Specifications

CIRCUIT IDEAS
warded by the author, the transformers
X1, X2, and X3 primaries as well as
switch S1 were connected after the con-
tacts of contactor. As a result energisation
of contactor was not feasible. Even when
switch S1 was shifted to a ‘live’ phase,
relay RL1 (as well as contactor C) was
energising/de-energising in quick suc-
cession during single-phasing and caus-
ing sparking—for obvious reasons. Hence
the circuit was suitably modified at EFY
(as presented).
2. The relay was also changed from
12V to 6V rating, as 12V relay was not
energising properly with single-phasing.
3. Proper polarity of the transformer
connections has to be ensured in the
above circuit. To determine proper po-
larity, connect the primary ends which
are eventually to be connected to three
phases, to any single phase (the other
ends are connected to neutral). Now pro-
ceed to connect secondaries of two of the
three transformers in series and mea-
sure the AC ouput across the uncon-
nected ends. This should be double (24V
AC) of the individual secondary output
(12V AC). If it is not so, reverse one of
the two secondary connections to get the
required output. Similarly, connect the
third transformer secondary in series
with the other two secondaries. The out-
put across the unconnected ends should
now be treble (36V AC). If it is not so,
reverse the connections of the third sec-
ondary. Now shift the primary ends
(connected to single phase) to each of
the three phases, as shown in the fig-
ure. The voltage across points P1-P2 will
be nearly zero if all three phases are
present.
—Technical Editor
54