TRANSFORMER TESTS VOLTECH AT3600 USER MANUAL
PAGE 7.1.56. VPN 98 - 024 TESTS AND TEST CONDITIONS
1.34. MAGI - MAGNETIZING CURRENT
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.
Magnetising current and open circuit voltage are the preferred tests for line frequency
transformers, designed to operate over the full extent of the B-H curve, including the
non-linear regions.
(For other transformers, such as pulse transformers and those used in switched mode
power supplies, inductance and turns ratio are the preferred tests.)
MEASUREMENT CONDITIONS
When measuring magnetising current, you should normally program the test to apply
the highest working voltage at the lowest working frequency to the primary winding.
In the case of a transformer with a split primary, the test can be conducted equally
well by energising just one of the primary windings, as opposed to the two in series.
The expected current will be greater for the single winding, rising in proportion to the
turns ratio:
I
A
= I
AB
x (N
AB
/ N
A
)
where
I
A
= The current to be specified when testing with winding A
I
AB
= The current for windings A and B in series
N
A
= The number of turns on winding A
N
AB
= The number of turns on A and B in series
(As an alternative, the formula above can be written using the voltage ratio between
the two windings, rather than the turns ratio.)
In principle, you may measure the magnetising current using any winding, or any
series combination of windings, with the current limit adjusted according to the
formula above, because the Ampere-turns required to magnetise a transformer to a
given flux level is independent of which winding is used.
In practice, the magnetising current waveform may have a transient component
following the switch-on of the test voltage. To give you repeatable accurate results,
the measurement does not start until any transient has settled. In addition, to give you
the quickest test execution time, the AT3600 uses a switch-on sequence which
minimises such transient effects.
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