Dynamic brake blending for an inverter propulsion system

In a vehicle control system including an a-c electric traction motor, an inverter having d-c terminals connected to a d-c power source and a-c terminals connected to the motor, regulating means responsive to a torque command signal for controlling the invertor, and mechanical means coupling the motor in driving relationship to a wheel of the vehicle, an improved electrical braking system comprising:

• a. a plurality of resistors;
• b. a plurality of switches, each of said switches being serially connected to a corresponding one of said resistors, to form a resistor-switch combination, each of said resistor-switch combinations being connected in parallel with the power source;
• c. means for determining the actual braking torque of the motor;
• d. means connected for comparing the actual braking torque to the torque command signal and for producing a torque error signal;
• e. filter means connected to an output terminal of said comparing means, said filter means being effective to block transient signals appearing at the output terminal of said comparing means;
• f. multiplying means having a first input terminal connected to the output terminal of said filter means and a second input terminal connected to receive a signal proportional to the rotational velocity of the motor armature, said multiplying means producing, at an output terminal thereof, an output signal proportional to the product of said two input signals;
• g. dividing means having a first input terminal connected to the output terminal of said multiplying means and second input terminal connected to receive a signal proportional to the voltage at the d-c terminals of the inverter, said dividing means producing at an output terminal thereof an output signal proportional to the dividend of said signals at said two input terminals;
• h. first comparison means having a first input terminal connected to the output terminal of said dividing means and a second input terminal connected to receive a predetermined current threshold signal, said comparison means producing an output signal at an output terminal thereof when the first input signal has a greater magnitude than said current threshold signal;
• i. second comparison means having a first input terminal connected to receive a signal proportional to the voltage of the d-c power source and a second input terminal connected to receive a voltage threshold signal proportional to a predetermined minimum desired d-c source voltage, said comparison means producing an output signal when said d-c power source voltage signal is less than said voltage threshold signal;
• j. a logic circuit connected to receive said output signals from said first and second comparison means, said logic circuit producing a first output signal in response to signals from said first comparison means and a second output signal in response to signals from said second comparison means;
• k. counter means connected to receive said first and second signals from said logic means, said counter means being connected to count in a first direction upon receipt of said first signal and in a second direction upon receipt of said second signal, said counter means having a plurality of output terminals for providing a binary representation of a count stored in said counter;
• and
• l. means connecting the output terminals of said counter to said switch means whereby said switch means are logically controlled by the count in said counter.