Dynamic behavior and stability of energy efficient electro-magnetic suspension of rotors involving time delay

Energy efficient contact less suspension of high-speed rotor shafts in application areas such as power-plants and Flywheel Energy Storage Systems (FESS), can be achieved through Active Magnetic Bearings (AMBs). Inevitably, there exists time delay in sensing the rotor displacement and actual application of control force by electromagnets, thus leading to system instability. This paper therefore analyses stability of AMBs, with constant time delay. FESS is modelled as a rigid rotor-AMB system. Performance of two different control laws, (i) PID and (ii) a recently introduced Four-Element (FE) law, are compared. The governing equations are transformed to frequency domain, to exploit Pade's approximation in modeling time delayed value of feedback signal and stability analysis is carried out by analyzing the system poles. This methodology has been verified using experimental results from the existing literature. Numerical simulation of delay differential equations is also used to validate analytical results. The FE control law shows promise in tolerating delay, as compared to conventional PID controller thus leading to AMB system with higher possible spin speed and relaxed sampling constraints on AMB system instrumentation. One of the prospective applications of this work is in design of flywheel energy storage systems for an electric vehicle. •Time delay in Active Magnetic Bearing (AMB) system is inevitable.•Delayed control force from AMB on rotor may cause instability.•Stability analysis of 4-DOF rotor AMB system with constant time delay is reported.•For the first time, such analysis is done with AMB using integral control action.•Verification with experimental results and numerical simulation is carried out.