Transformer powering Heating Element

[njohnston]

This one has me puzzled... using a variable power pack to heat an element - the powerpack is way overrated for the element (120A @ 12V secondary of transformer) so decided to build a smaller version.

To get the element to about the right heat (glowing red) i set the large PP to about 25A and measuring voltage at ends of element it is about 8.5-9V...

When i try the same with the smaller PP, setting it to 25A does not heat the element enough... even when smaller PP is turned up fully, giving roughly 31A @ 12V the element does not glow red or heat enough...

Thought it may have been the different Phase-angle controllers used in different PPs, but eliminated this by running smaller transformer off both Phase angle controllers... same result.

By no means an electrical expert but surely more wattage (P=IV) should generate more heat (i know this is not how you calculate heat energy but still...) or am i missing something blindingly obvious - feel free to embarrass me with a simple answer or explanation

 

[Baluncore]

It may be that the heating element has very low resistance when cold, needs more current and so saturates the transformer, but once hot it takes less current to keep it glowing.
What material is the heating element made from?

 

[njohnston]

unfortunately i am not sure of the material - i 'think' it is a nichrome strip...

will read up on saturation of transformers - that is a possibility - is there any method of determining this? would a saturated transformer give steady volt/amp readings? will try ramping up voltage more slowly to see if this has an effect...

 

[jim hardy]

Do you know who built the power pack? Sometimes there's a manual online...

And what kind of meter are you using to measure volts and amps? Some are fooled by non-sinewave like produced by chopper supplies.

and -

(P=IV) should generate more heat (i know this is not how you calculate heat energy but still...)

works fine for a resistive load provided you can get RMS readings.

Thought it may have been the different Phase-angle controllers used in different PPs, but eliminated this by running smaller transformer off both Phase angle controllers... same result.

Are you feeding phase width modulated AC into a transformer? Does it hum much?
Place a meter across the transformer primary and verify there's hardly any DC. Asymmetric wave applied to primary can really foul things up, and dc content is a quick check for that. More than a couple volts DC is cause for concern
That'd be a symptom of 'saturation', though from a different cause than Baluncore's equally plausible overcurrent.



old jim

 

[NascentOxygen]

This one has me puzzled... using a variable power pack to heat an element - the powerpack is way overrated for the element (120A @ 12V secondary of transformer) so decided to build a smaller version.

To get the element to about the right heat (glowing red) i set the large PP to about 25A and measuring voltage at ends of element it is about 8.5-9V...

When i try the same with the smaller PP, setting it to 25A does not heat the element enough... even when smaller PP is turned up fully, giving roughly 31A @ 12V the element does not glow red or heat enough...

Just to clear this up from the start: are these DC voltages and amperes? You measured these using a meter set to DC?

 

[meBigGuy]

Makes no sense, so there is missing information.

You need to measure the voltage across the element hot and cold with both supplies. Be nice if you could measure the current also. (even nicer if you measure the waveform)

Knowing whether it is an AC or DC supply also helps.

If you actually "force" 25 amps RMS or DC through the element, it will heat up and show the same DC or RMS voltage drop regardless of the supply. If the supply is non sinusoidal, meters may fool you.

When you say "giving 31A at 12V, what does that mean? How are those numbers determined?

 

[njohnston]

ok, transformer is AC 240v primary 12v secondary... i measured voltage across the element with an ordinary fluke meter set to V (does both AC/DC) and measured amps with the same meter at the point on the cable just before it connects to the element (meter has notch in the end to measure amps)... both powerpacks have CT's and Ammeters on secondary side as well...
Unfortunately have no equipment to measure waveform.

phase width modulated - is this the same as phase angle controlled? if so, yes that is what transformer is fed with and yes, it does hum quite a bit when at the lower end of the pot scale - but both transformers do this.

Have no idea who built first PP and transformer in it has no information or nameplate - only "max 120a"

 

[NascentOxygen]

ok, transformer is AC 240v primary 12v secondary... i measured voltage across the element with an ordinary fluke meter set to V (does both AC/DC) and measured amps with the same meter at the point on the cable just before it connects to the element (meter has notch in the end to measure amps)... both powerpacks have CT's and Ammeters on secondary side as well...

Okay. Can I correctly summarize your tests like this: you measued using a meter approx 25A when you measued 8.5V-9V? Then, with different hardware, you measued approx 31A when you measued approx 12V?

