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Active magnetic bearings (AMBs) offer an alternative to conventional ball bearings. Their main advantages stem from the lack of physical contact between the rotor and the stator, and their potential for active disturbance rejection. AMBs are indispensable for high-speed electromechanical flywheel batteries, which have been proposed recently as a replacement of chemical batteries on-board orbiting satellites. In order to be competitive with chemical batteries, these flywheel batteries must operate at very high speeds and have low dissipative losses. Low-losses imply AMB operation in low flux or current bias. In this paper we analyze the disturbance rejection properties of a recently proposed low-bias AMB controller subject to an imbalance force. We use describing functions to analyze and predict the magnitude of position oscillations of the AMB under the imbalance force.