Effects of Magnetic Saturation on Optimal Efficiency Reference Generation for the Wound Rotor Synchronous Machine

Minimization of electrical losses throughout full machine operation is essential for applications of modern drives with wide operating ranges such as vehicle drivetrains. Producing optimal reference currents that minimize copper loss and core loss for a combination of torque and speed is generally a difficult problem to solve analytically given the many non-linearities in a machine. The wound rotor synchronous machine’s magnetic behavior changes between zero torque and rated torque. The machine considered in this study has a saliency (L<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</inf> > L<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</inf>) at zero torque and a saliency (L<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</inf> < L<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</inf>) at rated torque. An affine magnetics model is created at each of these two points. Using the zero torque inductance approximation to predict the flux at the peak torque operating point leads to an error of 278 μVs or 94% for d-axis flux and 101 μVs or 80% for q-axis flux. Additionally, use of a constrained nonlinear optimization solver results in different sets of optimal efficiency reference currents for the two models.