Signatures of electrical faults in the force distribution and vibration pattern of induction motors

This paper presents a method for determining the signatures of electrical faults in the airgap force distribution and vibration pattern of induction machines. The monitoring of faults is achieved through measurement of the vibrations of the stator frame. The radial electromagnetic force distribution along the airgap, which is the main source of vibration, is calculated and developed into double Fourier series in space and time. Finite element simulations of faulty and healthy machines are performed; they show that the electromagnetic force distribution is a sensible parameter to the changes in the machine condition. The computations show the existence of low-frequency and low-order force distributions acting on the stator of the electrical machine when it is working under fault conditions. The simulation results are corroborated by vibration measurements on an induction motor with implemented broken bars and an inter-turn short circuit. The measurements and simulations show that low-frequency components of the vibrations can be used as identifiable signatures for condition monitoring of induction motors. The determination of the vibration frequency corresponding to a given electric fault in a given machine can be achieved through numerical simulations of the magnetic field and electromagnetic forces in the cross-section of the machine, without need for complex structural analysis.