TRANSFORMER TESTS VOLTECH AT3600 USER MANUAL
PAGE 7.1.8. VPN 98 - 024 TESTS AND TEST CONDITIONS
1.5. LS OR LP - INDUCTANCE
(SERIES OR PARALLEL CIRCUIT)
WHERE USED
The AT3600 offers two basic alternative ways to confirm that the transformer has
been assembled properly, with the appropriate number of primary and secondary
turns, the right grade of magnetic material for the core, and the correct air gap if
required.
When testing signal, pulse and switched mode power transformers, where the normal
operating conditions require only small excursions of the B-H curve, never extending
beyond the linear regions, a measurement of primary inductance, together with a
turns ratio test is used.
(For line frequency transformers, designed to operate over the full extent of the B-H
curve, including the non-linear regions, the preferred method is to use a magnetising
current test applied to a primary winding, followed by an open-circuit voltage test for
every other winding.)
MEASUREMENT CONDITIONS
To measure inductance, the tester applies an ac voltage across the selected winding; it
then measures the voltage across and the current through the winding using harmonic
analysis. The measured voltage is divided by the current to obtain a complex
impedance and the inductance is calculated.
The test signal can have a frequency in the range 20Hz to 1MHz, and an amplitude
from 1mV to 5V.
Generally, it is not necessary to measure the inductance at the normal operating
conditions of the transformer, which could involve, for example, voltage levels of
hundreds of volts. This is because the B-H curve can normally be assumed to be
linear in the operating region, and the inductance measured at a low level represents
the inductance that will appear in use.
Also, it can usually be assumed that the inductance value does not vary significantly
with frequency. Therefore, although high frequencies are available with the tester,
measurement frequencies above a few hundred kilohertz should be used with caution.
This is because the errors caused by the stray inductance and capacitance of your
fixture may become much more significant at these frequencies. Compensation can
be used to eliminate these errors.
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