Detecting load unbalance and shaft misalignment using stator current in inverter-driven induction motors

Abstract

This paper examines the effect of changing the input frequency of the induction motor with an adjustable speed drive on the detection of mechanical fault conditions, such as load unbalance and shaft misalignment. The mechanical force resulting from these fault conditions is highly dependent on the rotational speed of the motor. When the motor is run at low speeds, the mechanical fault produces less force, resulting in smaller frequency harmonics in the stator current. Nevertheless, results show that the fault signatures are evident in the stator current spectrum even when the drive input frequency is very low. In addition, the drive frequency harmonics do not affect the fault detection, since the frequency components of interest are much lower than the drive frequencies. It is shown that the tested fault conditions are detected in an induction motor running at a rotational speed of as low as 150 rpm. Mechanical fault conditions are detected using a simple, low-cost algorithm that utilizes a single phase of the stator current The employed algorithm could be easily incorporated into the drive at no additional cost.