C H A P T E R 1 4 - A T E D I T OR
AT5600 User Manual 98-119 issue 14 Page 328
Choosing the test limits
The limits may be entered as:
1. A nominal value together a percentage tolerance (%)
Enter the nominal Primary to Secondary ratio, together with the limits as
positive and negative percentage values.
2. An absolute number of turns (#)
When limits are set as +/- an absolute number of turns, the tester converts
these limits to the % type in the following way:
= +/- (Pri Limit + Sec Limit) x 100%
(Pri Turns Sec Turns)
(You can see the exact result of this conversion by clicking on % limits in the
Editor dialogue once the absolute, #, limits have been entered.)
For example, take a typical Switched Mode Power Supply transformer with
windings in the ratio of 200:10.
Typically, a prior measurement of inductance will have confirmed the correct
number of primary turns have been fitted with the correct cores. Setting absolute
limits of 0 on the primary and 1 on the secondary will result in limits of;
Nominal Ratio: 200/10 = 20
Ratio + 1 sec turn = 18.18
Ratio –1 sec turn = 22.22
Limits calculated by tester = (0/200 + 1/10) x 100%= +/- 10%
The equivalent ratio limits used by tester are:
Upper limit = 22. Lower Limit = 18
Which equates to the actual ratios with a one turn error on the secondary.
In general, the voltages obtained during any voltage turns ratio test will not equate
exactly to the theoretical calculation of Np / Ns, because, for example, of the
relative positions of the windings with respect to each other and the core.
General Rule
When setting turns ratio limits in absolute terms, use a limit of ½ turn less than
the number of turns you want to detect. In this case set a limit of 0 (pri) and ½
(sec). This will result in equivalent limits of 19 and 21. Ideal for detecting an error
of 1 turn or more.
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