Investigations on Failure of Electromagnetic Brake units of Motors used in Critical Applications of Sodium Cooled Fast Breeder Reactor

In sodium cooled Fast Breeder Reactors; power control is achieved through control rods which are driven by Induction motor with Electromagnetic brake. The motor will be energised upon release of brake unit. The brake coil will be energised to release the brake. The coil is of 48 V DC. If the brake is not released properly due to low flux developed in the brake coil, inter-turn short, liner wear; then there is a chance of Induction Motor getting damaged due to high frequency Surge generated due to interruption of locked rotor current. Hence the performance of brake unit in this application is highly critical. Voltage build-up time for brake power supply is checked. Magnetic field developed by various EM brake units has been measured and plotted as a function of applied voltage. Brake coil has been modelled in FEMM and Electromagnetic Finite Element Analysis has been carried out for determining the number of turns and fill factor of the coil. Brake unit has been dismantled and the effect of brake liner and the friction plate on pulling force of coil is observed. The same has been simulated in FEMM for computing the EM force on movable armature (disk) at various distances from the brake coil. Experiments and simulation results were used for diagnosing the cause of non-release of the EM brake. This paper discuses various causes of failure to the brake unit and to avoid recurrence of similar failure event.