OVERLOAD PROTECTIVE CIRCUIT FOR MOTOR

PURPOSE:To allow operation in the vicinity of limit load by constituting the protective circuit for motor having different overload limit of a combination of a plurality of integrating circuits having different time limit characteristics thereby obtaining optimal time limit characteristics. CONSTITUTION:A speed error amplifying circuit 1 generates a current command (a) for a motor from a speed command and a speed feedback signal, an overload protective circuit 2 detects overload of the motor from the current command (a), and an alarm signal processing circuit 4 processes an overload alarm signal (b) to produce a signal (c) for interrupting a motor drive circuit 5. The current command (a) is converted through an absolute value circuit into a signal of unidirectional characteristics which is fed to three integrating circuits A, B, C having time constants A, B, C. Since the time constants A, B, C are set such that the transistors are turned ON with different time limit characteristics, an overload alarm signal (b) makes a transition from H to L at such timing as the current (i) turns the transistors ON in respective regions by turning all switches On when the load limits are different.