DEVICE FOR PROTECTING THREE-PHASE MOTORS Russian patent published in 1994 - IPC

FIELD: electric engineering. SUBSTANCE: device has current transformers 1-3, units 4-6 for forming rectified voltages, units 7 and 8 for selecting minimal and maximal rectified voltages, overload checking member 9 made in form of bridge circuit formed by resistors 12-14 and stabilizing diode 15. The first input of integrator-adder is connected across a diagonally opposite pair of junctions of the bridge circuit. Integrator-adder 16 has three inputs of the base of resistors 20-22. The second input of integrator-adder 16 is connected to output of unit for selecting minimal rectified voltage. Output of signals integrator-adder 16 is connected with input of actuating member 11 through reacting member 10. The third input of signals integrator-adder 16 is connected to medium point of additional divider made of resistors 17 and 18 through isolating diode 19. If short-circuit takes place isolating diode 19 opens in the motor and quick charge of capacitor 24 is provided with time constant 22, 24. After that reacting member operates and actuating member and the motor turn off. EFFECT: improved protection. 2 dwg