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An electric drive system comprises a generator, a traction motor, a brake resistor, a bus, and a control unit. The generator, the traction motor, and the brake resistor are coupled electrically to the bus. The control unit is configured to determine a pulse-width-modulation duty cycle for the brake

An electric drive system comprises a generator, a traction motor, a brake resistor, a bus, and a control unit. The generator, the traction motor, and the brake resistor are coupled electrically to the bus. The control unit is configured to determine a pulse-width-modulation duty cycle for the brake resistor (“brake duty”) and control operation of the brake resistor according to the brake duty, wherein the brake duty can be a value intermediate of constant OFF and constant ON. A method of operating the electric drive system is also disclosed.

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1. An electric drive system, comprising: a generator,a traction motor,a brake resistor,a bus, the generator, the traction motor, and the brake resistor coupled electrically to the bus, anda control unit configured to determine a pulse-width-modulation duty cycle for the brake resistor (“brake duty”)

1. An electric drive system, comprising: a generator,a traction motor,a brake resistor,a bus, the generator, the traction motor, and the brake resistor coupled electrically to the bus, anda control unit configured to determine a pulse-width-modulation duty cycle for the brake resistor (“brake duty”) and control operation of the brake resistor according to the brake duty, wherein the brake duty can be a value intermediate of constant OFF and constant ON, and the control unit is configured to determine the brake duty dependent on an amount of power desired to be consumed by the brake resistor (“desired power consumption” or “DPC”) and a power consumption capacity of the brake resistor (“PCC”) according to the equation: brake duty=DPC/PCC, wherein the brake duty is constant OFF if an actual voltage of the bus (“actual bus voltage”) is less than a predetermined lower bus voltage and is constant ON if the actual bus voltage is greater than a predetermined upper bus voltage, and is intermediate constant OFF and constant ON if the actual bus voltage is between the predetermined lower bus voltage and the predetermined upper bus voltage. 2. An electric drive system, comprising: a generator,a traction motor,a brake resistor,a bus, the generator, the traction motor, and the brake resistor coupled electrically to the bus, anda control unit configured to determine a pulse-width-modulation duty cycle for the brake resistor (“brake duty”) and control operation of the brake resistor according to the brake duty, wherein the brake duty can be a value intermediate of constant OFF and constant ON, and the control unit is configured to determine the brake duty dependent on an amount of power desired to be consumed by the brake resistor (“desired power consumption” or “DPC”) and a power consumption capacity of the brake resistor (“PCC”) according to the equation: brake duty=DPC/PCC, wherein the control unit is configured to determine the desired power consumption dependent on a difference between an estimated power available from the bus and a non-zero reference power of the bus (“power difference” or “Pdiff”), and a motor power predicted to be applied to the bus by the traction motor (“predicted motor power” or “Pmot”) according to the equation: DPC=Pdiff−Pmot, such that, with respect to the predicted motor power, supply of electric power to the bus is negative and removal of electric power is positive. 3. An electric drive system, comprising: a generator,a traction motor,a brake resistor,a bus, the generator, the traction motor, and the brake resistor coupled electrically to the bus, anda control unit configured to determine a pulse-width-modulation duty cycle for the brake resistor (“brake duty”) and control operation of the brake resistor according to the brake duty, wherein the brake duty can be a value intermediate of constant OFF and constant ON, and the control unit is configured to determine the brake duty dependent on an amount of power desired to be consumed by the brake resistor (“desired power consumption” or “DPC”) and a power consumption capacity of the brake resistor (“PCC”) according to the equation: brake duty=DPC/PCC, wherein the control unit is configured to determine the desired power consumption dependent on a difference between an estimated power available from the bus and a non-zero reference power of the bus. 4. An electric drive system, comprising: a generator,a traction motor,a brake resistor,a bus, the generator, the traction motor, and the brake resistor coupled electrically to the bus, anda control unit configured to determine a pulse-width-modulation duty cycle for the brake resistor (“brake duty”) and control operation of the brake resistor according to the brake duty, wherein the brake duty can be a value intermediate of constant OFF and constant ON, and the control unit is configured to determine the brake duty dependent on an amount of power desired to be consumed by the brake resistor (“desired power consumption” or “DPC”) and a power consumption capacity of the brake resistor (“PCC”) according to the equation: brake duty=DPC/PCC, wherein the control unit is configured to determine the desired power consumption dependent on an actual voltage of the bus, a non-zero reference voltage of the bus, and a capacitance of a DC link capacitor coupled electrically to the bus. 5. An electric drive system, comprising: a generator,a traction motor,a brake resistor,a bus, the generator, the traction motor, and the brake resistor coupled electrically to the bus, anda control unit configured to determine a pulse-width-modulation duty cycle for the brake resistor (“brake duty”) and control operation of the brake resistor according to the brake duty, wherein the brake duty can be a value intermediate of constant OFF and constant ON, and the control unit is configured to determine the brake duty dependent on an amount of power desired to be consumed by the brake resistor (“desired power consumption” or “DPC”) and a power consumption capacity of the brake resistor (“PCC”) according to the equation: brake duty=DPC/PCC, wherein the control unit is configured to determine the desired power consumption dependent on a motor power predicted to be applied to the bus by the traction motor (“predicted motor power”). 