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  1. #11
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    Default Re: Plug-in transformer numbers?

    I agree it is good information too, except for one line "Using too big a transformer only reduces it's efficiency, ". That is not a given, there are too many factors when determining a transformers efficiency so that may or may not be true. Because the OEM transformers are usually "minimally sized" for the job, their load losses are pretty high. Load losses are exponential so going up in capacity by 25% can reduce load losses by as much as half, but usually at an increase of 25% in no load losses. No load losses usually account for about 20% of the total losses.

    Total efficiency has to take in the no load losses which are 24/7 losses and the load losses, which although are higher are also dependent on duty cycle. If the transformer is under load 24/7, then the load losses are more important than a transformer that is under load for say only a few hours a day. In this case, I'd guess that the transformer is under load whenever the alarm system is on so it probably is closer to 24/7.

    Materials, design and construction also play a big part in the efficiency. Better core material decreases no load losses and winding material (copper vs aluminum) wire size and design affect the load losses.

    An indication of the transformers efficiency is the amount of heat that it gives off. That is total heat, not just the surface temperature of the transformer. All the electrical losses are in the form of heat.
    Last edited by keith3267; 12-08-2013 at 02:44 PM.

  2. #12
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    Default Re: Plug-in transformer numbers?

    Keith, in this case I don't think it will be much of an issue.

    I've seen unloaded utility transformers get so hot you could fry an egg on them but this falls in the doorbell transformer size and is probably current limited by a break in the magnetic circuit.
    Last edited by The Semi-Retired Electric; 12-09-2013 at 11:19 AM.
    Good Luck from Columbiana, Alabama
    Maurice Turgeon, Hidden Content

  3. #13
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    Default Re: Plug-in transformer numbers?

    I can't imagine an unloaded utility (distribution?) transformer getting that hot unless it has gone into ferro-resonance, or it has a coil defect. (Or someone put the tap in one of the non rated positions with the DV switch in the parallel position.)

  4. #14
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    Default Re: Plug-in transformer numbers?

    Quote Originally Posted by keith3267 View Post
    I agree it is good information too, except for one line "Using too big a transformer only reduces it's efficiency, ". That is not a given, there are too many factors when determining a transformers efficiency so that may or may not be true. Because the OEM transformers are usually "minimally sized" for the job, their load losses are pretty high. Load losses are exponential so going up in capacity by 25% can reduce load losses by as much as half, but usually at an increase of 25% in no load losses. No load losses usually account for about 20% of the total losses.

    Total efficiency has to take in the no load losses which are 24/7 losses and the load losses, which although are higher are also dependent on duty cycle. If the transformer is under load 24/7, then the load losses are more important than a transformer that is under load for say only a few hours a day. In this case, I'd guess that the transformer is under load whenever the alarm system is on so it probably is closer to 24/7.

    Materials, design and construction also play a big part in the efficiency. Better core material decreases no load losses and winding material (copper vs aluminum) wire size and design affect the load losses.

    An indication of the transformers efficiency is the amount of heat that it gives off. That is total heat, not just the surface temperature of the transformer. All the electrical losses are in the form of heat.
    You're on target Keith, and thanks Maurice- this part of 'electrical' is in my realm of knowledge; I'll leave the line-side to you

    Materials, design, and construction all do play a part, but these types of transformers are usually designed for only one goal: cheapness. Given that, you get cheap stamped plates of poorly refined recycled metal with poorly chamfered edges, barely adequate insulation thickness on the minimally-sized windings, and wound to minimize the length or wire needed to get the voltage wanted even when better winding designs exist. Five cents on one unit may not matter, but multiplied by many thousands of units.... well, you get the picture Thus my generalizing. Besides, at best were talking such small numbers of current usage that when you compare the cost of a factory replacement part versus swapping out with a 'freebie', whatever efficiency you might lose or gain is insignificant in the average household environment. Here, I think the no-load losses would be the most significant use factor, but I'll also bet that the original equipment manufacturer did not even consider this as a design factor. Without knowledge and equipment available to most DIY'ers, they could not determine efficiency anyway so it becomes a moot point.

    Deep in storage I have an old 60A transformer-type power supply from an Airstream travel trailer (120VAC to 12VDC). I can't tell you it's efficiency but I can tell you that moving it around would qualify for weight training at a gym! One of these days I need to get on with my plans to build a better rectifier/filtering circuit for it so I can use it with my radio gear. Man, you should hear that old thing hum when you power it up! Now where is my surplus gaussmeter, did I leave it behind when I worked on that minesweeper a couple years ago?

    Phil

  5. #15
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    Default Re: Plug-in transformer numbers?

    Quote Originally Posted by keith3267 View Post
    I can't imagine an unloaded utility (distribution?) transformer getting that hot unless it has gone into ferro-resonance, or it has a coil defect. (Or someone put the tap in one of the non rated positions with the DV switch in the parallel position.)
    The transformer I'm speaking about was a 480V dry type. It supplied 120V area lighting to a chemical plant. When the plant when "cold iron" for maintenance the PF went from .8 to .2 and this transformer go too hot to touch.

    It was not defective or wired incorrectly.
    Good Luck from Columbiana, Alabama
    Maurice Turgeon, Hidden Content

  6. #16
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    Default Re: Plug-in transformer numbers?

    That sounds like ferro resonance.

  7. #17
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    Default Re: Plug-in transformer numbers?

    Quote Originally Posted by keith3267 View Post
    That sounds like ferro resonance.
    I'm with Keith on this. Inductive electricity is a lot more complex subject than what we usually deal with here with many more variables to consider. Add some RF components in the vicinity and you can get some really weird effects happening (and occasionally some acrid smoke too!)

    Phil

  8. #18
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    Default Re: Plug-in transformer numbers?

    It could have been ferro-resonance but there was no capacitance load and happened 30 years ago so I can't look into it or even remember the details.

    Iron losses do occur in transformers, even in doorbell types with current limited design. Usually not a problem.
    Good Luck from Columbiana, Alabama
    Maurice Turgeon, Hidden Content

  9. #19
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    Default Re: Plug-in transformer numbers?

    Ferro-resonance occurs when there is no load at all, or a very small one. There are a number of theories on what causes it or even what it is, but no general consensus. It is just a phenomenon that occurs occasionally on an unloaded transformer, usually when fed with a long underground cable. It is agreed that the interwinding and interlayer capacitance along with the capacitance of the feed cables are the source, but that is where agreement seems to end. When in ferro-resonance, the transformer generally hums pretty loud.

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