Control circuit for a D.C. motor

A circuit for controlling the temperature of a d.c. motor having an armature and a field winding, comprising:

• a chopper control circuit connected in series with the field winding and armature;
• voltage sensing means for sensing the resistive voltage drop across the field winding due to current flow in the winding;
• current sensing means for sensing current in the field winding, said current sensing means including means for deriving a voltage proportional to said current having a current shunt connected in series with the chopper control circuit;
• comparing means for comparing the sensed voltage across the field winding with the voltage proportional to said current and providing an output signal when the comparison indicates that the temperature of the field winding exceeds a predetermined level;
• and
• a sample and hold circuit operable to sense and retain a voltage proportional to the voltage across the current shunt during each "on" period of the chopper control circuit and to supply the retained voltage to the comparing means, the output from the comparing means limiting the mark-space ratio of the chopper control circuit and controlling the armature current, the chopper control circuit reducing the armature current of the motor in response to receiving the output signal from the comparing means.