Bias Flux Compensation in the 'Side-By-Side' Combination Radial/Axial Homopolar PM-Biased Active Magnetic Bearing

‘Side-By-Side’ (SBS) Combination Radial/Axial Homopolar PM-Biased Active Magnetic Bearing design has been demonstrated to be an attractive solution for many applications requiring magnetic bearings. One of the key parts of this design is a bias flux compensation coil, which is needed to maintain a constant bias flux level under various axial loads and positions of the rotor. This is important, because variations in the bias flux in the SBS combo bearing affect operating parameters of both the radial and axial channels, including actuator gains and negative stiffnesses. Besides complicating the controls, changes of the bias flux may result in losses of the load capacities, which cannot be recovered thru improving the control algorithms. Previously, the results of testing a standalone SBS combo bearing on a test rig have been presented. These included demonstrations of the effectiveness of the compensation for the effects of the axial control current on the overall bias flux. Those results showed good agreement with theoretical predictions, however, no analytical details of the compensation coil operation were given. The current paper presents the theoretical basis of the bias flux compensation. In addition, effects of the bias flux compensation on the properties of the magnetic suspension are demonstrated using a fully operational machine – a 200kW power range turbocompressor equipped with a SBS combination bearing.