TRANSFORMER LOADED VS UNLOADED

[fifty60]

I have two transformer branch circuits. One is a redundant fan motor for the other one, so that only one fan will be active at a time. There are no other loads on the respective transformer circuits.

At all time, both transformers will have line voltage, but only one transformer at a time will have any load current. What will be the current on the primary of the transformer that has the motor that is not in use?

Is there anymore than a negligible amount of primary current on a transformer when it is unloaded?

 

[gar]

150511-2352 EDT

You never or very seldom provide any feedback after people provide you some answers.

For your present question I suggest that you make measurements and answer your own question.

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[Besoeker]

I have two transformer branch circuits. One is a redundant fan motor for the other one, so that only one fan will be active at a time. There are no other loads on the respective transformer circuits.

At all time, both transformers will have line voltage, but only one transformer at a time will have any load current. What will be the current on the primary of the transformer that has the motor that is not in use?

Is there anymore than a negligible amount of primary current on a transformer when it is unloaded?

Figures I have for 400kVA 3-phase unit are 2% no load current and 0.3% no load losses.
I don't know if this helps you.

 

[fifty60]

Generally after I get a lot of feed back I am torn between responding "Thanks everyone" and not saying anything. I was kind of under the impression that a "Thanks everyone" would needlessly bump the thread, and that may make some people mad. It probably is best to say thanks and the question has been resolved.

For this quesiont, unfortunately I have to size the service months before I actually have the machine built. I would think the no load current would be similar to what Besoeker described, somewhere in the low percentile range. I will measure it when the time comes, but was trying to confirm my understanding now.

 

[gar]

150512-1012 EDT

fifty60:

Thanks is not the kind of response I am talking about. You have a problem, ask a question, and there are various responses. The kind of response that would be useful from you would be --- the problem was not solved, such and such was the solution or a partial solution, none of the suggestions were a solution, or additional research found a solution.

A small 175 VA transformer with a double bobbin construction using standard transformer iron with an EI structure has a no-load power input of 4 W at 110 V, 5 W at 120 V, and 6 W at 130 V. At 120 V this is about 3 % of the VA rating. Just sitting idle this is a big power loss.

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[Besoeker]

150512-1012 EDT

fifty60:

Thanks is not the kind of response I am talking about. You have a problem, ask a question, and there are various responses. The kind of response that would be useful from you would be --- the problem was not solved, such and such was the solution or a partial solution, none of the suggestions were a solution, or additional research found a solution.

A small 175 VA transformer with a double bobbin construction using standard transformer iron with an EI structure has a no-load power input of 4 W at 110 V, 5 W at 120 V, and 6 W at 130 V. At 120 V this is about 3 % of the VA rating. Just sitting idle this is a big power loss.

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That might well be case. It isn't often I've had to be concerned with the efficiency of transformers of that rating. Typically we would use 500VA as a control circuit transformer in a several hundred kW variable speed drive.
That's what the 400kVA example I gave was for.
Of course we have to provide overall drive efficiency figures. But minor losses such as in a 500VA transformer get lost in major component tolerances .

 

[gar]

150512-1644 eDT

fifty60:

In your thread "Wetting Current/PLC Input" you never came back with any feedback on how you solved your problem, and what thinking you used to arrive at a solution.

For this present thread you have not mentioned how large your fan motor loads are. If I asume 1000 W and that the fan load is continuous and the transformer is 1 kVA, possibly smaller than needed, then there is 30 W of wasted power. Per year this is about 0.03 * 8766 = 263 kWH. At $0.16 / kWH the waste cost is 263 * 0.16 = $42. Over 10 years $420. Will this pay for switching the primary instead of the secondary?

By comparison a new highly efficient refrigerator probably only averages about 60 W.

When trying to reduce energy use and costs it is important to consider various approaches. Energy use was not your question, but it is important to consider.

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[fifty60]

I will make a better effort at offering up conclusions to my posts. Generally, I get the answers I need from the post and other research/resources and drive on. I understand how this can make it all feel a little open ended.

The fan load is slightly less than 500W, and the general purpose transformer I will use is 1KVA. Where do you get the 30W of wasted power from? Is that a general number when using a 1KVA transformer? I am not completely green anymore, but I do feel like I tend to oversize transformers and from what you are saying there is a $ cost associated with this oversizing.

Normally, i would have one transformer, but for this system I have a primary and a redundant system. The redundant system load will not be energized normally, but the primary will always be energized.

 

[gar]

150514-211 EDT

fifty60:

When you ask questions that get envolved with various answers, you gradually add new information, possibly get other information from other sources, and possibly discover a better way to present your problem, then readers want to be able to know about the end results, good, bad, or whatever to correlate your questions with the suggestions and knowledge you gained to help them solve similar or other problems.

On transformers. You need to find a college textbook that covers the theory of transformers and study it.

A teaching technique that one of my old professors used was: You ask him a question and he would tell to go back and describe in detail what you thought was the solution and how you arrived at that conclusion.

He was the author of a book on "The Fundamentals of Engineering Electronics" first published in 1937, and later in 1952. This was largely on the theory of electron tubes and gaseous discharge. During WWII he worked on the development of the proximity fuse. After the war he was a driving force in the formation of the Willow Run Laboratories. They did early work in rockets and instrumentation to study the upper air atomosphere. Willow Run developed side-looking radar. See http://www.holography.ru/histeng.htm for a brief comment on Emmett Leith and Juris Upatnieks and Holography.

In the history section of http://en.wikipedia.org/wiki/Synthetic_aperture_radar is a long discussion on the Michigan development work on side-looking radar.

Transformers: In an unloaded condition the major power losses are hysteresis and eddy current. To some extent these scale by weight (size). Thus, I took my measurement of loss in the 175 VA transformer and extropolated to 1000 kVA. The 3% produces a value of 30 W,

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[kwired]

150512-1012 EDT
A small 175 VA transformer with a double bobbin construction using standard transformer iron with an EI structure has a no-load power input of 4 W at 110 V, 5 W at 120 V, and 6 W at 130 V. At 120 V this is about 3 % of the VA rating. Just sitting idle this is a big power loss.

.

What is actual VA input with no output load, I imagine power factor is pretty poor when there is no load with most transformers.

 

[gar]

150515-1948 EDT

kired:

At 123 V the PF unloaded is 0.22 .

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