Specifications

CIRCUIT IDEAS
causes transistor T2 to cut off
and relay RL1 is not energised.
When the telephone handset
is lifted, the voltage across points
X and Y falls below 12 volts and
so zenor diode D5 does not con-
duct. As a result, base of transis-
tor T1 is pulled to ground poten-
tial via resistor R4 and thus is cut
off. Thus, base of transistor T2
gets forward biased via resistor
R5, which results in the energisation of
relay RL1. The tape recorder is switched
‘on’ and recording begins.
The tape recorder should be kept
loaded with a cassette and the record
button of the tape recorder should re-
main pressed to enable it to record the
conversation as soon as the handset is
lifted. Capacitor C2 ensures that the re-
lay is not switched on-and-off repeat-
edly when a number is being dialled in
pulse dialing mode.
PRAVINCHANDRA B. MEHTA
SIMPLE AND ECONOMIC SINGLE-
PHASING PREVENTOR
T
hree-phase motors and other ap-
pliances are widely used in all
sectors of industry. These appli-
ances are prone to damage due to single
phasing. Apart from damage to the costly
apparatus, it may also cause a produc-
tion loss. Many circuits of single phasing
preventor (SPP) are avail-
able but the circuit sug-
gested here is very simple
and economical.
Easily-available mains
step-down transformers
X1, X2, and X3 (230V AC
primary to 0-12V, 500mA
secondary rating) are used
with their primaries con-
nected in star mode and
secondaries in open delta
mode. The characteristic
of this type of connection
is that when three-phase
balanced input is applied
to the primaries, no out-
put across open delta sec-
ondaries will be available.
But in case of major un-
balance or single-phasing,
some voltage, called re-
sidual voltage, is induced
in the secondaries across
points 1 and 6 shown in
the circuit diagram. Three-phase supply
is given to apparatus (load) through
contactor C. While the primaries of trans-
formers X1, X2, and X3 are connected
ahead of the contactor. The contactor
can be energised via N/C (normally
closed) contacts of relay RL1 by pressing
switch S1.
As soon as single-phasing or major
unbalance occurs, 8 to 12 volts are in-
duced across point P1-P2, which after
rectification operates relay RL1. As a
result, the supply to contactor coil is cut
off and it de-energises, thereby protect-
ing the apparatus. Lamp L(d) is also lit
up (unless B phase has failed), indicat-
ing that SPP has operated. Lamps L(a),
L(b), and L(c) indicate the healthiness of
three phases R, Y, and B. After resump-
tion of the balanced 3-phase supply, the
contactor will automatically energise
(with S1 closed) and supply to the appli-
ance will be resumed.
Notes: 1. In the actual circuit for-
S.C. DWIVEDI
53