Dead simple, but....

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peter d

peter d

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Here's a good one to ask your fellow coworkers. I expect one might get a variety of responses to this question:

Which consumes more power, a 5KW a/c unit operating at 120 volts, or a 5KW a/c unit operating at 240 volts?

(Let's ignore the obvious voltage drop issues. :)
 
Re: Dead simple, but....

Thats a tough one for some Pete. ;)

I have 150 KW generator, how many amps can I get out of it? ;)
 
Re: Dead simple, but....

I don't know the number of times over the years that I've had intelligent industrial folks ask me if they could change their power systems to 480 v to cut their power bills in half.
 
Re: Dead simple, but....

Something I find interesting is the question of which weighs more, a lb. of gold or a lb. of lead? Since lead is weighed in avoirdupois weight and gold is weighed in troy weight, the lead would weigh more. avoirdupois weight is 16 oz. to the pound and troy weight is 12 oz. to the pound. Therefore, even though a troy oz. weighs more than an avoirdupois oz., a pound of lead is heavier. :D
 
Re: Dead simple, but....

Charlie: That tricky question is more entertaining if you ask about a pound of feathers and a pound of gold. The answer is a pound of feathers, for the reason you have described. But people tend to recognize that they are getting sucked into the belief that we are trying to trick them with the obvious: gold is heavier than feathers. Then they recognize the "pound is a pound" trick, and proudly declare that the two are the same. You win bigger when you tell them that a pound of feathers is heavier than a pound of gold. ;)
 
Re: Dead simple, but....

Originally posted by peter d: Which consumes more power, a 5KW a/c unit operating at 120 volts, or a 5KW a/c unit operating at 240 volts?
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That's a deeper question than meets the eye. Bob (iwire) touched on the central issue, when he asked about the amps from a generator, without telling us the voltage. The correct answer to Bob's question and the correct answer to yours are exactly the same: "There is not enough information given to determine an answer." In your case, we would need to know the power factor of both machines, in order to calculate the KVA of each machine. Then, and only then, would we be able to tell which draws more power.

{Edited to add: Anyone who answered, "A KW is a KW, so both machines draw the same power" will have fallen into the same trap as those who said, "a pound is a pound, so a pound of feathers weighs the same as a pound of gold."}

Originally posted by peter d:(Let's ignore the obvious voltage drop issues.)
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If we do that, we might get the answer wrong. The voltage drop along the conductors will have an impact (however slight) on the degree to which the current lags the voltage, as seen at the power source. The characteristics of the conductors will influence the overall power factor.

[ August 24, 2005, 10:59 AM: Message edited by: charlie b ]
 
Re: Dead simple, but....

Geez Charlie, you had to take the fun out of that one, didn't you? :(

OK, let's make some assumptions and changes to the original problem then. This machine is one and the same. It's a multi-tap unit so that we have the ability to connect it to a 120 or 240 volt source.

Let's also assume that the unit is only 30 feet away from the source, so voltage drop would be so marginal as to not affect the outcome.
 
Re: Dead simple, but....

Or just forget about inductive loads entirely.

So which consumes more power, a 2KW electric heater (without fan motor :D
 
Re: Dead simple, but....

OK. Now you're talking "apples and oranges." By that I mean that both are fruits, and both are weighed on the avoirdupois scale. So they are the same. :D

So if there is still a trap out there, I will boldly step into it. Both of these heaters will draw the same amount of power. There. I am in it now. The ball is back in your court. :p
 
Re: Dead simple, but....

The origional question was
Which consumes more power, a 5KW a/c unit operating at 120 volts, or a 5KW a/c unit operating at 240 volts?
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Since the question uses "KW" units, the answer is
they both use the same "power" in KW. The question did not ask for the apparent power in KVA and thus there is enough information to answer the question that both use the same 5 KW.
 
Re: Dead simple, but....

The question was, "Which consumes more power. . . ?" The question was not, "Which consumes more 'real power" . . . ?

The two machines will consume KW and KVAR, so you can't answer the question without knowing KVAR (or equivalently, without knowing power factor).
 
Re: Dead simple, but....

Ok, let's go a little further.

Will either system "draw" power or will either system "consume" power or...will both systems convert power?
 
Re: Dead simple, but....

For the a/c example, I did mean 5 KW true power. I hadn't even thought of complicating the problem that much by introducing true and apparent power elements. It was just a simple mind exercise to get people to think about the Ohms Law relationships.

Many electricians have trouble understanding reactive power, myself included. It took quite a while before I had it under control. If we use this theortetical multi tap a/c unit, then we are dealing with the same power factor and the same true power, and the example stays simple. :)
 
Re: Dead simple, but....

Originally posted by bthielen:
Ok, let's go a little further.

Will either system "draw" power or will either system "consume" power or...will both systems convert power?
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Ok, I'll take a shot. I'll go with conversion and consumption.I think we have 2 laws of thermodynamics in play.

First, the consumption: no machine is 100% efficient, so some of the electrical energy is being wasted. All of the energy "consumed" by the motor is not being transferred via the shaft; some of it is heating up the windings of the motor. The electric heater is much more efficient, but there is still a very small amount of loss.

Second, the conversion: Energy can neither be created nor destroyed, only converted. So the heater is convering electrical energy into heat, and the a/c unit is converting electrical engergy into mechanical energy to move heat from one place to another.
 
Re: Dead simple, but....

Originally posted by peter d: If we use this theoretical multi tap a/c unit, then we are dealing with the same power factor and the same true power, and the example stays simple.
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I am not sure that is true. In order for the same motor to work on either 120 volts or 240 volts, there has to be some internal means of switching from one source to the other. I don't know how this is actually achieved. I suspect that it is either a second set of motor windings or an internal transformer (e.g., that is connected only when the 240 source is selected, and that steps the voltage down to 120 volts internal to the A/C unit). However it happens, there is going to be a difference in the inductive reactance of the two operating modes. That means the 120 volt mode and the 240 volt mode will have different values of power factor.

There are a number of "classic" questions that appear to be simple, but that are actually quite "entertaining." Charlie mentioned one of them ? the pound of gold and pound of lead (or feathers) question. Another is this: "Who is buried in Grant's tomb?" I've seen the tomb. The correct answer is not "Grant."
 
Re: Dead simple, but....

Originally posted by charlie b:
I don't know how this is actually achieved. I suspect that it is either a second set of motor windings or an internal transformer (e.g., that is connected only when the 240 source is selected, and that steps the voltage down to 120 volts internal to the A/C unit).
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Wow, what a can of worms. I will definitely stick with the heater example only. At least I understand that one fully. :(

As far as the motor is concerned, the voltage change is achieved by changing the windings from a series to parallel connection, or the reverse, at least with 3 phase motors. Hence the 240/480 volt rating. For single phase motors I am pretty sure it is the same.

Let me try to approach this from a differnt angle. I will try explain it the best way I can. We have an a/c unit that must do a certain amount of work to cool a certain amount of space to a cetain temperature. To achieve this we must "input" a certain amount of electrical energy. Whether we operate the unit at 120 volts or 240 volts, the a/c unit must still do the same amount of work, and our power "consumption" should be relatively the same. So I was incorrect to assume that the consumption would be identical, but I have trouble seeing how the differnce would be more than marginal, either.

And now that we are getting into the fundamentals of motors, isn't the inductive reactance of a motor a moving target anyway?
 
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