Demagnetization faults are troublesome because they have a profound impact on the overall performance of permanent magnet synchronous motors (PMSMs). This work presents and verifies experimentally a system to detect such faults which is based on the measure of the zero sequence voltage component (ZSVC). The proposed method is also appropriate for inverter fed machines and is particularly useful when dealing with fault tolerant systems. A fault severity index which allows quantifying the harshness of such faults is also proposed and its behavior is analyzed from experimental data. Features of the proposed method include low computational burden, simplicity and high sensitivity. Experimental results conducted at different speed and load conditions show the potential of the proposed fault severity index for online diagnosis of demagnetization failures.