Voltech ATi User Manual page 364

T T
ESTS AND EST
P 7-66
AGE
C
ONDITIONS
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 errors of one turn or more.
What happens if I set absolute limits for both primary and
secondary windings?
If you want to detect up to a ten-turn error on the primary and one-
turn on the secondary, in the above example, set 9½ (pri) and ½
(sec). The tester uses the equivalent ratio limits: 21.95 and 18.05.
Setting both limits WIDENS the limit to allow for an error of ten
turns on the primary SIMULTANEOUSLY with one turn on the
secondary.
I.e., to allow 190:11 (17.27) and 210:9 (23.33).
Notice that these limits will NOT detect just a one-turn error on the
secondary because you have now chosen limits to allow for errors
on both primary and secondary, resulting in a broader tolerance on
the absolute ratio.
V AT U
OLTECH I SER
M
ANUAL