Safety control device of commutator motors

Claim 1

A safety control device for a speed control circuit of a commutator motor having an armature and a stator and including an A.C. power source having predetermined power source periods, said motor being energized with a supply current from the A.C. power source and being provided with circuit means which cause a free wheel current to be produced in the motor by the inductance of the motor, said free wheel current flowing in the armature and the stator of the motor, a motor speed controller, a comparator circuit comparing a voltage due to an operated amount of the motor speed controller with a voltage due to an actual rotation speed of the motor, an ignition circuit responsive to an output of the comparator circuit and producing a signal, and semiconductor control elements responsive to the signal from the ignition circuit to make a full wave phase control of the motor supply voltage to thereby compensate for rotational variations of the motor, the control device comprising armature current detecting circuit means coupled to the motor armature and operative to detect successively, all through the power source periods and with the advance of the power source periods, the presence and absence of the sections of time series in which the supply current and the free wheel current flow in the armature of the motor; voltage maintaining circuit means connected to said detecting circuit means and being initially set to a level below a predetermined amount of voltage at the time of application of the power source to the motor, said maintaining circuit means being operative to progressively accumulate the voltage above the level in a lapse of time and to decrease the voltage when the supply current and the free wheel current flow in the armature of the motor or to increase the voltage when the supply current and the free wheel current cease to flow in the armature of the motor and to maintain the voltage over at least one cycle of the power source; and bistable circuit means connected to said ignition circuit and operative for nullifying said ignition circuit when said maintaining circuit means has a voltage below the level of said predetermined amount of voltage, said bistable circuit means switching the ignition circuit into an effective condition as said maintaining circuit means comes to accumulate the voltage above the predetermined level while no speed designation is made to the speed control circuit of the motor.