30 amp variac instead of an autoformer?

pianotuna wrote:
Hi all,

Just to make life more interesting they also make a 5000 watt model.

Simran AR5000

Yes that would do it. I did not read the specs, but am guessing that the output voltage is automatic? But restricted to only 110 volts, while I prefer 120 volts for my motors.

I plugged in my refrigerator to the power post directly, as the refrigerator will work fine on anything from 90 - 119 volts, but hates boosted voltage that will increase amperage through the electric heat element, and thus possibly overheat the heating element and can lead to cracked tubing on the very expensive refrigerator. .

But my air conditioner likes 120+ voltage. While I will run it at 105 volts, I know how to replace that compressor without buying a whole new unit. At 110 volts, I will get out my 12 volt voltage booster. I bought a 40 amp rated 12 volt coil buck/boost transformer from Grainger.com and wired it to boost the incoming voltage by 12 volts.

So 120 in = 132 volts output. Fine in my case, as the TV really did not care, and when I turned on the A/C compressor, then the voltage dipped to 118 volts due to the 300 foot distance from the ranch house I was parked near for 4 years!

No you do not need to monitor the variac, just check the output voltage every hour or so. If it gets to 128 - 135 volts while the compressor cycles off on a cool day, that is OK for my application. (I ran 132 volts into my RV for all summer months for 4 years, and the TV never complained.) I did not run my inverter / charger at all during summer, I have solar charger 415 watts.

If the output from the variac is over say 125 volts, and you wish to reduce it a bit, most say to shut off the load, then move the control knob, and then turn on the load. I suppose if there is a 50 watt light bulb, and 0.5 amp load, then you can turn the dial under load, but I would not! If under 110 volts while under load, stop it, turn the dial, leave the compressor off for at least 5 minutes, and then turn it back on.

AS for input amperage. It is very easy to calculate and get away with 'consuming' 35 amps into the transformer and have 30 amps to the RV cord and on to the 30 amp main panel in the cool RV. Just use a 50 to 30 amp reduction adapter, and plug into the much larger 50 amp plug - more brass in the plug means less voltage loss across the terminals.

So here is a extreme example of my booster. It is actually 500 watts and has a pair of 40 amp coils at 12 volts each, isolated from the input voltage. I have a 3 way switch, so I can put 1 or both 12 volt coils inline with the output 30 amp wire.

While in Bowling Green KY, at Big Bend RV park and race track, I was in the pits, hooked up to a 15 amp receptacle, hooked up to #10 wire going to overhead #2 wires, with no fuses in site. My guess is someplace there would have been 150 amp (or 200?) fuses protecting the overhead wires. I had my 50' long #14 cord plugged into the 15 amp receptacle. Then a 25' long #12 cord, then a 15 - 30 amp adapter, and my 25' long #10 cord. Then the voltage booster, then the 25' long factory cord, and 10 more feet of #10 into the electrical panel. With the A/C running I was getting about 112 volts on 12 volt boost. AS others turned on their A/C units, the voltage dropped to only about 105, so I shut everything off, unplugged, switched to the +24 volt setting, and plugged back in. I now had 115 volts while the A/C was on. My guess was the input voltage was around 90? at the end of all those extension cords. Who knows how many amps, but only my A/C was running, and perhaps a 1 amp tv. (remember solar kept the battery full).

Also living 300+ feet from the main house would put a strain on the A/C if I did not boost it. I would see 132 volts on my wall mounted digital voltmeter all the time when the A/C is off. It does not bother me. . .

Yes I would use the 30 amp variac. Hopefully with 100 volts input it will put out a little over 113 volts, and that will be enough. This 10% boost means that input amperage will be about 11% more than the output amperage. So plan accordingly, and have a 50 to 30 amp reducer adapter handy.

No I do not worry about 130 volts going into my TV set, it never bothered it in 4 years at the ranch. The TV also works great when there was only 95 volts to it as well!

Good luck,

Fred.