circuit is very simple and low-cost and does not depend
on phase sequence. In case of three-phase motors, if
the motor is already running then it will continue to
run on the remaining two phases even if one of the phases
fails, but it will draw heavy current and may burn.
The circuit shown here keeps track of the presence of
all three phases, and if any of the phases fails, it
activates the relay, thus preventing the motor from
running on two phases and causing damage. During the
presence of a phase, the LDR resistance is low and thus
the voltage at non-inverting terminal of IC LM308 (used
here as a comparator) is low compared to the voltage
set at its inverting input terminal with the help of
preset VR1. As a result, the output of the comparator
is low and the relay remains de-energised. Now if any
phase fails, the corresponding LED stops glowing, so
the concerned LDR resistance as also the voltage drop
across it increases. As a result the potential at non-inverting
terminal becomes higher than that at inverting terminal.
Thus the output of comparator goes high, which energises
the relay through driver transistor T1. The contacts
of the relay can be suitably used for tripping the motor.
Note: The LED and LDR combination should be packed facing
each other in a housing such that there is no leakage
of light. Please note that the voltage at the inverting
input of LM308 is set at a higher value than that of
non-inverting terminal (with all phases present).