Autoformer pros and cons

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Ned

Ned

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I wouldn't use an autoformer, as it will increase the current drawn as the voltage is raised, further exacerbating the problem.  Autoformers cause more problems than they solve and I recommend against their use under any conditions.
 
Ned said:
I wouldn't use an autoformer, as it will increase the current drawn as the voltage is raised, further exacerbating the problem.  Autoformers cause more problems than they solve and I recommend against their use under any conditions.
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I agree with Ned on the use of autoformers. IMHO they should never be used.
 
I have to disagree concerning the effect of using an autofromer, but do not wish to debate the issue here, since it can only lead away from the original question.   
 
I would be interested in following the autoformer discussion if you take it to a new thread.
 
Unfortunately, such discussions can get clouded by myth and emotion, but the link is in my prior message.
 
I wonder about the concern of the addl current draw (using an autoformer) exacerbating the issue for other campers.  It occurs to me that "assuming" the utility company is delivering proper power,  then the only campers affected would be those down line from the high joint that is causing the problem.  Further, if the campground distribution system consists of more than on branch, and the high join causing the problem is on one of the multiple legs, then only the campers on that leg should be affected.

Am I missing something here? 
 
No you are not missing anything basically it comes down to there is no free lunch.  While an autoformer may raise the voltage for the person using it somebody is going to get less.  I stand by my position that autoformers should not be used.
 
geodrake said:
Another consideration would be to use an Autoformer in addition to the extn cords.  It would provide a boost making up for the IR drops in the cords.  A bit expensive, but then you would have it on the road too.  Camping world has one http://www.campingworld.com/shopping/product/autoformers/3251
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If the problem is excessive voltage loss in the extension cord  ... Use of an autoformer might cause overheating of said cord

A dangerous situtation.  This is the one time when I would advise against them

As I said elsewhere.. When I pay my rental fee there is an implied contract to provide proper power, either 30 or 50 amps depending on the site.. Thus if an autoformer by boosting the voltage causes problems elsewhere in the park.

That is, in fact, the fault of the park owner for providing shoddy wiring.. Not my fault, and not my problem (Unless I'm the "Elsewhere in the park"

Some parks.. > Do not have proper power
 
In this case Beardog is asking about the power supply at his own house, for his RV alone, so the effect on others, if any, is irrelevant.  He asks a very legitimate and straight-forward question about the use of extensions, so lets not go far afield with tangential issues.

Nor would I leap to the conclusion that an autoformer is needed here - it hasn't even been established if there is a problem.

There MAY be some voltage drop, perhaps a couple volts in each length of extension cord. That is not a problem as long as the supply voltage is decent  in the first place.  If it starts out at 116-120v, then a drop to 110 is acceptable. If it starts out at 110-112 and suffers a 6v drop, the result  it is not OK.  Measurement is the only way to make a meaningful determination.
 
It does no good at all to put an Autoformer at the end of a long extension cord.  The Autoformer draws extra current to provide it's boost in voltage, which increases the voltage loss along the cord.  If you're the only user on the circuit, the increased voltage loss will be exactly the same as the boost provided by the Autoformer.

In other words, you'll wind up with exactly the same voltage at the rig, but you'll consume 10% more power to get it there.
 
Thanks Lou, that's just the problem I was attempting to point out.  TANSTAAFL :)
 
Lou is correct if considering only resistive loads, but ....

The time folks are concerned about low voltage is when they want to run their a/c and, in those cases, low voltage will result in increased current flow. This is the reason manufacturers tell us not to run the a/c units under low voltage conditions. Using an autoformer to increase the voltage at the RV to run a/c will result in reduced current load on the campground's service.

I don't have time to explain this more fully right now, but it's basic AC (electrical) theory. I also don't have good/comprehensive references handy, but I understand that IEEE Standard 141-1993 would be a good starting point for a disbeliever. Hopefully, I'll get the references together by the time I get to finish writing my article (currently in draft).

