Transformer Turns Question

I think you'll have so much leakage inductance that the number you get will be very rough.

Other methods:

• estimate the volume of wire, measure it's diameter, get the length, then estimate the turns from the area.

• Weigh the thing, estimate the weight due to copper, measure the diameter, get the length, ditto.

D'oh. Measure the DC resistance, measure the wire diameter, use the conductivity of copper or a copper wire table to estimate the length ...

Whoa! "leakage inductance"?? Where's that coming from already? Concentrically-wound transformers are a perfectly valid topology AFAIAA.

Not so simple. The windings aren't single layer; they're stacked about

That can work. It might be better to drive the relay coil from a signal generator and measure the added winding voltage as the secondary.

For more accuracy, measure the DC resistance and the inductance of the relay coil, pick a good frequency, and do the math to account for the copper loss. Or Spice it.

Don't expect an exact turns count, but you should be able to get into the ballpark. If you "close" the relay (push the armature down) there will be less flux leakage error and you might get within 1%.

Certainly *safer* I guess!

Not sure what you mean by "a good frequency" - how do I determine that? I also thought maybe pulling the iron core out (if possible) and replacing it with a ferrite rod in case saturation might be an issue?

I would add more than ten turns. Why? I don't know, I just feel you would have more accuracy with 100 or 200 turns.

Mikek

Well, you need to do a little math and maybe some measurement. The frequency should be high enough that the voltage drop in the inductance is above the voltage drop in the copper resistance. And stay below the self-resonant frequency.

A nonlaminated iron core in a DC relay will have a lot of loss at high frequencies, too.

If you are willing to disassemble the relay, you could use a better, closed core. Or unwind and count!

Just curious, why do you want to know the number of turns?

If you drive it from both sides, and compare the ratios, maybe you could compensate for that?

If I were doing it , I would put the 10 turns in series with the original winding and apply AC. Measure the voltage across the original winding and measure the voltage across the ten turn winding. And get the ratio of the o riginal winding voltage to the ten turn voltage and multiply that by ten to get the number of turns on the original winding. Digital multimeter ought to work. Will not be perfect, but should be close.

That may be what you said, but I did not understand you to say put the wind ings in series.

Dan

That is true, but you said it was a relay coil.

Dan

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Good advice. And place the turns right on top of the existing coil, and use a much lower frequency and watch the excitation voltage because what you will have is a high turns count step up ratio and you could easily short turns in the coil, especially if it was 'scatter wound'.

Assuming a ten volt 60Hz excite on a 100 turn primary, the secondary would be at 500 volts. It is a 50 to 1 ratio.

So, one could easily determine what the actual turns count is by simply reading the 'secondary' voltage and the ten volts into 100 turns makes all the math easier too. Keeping the freq low is better as well, especially if it is as was said a non-laminated steel core.

Don't answer that, Chris. It's a trap. ;-)

Measure the inductance

Take 100 turns off

Measure the inductance again

Do some math which will show the original inductance

Cheers

Klaus

is

l winding and apply AC. Measure the voltage across the original winding an d measure the voltage across the ten turn winding. And get the ratio of the original winding voltage to the ten turn voltage and multiply that by ten to get the number of turns on the original winding. Digital multimeter oug ht to work. Will not be perfect, but should be close.

Supply the overwind with enough DC current to make the relay just close.

Add 50% to this number.

Compare this with the rated DC current for the relay.

The ratio gives the turns ratio.

Eg: relay closes at 2A with 10 turns, plus 50% = 3A

So the relay will operate reliably at 30A with one turn.

Say the nominal relay current is 24mA, ie 12V into 500ohms.

... Phil

But how accurate is the 50% derating?

Instead of comparing to the rated current, you can compare to the minimum current. The minimum current of one coil to the minimum current of the other.

This is a surprisingly great thread. It reminds me of how many ways there are to use a barometer to measure a building.

Or temporarily add 100 turns and you don't have to mess up the original.

snip

Transformer turns to voltage ratio is the most accurate and the most precise. 10 turns is too few and results in too high a ratio. 100 is far better and with a mere 10Volt excitation even makes the math easier.

I'll add to that, if you made two measurements, one using 100 turns and one using 200 turns, and combined that data correctly that would be even bette r.. cancel the effects of leakage inductance etc.

also you proable want to drive the 1000's turn coil from a low Z and measur e the induced V in the small coils with a high Z load. Sweep the freq and see if there are any freq effects to look out for. Too high and you will b e near resoances. I can't think of any isses beig too low???

I'll let someone smarter than me figure out the math for combining the two measurments in the optimal way.

Mark