6. The electric drive system of claim 5, wherein the control unit is configured to determine the predicted motor power dependent on a motor torque setpoint of the traction motor and a rotational speed of the traction motor. 7. An electric drive system, comprising: a generator,a traction motor,a brake resistor,a bus, the generator, the traction motor, and the brake resistor coupled electrically to the bus, anda control unit configured to determine a pulse-width-modulation duty cycle for the brake resistor (“brake duty”) and control operation of the brake resistor according to the brake duty, wherein the brake duty can be a value intermediate of constant OFF and constant ON, and the control unit is configured to determine the brake duty dependent on an amount of power desired to be consumed by the brake resistor (“desired power consumption” or “DPC”) and a power consumption capacity of the brake resistor (“PCC”) according to the equation: brake duty=DPC/PCC, wherein the control unit is configured to determine the power consumption capacity of the brake resistor dependent on an actual voltage of the bus and a resistance of the brake resistor. 8. A method of operating an electric drive system, the electric drive system comprising a generator, a traction motor, a brake resistor, and a bus, the generator, the traction motor, and the brake resistor coupled electrically to bus, the method comprising: determining a pulse-width-modulation duty cycle for the brake resistor (“brake duty”), andcontrolling operation of the brake resistor according to the brake duty, wherein the brake duty can be a value intermediate of constant OFF and constant ON, and determining the brake duty comprises determining an amount of power desired to be consumed by the brake resistor (“desired power consumption” or “DPC”) and a power consumption capacity of the brake resistor (“PCC”) according to the equation: brake duty=DPC/PCC, wherein determining the desired power consumption comprises determining a difference between an estimated power available from the bus and a non-zero reference power of the bus (“power difference” or “Pdiff”), and a motor power predicted to be applied to the bus by the traction motor (“predicted motor power” or “Pmot”) according to the equation: DPC=Pdiff−Pmot, such that, with respect to the predicted motor power, supply of electric power to the bus is negative and removal of electric power is positive. 9. A method of operating an electric drive system, the electric drive system comprising a generator, a traction motor, a brake resistor, and a bus, the generator, the traction motor, and the brake resistor coupled electrically to the bus, the method comprising: determining a pulse-width-modulation duty cycle for the brake resistor (“brake duty”), andcontrolling operation of the brake resistor according to the brake duty, wherein the brake duty can be a value intermediate of constant OFF and constant ON, and determining the brake duty comprises determining an amount of power desired to be consumed by the brake resistor (“desired power consumption” or “DPC”) and a power consumption capacity of the brake resistor (“PCC”) according to the equation: brake duty=DPC/PCC, wherein determining the desired power consumption comprises determining a difference between an estimated power available from the bus and a non-zero reference power of the bus. 10. A method of operating an electric drive system, the electric drive system comprising a generator, a traction motor, a brake resistor, and a bus, the generator, the traction motor, and the brake resistor coupled electrically to the bus, the method comprising: determining a pulse-width-modulation duty cycle for the brake resistor (“brake duty”), andcontrolling operation of the brake resistor according to the brake duty, wherein the brake duty can be a value intermediate of constant OFF and constant ON, and determining the brake duty comprises determining an amount of power desired to be consumed by the brake resistor (“desired power consumption” or “DPC”) and a power consumption capacity of the brake resistor (“PCC”) according to the equation: brake duty=DPC/PCC, wherein determining the desired power consumption comprises determining the desired power consumption dependent on an actual voltage of the bus, a non-zero reference voltage of the bus, and a capacitance of a DC link capacitor coupled electrically to the bus. 11. A method of operating an electric drive system, the electric drive system comprising a generator, a traction motor, a brake resistor, and a bus, the generator, the traction motor, and the brake resistor coupled electrically to the bus, the method comprising: determining a pulse-width-modulation duty cycle for the brake resistor (“brake duty”), andcontrolling operation of the brake resistor according to the brake duty, wherein the brake duty can be a value intermediate of constant OFF and constant ON, and determining the brake duty comprises determining an amount of power desired to be consumed by the brake resistor (“desired power consumption” or “DPC”) and a power consumption capacity of the brake resistor (“PCC”) according to the equation: brake duty=DPC/PCC, wherein determining the desired power consumption comprises determining a motor power predicted to be applied to the bus by the traction motor (“predicted motor power”). 12. The method of claim 11, wherein determining the predicted motor power comprises determining the predicted motor power dependent on a motor torque setpoint of the traction motor and a rotational speed of the traction motor. 13. A method of operating an electric drive system, the electric drive system comprising a generator, a traction motor, a brake resistor, and a bus, the generator, the traction motor, and the brake resistor coupled electrically to the bus, the method comprising: determining a pulse-width-modulation duty cycle for the brake resistor (“brake duty”), andcontrolling operation of the brake duty resistor according to the brake duty, wherein the brake duty can be a value intermediate of constant OFF and constant ON, and determining the brake duty comprises determining an amount of power desired to be consumed by the brake resistor (“desired power consumption” or “DPC”) and a power consumption capacity of the brake resistor (“PCC”) according to the equation: brake duty=DPC/PCC, wherein determining the power consumption capacity of the brake resistor comprises determining the power consumption capacity dependent on an actual voltage of the bus and a resistance of the brake resistor.