DRIVING DEVICE FOR MOTOR

PURPOSE:To make it possible to keep on operating a motor without stopping the operation of a production line, etc., by continuing the operation in detecting the signal loss signal, and at the same time in giving the speed command signal in substitution when the motor speed signal is lost. CONSTITUTION:When the signal loss signal is detected by a speed detection signal loss detecting device 3, the point of contact 9a is released to open the speed control loop. At the same time, points of contact 9a and 9c are closed, the past data of a speed detection signal 2 are outputted as a rotating speed command 7, a torque command 12 is outputted multiplying by the past data of a torque command 6 to a multiplier 8 multiplied by a multiplying factor K in consideration of the detection delay, a vector computing element 4 is passed through, an inverter 5 is controlled and a motor 10 is driven. In a digital type control unit, as to the past data, such a method is adopted as preserves the current value of the rotating speed of the motor 10 in a register 13 every time and discards outdated data one after another.