Phase failure relay

Phase failure relay

This 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).