Dynamic Eccentricity Induced in Induction Motor Detected by Optical Fiber Bragg Grating Strain Sensors

Mechanical and electrical forces of a three-phase induction motor (TIM), in operation, can cause strain in the stator, which may lead to frustrating vibration problems. These problems may greatly reduce reliability and ultimately lead to increased maintenance cost. This paper shows the method to overcome this problem by carrying out the dynamic strain measurement analysis in a TIM running at no-load condition, using an optical fiber sensor. The dynamic strain induced by mechanical and electrical forces was measured by using fiber Bragg gratings (FBGs), installed inside the motor, in between two stator teeth. The electromagnetic immunity and reduced size of the FBGs render them suitable for this application. The frequency components of the strain in TIM were predicted using a model of the forces present in the air gap, considering static and dynamic rotor eccentricities. Two TIM motors, one of 2-poles and the other of 4-poles, and each equipped with two FBGs strain sensors, were used. The sensors were positioned between two stator teeth, separated by 120° from each other in the 2-pole motor and 90° in the 4-pole motor. The experimental analysis shows that the frequency components of the dynamic force spectrum presented by the two motors are the same as those predicted theoretically. Rotor eccentricity was achieved by attaching an unbalanced load to the rotor shaft. The amplitude of vibration at rotor rotational frequency increased when the unbalanced load on the rotor shaft increased. The measurement presented in this paper can be used to determine some TIM parameters, such as the effect of the motor power supply on its vibration signal. It can also be used as a potential maintenance tool for the continuous monitoring of the TIM with high sensitivity.