Broken Bar Fault Detection in Induction Motor by Using Optical Fiber Strain Sensors

In this paper, the dynamic strain measurement of a four-pole induction motor stator operating at 75% and 100% of the rated load is used to detect a broken rotor bar. Four fiber Bragg grating (FBG) strain sensors are installed inside the motor, between two stator teeth. Electromagnetic immunity and decreased size of the FBGs make them suitable for this application. When a broken bar is present, some characteristic frequencies in the stator strain spectrum appear. The values of these frequencies are proportional to the motor slip. Instrumentation allows the diagnosis of the broken bar in two different regions of the stator strain frequency spectrum: first, near the rotational frequency, and second, close to twice as much the supply frequency. Two different supply conditions are presented, feeding the motor from an electrical grid and by using a variable-frequency driver. The experimental results indicate that the broken rotor bar can be detected in all supply situations by using the two regions of the stator strain spectrum. By using the method presented in this paper, the broken rotor bar can be detected, even with a harmonic distortion in voltage. This is an advantage over the classic motor current signature analysis method.