Voltech AT3600 User Manual page 291

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OLTECH SER
1.6. LSB
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With the LSB and LPB tests, the AT3600 offers two further ways to confirm that the
transformer has been assembled properly, with the appropriate number of turns, the
right grade of magnetic material for the core, and the correct air gap if required.
These tests would normally be used for transformers designed to be used in
applications where a large dc current is flowing through one or more of the windings.
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To measure inductance with bias, the tester firstly applies the programmed bias
current to the winding under test. When this has stabilised, it then applies an ac
voltage across the selected winding, and measures the voltage across and the current
through the winding using harmonic analysis. The measured voltage is divided by
the harmonic current to obtain a complex impedance and the inductance is calculated.
The bias current can be programmed in the range 10mA to 400mA. The (ac) test
signal can have a frequency between 20Hz and 1MHz, and an amplitude from 1mV to
5V.
Usually, the signal frequency and voltage are chosen so that, with the expected
inductance, the resulting signal current is less than 20% of the bias current.
Recommended test signals generally would be the same as given in section 7.1.8 for
the normal inductance test. However, if the recommended levels correspond to too
high a signal current, then use a corresponding smaller test voltage.
It is not normally recommended that the test frequency be increased in an attempt to
reduce the signal current, as this may lead to other problems, such as those caused by
parasitic inductance and capacitance of the test fixture, and the self-resonant
frequency of the transformer itself.
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