Development of Active Magnetic Bearings for a Vertical Centrifugal Pump Rotor

Vertical centrifugal sodium pumps are used for circulation of coolant in the primary and secondary circuits of fast reactors for nuclear power plants. Lubrication oil leak from the conventional top bearings in these pumps can cause reactivity changes and result in extended reactor shutdown. One alternative to the conventional bearing is the Active Magnetic Bearing (AMB), which operates without contact and therefore needs no lubrication system. This paper discusses the work done to develop, on a small scale, active magnetic thrust and radial bearings to suit a small sized vertical centrifugal sodium pump. The rotor assembly of this pump weighs approximately 105 kg and the pump is designed to operate in the speed range of 20% to 100% of the rated speed of 2900 rpm. As the shaft diameter and span between bearings is fixed in the existing vertical centrifugal sodium pump some of the critical speeds are in the operating range. The radial magnetic bearing stiffness was designed in such a way that the rotor critical speed within the operating range is largely rigid mode. The thrust magnetic bearing is designed to carry the full rotor weight in vertical direction and levitate the rotor. Proper attention has been paid to design the thrust and radial magnetic bearing rotor to reduce the eddy current loss. Operation of the pump rotor in air has been demonstrated up to full speed using both analog and digital controller based on direct output feedback control scheme. Experimental results show that the vibration level at the bearing location is within 20% of the nominal gap, which is within the allowable limit.