C H A P T E R 7 – T E S T S A N D T E S T C O N D I T I O N S
AT5600 User Manual 98-119 issue 14 Page 150
7.35. MAGI - Magnetizing Current
Where Used
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.
Magnetizing 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 magnetizing 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 energizing 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 magnetizing 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 magnetize a transformer to
a given flux level is independent of which winding is used.
In practice, the magnetizing 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 AT5600 uses a switch-
on sequence which minimizes such transient effects.
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