Control designs for low-loss active magnetic bearings

Highly efficient electromechanical flywheel batteries (FWBs) require the use of low-loss active magnetic bearings (AMBs). Since the losses in the FWB, and in AMB itself, are proportional to the square of the electromagnet flux, it is imperative to minimize the bias flux (or current) customarily used in the AMB control design. The low-bias requirement impacts the control design in two ways: (1) the constraint imposed between the electromagnets which compose an AMB control axis must be modified to handle zero-bias operation (2) the standard (necessarily nonlinear) control laws used for voltage-mode low-loss AMB operation´ must be modified to avoid control algorithm singularities. This paper illustrates the experimental implementation of a generalized complementary flux constraint to address the first impact. This constraint relies on a DC flux estimation technique that does not use unwieldy Hall-effect sensors. Furthermore, passivity theory is used to remove the zero-bias control law singularity. Experimental evidence supports this claim from a time and frequency domain perspective