Voltech AT3600 User Manual page 302

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In some transformer designs, the turns ratio between a primary winding and a
secondary winding is not as important as the ratio between different primaries or
different secondaries.
To make the most accurate measurements in such cases apply the test signal to the
primary winding and measure the turns-ratio from primary to one of the secondaries.
Then, leaving the primary energised as above, measure the turns ratio between the
secondaries.
Next, energise a secondary winding (possibly at a different voltage and / or frequency
depending on its inductance) and measure the turns ratio between the various
primaries.
In this way windings which should be matched are treated equally during the test.
S
PECIFYING THE
When specifying turns ratio tests, it is preferable to avoid limits which are
unnecessarily tight, and which may therefore lead to measurement difficulties.
For example, if two equal secondary windings should have 10 turns each, the ratio
should be 1:1. One turn in error would produce a ratio error of 10% or -10% (i.e.
11:10 or 10:11), and therefore limits of +5% and -5% would be suitable to detect the
error.
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