TRANSFORMER TESTS VOLTECH AT3600 USER MANUAL
PAGE 7.1.10. VPN 98 - 024 TESTS AND TEST CONDITIONS
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.
This can be a problem when trying to compare measurements made on
commercially available impedance bridges, or component testers, with
measurements made using the AT3600. The test signal in such bridges is
usually determined within the instrument, and is often at a fixed frequency and
at a voltage level which is not guaranteed to be constant for all value of
inductance.
Usually, if the actual test conditions of the bridge can be determined, and the
tester is then programmed to deliver the same test conditions across the
inductance the results will then agree. (See also the comments below on
differences caused by the choice of equivalent circuit)
D) EQUIVALENT CIRCUIT
Inductance is always measured as part of a complex impedance, the result being
expressed in terms of either a series or parallel equivalent circuit. Note that, for
any given winding, the inductance values for the two circuits are not
necessarily the same. This should be born in mind when specifying the test
limits.
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