Bus voltage stabilization in powertrain having electric starter system with polyphase brushless starter motor

A method for stabilizing voltage on a direct current (DC) voltage bus in a system having an internal combustion engine, a power inverter module (PIM) electrically connected to an alternating current (AC) voltage bus and a direct current (DC) voltage bus of a powertrain, a polyphase/AC brushless starter motor electrically connected to the PIM via the AC voltage bus and selectively coupled to the engine during a requested engine start event, and a controller programmed with voltage stabilization logic that includes a proportional-integral (PI) torque control loop, the method comprising:

• responsive to the requested engine start event, determining the voltage level of the DC voltage bus using an electrical sensor;
• comparing the voltage level to a calibrated minimum voltage using the controller;
• responsive to the voltage level being less than the calibrated minimum voltage:
• determining, using the PI torque control loop, a motor torque of the starter motor;
• and
• transmitting the motor torque to the starter motor as a commanded starting torque to thereby crank and start the engine;
• wherein the commanded starting torque has a value that limits inrush current to the starter motor.

A powertrain comprising:

• an internal combustion engine;
• a transmission connected to the engine;
• a direct current (DC) voltage bus;
• an alternating current (AC) voltage bus;
• a power inverter module (PIM) electrically connected to the AC voltage bus and the DC voltage bus;
• a polyphase/AC brushless starter motor electrically connected to the PIM via the AC voltage bus, wherein the starter motor is selectively coupled to the engine in response to a requested engine start event;
• an electrical sensor connected to the DC voltage bus and configured to output a signal indicative of a voltage level of the DC voltage bus;
• and
• a controller in communication with the electrical sensor and programmed with voltage stabilization logic that includes a proportional-integral (PI) torque control loop, wherein execution of the voltage stabilization logic in response to the requested engine start event, when the voltage level of the DC voltage bus is less than a calibrated minimum voltage, causes the controller to determine a starting torque of the starter motor using the PI torque control loop, and to transmit the starting torque to the starter motor as a torque command to thereby crank and start the engine;
• wherein the starting torque has a value that limits inrush current to the starter motor.