In this work, an indirect method for the detection of voltage unbalance in electrical engines using an optical fiber sensor is presented. The in-line fiber etalon structure has been designed and used to detect 0-5% voltage unbalances in the frequency range up to 500 Hz. The interferometric sensing mechanism of the sensor allows a high sensitivity with the additional advantages of EMI immunity and high dielectric isolation. In order to prove this claim, a 1.5 kW squirrel cage induction motor is unbalanced and its vibration analyzed. 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 analysis proposed. This scheme can be applied to reduce the oscillating electromagnetic torque, power losses and temperature rise due to unbalance, and leading to a higher efficiency and the extension of the life of the machine