The Test Conditions for Inductance Measurements
Wherever possible, this table should be used for all inductance tests. The
inductance range should be chosen based on minimum value of inductance
expected. When choosing the test conditions, the following potential problems
should be considered:
Current levels
The upper voltage limits should be chosen to give a maximum current level of
about 50mA RMS. for the lowest inductance expected. In some cases, this
current may cause core saturation, and a lower voltage should be used. The
minimum voltage level must be chosen so that the test current does not become
so low that it cannot be sensibly measured. The lower voltage limits in the table
above always give test currents higher than 3µA RMS.
Self-Resonant Frequency
At lower frequencies, the capacitance of the windings can normally be ignored
because its impedance is much higher than that of the inductance. However, at
very high frequencies, this is not so, the capacitance dominates, and inductance
cannot be measured. The self-resonant frequency of the transformer is the
change-over point between these two regions.
Normally to get a good measurement of inductance, the test frequency should be
less than 20% of the resonant frequency of the transformer.
In general, high values of inductance will have a high inter-turn capacitance and
hence a low resonant frequency. Where there is a choice of test frequencies
always use the lower value, to minimize any problems due to self-resonance.
Non-linear inductance
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