Author Topic: Center tap transformer - measuring inductance  (Read 430 times)

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Offline justinjjaTopic starter

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Center tap transformer - measuring inductance
« on: June 03, 2023, 03:28:15 pm »
I'm measuring the secondary of a ~1kw 60hz transformer,
between the 2 secondary legs I get 0.22H, seemed reasonable.
But measuring between each leg and the center tap and I get no inductance, just a small resistance.

Is that normal?
I was expecting to get 0.11 H between each leg and the center tap.

I was thinking it was the meter, so I hooked up an AC voltage and confirmed the meter isn't wrong, rather my understanding must be wrong.
 

Online Kleinstein

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Re: Center tap transformer - measuring inductance
« Reply #1 on: June 03, 2023, 03:43:24 pm »
With a coil the inductance is proportional to the number of turns squared. So the expected inductance is 0.055 H for each half. An iron core may be slightly nonlinear and the inductance reading may not be that accurate.
 

Offline justinjjaTopic starter

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Re: Center tap transformer - measuring inductance
« Reply #2 on: June 03, 2023, 04:01:08 pm »
Oh right,
The meter reads inductors an order of magnitude smaller than 0.055H,
Seems like I should still at least get something?
 

Offline Jwillis

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Re: Center tap transformer - measuring inductance
« Reply #3 on: June 03, 2023, 05:13:32 pm »
Just a shot in the dark here. Does your meter have both an Auto function and manual function? I know with mine that sometimes when values are very low, I can get resistance readings from inductors when in Auto mode but get proper inductance reading in manual mode. You can get weird readings as well if the frequency is set to low or to high.
Also try doing calibration before doing your test. This will often settle down meters that get a bit erratic.
Inductance changes with frequency. Some inductors are sold to have a specific value at a specific frequency. You can find some with the same inductance values but are at different test frequencies. The standard 1kHz test does not always work. For laminated transformers its best to test at the lowest frequencies of 100 or 120Hz.

Hope this helps some.
 

Offline justinjjaTopic starter

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Re: Center tap transformer - measuring inductance
« Reply #4 on: June 03, 2023, 06:49:56 pm »
Figured out my issue, was testing in series with a light bulb at 120v 60hz (lol)
Treating the bulb as a resistor in my calculations,
But really a small change in voltage was changing the bulb's resistance.

Using a 1k resistor and I get  14mH for the full transformer and 2.8mH between the center tap.

EDIT:
Oh my I'm using one of those cheap T4-LCR meters, so it can't be trusted for any level of accuracy.
« Last Edit: June 03, 2023, 06:51:33 pm by justinjja »
 


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