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Old 03-12-2019, 09:02 AM   #1
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Need Electric Engineers help.

I wish to get the skinny on Autotransformers. I'm just a dumb retired electrician of 50 years or so. 35 as a Master. I have been told over the years that an autotransformer DOES NOT use park electricity.

This from Hughes autotrans web site:
The Autoformer DOES NOT take power from the park. It does not affect the park or source voltage, or make electricity. What it DOES is change the voltage (amperage relationship), lowering the amperage requirement by raising the voltage. Since appliances run better on higher voltage with lower amperage, less overall power is used from the park, and better service is enjoyed from your RV.

My simple brain says: Raise the voltage, lower the amperage and in turn lower the KWH. Am I wrong in my thinking?

There seems to be a lot of naysayers on this site that say YES.

I don't need answers from DIY guys. For the first time in my electric career I'm looking for an engineers advice
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Old 03-12-2019, 09:40 AM   #2
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Without going into gory details it is simply a transformer and is trading amps for volts.



DOES NOT use park electricity. This is a confusing statement The device itself does not consume park power ( except for a small inefficiency) but passes slightly altered park power to your RV.

It does not affect the park or source voltage, or make electricity.
What it DOES is change the voltage (amperage relationship), lowering the amperage requirement by raising the voltage. This is a correct statement. A device in your RV ( ie microwave) may use 1000 watts. By increasing the voltage it lowers the current needed to generate 1000Watts. This is important as items like ACs do not like running on reduced voltages as they use higher current and heat up. Since the park is charged for electricity based on the current used, this has no impact on the park side of the circuit. This is the same principal as is used for power lines. By distributing power at 100,000V the amount of current running through the wires is relatively low. If the same amount of power was run through the lines at 1,000V the wires would likely melt.
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Old 03-12-2019, 09:43 AM   #3
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Cavie, while I am not truly an engineer, I have 50+ years working with electronics. Design, build, etc. As an electrician, you are familiar with P=I x E. The autoformer do use park power. Power is simply, watts. What they do is to take volts and amps in, and put volts and amps back out. Increase one, lowers the other, but P, or watts remain the same. (Assuming the Hughs AF is 100% efficient)

That said, if you have low voltage in, the unit takes it and changes it to higher voltage out. While the higher voltage out means the trailer will draw less current on that side of the AF. The higher voltage on the output does not come freely. The AF will draw higher current on the input side to compensate, thus reducing the current and voltage available to everything else on the input side. Power in watts, though, may not be affected as much.

As I said above, Watts may be pretty close to the same, but since the AF draws more current at less voltage in the input side, that current draw quite often leads to lower voltage elsewhere on the park grid, increasing the current draw for everyone else, thus further lowering the voltage.

While I am not familiar with the Hughs autoformer, that is how the AF’s I have used work. My question that you answered on another post, was questioning whether the Hugh’s AF had incorporated the same attributes as the P.I. Surge protector, which you answered to me. Thanks for that.

Hope this helps.
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Old 03-12-2019, 09:44 AM   #4
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And it looks like I got beat to the punchlines by Flybob!
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Old 03-12-2019, 10:05 AM   #5
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Quote:
Originally Posted by Flybob View Post
DOES NOT use park electricity.
Would the statement be clearer if it said:
Does not increase the amount of park electricity consumed?

Quote:
Originally Posted by Flybob View Post
It does not affect the park or source voltage, or make electricity.
Does this imply that it does not FURTHER LOWER the voltage available to other campers?

Inquiring minds need to know...
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Old 03-12-2019, 11:04 AM   #6
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There is a lot of misinformation about what an Autoformer does. It's pretty much as FlyBob said, with a few caveats.

Basically it increases the voltage to your RV (or whatever load it's feeding) in a low voltage supply situation, thereby lowering the output current required to produce the required power (Watts). The Hughes unit (and a few others) has some circuitry built in that regulates the output voltage to ~120 volts regardless of what the incoming power is (within reason). If the incoming power is at or above 120 volts the autoformer is disconnected and doesn't provide any boost.

The autoformer doesn't lower the voltage to other users, but in a park where the power is really bad, it will draw more current in order to maintain the proper output voltage, so it can draw down the voltage to other users. But, if the incoming power is that bad, the whole user base is going to have issues anyway whether the autoformer is there or not.
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Old 03-12-2019, 11:26 AM   #7
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SailorSam, it may lower the voltage a bit to others due to the increased current draw. Depends on the voltage and current of grid connected to.
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Old 03-12-2019, 11:40 AM   #8
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P=IxV
Power in watts
I in amps
V in Volts

so if something draws 10 amps at 100 volts it is using 1000 watts
then we raise the voltage to 120 volts still use 1000 watts I = P/V 1000/120 = 8.33 amps
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Old 03-12-2019, 12:10 PM   #9
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Quote:
Originally Posted by kcmusa View Post
P=IxV
Power in watts
I in amps
V in Volts

so if something draws 10 amps at 100 volts it is using 1000 watts
then we raise the voltage to 120 volts still use 1000 watts I = P/V 1000/120 = 8.33 amps
This is basically correct on the RV side of the transformer. On the park side of the transformer, it is essentially 100V @ 10A. These discussions are increasing due the fact that RVs are getting larger with a higher number of high wattage devices much faster than the facilities are improving their infrastructure. the number of ACs has increased from one to two to three and small or no microwaves have changed into larger convection oven combos. Many parks have wiring that was designed when a 30A RV was considered high demand. This is especially true for state and federal campgrounds.
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Old 03-12-2019, 12:31 PM   #10
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The auto transformer is just a multi tap transformer with selectable taps on the input and selectable taps on the output. It chooses which taps to use in an undervoltage or overvoltage condition. It of course does not change the power that is used for the most part.

As far as the autotransformer causing more current to be drawn from the park, it depends on the load type. Some loads will draw more current in an undervoltage condition and others will linearly decrease.

Assuming the transformer preserves the V A relationship at its input and output, the power meter at the park won't see a difference in their power usage at their main meter. Their main meter measures a power in watts as it has a way to use the volts and the current to measure the actual power.
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