The most electrical energy in the world is produced by synchronous generators. Therefore, it has always been important to condition monitor the power generators. One of key parts of the brushless synchronous generator is rectifier that is installed on the shaft of the generator which rotates in operational mode. As a result, it is difficult to access the rectifier for condition monitoring. In this paper, a method is proposed to detect the occurred faults on the rectifier using three-phase terminal voltage of the brushless synchronous generator. In order to evaluate the method, the open-circuit fault on one diode and the open-circuit fault on two diodes which leads to opening one phase of the armature winding in exciter generator are detected in an experimental set. In the method, the relative wavelet energy along with some statistical features of the measured signals are extracted. Besides, K Nearest Neighbors (KNN) is used for fault classification.