Monday, September 25, 2023


RVelectricity – Hughes Autoformer testing – Part 2

By Mike Sokol

Dear Readers,
If you subscribe to my monthly RVelectricity Newsletter you’ve already read the first part of my Hughes Autoformer test. If you haven’t seen it, please read Part 1 HERE. For you casual observers this information may not sink in right away. But for any readers who have an electrical background this is going to read like a mystery novel.

Disclaimer: Hughes has not offered to pay me for this test, and I neither asked for nor received any compensation. But Hughes did send me a 30-amp version of the Autoformer for testing with the understanding that I have 100% control over all tests and publication of my data. I have offered to share my results with them after the testing is done. So you can be assured that whatever data I gather and conclusions I come to are unbiased. In short, I don’t have a horse in this race. It’s just an interesting engineering question to answer.

What’s goin’ on?

Well, here’s the reason why I’ve set up this test in the first place. Hughes has a product they call an Autoformer, which is basically a low-voltage/high-current transformer (10 volts or so at 30 or 50 amperes) that’s connected in buck/boost mode with a relay to be able to boost low voltage from a campground pedestal up to a higher level.

Why is this needed?

The theory is that since air conditioner compressors draw more current as the AC voltage decreases, if the campground pedestal gets down to 100 volts or so, then you’ll burn up the compressor in your expensive rooftop air conditioner. The Hughes Autoformer product boosts any low voltage from the campground pedestal by 10% or so, which they claim keeps air conditioners cool and safe.

What’s the controversy?

Well, any transformer that can boost the voltage does it with an equal increase in amperage draw from the power source. So boosting the voltage going to a camper by 10% would also increase the current draw on the pedestal (and the campground) by a 10% increase in amperage. That’s exactly how it works for resistive loads like your water heater’s electric element.

This basic equation has caused the NEC to ban booster transformers of any kind in campgrounds. Their theory is that a voltage boosting transformer uses additional power that can further stress an already overloaded campground electrical system. Yes, as of the 2020 NEC, Hughes Autoformers (and any other voltage boosting transformers) are a code violation in U.S. campgrounds.

What is Hughes’ theory?

However, rotating inductive loads like air conditioner compressor motors have a more complex volt/amp relationship dependent on motor load, Locked Rotor Amps (LRA), flux field slip angle, etc.

So the position from Hughes is that their Autoformer product will reduce the overall load on the campground because their product increases the efficiency by boosting the voltage going to the air conditioner, which in turns draws less current. They say it may actually decrease the overall current needed from the campground even adding in the 10% extra amperage loss caused by the transformer voltage boosting action.

Why am I performing this first test?

Well, first to know what the Hughes product does I needed to get some baseline data on exactly how air conditioners operate at low voltage. So I did the simplest test possible, without their Autoformer in the circuit.

However, there’s one unaccounted variable in this first proof of concept test. Since I was just finishing up a round of experiments with a SoftStartRV™ still connected to my Dometic Penguin II air conditioner, that’s what I plugged into my big 3,000-watt VARIAC® for the dry run.

Here are the amperage measurements with the compressor running at the following voltage steps without the Autoformer, but with a SoftStartRV connected. Note that the air temp for the air conditioner was in the high 60s at the time of this first test. 

  • 120 volts = 12.06 amps
  • 115 volts = 11.15 amps
  • 110 volts = 10.86 amps
  • 105 volts = 10.90 amps
  • 100 volts = 10.93 amps
  • 95 volts = compressor shut down

What the heck?!!! Even though all my EE books and urban myths say that air conditioner compressors will draw more amperage as the voltage goes down to 100 volts, that’s the opposite of what I’m seeing with this first test. While it’s drawing just over 12 amperes at 120 volts, when I run the VARIAC down to 100 volts, the current DECREASES to 10.93 amperes instead of INCREASING! That’s a 10% DECREASE in current when the voltage is reduced from 120 volts down to 100 volts.

