On the detection of missing magnetic wedges using impedance analysis

In this paper, a method for detecting missing magnetic wedges in induction motors supplied both direct-online and via variable-speed-drive is presented. The proposed algorithm is based on spectrum analysis of an impedance calculated using measured stator current and voltage signals. The main idea of proposed algorithm is to first normalize current and voltage signals in order to remove variations due to non-stationary operating conditions. Once normalized, the signals are processed to obtain the frequency spectrum of the impedance for each power phase and, subsequently, specific differences between phases are analyzed to identify faults. To verify these algorithms three induction motor cases were examined. Two were supplied direct-on-line and one supplied by variable-speed-drives. In the case of direct-online-supply one of the motors was known to be healthy and the other was known to have missing magnetic wedges while in the case of variable-speed-drive-supply one motor was known to have missing magnetic wedges. Measurements were recorded and processed using an ABB portable condition monitoring tool dedicated for electric motors. Experimental investigations show that the method successfully identifies defective cases.