 

[njohnston]

yes - fluke meter measured 25a @ 8.5-9v with large pp attached to element (and pot turned up slowly until 25a showed on pp ammeter) - glowing dull red, nice heat. fluke measurements taken about 1 minute after switch on.

with same fluke meter measured 31a @ 12v with smaller pp attached to element (and pot turned up fully)- no glow, not hot enough. fluke measurements also taken about 1 minute after switch on.

i may be able to source a larger rated transformer next week and will try it as a replacement to see what happens (assuming the input current is within the rating of the phase angle controller).

 

[Baluncore]

It would be good to avoid using a phase angle power controller before a transformer designed for power line frequency. The voltage transformation should be done at a much higher frequency.

Have you considered using a switching power supply?

Also consider the smallest and lightest available “inverter” or “caddy” welder for stick, TIG, MIG or plasma cutting. Turn off the pilot arc. No heavy transformer, DC voltage output is adjustable up to about 32V, current out adjustable limit, up to about 120A. New cost is now about US$100. Visit your local welding supplier or hardware and tool store to see what is available.

 

[jim hardy]

i measured voltage across the element with an ordinary fluke meter set to V (does both AC/DC) and measured amps with the same meter at the point on the cable just before it connects to the element (meter has notch in the end to measure amps)... both powerpacks have CT's and Ammeters on secondary side as well...
Unfortunately have no equipment to measure waveform.

okay, understand.
You have plenty of good help on board .

Just for my curiosity - did you check for DC voltage in the PP's output, which would be across the transformer's high side ? Fluke on DCV should read a fraction of a volt.

phase width modulated - is this the same as phase angle controlled?

Yes. Thanks . Have a better idea now of your setup.

Have you a photo of that powerpak?

(meter has notch in the end to measure amps)...

now THAT's interesting - what's its model number ? I'd like to read up on it.

 

[jim hardy]

Aha think i found it Fluke T5 ?

its spec sheet says 45 to 66 hz for AC and it's average responding
http://www.myflukestore.com/pdfs/ca...fluke_t5_1000_electrical_tester_datasheet.pdf

Not indicting it, just exploring its limits.
I assume you're working at 50 or 60 hz.

old jim

 

[Simon Bridge]

I've been sitting out. There are a bunch of questions that occur to me from previous discussion.

It appears: the PSU has dial on the front reading "amperes" ... the voltage supplied is adjusted by a knob on the front until the dial reads 25A and the voltage across the element is measured using a digital (Fluke-brand) multimeter.

With the first PSU the voltage is reported to be 8.5-9V on a meter set to DCV.
It is noticed that the element glows red hot (time to do this?)

The range reported suggests either the result of multiple tries or fluctuations in the reading on the meter.
Or maybe it starts out at 9V and drops to 8.5V as the element heats up?
Note: Fluke make meters to read down to mV resolution. My cheapo Fluke has a 0.1V resolution on the DCV scale. - so I guess the variation does not reflect the resolution of the voltmeter scale.

The experiment is repeated for a second PSU, 25A measured at the terminals, the voltage across the element is not reported.
It was observed that the element does not heat up.

Clearly there is something different about the two PSUs.
We are told that the second one has deliberately been made smaller than the first one.
In what way? What does "small" mean in this context?

I'm guessing there is more different about the design than physical size or max power rating.
This is where you will find your answer.

i.e. if they operate at different frequencies, and the element includes a reactance, then there would be a difference in the heating profiles as the impedences will be different. Looking more closely at the element will also yield clues ... perhaps it is not a simple element?

More data is needed, and more careful data.
You could find the V vs I graph for the element, taking care that the parameters will change with temperature.
Compare the profiles for the different PSUs.

Try the experiment with a different component - i.e. a stock resister, and an RL circuit.
The RL circuit will give you some information about the waveform at least.

More careful data is needed.
Perhaps use the ACV scale?
(We are all assuming the output of the PSU is some sort of wave - but it could be that it is supposed to be rectified and there is a ripple...)
Ideally get hold of an oscilloscope.

Where did you get the big PP? (Did you build the smaller one yourself?)
i.e. is it designed for a specific use?

Where did you get the element from?
i.e. is it part of a larger object like a soldering iron or a toaster?

The use of a phase-angle converter is odd to me - but I live in a single-phase country so their need is not familiar to me. Are the PPs are all 3-phase wiring?

If you have not opened the large PP, do so (unplug and wait a bit after unplugging, and observe workplace safety procedures pertaining high voltage) and compare the design with the smaller one.

