Variable flux machine is a new class of permanent magnet machines that combine the flux controllability in wound field synchronous machines, and the high efficiency due to permanent magnets. An AlNiCo based variable flux machine whose air-gap flux is varied by applying a direct axis current pulse is considered in this work.

This thesis reviews the dq model of permanent magnet machines, which is then modified to account for flux controllability in variable flux machines. The existing inductance measurement methods are analyzed, and a vector control technique that is fast in the measurement of current dependent inductances at various magnetization level is proposed. The accuracy of the method is assessed by comparing it with the flux linkage calculation method.

The variable flux machines are designed with small air-gap lengths to reduce the magnet thickness and the magnetization current. Due to this reason, saturation and cross-magnetization effects causes changes in the flux produced by one coil, when current in other coil is changed and vice versa. If the cross-magnetization effects are neglected, accurate control of the magnet flux would require a slower current controller. This would need a longer duration pulse to change the magnetization level. The longer the pulse duration, the more severe is the torque ripple. For high-performance drives, consideration of cross-magnetization effects is required. Therefore, this thesis proposes a vector controlled technique to evaluate the cross-magnetization effects in variable flux machines. The proposed technique is able to measure the self and cross-coupled inductances at any magnetization level using an existing drive. The dq model of variable flux machine is then modified to account for the cross-magnetization effects.

The cross-coupled inductances are used to evaluate the static torque-angle characteristics of the machine. Two methods using the vector control technique are presented to experimentally measure the static torque-angle characteristics. The benefit of using a vector control drive is that the effects of heating and unequal phase resistances are taken care automatically. The measured torque-angle characteristics are compared with the calculated and finite element method simulation ones. Finally, the same vector control technique is used to evaluate the torque ripple performance of the VFM. This technique is not only useful in evaluating the designed machine, but it is also useful to evaluate design software.