Measuring Transformer Inductance

[Newmarket]

Hi all.
I have a beefy output transformer. Wanting to measure the inductances.
I should know this but I don't ! Should I short out the other windings when measuring a winding to get sensible results ?

 

[Newmarket]

 

[Murdock]

You don't need to short the other windings as far as I know. That way you would measure leakage inductance.
Do you have a LCR meter?

 

[user 37518]

Maybe CJ can offer good pointers. I guess that if you only want to know the primary inductance you would want to measure without a load, so you can measure only the primary inductance without the effect of the mutual inductance between the secondary coil.

 

[dmp]

Yes, leave the other windings open. Measure at 100 or 120 Hz and you can compare to other measurements posted around here.

Frequency response however may be more important as you'll see many unknown brand transformers that roll off beyond 10-15kHz.

And if I'm reading that right it is a 10k:10 ohm ? allowing 5 mA DC? Guess it is a push-pull or single ended speaker output transformer?

 

[Newmarket]

You don't need to short the other windings as far as I know. That way you would measure leakage inductance.
Do you have a LCR meter?

Yes. I have use of LCR meters. I thought I had put an image of one in addition to the tx but seems not so putting it below.

 

[Newmarket]

btw reason for asking is that I was getting unrealistic readings with windings open. But I'll check again tomorrow in case I was getting the setup wrong.

 

[Bo Deadly]

There are other ways to measure inductance.

For example, you have to figure the inductance is in the ball-park of 10H. So if you use a noise / fn generator make an LC with a 1nF capacitor, you should get a big peak in the 1kHz range. Find the exact peak frequency and then plug that into a calculator like this one:

(Sample)RLC Low-pass Filter Design Tool - Result -

Using the DCR for R, 1n for C, fiddle with L until it matches the measured peak. Then you know L.

 

[arensattic]

Hallo all,

I have used the following procedure with some success for determining the inductance values of unknown transformers. In my attic, I have an ancient Hewlett Packard HP-200CD audio generator (valve generator with 600 ohm output). I used this output to drive a parallel circuit of the unknown L in parallel with a 1 µF C via a 33k series resistor. Noted the frequency at which voltage across the LC circuit peaked, and calculated L with the well-known formula for parallel resonance. I left all secondary windings open, since short-circuiting these windings resulted in erroneous (nonsense) values for L.

Another method (for measuring inductance values <= 10H) is described on my website.
See Inductance Meter.

Best regards,

Aren

 

[trobbins]

The OT rating plate indicates it is for single-ended amplifier, given the 5mA DC rating for the primary, and the tapped primary of 2.5k and 10k. Did you get a valid inductance reading with your LCR meter? If not (perhaps due to a low AC excitation voltage), and if you want a measurement with 5mAdc, then the way I make that measurement is in link:
https://dalmura.com.au/static/Choke measurement.pdf

 

[Newmarket]

No. I ran into trouble in that meters (I have access to a couple) returned some negative values. Indicating that it's seeing capacitance as the dominant reactive impedance ???

 

[trobbins]

Does your LCR meter allow you to choose frequency? The bulk capacitance from the winding may mask the inductance at 1kHz and above. Apart from the link I gave, another method is to put 5 to 12Vac (whatever is available for you) at mains frequency across the primary, but through a sensitive ac current meter, and measure the AC current level through the primary - that was the way they did it in the 1940's with the AVO 7 meter.

 

[Newmarket]

Thanks. Yes one of them - Hameg HM8118 - allows to set frequency and level. I'll have a play when I get a chance. Maybe Friday afternoon.

 

[abbey road d enfer]

The big issue with LCR meters is they don't have enough juice to operate the core at its intended flux, which results in lower value than normal.
The winding should be driven at a level similar to that it will be submitted to in normal operation.
A simple method consists in driving the winding at nominal level while measuring the current in a low value sensing resistor, as suggested by trobbins.

 

[CJ]

Hook it up to a lamp cord

Put a ammeter in series.

Use power strip so you have a circuit breaker.

Measure current.

If using 120 AC then use this formula.

Primary inductance is


120/6.28 x 60 Hz x I or simply

Henries = 0.318/I

Example 1 ma will equal 318 Henrys

Be careful you do not hook the 120 up to the secondary.


There is also a small fuse in your ammeter.

A Variac is nice but I use the wall socket on a regular basis.

Using 120 will simulate real world levels and frequency.

Inductance will be higher at 20 Hz but not by much in a gapped core like you have.

 

[abbey road d enfer]

Hook it up to a lamp cord

Put a ammeter in series.

Do I understand that you mean powering the xfmr with 120Vac, with only the ammemter and a circuit breaker for protection?

Be careful you do not hook the 120 up to the secondary.

Isn't it somewhat contradictory with your first statement?

Using 120 will simulate real world levels and frequency.

In the case of this specific xfmr, yes, but the principle cannot be extended to just any xfmr.

 

[Newmarket]

Thanks for the input. I forgot to take it back into work to try measuring on the bench last week so will try and remember to do so this week as time allows.

 

[CJ]

Of course use insulated alligator clips and secure the wires ,

Kit Carson would charge into a camp of 30 Indians with just himself and his buddy to get his stolen horses back. How dangerous is that compared to plugging in a transformer?

We have gotten a little soft over the years me thinks.

https://www.nationalgeographic.com/...-yosemite-el-capitan-explore-through-the-lens

 

[abbey road d enfer]

Of course use insulated alligator clips and secure the wires ,

Operator safety is not my concern here. I'm just warning that you can't do it for measuring the secondary inductance (rated at 2.5/10 ohms.

 

[Newmarket]

Thanks again for the input here.
For clarity wrt safety - I'm very aware of this. My work environment may involve voltages upto 70kV so safety is clearly a priority.
Also, regardless of personal safety, I don't want to be the guy that trips out the lab power with kit on test etc - esp when it's a personal project.
I'll try the suggested LCR Meter method before getting mains voltage involved directly.