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A powertrain system includes an electric motor/generator unit. Upon detecting an open high-voltage switch associated with a high-voltage DC electrical bus, low-voltage electrical power is employed to energize the high-voltage DC electrical bus, control parameters are adjusted to operate the electric

A powertrain system includes an electric motor/generator unit. Upon detecting an open high-voltage switch associated with a high-voltage DC electrical bus, low-voltage electrical power is employed to energize the high-voltage DC electrical bus, control parameters are adjusted to operate the electric motor/generator unit in a fault tolerant electric generation mode, an internal combustion engine is operated to spin the electric motor/generator unit, and the electric motor/generator unit is operated in the fault tolerant electric generation mode.

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1. A method for operating a powertrain system including an electric motor/generator, comprising: determining a first torque command for the electric motor/generator responsive to an operator torque request;determining a regenerative torque command for the electric motor/generator responsive to a com

1. A method for operating a powertrain system including an electric motor/generator, comprising: determining a first torque command for the electric motor/generator responsive to an operator torque request;determining a regenerative torque command for the electric motor/generator responsive to a commanded voltage across a high-voltage DC bus and a monitored voltage across the high-voltage DC bus;determining a speed-related torque command for the electric motor/generator responsive to a commanded rotational speed for the electric motor/generator;upon detecting an open high-voltage switch associated with a high-voltage DC electrical bus, operating the electric motor/generator in a fault tolerant electric generation mode comprising: employing low-voltage electrical power to energize the high-voltage DC electrical bus;adjusting control parameters to operate the electric motor/generator in the fault tolerant electric generation mode;operating an internal combustion engine to spin the electric motor/generator; andoperating the electric motor/generator in the fault tolerant electric generation mode responsive to the regenerative torque command; else:operating the electric motor/generator responsive to the speed-related torque command in response to one of a commanded speed and a preferred speed profile and a measured rotational speed of the electric motor/generator when operating responsive to a command to control rotational speed of the electric motor/generator; oroperating the electric motor/generator responsive to the first torque command. 2. The method of claim 1, wherein employing low-voltage electrical power to energize the high-voltage DC electrical bus comprises transferring electrical power originating from a low-voltage battery across pre-charge circuits to electrically charge a DC link capacitor electrically connected across the high-voltage DC electrical bus. 3. The method of claim 1, wherein employing low-voltage electrical power to energize the high-voltage DC electrical bus comprises transferring electric power originating from a low-voltage battery to energize the high-voltage DC electrical bus. 4. The method of claim 3, wherein transferring electric power originating from the low-voltage battery to energize the high-voltage DC electrical bus comprises transferring electric power originating from the low-voltage battery across pre-charge circuits to energize the high-voltage DC electrical bus. 5. The method of claim 1, wherein operating the electric motor/generator unit in the fault tolerant electric generation mode comprises operating the electric motor/generator unit to only generate electric power and not operating the electric motor/generator unit to generate torque. 6. The method of claim 1, wherein the commanded power is determined in response to a commanded voltage across the high-voltage DC electrical bus and a monitored voltage across the high-voltage DC electrical bus. 7. The method of claim 1, wherein adjusting control parameters to operate the electric motor/generator unit in the fault tolerant electric generation mode comprises adjusting maximum allowable time-rate changes in commanded direct and quadrature-axis currents associated with controlling the electric motor/generator unit. 8. The method of claim 7, wherein adjusting the maximum allowable time-rate changes in commanded direct and quadrature-axis currents associated with controlling the electric motor/generator unit comprises increasing the maximum allowable time-rate changes in the commanded direct and quadrature-axis currents when operating in the fault-tolerant electric generation mode. 9. The method of claim 1, further comprising operating the internal combustion engine to generate tractive torque output from the powertrain system. 