Unbalance and harmonics detection in induction motors using an optical fiber sensor

In this work, a new method for the detection of the negative effects of a particular unbalanced voltage and inverter harmonics on the performance of an induction motor using fiber sensors is proposed. Supplying a three-phase induction motor with unbalanced voltages causes an oscillating electromagnetic torque that generates vibrations, increased losses, efficiency reduction, and an extra temperature rise that leads to a reduction on insulation life of the machine. A new in-line fiber etalon accelerometer has been designed to detect these vibrations in the range DC-500 Hz. The in-line fiber etalon scheme used provides high robustness and stability, giving enough sensitivity to monitor the low-frequency and low-amplitude oscillations in the stator of the machine that exist in a voltage unbalance situation. To prove this claim, a 1.5-kW squirrel cage induction motor is analyzed under different unbalance levels. It is shown that a precise unbalance factor can be detected without accessing to the electric part of the machine and an accurate monitoring can be obtained using the high-resolution analysis proposed.