers due to the perpetually increasing current draw.

So to restate your assumptions:

Output needs 120 volts x 10 amps = 1200 watts
Input to autoformer needs 100 volts times 12 amps = 1200 watts
Increase in input current is = 12 amps - 8.33 amps = 3.67 amps