240V transformer on 277V line?

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ian
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240V transformer on 277V line?

Post by ian » Thu Oct 11, 2007 1:19 pm

I think I know the answer to this but I wanted to run it by the smart guys here.

I want to power a 240V transformer with a 277V feed.

If I use a resistor I guess I'd power the transformer at no load, calculate the current on the primary and select a resistor to drop 37V. Since it's only a 5VA transformer running at partial load a load variance shouldn't drop the voltage too much more on the primary. I'm just guessing though.

I think a better way would be two 37V zeners back to back dropping the voltage to 240V. But what about the resulting characteristics? I know there'd be "flat" 0V gaps between the sine waves due to the 37V reduction. Can this be done reasonably? Are there any concerns?
Will it work?

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cheapNdisgusting
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Ean - STOP

Post by cheapNdisgusting » Thu Oct 11, 2007 1:47 pm

I'm an electrician and wouldn't advise doing anything like that. A 240 V transformer is rated for 240. Higher voltages like this are dangerous to to mess with. In layman's language, the extra 37 V goes into heat. What voltage did you want to end up with?

Also, 277 V is 1/3 of a complete phase, since it comes from a 277/480V transformer connected in a wye connection.
It is a comercial/industrial voltage.
cNd

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Post by jwax » Thu Oct 11, 2007 1:52 pm

Resistor selection should be done at full load.
What does the transformer secondary go to?
Where the blazes are you coming up with 277 VAC in the first place?

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Post by ian » Thu Oct 11, 2007 2:06 pm

It's a common voltage for lighting applications in industrial areas.

I'm trying to control lighting and the available voltage is sometimes ONLY 277V.

If I measure the resistor at full load and drop the voltage 37V then at no load there might not be any drop, overloading the 240V transformer.

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cheapNdisgusting
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Post by cheapNdisgusting » Thu Oct 11, 2007 3:27 pm

For the application you are talking about, reliability sounds important. A 277/24vac transformer is not a "pricey" item. I'm assuming you will use 24vac to operate a contactor. The transformer will cost 1/10 what the contactor will.

Do it properly, lights are dependable, nobody gets hurt, you look good.
cNd

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Post by Robert Reed » Thu Oct 11, 2007 4:04 pm

Have you considered adding a small "bucking" transformer to the supply line ?

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Post by ian » Thu Oct 11, 2007 4:10 pm

Thanks for the lovely replies, anybody care to answer the origional question?

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Post by jwax » Thu Oct 11, 2007 5:18 pm

Which one? You had four questions.

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Post by Robert Reed » Thu Oct 11, 2007 8:34 pm

I thought your intent was to reduce 277VAC to 240VAC. I am now assuming that you want to do this by zener action only. Is that correct?

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haklesup
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Post by haklesup » Thu Oct 11, 2007 10:32 pm

WRT back to back zeners in series with the primary and 277V mains.

As long as the current in the primary is less than the max allowed for the diodes you use, it should work but not terribly efficiently. You need to stay under the zener current and the forward current limitations and have adequate wattage for the current passed through.

At 5VA and 240 V you get about 20mA max and with 37V drop, thats 0.79W of heat in the Zener. You can get by with a 1W or larger device with ample airflow. More watts if in a hot cabnet.

Yeah, the AC waveform at the secondary will be similarly distorted (clipped sine) and whether that makes a difference depends on what you do with it.

I don't think applying an extra 37V to the primary of a 240V transformer woululd do any harm. The dielectric withstanding voltage must be far higher than 15% over operating V. All you would get is a proportionately larger voltage at the secondary which might be easier to deal with than at the primary.

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MrAl
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Post by MrAl » Thu Oct 11, 2007 11:07 pm

Hi there,

In addition to the other good replies so far...


How well a 240v primary transformer works at 277v depends mostly
on how it was designed. Most power line transformers are designed
to take an input at least 20 percent higher then nominal because
the power line has a max spec of plus or minus 15 percent. This means
the transformer will most likely work at 277v but you will have to keep
an eye on it just in case that line goes 15 percent high.
20 percent high on a 240v line works out to about 288 volts, which
is what the transformer can most likely take without excess heat,
so if the 277v line goes as high as 15 percent higher then the
transformer will have to take about 319 volts.
The test therefore would be to input 319 volts and connect the load
and see how hot the transformer gets.

