1.  Field of the Invention
The present invention relates to a method and apparatus for controlling an electric motor using star modulation. 
2.  Description of the Related Art
Electric motors are used to drive a variety of vehicular systems, such as electric assist steering systems, for example, which are well known in the art.  In such systems, an electric assist motor, when energized, provides torque assist to aid the driver in turning steerable wheels of the vehicle.  The electric assist motor is typically controlled in response to both steering torque applied to the vehicle steering wheel and measured vehicle speed.  A controller monitors steering torque and vehicle speed and controls a drive circuit to control current applied to the electric assist motor.  Such drive circuits can include FETs (field effect transistors) or other forms of solid state switches operatively coupled between the vehicle battery and the electric assist motor.  Motor current is controlled by pulse width modulation of the FETs or switches. 
An electric assist steering system can use a permanent magnet AC (PMAC) motor.  The PMAC motor is a three-phase motor.  The FETs would be connected to provide three-phase voltage/current to each phase (A, B, C) of the PMAC motor.  The star voltage of the PMAC motor is the sum of the A, B and C phase voltages of the motor.  The star voltage is measured at the wye of the motor, where the motor phases are connected to system, i.e., battery ground. 
Motor control methods incorporate current control algorithms which assume that the star voltage of the motor remains constant or uncontrolled.  In reality, the star voltage can vary due to a variety of factors, such as impedance variations caused by variances in the windings of the individual the motor phases.  Assuming a constant star voltage in a control arrangement can lead to overmodulation.  For example, if the star voltage is assumed to be zero and is actually higher, pulse-width-modulation of the FETs to supply a desired voltage to any of the motor phases would actually cause a higher voltage to be supplied to the phases.  As a result, the sinusoidal voltages produced via pulse-width-modulation of the FETs would be clipped. 
Overmodulation may also lead to variations in the star voltage as a function of rotor angle.  As a result, the voltage available for each phase may vary depending on operating conditions.  This can lead to phase-to-phase distortion, and may cause the phase-to-phase voltages and currents to become non-sinusoidal.  Non-sinusoidal phase-to-phase voltages and currents may produce unwanted torque ripple and/or acoustic noise.