Method and apparatus for active voltage control of electric motors

1. A method for controlling an inverter, the inverter operable to provide an output voltage to an electric motor, the method comprising:

determining if a commanded output frequency of the inverter does not exceed a first pre-determined threshold;

determining, if the commanded output frequency does not exceed the first pre-determined threshold, if a commanded stator current of the inverter exceeds a second pre-determined threshold;

producing a first command if the commanded stator current exceeds the second pre-determined threshold, the inverter receiving a first gate voltage in response to the first command and providing the output voltage in response to the first gate voltage; and

producing a second command if the commanded stator current does not exceed the second pre-determined threshold, the inverter receiving a second gate voltage in response to the second command and providing the output voltage in response to the second gate voltage, the first gate voltage greater than the second gate voltage.

2. A method according to claim 1 , wherein the inverter has an inverter temperature and is operable with a direct current (DC) supply voltage; and wherein, prior to the step of determining if a commanded output frequency of the inverter does not exceed a first pre-determined threshold, the method further comprises:

determining if the DC supply voltage is not elevated; and

determining if the inverter temperature does not exceed a pre-determined temperature; and

wherein the step of determining if a commanded output frequency of the inverter does not exceed a first pre-determined threshold comprises determining, if the DC supply voltage is not elevated and if the inverter temperature does not exceed a pre-determined elevated temperature, if the commanded output frequency of the inverter does not exceed the first pre-determined threshold.

3. A method according to claim 2 , wherein the step of determining if the inverter temperature does not exceed a pre-determined temperature comprises determining if the inverter temperature does not exceed an elevated temperature, the elevated temperature based on a pre-determined dissipation of one or more losses associated with the inverter at the elevated temperature.

4. A method according to claim 1 , further comprising:

producing the second command if the commanded output frequency does not exceed the first pre-determined threshold; and

producing the second command if the commanded stator current exceeds the second pre-determined threshold.

5. A method according to claim 1 , wherein the inverter has a first current capacity based on the first gate voltage; and wherein the step of producing the first command comprises increasing the first current capacity to a second current capacity, the second current capacity based on the second gate voltage, the first current capacity greater than the second current capacity.

6. A method according to claim 1 , wherein the step of determining if a commanded stator current of the inverter exceeds a second pre-determined threshold comprises comparing an amplitude of the commanded stator current to a threshold current, the threshold current having a hysteresis.

7. A method according to claim 1 , wherein the step of determining if a commanded output frequency of the inverter does not exceeds a first pre-determined threshold comprises determining if the commanded output frequency does not exceed about 3 Hz.

8. An inverter for providing an output voltage to an electric motor and configured to receive a commanded output frequency and a commanded stator current, the inverter comprising:

a gate drive circuit having an input and having an output configured to couple to a switch network, the gate drive circuit configured to provide a gate voltage to the switch network, the switch network operable to produce the output voltage in response to the gate voltage; and

a controller coupled to the input of the gate drive circuit, the controller configured to:

modify the gate voltage based on a speed of the electric motor;

determine if the commanded output frequency does not exceed a second first pre-determined threshold;

determine, if the commanded output frequency does not exceed the first pre-determined threshold, if the commanded stator current of the inverter exceeds a second pre-determined threshold;

produce a first command if the commanded stator current does not exceed the second pre-determined threshold, the switch network receiving a first gate voltage in response to the first command and providing the output voltage in response to the first gate voltage; and

produce a second command if the commanded stator current exceeds the second pre-determined threshold, the switch network receiving a second gate voltage in response to the second command and providing the output voltage in response to the second gate voltage, the second gate voltage greater than the first gate voltage.

9. An inverter according to claim 8 , wherein the controller is further configured to increase the gate voltage if the speed of the electric motor is less than a pre-determined threshold.

10. An inverter according to claim 8 , wherein the controller is further configured to increase the gate voltage if a rotational speed of the electric motor is less than about 3 Hz.

11. An inverter according to claim 8 , wherein the controller is further configured to:

produce the first command if the commanded output frequency exceeds the first pre-determined threshold; and

produce the first command if the commanded stator current exceeds the second pre-determined threshold.

12. An inverter according to claim 8 , wherein the gate drive circuit is configured to receive a positive supply voltage and a common supply voltage; wherein the controller is further configured to increase the positive supply voltage to the gate drive circuit if the speed of the electric motor is less than a third pre-determined threshold; and wherein the gate drive circuit is configured to increase the gate voltage when the positive supply voltage increases.