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A retarding system for an electric drive machine (100) includes a direct current (DC) link (312), at which a DC voltage is developed, disposed between a rectifier (206) and an inverter (208). A first contactor switch (216) electrically communicates with a first rail of the DC link (312), and a secon

A retarding system for an electric drive machine (100) includes a direct current (DC) link (312), at which a DC voltage is developed, disposed between a rectifier (206) and an inverter (208). A first contactor switch (216) electrically communicates with a first rail of the DC link (312), and a second contactor switch (216) electrically communicates with a second rail of the DC link (312). A first resistor grid (214) is connected in series between the first contactor switch (216) and the second contactor switch (216). The first resistor grid (214) dissipates electrical energy in the form of heat by conducting a current between the first rail and the second rail of the DC link (312) when the first contactor switch (216) and the second contactor switch (216) are closed.

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1. A retarding system for an electric drive machine having an engine that powers a generator, a rectifier connected to the generator, an inverter, and at least one drive motor connected to the inverter, the retarding system comprising: a direct current (DC) link connected to the rectifier and the in

1. A retarding system for an electric drive machine having an engine that powers a generator, a rectifier connected to the generator, an inverter, and at least one drive motor connected to the inverter, the retarding system comprising: a direct current (DC) link connected to the rectifier and the inverter and including a first DC rail at a first voltage and a second DC rail at a second voltage;a first contactor switch in electrical communication with the first DC rail;a second contactor switch in electrical communication with the second DC rail;a first resistor grid connected in series between the first contactor switch and the second contactor switch;a controller disposed to selectively actuate the first contactor switch and the second contactor switch to conduct a waste current through the first resistor grid between the first DC rail and the second DC rail, the waste current being generated by the at least one drive motor when a motion of the electric drive machine is retarded;an inverter circuit disposed in parallel electrical connection with a portion of the first resistor grid having a resistance that is less than a grid resistance of the first resistor grid, wherein the inverter circuit is configured to control a frequency of an alternating current (AC) voltage being generated in response to a blower command provided by the controller, and wherein the inverter circuit is arranged to convert a DC voltage that is present across the portion of the first resistor grid into the AC voltage;an AC motor electrically connected to the inverter circuit and configured to operate at a variable speed in response to the frequency of the AC voltage;a blower connected to the AC motor;a chopper circuit connected to the first DC rail, and a second resistor grid connected between the chopper circuit and the second DC rail, wherein the chopper circuit is arranged to selectively control a voltage difference applied to the second resistor grid; anda switchable diode disposed in series with the second resistor grid and in parallel with the chopper circuit between the first DC rail and the second resistor grid, the switchable diode being selectively connectable between the second resistor grid with the first DC rail continuously to equalize the first and second voltages and thus deactivate the DC link during service;wherein the blower is arranged to provide convective cooling of the first resistor grid when the AC motor is operating. 2. The retarding system of claim 1, wherein the voltage difference is between a value of zero and a difference between the first voltage and the second voltage. 3. The retarding system of claim 1, further including a blower housing disposed around the blower and the first resistor grid. 4. A machine, comprising: an engine having an output;a generator connected to the output of the engine and disposed to provide a generator output;a rectifier connected to the generator;an inverter connected to the rectifier;a direct current (DC) link connected to the rectifier and the inverter defining a first rail and a second rail;at least one motor connected to the inverter, the at least one motor connected to at least one drive wheel;a first contactor switch connected to the first rail of the DC link;a second contactor switch connected to the second rail of the DC link;a first resistor grid connected in series between the first contactor switch and the second contactor switch;an inverter circuit disposed in parallel electrical connection with a portion of the first resistor grid having a resistance that is less than a grid resistance of the first resistor grid, wherein the inverter circuit is arranged to convert a DC voltage that is developed across the portion of the first resistor grid into an alternating current (AC) voltage having a variable frequency, wherein the variable frequency is determined based on a command signal provided by an electronic controller associated with the machine;an AC motor electrically connected to the inverter circuit and disposed to operate at a variable speed by receiving the AC voltage;a chopper circuit connected to the first DC rail, and a second resistor grid connected between the chopper circuit and the second DC rail, wherein the chopper circuit is arranged to selectively control a voltage difference applied to the second resistor grid; anda switchable diode disposed in series with the second resistor grid and in parallel with the chopper circuit between the first DC rail and the second resistor grid, the switchable diode being selectively connectable between the second resistor grid with the first DC rail continuously to equalize the first and second voltages and thus deactivate the DC link during service;wherein a flow of power is defined from the generator, to the rectifier, to the inverter, and to the at least one motor when the machine is in a propel mode, andwherein a waste power flow path is defined, at least partially, from the at least one drive wheel, through the at least one motor, the inverter, the first contactor switch, the second contactor switch, the first resistor grid, the inverter circuit, and the AC motor when the machine is in a retarding mode, the machine further including a blower connected to the AC motor operable to provide convective cooling to the first resistor grid. 5. The machine of claim 4, wherein the chopper circuit selectively controls a voltage developed across the second resistor grid between a value of zero and a DC link voltage when the machine is in the retarding mode. 6. The machine in claim 5, wherein the second resistor grid is coupled with the first rail when the switchable diode is actuated. 7. The machine of claim 4, wherein the electronic controller is further disposed to selectively actuate at least one of the first contactor switch, the second contactor switch, the rectifier, the inverter, and the AC motor. 8. The machine of claim 4, wherein the generator is a three-phase alternating current (AC) synchronous generator having a brushless, wound rotor. 9. A method of retarding an electric drive machine having a DC link voltage present between a first DC rail and a second DC rail, comprising: determining a magnitude of a retarding command;comparing the magnitude to a threshold value; andactivating two contactor switches and conducting a current through a resistor grid while the magnitude exceeds the threshold value, further comprising activating a blower powered by an alternating current (AC) motor at a selectively variable speed that depends, at least in part, on the magnitude of the retarding command, the blower disposed to receive AC electrical power from an inverter circuit that is electrically connected to a portion of the resistor grid having a resistance that is less than a grid resistance of the resistor grid;providing a chopper circuit connected to the first DC rail, and a second resistor grid connected between the chopper circuit and the second DC rail, and selectively controlling a voltage difference applied to the second resistor grid with the chopper circuit; andproviding a switchable diode disposed in series with the second resistor grid and in parallel with the chopper circuit between the first DC rail and the second resistor grid, and selectively connecting the second resistor grid with the first DC rail continuously to equalize the first and second voltages and thus deactivate the DC link during service. 10. The method of claim 9, wherein activating the two contactor switches includes ensuring that the two contactor switches are activated at about the same time. 11. The method of claim 9, further including controlling the voltage of the DC link by controlling the current conducted through the second resistor grid with a selective duty cycle command of the chopper circuit. 12. The method of claim 9, further including operating an electronic controller that is disposed to receive the retarding command, determine the magnitude of the retarding command, activate the chopper circuit, activate an inverter to regenerate at least one traction motor, and activate the two contactor switches. 13. The method of claim 9, further including generating electrical power with at least one traction motor, the electrical power tending to increase the voltage in the DC link when the electric drive machine is in a retarding mode.