Electric power system for controlling an electric machine

1. A method for controlling a front-end converter, wherein the front-end converter is electrically connected to an electric inverter electrically connected to an electric machine via a high-voltage DC bus, and wherein a high-voltage DC power source is disposed to supply electric power to the front-end converter, the method comprising:

monitoring, via a controller, the electric machine;

determining a reference current based upon the electric power supplied from the high-voltage DC power source;

determining a motor current based upon the monitoring of the electric machine;

determining a feed-forward current based upon the motor current and the monitoring of the electric machine, including:

determining an average motor current,

determining a rate limiter term associated with a command to operate the front-end converter in one of a linear mode and a six-step mode, and

determining the feed-forward current based upon the motor current, the average motor current and the rate limiter;

determining a first duty cycle based upon the reference current, the motor current and the feed-forward current;

determining a feed-forward duty cycle based upon the monitoring of the electric machine;

determining a second duty cycle based upon the feed-forward duty cycle and the first duty cycle; and

controlling operation of the front-end converter based upon the second duty cycle.

2. The method of claim 1 , wherein determining a reference current based upon the electric power supplied from the high-voltage DC power source comprises determining the reference current based upon a difference between a voltage potential of the high-voltage bus and a commanded voltage for the high-voltage bus.

3. The method of claim 1 , wherein determining the feed-forward current based upon the motor current, the average motor current and the rate limiter comprises determining the feed-forward current based upon only the motor current when the rate limiter term is associated with a command to operate the front-end converter in the linear mode.

4. The method of claim 1 , wherein determining the feed-forward current based upon the motor current, the average motor current and the rate limiter comprises determining the feed-forward current based upon the average motor current when the rate limiter term is associated with a command to operate the front-end converter in the six-step mode.

5. The method of claim 1 , wherein determining a motor current based upon the monitoring of the electric machine comprises:

determining electric current supplied to the electric machine;

determining a motor torque command, motor speed, a battery electrical potential and a bus electrical potential;

determining commanded duty cycles for controlling the front-end converter based upon the motor torque command, the motor speed, the battery electrical potential and the bus electrical potential; and

determining the motor current based upon the electric current and the commanded duty cycles.

6. The method of claim 1 , wherein determining a feed-forward duty cycle based upon the monitoring of the electric machine comprises:

determining a motor current;

determining an average motor current;

determining a ripple current based upon a difference between the motor current and the average motor current;

determining a rate limiter term associated with a command to operate the front-end converter in one of a linear mode and a six-step mode; and

determining the feed-forward duty cycle based upon the ripple current and the rate limiter term.

7. The method of claim 6 , comprising determining the feed-forward duty cycle based upon the ripple current and the rate limiter term only when the command to operate the front-end converter comprises a six-step mode.

8. The method of claim 6 , wherein the feed-forward duty cycle is zero when the command to operate the front-end converter comprises a linear mode.

9. The method of claim 1 , wherein determining a second duty cycle based upon the feed-forward duty cycle and the first duty cycle comprises adding the feed-forward duty cycle and the first duty cycle.

10. An electric power system, comprising:

a front-end converter electrically connected to an electric inverter electrically connected to an electric machine via a high-voltage DC bus;

a capacitor electrically connected between positive and negative conductors of the high-voltage DC bus;

a high-voltage DC power source disposed to supply electric power to the front-end converter;

a controller in communication with the front-end converter, the electric inverter, the electric machine, and the high-voltage DC power source, and operatively connected to the front-end converter and the electric inverter, the controller including an instruction set, the instruction set executable to:

monitor operation of the electric machine;

determine a reference current based upon a magnitude of electric power supplied from the high-voltage DC power source;

determine a motor current based upon the operation of the electric machine;

determine a feed-forward current based upon the motor current and the operation of the electric machine;

determine a first duty cycle based upon the reference current, the motor current and the feed-forward current;

determine a feed-forward duty cycle based upon the monitoring of the electric machine, including the instruction set executable to:

determine an average motor current,

determine a rate limiter term associated with a command to operate the front-end converter in one of a linear mode and a six-step mode, and

determine the feed-forward current based upon the motor current, the average motor current and the rate limiter;

determine a second duty cycle based upon the feed-forward duty cycle and the first duty cycle; and

control operation of the front-end converter based upon the second duty cycle.

11. The electric power system of claim 10 , wherein the instruction set is executable to determine the reference current based upon a difference between a voltage potential of the high-voltage bus and a commanded voltage for the high-voltage bus.

12. The electric power system of claim 10 , wherein the instruction set is executable to determine the feed-forward current based upon only the motor current when the rate limiter term is associated with a command to operate the front-end converter in the linear mode.

13. The electric power system of claim 10 , wherein the instruction set is executable to determine the feed-forward current based upon the average motor current when the rate limiter term is associated with a command to operate the front-end converter in the six-step mode.

14. The electric power system of claim 10 , wherein the instruction set is executable to:

determine electric current supplied to the electric machine;

determine a motor torque command, motor speed, a battery electrical potential and a bus electrical potential;

determine commanded duty cycles for controlling the front-end converter based upon the motor torque command, the motor speed, the battery electrical potential and the bus electrical potential; and

determine the motor current based upon the electric current and the commanded duty cycles.

15. The electric power system of claim 10 , wherein the instruction set is executable to:

determine a motor current;

determine an average motor current;

determine a ripple current based upon a difference between the motor current and the average motor current;

determine a rate limiter term associated with a command to operate the front-end converter in one of a linear mode and a six-step mode; and

determine the feed-forward duty cycle based upon the ripple current and the rate limiter term.

16. The electric power system of claim 10 , wherein the instruction set is executable to determine the feed-forward duty cycle based upon the ripple current and the rate limiter term only when the command to operate the front-end converter comprises a six-step mode.

17. The electric power system of claim 10 , wherein the instruction set is executable to determine wherein the feed-forward duty cycle is zero when the command to operate the front-end converter comprises a linear mode.

18. The electric power system of claim 10 , wherein the instruction set is executable to determine a second duty cycle based upon the feed-forward duty cycle and the first duty cycle by numerically adding the feed-forward duty cycle and the first duty cycle.