Note: The above does not include the nominal (1A or less) quiescent current draw of the autoformer; However, this is smaller than the current reduction resulting from using an autoformer to power the a/c unit(s). So you really can enjoy the comfort of your a/c while hooked up in a campground with low voltage, and you won't be negatively affecting other RVers.
 
So if every one uses an autoformer in a campground with low voltage, and runs their air conditioners, it will solve the power problem.  I guess there really is a free lunch.
 
Lou is correct if considering only resistive loads, but ....
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Uh, no actually I was wrong...

Operating a step-up transformer will increase the amount of voltage LOST ALONG THE LINE by the percentage it boosts the voltage.  If you're going from 120 volts no-load to 100 volts under load, that's a line loss of 20 volts.  Increasing the line current by 10% (as the autoformer does to give it's 10% voltage increase) will cause an additional 10% of line loss.  Instead of dropping 20 volts along the line you'll lose 22 volts.

You'll still get the 10% voltage boost from the autoformer.  Instead of the original 100 volts under load you'll have 110% of 98 volts, or 108 volts.

So you will still have a net voltage gain with the autoformer.  Gary and Tom are right, I was wrong.

Sorry about that!
 
And you'll still draw more current from the source than if the autoformer weren't used to power an increased load.  The reduction in current is only true IF you have the same load with and without the autoformer.
 
The reduction in current is only true IF you have the same load with and without the autoformer.
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That's correct Ned. Obviously, adding load, such as one or more a/c units, will increase current on the campground supply. That in turn will, of course, increase voltage drop along the line.
 
I think it is obvious that turning off the a/c reduces the current draw on the campground supply. However, most people aren't doing that, even if the voltage goes dangerously low.

The power draw of the a/c compressor is the same regardless of voltage. If it needs 1500 watts to function, it will draw 1500 watts no matter what. For any given appliance, we can assume for simplicity's sake that power (watts) = voltage x amps, so it is evident that reduced voltage causes an increase in amps in order to keep the power constant. The autoformer's output has to be 1500 watts to run the a/c, and it requires an input of 1500 watts (plus any overhead due to conversion inefficiency) to do so. For example, if the input is 100V x 15A and the autoformer has a 10% voltage boost, the output is 110V x 13.6V.   It doesn't require any more power (except for inefficiency) to run the a/c than if it were not used at all. If there was no autoformer, the a/c would still be drawing 1500 watts or 15A @100V.  No magic and no free lunch, but your equipment is protected from the overheating that results from very low voltage.

Call me selfish, but I don't see any reason why I should turn off my a/c to enable somebody else in the park to have enough voltage to run their a/c without risk of overheating. Are they more entitled than I?  As John says, there is an implied covenant when you rent a site with 30A or 50A electric and you are entitled to use the power you paid for.
 
RV Roamer said:
Call me selfish, but I don't see any reason why I should turn off my a/c to enable somebody else in the park to have enough voltage to run their a/c without risk of overheating. Are they more entitled than I?  As John says, there is an implied covenant when you rent a site with 30A or 50A electric and you are entitled to use the power you paid for.
Click to expand...

Actually the power draw of an A/C compressor does, to some extend, depend on voltage.. Has to do with the power factor.  A very good system allows full voltage to the compressor during start up but backs down once it's running to control the power factor,  Google "Green Plug" and you may find more info on that...(Have not checked, used to own such a device till Detroit Edison destroyed it one day)

As for the quoted text

My position is this: When I paid for the site rental the compground has some responsibilities.. One is to deliver proper power

If they can not do that it's THEIR problem.. If I have the ability to correct it it's THEIR problem

So I won't call you selfish.. I call you reasonable.  I call the campground that makes you take corrective action negligent and if any one can't use their A/C that's between them and the campground. NOT your problem.
 
I know I ignored power factor, but the power factor is a constant in this discussion - it's the same a/c compressor, with the same power factor, whether the autoformer is present or not.  For the purposes of illustrating what happens with an autoformer on the power cord, I felt it was safe to ignore it and doing so greatly simplifies the discussion.
 
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