Simplest Single Phase Preventor Circuit for Three Phase Motor Potection

 For a 3-phase induction motor,Lit is necessary that all the three phases of supply are present while it is on load.
When any one of the fuses goes out, or a phase is missing, the motor will continue to run with two phases only, but it will start drawing a huge current for the same load.

This high current may ruin the motor, unless switched off immediately. A single phase preventor circuit avoids such a mishap. With this circuit the motor will not run, unless all the three phases are present. In a 3-phase supply, the voltages are 120 degrees apart from each other. Thus the addition of three phases gives zero voltage. lf any one of the phase goes, voltage present at the summing point equals half the line voltage. ‘ In this circuit, the three phases (R, Y, B) are connected to line neutral, which in turn is connected to the ground of the   circuit.

When all three phases are present, voltage at point D is zero. So potential at pin 3 of 1C 741 is also zero, but voltage at pin 2 is nearly 4V. Here 741 is used as a comparator and the voltage at pin 6 is zero. Hence the relay cannot operate. When a phase goes out, voltage at point D goes up to about halt the line voltage. This voltage is divided by 150k and 50k resistors. The voltage at pin 3 is about 8V when 50k potentiometer is properly adjusted. The voltage at pin 6 is about 12V. This base voltage can drive the relay into operating condition. So, the relay would operate when any of the phases goes out.  This relay, when used in the control circuit of the 3-phase motor, or with a circuit breaker, would switch the power oft on operation.