Symmetrical stator flux orbits for direct flux and torque controlled drives

In this paper a solution is given for the problem of asymmetrical flux orbits, inherent to the field acceleration method (FAM) which limits the stator flux amplitude between two levels in an induction motor. It is shown that the allowable stator flux band ΔΨ has a number of optimal values which generate the smallest radial ripple (reactive power ripple) resulting in polygons with 6,18,30,42…etc. sides. The problem of asymmetrical flux orbits originates in the uncertain `position´ of the flux vector Ψ&oarr; at the start of a segment of 60°. The proposed solution consists of a radial-band controller that measures the flux position 6 times per cycle and adapts the band for ΔΨ accordingly. Within 2 cycles the orbit shape is stable and symmetrical at all possible frequencies and accelerations. Such an orbit eliminates the FAM problem of inter- and even-current harmonics, which can cause low frequency torque and speed changes, especially for the coarse orbits of sub-kHz high power VSIs. Experimental results of a DSP controlled small 2.2 kW setup are given