Need Electric Engineers help.

[cavie]

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

[Flybob]

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.

[Paulie1138]

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.

[Paulie1138]

And it looks like I got beat to the punchlines by Flybob!

[SailorSam20500]

Originally Posted by Flybob

DOES NOT use park electricity.

Would the statement be clearer if it said:
Does not increase the amount of park electricity consumed?

Originally Posted by Flybob

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...

[Bama Rambler]

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.

[Paulie1138]

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.

[kcmusa]

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

[Flybob]

Originally Posted by kcmusa

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.

[babock]

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.

[BriaBeck]

But what about efficiency losses in the auto(trans)former?


TNSTAAFL. Assuming the autoformer is 90% efficient... would the park not see 10% more wattage for the same camper using the autoformer?

[Wilco]

I only take exception with the following statement from Hughes autotrans; "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."

The overall power used from the park will be equal or slightly higher.

1000 watts is still 1000 watts no matter how much you boost the voltage and transformer inefficiency will make the draw slightly higher than 1000 watts.

[babock]

Originally Posted by BriaBeck

But what about efficiency losses in the auto(trans)former?


TNSTAAFL. Assuming the autoformer is 90% efficient... would the park not see 10% more wattage for the same camper using the autoformer?

You don't understand transformers. Most are likely 98% efficient and in this case it's probably higher since the difference in the secondary and primary windings is 10%. If it was only 90% efficient and you had a 30A load going through it at 120V, the thing would be burning hot.

[BriaBeck]

Originally Posted by babock

You don't understand transformers. Most are likely 98% efficient and in this case it's probably higher since the difference in the secondary and primary windings is 10%. If it was only 90% efficient and you had a 30A load going through it at 120V, the thing would be burning hot.

No, I do understand transformers. I just used 10% (90%) as an example for easy math since I could find no efficiency rating for the Hughes Autoformer.

You said "the power meter at the park won't see a difference in their power usage at their main meter.", which is not entirely true. It may be a very small increase, but it is still there. Multiply that out by several units in use, it could become noticeable.

[Dp26]

The confusion is when one starts comparing the ramifications of someone actually being able to use what the outlet "can" provide, versus those who cannot run something (say an AC) because of their setup not providing enough volts, and leave some of their available power to others.

---


IMHO, the real issue is parks building just to (hopefully at least to) code, which allows for a capacity to be a certain % of max load as "enough" capacity. Fine for a house, such as mine, where I trip my own breaker if I go over what the incoming feed is. In an RV park, where there is any weather above (for arguments sake) 75deg weather, the park should assume closer to full usage (80% of breaker) for all sites - and probably more since some will run their main plug plus the 20amp. The problem is, campers do not want to pay what that would cost to build/upgrade to that infrastructure, unless it can be "folded" into existing rates. (See the threads complaining about separate electricity charges).

[Larry-NC]

Originally Posted by cavie

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

Well, Cavie, we can have some fun with this one for sure.

First of all, I have a BSEE in Electrical from University of Illinois (1968) and an MSEE from New Mexico State University (1973). I have another MS, too, but that's irrelevant. Details here.

One problem is with this statement:

Raise the voltage, lower the amperage and in turn lower the KWH. Am I wrong in my thinking?

Not all loads are as simple as that. Consider a regular old tungsten-filament incandescent lamp. When the filament is cold, the resistance is nearly zero. The hotter it gets, the higher the resistance. You raise the applied voltage, and the current drops a little more than proportionally, because the resistance goes up--it's not constant.

The engineering term is that the load has a positive temperature coefficient of resistance. Two young guys starting a business in a California garage, Bill Hewlett and Dave Packard, were looking for a way to stabilize the circuit in their first product, the HP200A audio oscillator, and hit on the idea of using this characteristic by including a small incandescent lamp in the feedback circuit, using this characteristic. The product was successful and the company survives today. You may have heard of it: Hewlett-Packard! Rest of the story.

The reason for the lecture above was to introduce the notion that some components do not have constant resistance (or reactance: inductance or capacitance).

Here's another example: We've all seen those "bricks" on the cord to laptops, TV, and other small/medium electronics. The proper name for that lump is a Switched-Mode Power Supply (SMPS). The worldwide ones in such common use today (100-260 VAC) convert a wide range of voltages to a fixed voltage. The fixed voltage load may draw a constant current, but the efficiency of the SMPS varies depending on the input voltage.

Here's another example:
This is an voltage vs. power at the input of an SMPS where the load is light and constant. If this were a constant resistance, the line would be straight; it's not.

The real question you are probably thinking about is the big loads: converter, refrigerator, and air conditioner. The converter could be an SMPS. Or it could be an old-style, transformer, rectifiers, and filter capacitors. Even the old style doesn't have constant resistance--think about it; at zero load there is current through the transformer primary even though there's no current (no power) out the secondary. (Even your home doorbell is like this.)

The others are all motors. This gets into motor types: synchronous, induction, etc. They probably all have different V-I characteristics. Let me know if you can't find them on Google and I'll take a look.

Larry

[babock]

Originally Posted by BriaBeck

You said "the power meter at the park won't see a difference in their power usage at their main meter.", which is not entirely true. It may be a very small increase, but it is still there. Multiply that out by several units in use, it could become noticeable.

It could even be less depending on the type of load being driven. An inductive load may draw more wattage at lower voltage.

EDIT: Saw Larry touched upon that.

[Bennington Camper]

Originally Posted by cavie

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

So the bottom line is Watts in equals Watts out.
You can juggle both sides of the power equation to produce the results you want, but in the end if you increase the voltage on one side, you increase the current draw on the other to make W=W.
Of course there will be ineffeciencies across the transformer (and an autoformer will be less effecient than a standard step up/down transformer) that will be wasted as heat.

[andymil]

“Raise the voltage, lower the amperage” doesn’t always apply. Raising the voltage on a fixed load will raise the current. An appliance like an air conditioner will draw more current when the voltage drops because of the characteristics of the compressor motor. It’s not really a fixed load. Not all electric powered equipment reacts the same way to voltage fluctuations.

[itat]