Magi - Magnetizing Current - Voltech AT5600 User Manual

C H A P T E R 7 – T E S T S
7.35.

MAGI - Magnetizing Current

Where Used
The AT5600 provides two main methods to verify that a transformer is correctly
assembled, ensuring the proper number of primary and secondary turns, the right
core material, and the correct air gap (if required).
Magnetizing current and open circuit voltage tests are preferred for line-frequency
transformers, as they operate over the full B-H curve, including non-linear regions.
These tests allow for performance verification at the actual operating voltage and
frequency, unlike conventional 5V LCR-style tests.
(For other transformer types, such as pulse transformers or those used in
switched-mode power supplies, inductance and turns ratio tests are generally
preferred.)
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:
where
= The current to be specified when testing with winding A
I
A
= The current for windings A and B in series
I
AB
= The number of turns on winding A
N
A
= The number of turns on A and B in series
N
AB
(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, which is the inrush effect.
To give you repeatable and accurate results, the measurement does not start until
any transient has settled.
AT5600 User Manual 98-119 issue 30
A N D T E S T C O N D I T I O N S
I = I x (N
A AB AB
/ N )
A
Page 168