10. The method of claim 1, further comprising operating the electric motor/generator responsive to the regenerative torque command when there is a command to operate the electric motor/generator in an electric power generation mode to charge a high-voltage energy storage system electrically connected to the high-voltage bus. 11. A method for operating a powertrain system including an electric motor/generator, comprising: determining a first torque command for the electric motor/generator responsive to an operator torque request;determining a regenerative torque command for the electric motor/generator responsive to a commanded voltage across a high-voltage DC bus and a monitored voltage across the high-voltage DC bus;determining a speed-related torque command for the electric motor/generator responsive to a commanded rotational speed for the electric motor/generator;upon detecting a fault associated with a high-voltage DC electrical bus resulting in disconnection of the high-voltage DC electrical bus from a high-voltage energy storage system, operating the electric motor/generator in a fault tolerant electric generation mode comprising: employing low-voltage electrical power to energize the high-voltage DC electrical bus;adjusting control parameters to effect operation of the electric motor/generator in the fault-tolerant electric generation mode;imposing limitations on the regenerative torque command, said limitations comprising a first torque limit associated with torque generating capability of the electric motor/generator, a second torque associated with closed-loop power control and a third torque limit associated with closed-loop voltage control;operating an internal combustion engine to transfer torque to the electric motor/generator unit and to generate tractive torque output from the powertrain system; andoperating the electric motor/generator in the fault-tolerant electric generation mode responsive to the regenerative torque command and the imposed limitations; else:operating the electric motor/generator responsive to the speed-related torque command in response to one of a commanded speed and a preferred speed profile and a measured rotational speed of the electric motor/generator when operating responsive to a command to control rotational speed of the electric motor/generator; oroperating the electric motor/generator responsive to the first torque command. 12. The method of claim 11, wherein employing low-voltage electrical power to energize the high-voltage DC electrical bus comprises transferring electrical power originating from a low-voltage battery across pre-charge circuits to electrically charge a capacitor electrically connected across the high-voltage DC electrical bus. 13. The method of claim 11, wherein employing low-voltage electrical power to energize the high-voltage DC electrical bus comprises transferring electric power originating from a low-voltage battery to energize the high-voltage DC electrical bus. 14. The method of claim 13, wherein transferring electric power originating from the low-voltage battery to energize the high-voltage DC electrical bus comprises transferring electric power originating from the low-voltage battery across pre-charge circuits to energize the high-voltage DC electrical bus. 15. The method of claim 11, wherein operating the electric motor/generator unit in the fault tolerant electric generation mode comprises operating the electric motor/generator unit to only generate electric power and not operating the electric motor/generator unit to generate torque. 16. The method of claim 11, wherein the commanded power is determined in response to a commanded voltage across the high-voltage DC electrical bus and a monitored voltage across the high-voltage DC electrical bus. 17. The method of claim 11, wherein adjusting control parameters to operate the electric motor/generator unit in the fault tolerant electric generation mode comprises adjusting maximum allowable time-rate changes in commanded direct and quadrature-axis currents associated with controlling the electric motor/generator unit. 18. The method of claim 17, wherein adjusting the maximum allowable time-rate changes in commanded direct and quadrature-axis currents associated with controlling the electric motor/generator unit comprises increasing the maximum allowable time-rate changes in the commanded direct and quadrature-axis currents when operating in the fault-tolerant electric generation mode. 19. A method for operating a powertrain system including an internal combustion engine and an electric machine, comprising: determining a first torque command for the electric machine responsive to an operator torque request;determining a regenerative torque command for the electric machine responsive to a commanded voltage across a high-voltage DC bus and a monitored voltage across the high-voltage DC bus;determining a speed-related torque command for the electric machine responsive to a commanded rotational speed for the electric machine;upon opening a high-voltage switch of a high-voltage DC electrical bus electrically connected between a power inverter module and the electric machine, operating the electric machine in a fault tolerant electric generation mode comprising: disabling gate drive pulses to an inverter electrically coupled to the electric machine;employing a low-voltage battery to energize a high-voltage bus capacitor of the high-voltage DC electrical bus;adjusting control parameters to operate the electric machine in a fault-tolerant electric generation mode, comprising determining a regenerative torque command comprising a commanded power divided by rotational speed of the electric motor/generator;starting the internal combustion engine to spin the electric machine; andupon the electric machine achieving a predetermined rotational speed: activating the inverter module; operating the electric machine in the fault-tolerant electric generation mode when the high-voltage bus capacitor achieves a predetermined voltage level responsive to the regenerative torque command; andcontrolling an auxiliary power module to electrically charge the low-voltage battery; else:operating the electric machine responsive to the speed-related torque command in response to one of a commanded speed and a preferred speed profile and a measured rotational speed of the electric machine when operating responsive to a command to control rotational speed of the electric machine; oroperating the electric machine responsive to the first torque command. 20. The method of claim 19, wherein the predetermined voltage level comprises a voltage level sufficient to magnetize a stator of the electric machine.