Everything you know is wrong…

I published this first round of data just to gauge interest with my readers, and boy did it generate a lot of discussion. If my data was correct, then there’s some other mechanism allowing the current to decrease with a lower voltage rather than increase like all the textbooks say. The only possible explanation would be the SoftStartRV behaving in a way that was unanticipated by its designer.

Now, it’s also possible there was a mistake in my measurement protocol, or perhaps the Dometic Penguin II has some sort of additional circuitry that allows the current to reduce gracefully rather than increase and burn up the compressor, or maybe there’s something else going on that I don’t understand. That’s what I”m going to figure out.

Second verse, with a chord change

So just yesterday I reconnected this test setup, but removed the SoftStartRV controller and reconnected the stock starting capacitor for the Dometic air conditioner. And you can see the pictures below for the current readings at 5 volt intervals. Finally, the air conditioner amperage was behaving exactly as predicted by every engineering textbook and theory that I’ve read over the last 50 years.

As you can see from this picture sequence, the amperage is now increasing from 14.21 amps to 18.62 amps as I decreased the voltage using a VARIAC from 120.8 volts down to 85.8 volts, just  as predicted by induction motor theory. Oh, when I tried to get down to 80 volts, the current shot up to 35 amps and the voltage dragged down to 76 volts right before the 25-amp fuse in the VARIAC blew with a big pop. Yes, I do blow a lot of fuses…

What does this mean?

Heck if I know. Is this from some Power Factor change I’m not metering for? Does the SoftStartRV modify the run current as well as the starting current? Did I just have a senior moment and write down the numbers incorrectly for Test #1? Or is there some other effect that can be exploited to help protect air conditioners in low voltage situations? And just where does the Hughes Autoformer fit into all of this? Stand by for my next experiment when I’m going to reconnect the SoftStartRV controller and repeat the exact same reduced voltage sequence.

To Infinity, and Beyond…

Now that my lab is fully functional I can do this type of experiment as well as a lot more important (and sometimes dangerous) ones that can help RV owners as well as the RV industry in general. I just don’t know what I’m going to find, which is the best type of science you can perform. I’m not trying to prove a theory, I’m trying to understand it and look for additional variables.

See you with more results next week. In the meantime, let’s play (and experiment) safely out there….

Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at For more info on Mike’s qualifications as an electrical expert, click here.

For information on how to support RVelectricity and No~Shock~Zone articles, seminars and videos, please click the I Like Mike Campaign.



    • Another test you could do: Since a starting capacitor is used (supposed) during the starting phase, then removed during the run phase, is to the remove the start capacitor from the unit (I have had several A/C that did not have a start capacitor, only a run capacitor). That would provide information if a start capacitor interacts during running. Alternately, adding a switch to remove the start capacitor (or soft starter) from the circuit from the running phase.

  1. Google Translate from the ISB Sola Basic website in Mexico. I have more if interested.

    Is a device consisting of power transformer and an electronic control circuit.
    All the equipment you have in your electrical system, even as low as 87 volts and from a no load operation up to 60 amps for Mod 8000 and 30amp for the 4000
    4000VA @ 120VAC        = 33.4 AMPS

    8000VA @ 120VAC        = 66.8 AMPS

  2. I see no way to contact you directly so will put some very basic stuff here. We have been traveling in Mexico for many years – winters. Perhaps 10 years ago I bought a voltage regulator in Mexico for about $250 US. In Mexico the standard voltage is 127. There are frequent surges. So we wanted to not only have a way to bring up voltage but to lower voltage. I have only seen one of these in the US on ebay which was way overpriced. Test?

