2.30 MAGI - Magnetizing Current
Magnetizing current is the term used to denote the total current that
flows into the primary of a transformer when the transformer is energized
at a specific voltage and frequency, with the secondaries open circuited.
Although known as magnetizing current it is actually the combination of
the current required to magnetize the core (I1) and the current
required to supply the losses in the core (I2)
The AT5600 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.)
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
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 AT uses a switch-on sequence, which minimises such
Specifying the Test Limits
The AT offers you two ways to specify the test limits: -
Using a true rms measurement
Using a mean-sense measurement, which is scaled to rms for sinewaves.
Generally the rms value would be used.
However, you may wish to use the second method if, for example, your test
limits have been established by a previous measurement on a low cost
multimeter which uses this technique.