By Mike Sokol
My Hughes Autoformer testing has begun. Here are details on how much additional current an air conditioner draws as the voltage goes below 100 volts. Hint: Everything You Know Is Wrong!
Since this is a rather complicated answer to what appears to be a simple question about Hughes Autoformers (and autotransformers in general), the results of my study will be presented in pieces over the next several weeks as I gather more information that’s been peer reviewed.
Note that while Hughes has sent me a demo unit at my request, they neither offered nor have I asked for any compensation for this study. That way it can be 100% unbiased. Hughes has also agreed to accept the results of my study, which will be sent to the NEC board for review.
Here’s Part 1, where I show how much extra current an air conditioner draws as the voltage is reduced.
But before I show you the data I’ve gathered so far we all need an understanding of the theory of operation. So here’s your primer on the how this is supposed to work.
While resistive loads behave predictably as the voltage goes up and down, air conditioners (and all inductive motors) are not that simple. To understand what’s happening we need a quick look at an ohm’s and watt’s law chart. For a resistive load like an electric water heater element, there’s a predicable relationship between voltage and power.
Since watts of power is equal to the voltage squared divided by resistance, as the voltage drops so will the power. So an electric heater element rated for, say, 1,200 watts at 120 volts will be reduced to 1/4 power at 1/2 voltage. So at 60 volts it will only produce 300 watts (1/4) of power. There’s nothing to cause it to try to draw more amperage as the voltage goes down. We’ll revisit this concept below.
Not so with inductive motors. You’re probably aware that they have something called a start winding and capacitor, along with a rating for LRA (Locked Rotor Amperage). What this means is that when the motor starts up it tries to draw a lot more amperage (maybe 400% to 800%) of running amperage, which we call Inrush Current. The reduction in amperage as the motor comes up to speed is due to something called Back EMF (Back Electromotive Force).
Basically, the motor itself acts like its own generator that’s 180 degrees out of phase with the incoming current. That reduces the amperage draw when the motor is up to speed with no load. When you load the motor, the slip angle increases, which draws more current. Reduce the voltage to below its design rating and it draws even more current.
So air conditioner (and refrigerator) compressor motors act differently than our basic water heating element (which is just a big resistor). This is what leads to the infamous extra current (and overheating) of an air conditioner’s compressor as voltage is reduced. The question is just how much and how fast does the current increase as the campground voltage decreases?
Enter the Autoformer
The Hughes Autoformer is basically an automatic low-voltage/high-current transformer connected in autotransformer mode via a relay and sensing circuit. So when the campground voltage gets below the set threshold, it performs a 10% voltage boost.
But what’s the cost?
Now, this 10% voltage boost comes at a cost of a 10% increase in current from the campground pedestal. But the real question should be: Is the extra 10% voltage provided by the Hughes Autoformer enough to offset the increased current draw from the pedestal due to the air conditioner’s extra current draw from running it at a low voltage? Hughes makes the claim that it does indeed reduce the air conditioner current enough to make up for its own 10% input amperage increase. That’s the subject of this entire test.
Well, the electric element in your water heater will indeed draw more current from the pedestal when the Hughes Autoformer kicks into boost mode. That’s a given, which follows Ohm’s and Watt’s laws exactly. It’s basically just a big resistor that responds to voltage changes without any crazy Back EMF generator action.
And you can’t damage a water heater element because there are no moving parts. As I noted in my explanation above, even if you drop the incoming voltage to 60 volts, it will happily keep producing 1/4 of it’s rated wattage forever.
However, there are some weird power factor effects from PWM (Pulse Width Modulated) power supplies in RV converters and battery chargers which are largely self-regulating as the campground voltage goes up and down. when the incoming AC voltage is higher the Pulse Width Modulator produces a pulse with less duty cycle. But when the AC voltage is lower, it automatically increases the duty cycle of the current pulse. That’s why PWM power supplies will tend to draw just enough extra current to make up for any reduced AC voltage. And they don’t need an Autoformer in the circuit at all to output the correct voltage. But they are indeed part of the campground current balance equation.
The big picture
So what does this mix of load types (resistive, inductive and PWM) mean to campsite amperage with or without voltage correction from the Hughes Autoformer? To know for sure, I’m building a test system one piece at a time and adding them together to see how the entire system works. Maybe Hughes is correct and their Autoformer is beneficial in reducing overall current in campgrounds with low voltage due to increased air conditioner current. And maybe the NEC is correct that any autotransformer (or Autoformer) creates an extra amperage load on the campground’s electrical system, increasing the stress on an already overstressed electrical grid. Who’s right and who’s wrong? I honestly don’t know for sure yet.
Here are the numbers….
Here are the numbers I’ve found so far showing how much extra current an RV air conditioner draws as the voltage is reduced. Next week I’ll put the Autoformer in the circuit and do a comparative voltage sweep. That’s going to set the direction of the next experiment where I add in a PWM (Pulse Width Modulated) battery charger to see how it reacts with and without the Hughes Autoformer connected.
- 3kVA (3,000 Watt) VARIAC
- Dometic Penguin II 15kBTU air conditioner
- SoftStartRV™ starter
- Southwire 14090T Tru-RMS digital voltmeter
- Southwire 25015T Tru-RMS clamp ammeter
These are the amperage measurements with only the fan running at the following voltage steps:
- 120 volts = 3.78 amps
- 115 volts = 3.71 amps
- 110 volts = 3.84 amps
- 105 volts = 3.97 amps
- 100 volts = 4.12 amps
- 95 volts = 4.22 amps
- 90 volts = 4.53 amps
This works just as I predicted, with a 10% increase in amperage when the voltage was reduced from 120 volts down to 100 volts. But this next set of measurements blew my mind. These are volt/amp readings with the compressor running. Note that this is the total combined current of the air conditioner fan and compressor, not just the compressor amperage.
Here’s the amperage measurements with the compressor running at the following voltage steps:
- 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.
Note that the compressor would drop out and refuse to run when I sent the air conditioner voltage down to 95 volts. Now that’s understandable, as Dometic may have some sort of low-voltage shutoff to protect itself.
Am I right or am I wrong?
So did I measure this incorrectly? I don’t think I made any mistakes, but I’ll double-check my work. Heck, I’m gonna triple-check my work with entirely different meters this week. I used high-end Southwire meters for this, but I have access to some top-shelf Fluke meters. Time to step up my game.
Is there some magic infused by the SoftStartRV™ controller? Definitely Not! Maybe it will help with starting the compressor at lower voltages, but it’s out of the circuit once the compressor is running. So there’s no magic there…
Has Dometic installed some sort of automatic current reduction circuit in the compressor? Nothing in their literature hints at that, but I’ve already sent these numbers to my engineering contact at Dometic for peer review.
Are all the text books wrong about induction motors having increased current draw at low voltage? I’m pretty sure not, since this engineering knowledge goes back 100 years; but perhaps it’s not as much of an effect as we’ve all assumed. Or maybe there’s another variable at play due to this being a compressor load. I just don’t know.
Much more on this drama next week, as I create alternate tests and try to qualify what’s happening. I LOVE it when measurements are unexpected. That’s true science.
So hold your breath until next week. But in the meantime, let’s play safe 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 Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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