Voltech ATi User Manual page 321

Notes
When choosing test conditions, the following potential problems should be
considered:
a) 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.
b) 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 changeover 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 minimise any problems due to
self-resonance.
c) Non-linear Inductance
Normally inductance measurements should be used for transformers where
the B-H characteristics are linear.
However, if inductance measurements are attempted for instance with line
frequency transformers where the core material is non-linear even at low
signal levels, the measured results can be highly dependant on the applied
test signal.
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