    ISB Voltage Regulators  

    Remember, whatever the input line voltage, you will have a better output voltage corrector, for example:
    IN     = OUT
    85    =  103
    90    =  109
    95    = 115
    100  = 121
    110   = 121
    120  = 120
    135   = 121
    140  = 125
    145  = 130
    150  = 134
    155  = 139

    • I’m wondering if this is some sort of ferroresonant transformer. If so, it’s extremely wasteful of energy since it uses nearly a constant amount of input power no matter how much output power it needs to provide. It works by over-saturating the transformer’s iron core when there’s a higher voltage than expected. This lowers the higher incoming voltage by distorting the waveform from its original sine wave. Something like this would never be UL listed for portable/mobile use, nor would it be allowed for campground use by the NEC. But Mexico has very lax electrical codes, as I’m sure you’ve witnessed over the years. I’ll try to get more info on this product, just for grins.

      • 40 years ago we used SOLA ferro-resonant transformers in Puerto Rico and the USVI to power electronic test systems. The mains power fluctuated terribly and it was the only way to have a barely constant voltage for the relatively sophisticated test consoles we used for our products. When I brought the first computer controlled test rack down in 1982, I ended up having to use an on-line UPS to keep the switching power supplies in the IBM PC’s and HP desktops we used to run the tests from blowing up from the terrible mains power! Good stuff you are doing here, keep it up!

  3. Mike, Great article. Although I have little electrical experience I do understand some of what you are saying. In addition I have installed a Soft start RV on my main AC unit and with these results I will be installing one on my 2nd AC unit. I knew the softstarts helped with starting and now appears to safe guard it from low voltage damage. Eagerly waiting for the next article.

  4. Wow.. I am fascinated with the first test results.. completely unexpected. Can’t wait to see your next round!
    I have had an Hughs Autoformer installed in my RV for the past 6 years.. so I have a vested interest in your findings!
    Thank you for creating and sharing all this.. I feel like I’m back in school all over again!
    BTW, I too, have a past in live performance and recording studio Sound Engineering.. I sure hope to meet up with you someday!

  5. I am highly interested in the final results of this series of tests.
    As you mentioned, the initial test was counter intuitive.
    Even with the soft-start in the picture, it is indeed an interesting result.
    One additional thought – is there any long term consequence to the AC unit operating at a lower voltage with the soft-start in the picture?
    As someone once said – verrry interesting!

  6. Simply outstanding. Thanks again Mike.

    I am unsure what I am seeing with my personal testing though. After installing my SoftStart, I’m seeing lower overall wattage draw from our low profile Coleman Mach 8 A/C. 1055watts, 11.43amps. The ambient air temperature was 65degrees. I set the thermostat down to 38degrees, just to force it to blow cold air. It did. But how could I possibly only use 1055watts, on a 1500watt to 1900watt Air Conditioner?

    My testing tools are the Kill-a-Watt meter plugged into either shore power. (I validated that the Kill-a-Watt meter with a 60 watt light fixture and it appears to be pretty accurate)

    My future test will include a clamp Amp meter with CarGenerator as well as the Kill-a-Watt tester.

    • I’ll keep adding on one more layer to this experiment next week, and report on my findings. Almost as fun as calibrating missile guidance systems, which is what I did in the ‘80s.

  7. How about using an IR thermometer and measuring the temperature of the compressor casing as part of your tests? You may need several minutes of run time at each voltage for any temp change to stabilize.

    I’m wondering if the motor efficiency is also decreasing (as it should) and that is generating excessive heat (in comparison to the E * I number). Excessive heat could contribute to air conditioner damage in addition to decreased A/C efficiency if the compressor is heating the coolant.

    In other words, does lower voltage cause some of the power draw to be converted to heat instead of mechanical efficiency?

    You did bring up a good non-obvious point. While everyone focuses on inductive loads other equipment is negatively affected by low voltage, just maybe not damaged. Longer water heating times, longer cooking times, cooler coffee (!), etc.

    • I not only have an IR thermometer and a thermocouple, I also have an infrared FLIR camera that takes pictures showing the temperature in false colors, just like weather maps. Stand by for more science…

  8. Did you contact the SoftStart designer and get his take on what you saw? They know exactly is in their circuit. On my brand new Mach3’s, I saw about 1 amp less running current after installation of the soft start. Mine had no starting capacitor.


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