The bottom line is that different PPs are driving the same device with different results - so there is something different about the PPs beyond what you have reported so far.

 

[MrSparkle]

It fundamentally makes no sense that a higher current produces less heat than a lower one. I can only assume that the output waveform is very screwy on the small PP leading to the fluke thinking it is producing a much higher current than it is.

 

[jim hardy]

With the first PSU the voltage is reported to be 8.5-9V on a meter set to DCV

I never saw a clarification of whether it's AC or DC volts he's reading.

i measured voltage across the element with an ordinary fluke meter set to V (does both AC/DC)

the Fluke T5 tells you whether it sees AC or DC voltage, without a clue in datasheet as to how it decides which to report if both are present. i assume he sees AC reported since he speaks of a transformer.
T5's current measurement is AC only though.
i also assumed he's got a T5 since it's the only thing on Fluke site that resembles his description.



http://www.fluke.com/fluke/m3en/electrical-testers/Electrical-Testers/Fluke-T5-600.htm?PID=74729
http://www.myflukestore.com/pdfs/ca...fluke_t5_1000_electrical_tester_datasheet.pdf

I also assumed his phase width modulation is delaying turn-on after each zero crossing just like a thyristor lamp dimmer

i could be all wet here .. .

awaiting clarification

won't meddle further

over and out,

old jim

 

[The Electrician]

If, and that's a big if, your measurements are correct, the only way that a current of 31 amps and 12 volts across the nichrome element are not heating as much as with the large PP, is that that the phase relation between the applied current and voltage are not favorable.

You've already said you don't have equipment to measure waveform.

You won't be able to solve this problem without some measurements beyond what you've already made.

The least expensive measurement device I can think of that would give you information about the phase relation between voltage and current is a wattmeter.

This meter:

https://www.amazon.com/dp/B00009MDBU/?tag=pfamazon01-20

will allow you to determine just how much power is being delivered to your heated element. It can also measure the power factor of the power delivered from the grid to each PP.

You can use it to measure the power supplied from the grid, and minus the (probably) insignificant power loss in the transformer, that will be the power heating your element.

You thus get a comparison of the actual power delivered by the small PP compared to that delivered by the large PP, which is what counts in comparing heating performance.

 

[jim hardy]

If the element isn't heating it's a safe bet 31 amps are not flowing through it.

So one question is, what could make a meter that is measuring a phase angle modulated AC waveform report higher than actual volts and amps ?
From T5 customer reviews at Amazon:

(Gripe)#2 is that if you had ever had to trouble shoot a lower voltage like 24v dc the meter will show the highest reading in which sometimes there are phantom voltages that will show like say 30 vac but negligible voltage on dc.

try an analog meter
http://www.homedepot.com/catalog/pr...ops - wasn't going to meddle. over and out.

 

[njohnston]

wow, away for weekend - didn't expect so many replies...

where to start... yes, used a T5 meter to measure AC output from PP... PP setups are identical apart from component ratings (also rewired larger pp equipment to feed smaller pp transformer to confirm that the transformer is the problem)...

from the replies i have received i believe the problem must be that i underrated the smaller transformer when building smaller pp due to inaccurate measurements and therefore inaccurate calculations...

my main reason for posting this thread was to find out why the element was not heating when (supposedly) higher voltages and amps were fed to it via smaller pp... the inaccurate readings from wonky waveforms or phantom voltages or just inaccurate meter would account for this...

i would like to get a hold of a wattmeter as suggested - just to confirm - but will have to ask around for one...

i think my next course of action will be to try the higher VA transformer (if i can borrow it) to see what happens... if problem still persists then i will try baluncores suggestion and try “inverter” or “caddy” welder for stick, TIG, MIG or plasma cutting.

hehe, i had no idea that there would be such a problem with this... thought i was simply scaling down an existing design - plus the literature on phase angle controller stated it was suitable for feeding transformers...

Many thanks to all who have contributed here - i now have a couple of things to try out and a better idea of where i went wrong in the first place...

 

[NascentOxygen]

i would like to get a hold of a wattmeter as suggested - just to confirm - but will have to ask around for one...

A wattmeter won't tell you anything useful. We already know it's getting fewer watts! The element's reduced temperature is as good a watt-gauge as we need.

The most useful data to explain your quandry would be the voltage waveform across the element, this when using each controller, to reveal how they differ. The explanation accounting for the different heating will be revealed within that essential piece of information, the voltage waveform.

So, if you are able to borrow equipment, try for an oscilloscope.