Three-phase AC motor phase loss protector

The circuit of this protector is simple and reliable, easy to make, and does not require complex debugging. It can be installed in one go. The circuit is shown in the figure.
　　Working Principle
　　Close the switch QS, press the start button SB2, and the three main contacts connected in series in the main circuit are closed through the B and C phases and the normally closed contacts of the relay K, the thermal relay FR, and SB1, and the AC contactor KM coil to form a loop, and the motor rotates. Release SB2, and the auxiliary contacts and main contacts of the AC contactor connected in parallel with SB2 are closed at the same time, and the KM coil is still energized... and self-locked. When you want to stop the machine, press SB1 to disconnect the contacts, and the auxiliary contacts and main contacts of the contactor are all disconnected, and the motor stops rotating after cutting off the power supply.
　　The capacitors from C1-C6 are connected in a star shape to produce a neutral point. When the motor is normal, the voltage at point M is approximately zero, which is consistent with the midpoint potential of the three-phase four-wire. There is no voltage output between M and N, and the relay K does not operate.
　　When any phase of the power supply line is missing (broken), the potential at point M increases.
　　After rectification by VD1-4 and filtering by C, the coil of K relay is energized and closes, cutting off the AC contact point to cut off the power supply to the motor, thus protecting the motor from burning out. If the motor is missing (broken), the phase time is within 1 second, and the highly sensitive relay K can act.
　　This circuit is universal for 0.1-50KW motors with star or triangle connection. If the motor capacity exceeds 30KW, a larger AC contactor should be selected.
　　Component selection
　　QS is a knife switch, HD13-200/3 can be selected; RF thermal relay is JR10-10; SB2 is LA-10 (green), SB1 model LA-10 (red). KM AC contactor is CJ0-20A, coil closing voltage is 380V; C1~C6 is 2.4μF oil-immersed capacitor, withstand voltage 630V; VD1-VD5 uses 1N4007; C4 is 150μF/50V electrolytic capacitor; K DC relay coil closing voltage is 24V, model JRX-13F, one set of contacts is enough. Other components are subject to the markings in the figure, without special requirements.
　　The author uses two in series, each group of 1.2 microfarads, a total of three groups, in order to increase the withstand voltage value and increase working reliability.