If you do use a resistor, with only 21ma a 1500 ohm 2 watt resistor would
probably be good enough. At full load the resistor will drop the required
voltage, and at no load it wont matter as much as the transformer,
although having a higher input voltage, will have less load so heating
will probably be ok.

As i said, the real test would be 319v for 24 hours at full load to
find out if this or any other solution would really work ok.

Most modern designs like this would include a thermal fuse anyway.
The thermal fuse would be physically touching the transformer maybe
between the winding and the transformer core. The thermal fuse blows
open if the temperature gets above the fuse temperature rating
for very long. 80 degrees C probably wouldnt be a bad rating for
this fuse. A standard fuse is also used on the input to the transformer
but in this case the current is quite low.

If you do use a resistor to drop voltage, it would also help to install
a thermal fuse in contact with the resistor. In case of overload
the resistor will heat up and blow the fuse. A flame proof resistor
here is a good idea too.

As a side note, as the input voltage to a transformer is increased the
excitation current increases, causing the copper and the core to
heat up more than at lower input voltage. At some point it becomes
impractical to use the transformer at that voltage.
If the transformer gets too hot it may melt the insulation used in between
the winding layers, which might cause breakdown and with it high current
draw.
LEDs vs Bulbs, LEDs are winning.

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Post by ecerfoglio » Fri Oct 12, 2007 6:13 am

As a side note, as the input voltage to a transformer is increased the excitation current increases, causing the copper and the core to
heat up more than at lower input voltage. At some point it becomes
impractical to use the transformer at that voltage.
The current that the transformer takes under no load conditions (excitation current) increases as you increase the voltage, but not linearly.

The transformer will take more current and run hotter, which leads eventually to an early failure.

If you can get a higher voltage transformer you can use it safely at a your (lower) voltage.

If you use a 440 V transformer at 277V the output will be 63 % of the rated value.

You must reduce the VA ratings accordingly, as the "real" maximum ratings are voltage and current, not VA

eg: A 440V / 36V - 8 VA transformer can be used as a 277V / 22.6 V - 5 VA one without overheating.
E. Cerfoglio
Buenos Aires
Argentina

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Post by dyarker » Fri Oct 12, 2007 6:39 am

Where are you doing this? At work? You might be cancelling the insurance (fire and personal injury) by doing a "lash-up" job.
Dale Y

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Post by ian » Fri Oct 12, 2007 9:04 am

It's for a lighting control based on a number of parameters such as occupancy, time periods, temperature, brightness, settings, etc.
It is expected there will be a production run and properly certified.
If we make 1,000 it's better to have 1 transformer that has a dual
primary for 115, 240, and drop the 277 so that 1 stocked part can
handle all situations.
Obviously, any little thing added multiplied by qty 1,000 adds up cost fast. There's no point in controlling a 30W bulb to save energy with a $200 device. Two back to back 5W zeners costs about $1.00.

Mr. Al
I called the transformer company and they said something like 20% was O.K. but I was concerned with failures over time if the transformer was just inside the reliability margins. I don't want a 1,000 of these things going in, then having to replace 300 of them.

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haklesup
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Post by haklesup » Fri Oct 12, 2007 10:34 am

If you make 1000 then you should just get a transformer that can handle 277 V out of the box. It should be no problem to get a multi tap primary designed to any spec you want. The NRE and setup charge will be easily outweighed by the quantity discount for 1000 or more units.

If this lighting control is hard wired, you don't have a cord and its no problem to get the installer to wire for the proper voltage on site. A corded appliance should be more convenient though.

See the 1N5368, it looks like it can handle the job. Zener current is 132mA
http://www.onsemi.com/pub/Collateral/1N5333B-D.PDF

Definately don't use a resistor if this lighting controller turns on or off or dims a light since the resulting change in current will result in a proportional shift in voltage drop across that resistor. Far less regulated than the zener but a transformer alone is the most reliable.

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