If you need to measure a transformer winding inductance in situ, while the transformer is all hooked up to its operating circuitry, that surrounding circuitry may interfere with getting the measurement done if there is a large capacitance present across the winding in question. An LC meter I once tried to use in that way simply could not ascertain a transformer winding's inductance because there was a large capacitance being reflected into that winding from the transformer's other windings.
However, there is a way out of this problem using the test fixture as sketched below.
This test fixture is an amplitude controlled LC oscillator where "L" is the unknown inductance and "C" that inductance's unknown shunt capacitance value.
The fixture is used by connecting the unknown winding, or coil, as shown, selecting C3a, then C3b, then C3c and then C3d and adjusting S2 and R15 in each case to obtain a linear sinewave output which will be AGC controlled to an amplitude of one volt peak. You'll find in most cases that this is not a critical thing to do. The AGC sets and holds the signal level quite easily.
For some unknown value pair of L and C, the inductance and capacitance at the "unknown" winding, we consider two selected capacitance values for which two oscillation frequencies are obtained. Hence the term "dual-resonance".
We will call these Cx and Cy yielding frequencies Fx and Fy. From the basic LC resonance equation, we obtain the following two equations:
L = ( (1/Fx)² - (1/Fy)² ) / ( 4 * pi² * ( Cx - Cy ) ) and C = ( Fy² * Cy - Fx² * Cx ) / ( Fx² - Fy² )
With four values of test capacitance selectable via S1, we get four test frequencies which we then use in six paired combinations to obtain redundant calculations of "L" and "C". If those redundant calculation values are essentially alike, we can have confidence that we've taken all of our readings properly.
The following are some typical results:
In the second case shown, trying to directly measure the 25.5 µHy in the presence of 0.86 µF could be quite problematic.
GWBASIC code for doing these calculations is as follows:
10 CLS:SCREEN 9:COLOR 15,1:PI=3.14159265#:DIM CT(4),FT(4):ON ERROR GOTO 180
20 PRINT "save "+CHR$(34)+"dualres.bas"+CHR$(34):PRINT
30 PRINT "save "+CHR$(34)+"a:\dualres.bas"+CHR$(34):PRINT:PRINT
40 A$=" ###### Hz ###### Hz ########.## uHy #.#### uF"
50 B$=" ###### Hz ###### Hz ####.###### mHy #.#### uF"
60 C$=" Averages = #####.### uHy #.##### uF"
70 FOR XX=1 TO 4:READ CT(XX):CT(XX)=CT(XX)*.000001:NEXT XX
80 DATA .0230,.0349,.0492,.1039:REM These CT values are the test fixture caps.
90 READ F(1),F(2),F(3),F(4):CSUM=0:LSUM=0
100 FOR XX=1 TO 4:F(XX)=F(XX)*1000!:NEXT XX
110 FOR X=1 TO 4:FOR Y=X+1 TO 4
140 PRINT USING B$;F(X),F(Y),L/1000,C:LSUM=LSUM+L:CSUM=CSUM+C
150 NEXT Y:NEXT X:PRINT USING C$;LSUM/6,CSUM/6:PRINT:GOTO 90
160 DATA 14.41,12.07,10.34,7.310:REM Coil 1 readings in kHz.
170 DATA 33.56,33.36,33.06,32.14:REM Coil 2 readings in kHz.
This code will analyze the frequency readings in each DATA line. Add more lines and you get more analyses. In each analysis, you get the six individual calculations and then you get the average values as a sanity check.
A word about the capacitance values in Line 80.
These somewhat odd looking capacitance values were originally entered as standard values of 0.022 0.033,0.047 and 0.1 µF. An air wound test coil was made whose inductance would not change with varying excitation and frequency readings were taken for that coil. These capacitance values were then hand tailored to make the six sets of calculated L and C values as much alike as possible.
One last point is that this test fixture needed to be inside a metal box. It was not possible to make it work properly when unshielded.