Voltech AT360, User Manual

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7-69
Where Used
The turns ratio by inductance test is used on transformers that have poor
coupling btween windings and where a turns ratio by voltage (TR) test may not
produce the expected result.
For example, transformers for electronic lighting ballasts are often deliberately
wound to have relatively large values of leakage inductance. During voltage
TR testing, some of the voltage will appear across the leakage inductance and
alter the result.
The TRL test is not affected by leakage inductance and other parasitic
components in this way.
Test Conditions
To measure turns ratio by inductance, two inductance measurements are
made - one on each winding. The same voltage (at the specified amplitude
and frequency) is applied to each winding in turn and the inductance of each is
determined from the measured voltage and current using harmonic analysis.
The turns ratio is then calculated as the square root of the ratio of the
inductances.
The inductance of a winding can often depend upon the flux density in the
core/windings. Since the flux density will depend upon the signal energising
the winding, it is important that both windings are energized at the same level.
This will ensure that both inductances are measured along the same region of
the B/H curve of the core and give a true ratio.
Choosing Test Conditions
The simplest method of setting the test conditions is to use the ‘Measure’
button of the PC Editor software. There are two other methods of inputting the
test parameters, one is to set the primary voltage and frequency and let the
editor set the secondary voltage, and the second is to set both voltages
manually.