Cleverly use LED lights as motor phase failure protectors

The circuit is shown in the figure below. When any phase in the three-phase circuit is broken, the balanced star point is broken, the small relay J is energized and energized, and the external control contact is actuated to immediately cut off the coil power supply of the main circuit AC contactor to protect the motor from phase failure.

The components are φ5mm high-brightness white LED (or yellow, green and red, corresponding to ABC three-phase), 1N4001 silicon diode, 2W22kΩ resistor, and small relay HH52P-L plug-in type with action indicator 5A AC220V or JZX-22F of Chint. Adjust the resistance value so that the forward working current of the LED is ≥15mA and the forward voltage drop is about 3.5V, so that it can emit light normally. More importantly, it is necessary to meet the pull-in current of the small relay, which is generally between 10~20mA. When any phase in the three-phase circuit is disconnected, the balanced star point is broken, and the zero point at 0 rises to a voltage of 190V. This 0 point is suspended and cannot be connected to any neutral line. This 190V voltage is added to both ends of the small relay J, which is greater than the minimum pull-in voltage, and it acts, and the red LED on the top of itself lights up.

These small components are cheap and good quality. A small buzzer can be added between point 0 and point N, that is, the two ends of the small relay J, or it can be connected to an external large alarm through the contact point. Sound, light, and motion can alarm at the same time.

We can install all these components in a relay to make a special phase-failure protection relay.