Driving circuit for DC motor

A DC motor drive circuit comprising

• speed command means for generating a command speed signal;
• speed detection means for detecting the actual speed of a DC motor;
• a thyristor-controlled full-wave reversible bridge including means, when in a power rectification mode, for converting an AC voltage from a power source to a DC voltage and for applying the DC voltage to the DC motor and, when in a power inversion mode, for feeding the DC motor armature current as AC power back to the power source;
• and
• control means for controlling the firing angle of the thyristors of said bridge so that a speed deviation signal corresponding to the difference between the command speed signal and the detected actual DC motor speed is made to approach a predetermined difference, and for shifting the operating mode of said bridge from the power rectification mode to the power inversion mode when the command speed undergoes an abrupt decrease in absolute value, and then back to the power rectification mode from the power inversion mode so that the DC motor will rotate at said predetermined difference from the command speed, said control means further comprising:
• polarity discrimination means for detecting a change in the polarity of the speed deviation signal after said bridge has switched to said power inversion mode after said abrupt decrease of the command speed;
• firing angle narrowing means responsive to said detected change in polarity of the speed deviation signal for narrowing the firing angle of said thyristors down to a reduced angle when said bridge is operating in the power inversion mode before changing back from the inversion mode to the rectification mode, said reduced firing angle being such as to ensure that the current through said motor is intermittently interrupted;
• current intermittence detection means for detecting at least the first said interruption of the DC motor armature current after the operation of said firing angle narrowing means, and for generating a signal indicative thereof, the generation of said signal taking place after the occurrence of a predetermined condition after said detection of the first interruption of the DC motor armature current;
• blocking means for blocking the firing of said thyristor bridge circuit for a prescribed period of time after the third of said three firing pulses has been generated;
• and
• means for changing over the operating mode of said thyristor bridge from said inversion to said rectification mode after the occurrence of said predetermined condition.