Braking mode control for a chopper controlled d-c electric motor

A control system for effecting smooth resistor staging during electrical braking of a chopper controlled d-c electric motor comprising:

• a. a plurality of resistor stages serially connected in a motor current path, each of said stages including a resistor and power switching means having normally open main contacts connected in parallel circuit arrangement with said resistor;
• b. a chopper circuit including dynamic braking means connected in said motor current path for providing a series impedance the magnitude of which can be varied between minimum and maximum limits;
• c. sensor means for sensing motor current in said path and for providing a feedback signal proportional thereto;
• d. means for supplying a reference signal proportional to a desired motor current;
• e. summing means for summing said reference signal and said feedback signal and for providing an error signal proportional to any difference in magnitude therebetween;
• f. control means connected to receive said error signal and responsive thereto to vary the impedance of said chopper circuit in a manner tending to reduce said error signal to a minimum value, said control means including means for selectively actuating the power switching means in said resistor stages when the impedance of said chopper circuit reaches said minimum limits, each power switching means being operative when actuated to effect closure of its respective main contacts to thereby short-circuit the corresponding resistor;
• g. detector means coupled to the main contacts of each power switching means for sensing operation thereof, said detector means providing an output signal during operation of said main contacts;
• and
• h. means responsive to said output signal for changing said error signal only during closing operation of said main contacts to a magnitude which causes said control means to adjust said chopper circuit impedance to a value intermediate said minimum and maximum limits.