A new method of current-based condition monitoring in induction machines operating under arbitrary load conditions

Fault conditions in induction machines such as eccentric air gaps and broken rotor bars result ire harmonics in the stator current of the motor which can be used to monitor the health of the machine. However, variations in the load torque at multiples of rotational speed typically have the same effect on the motor current spectrum. When monitoring a single phase of the stator current, this load effect can prevent the detection of a fault condition by producing current components that obscure those harmonics related to the condition of the machine. This paper presents a new method for removing the load effects from the monitored quantity of the machine. This is accomplished by comparing the actual stator current to a model reference value which includes the load effect. Simulation results illustrating the effects on the spectrum of the monitored quantity are given for both constant and eccentric air gaps when in the presence of an oscillating load.