2.34 MAGX - Magnetizing Current (External Source)
The AT 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.)
This version of the MAGI test uses an external source to provide the test
signal, which must be coupled to the tester via a Voltech AC Source
Interface (contact your supplier for details).
See the AC Source Interface user manual or the Editor help system for
details of how to configure external ac sources for use with an AT.
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: -
IA = IAB x (NAB / NA)
IA = The current to be specified when testing with winding A
IAB = The current for windings A and B in series
NA = The number of turns on winding A
NAB = 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 repeatable accurate results, the measurement does not start until
any transient has settled.
The test signal can have a frequency in the range 20Hz to 5KHz, and an
amplitude from 5V to 600V depending on the type of external source used.
The current rating of a single AC Source Interface is 10A rms.
See also the MAGI test which uses the ATs internal generator, which can
provide up to 270V ac at 25W.