Chopper type propulsion system with low speed electrical braking capability for traction vehicles

Improved means for effecting electrical braking of a traction vehicle equipped with a propulsion system having motoring and braking modes of operation, said propulsion system comprising a d-c traction motor that behaves as a generator during said braking mode of operation, said motor having an armature and a series field winding and said propulsion system further comprising a filter capacitor adapted to be coupled to a source of d-c electric power, a chopper, means for periodically turning on and turning off said chopper in response, respectively, to alternate first and second gating signals, means for connecting said chopper in series with said armature and field winding across said capacitor when the propulsion system is operating in its motoring mode, and free wheeling rectifier means connected in circuit with said armature and series field winding to conduct motor current during intervals when said chopper is turned off, wherein the improvement comprises:

• (a) means for supplying a variable control signal having a value that can vary between predetermined low and high extremes and that can traverse a predetermined intermediate value in between said low and high extreme values;
• (b) cyclically operative means connected to said control signal supply means for normally producing alternate first and second gating signals for respectively turning on and turning off said chopper, each of said gating signals having a relatively short predetermined duration, and the interval from the production of one of said second gating signals to the production of the succeeding first gating signal being referred to as the off time of said chopper, said cyclically operative means being so constructed and arranged that
• (i) whenever the value of said control signal is at its low extreme no gating signals are produced and said chopper is continuously turned off,
• (ii) when said control signal is in a predetermined range between said low extreme value and said predetermined intermediate value said second gating signals are produced at a predetermined constant frequency while said off time varies inversely with the value of said control signal and decreases toward a predetermined minimum as said control signal approaches said intermediate value, and
• (iii) whenever said control signal is between said intermediate value and said high extreme value said second gating signals are produced at an average frequency that varies inversely with the control signal value and decreases from said constant frequency toward zero as the control signal approaches said high extreme while said off time is maintained substantially equal to said predetermined minimum;
• (c) command means having alternative motoring and braking states;
• and
• (d) brake set up means operative in response to a motoring-to-braking state change of said command means for reconnecting the propulsion system to establish an armature current path comprising said field winding in series with first and second parallel branches, said first branch including said chopper and said second branch including said capacitor in series with said free wheeling rectifier means, and for reversing the polarity of the connection of said series field winding relative to said armature, said brake set up means being effective whenever said command means is in its braking state to maintain said armature current path and to maintain the reversed polarity connection of said field winding and armature.