This article proposes an online magnet fault detection for standard permanent magnet (PM) synchronous motors. The proposed detection is based on the magnet flux estimation with pulse width modulation (PWM) voltage measurement. This magnet fault detection can be easily implemented into existing motor drive microcontroller (MCU). On this basis, the PM motor flux waveform is analyzed considering both uniform demagnetization (UD) fault and local demagnetization (LD) fault. After that, the magnet flux is real-time estimated through the voltage-based flux observer. It is shown that the estimated flux magnitude is decreased if UD fault occurs. By contrast, high-order flux harmonics are resultant during the appearance of LD fault. Because the flux deviation reflected by either UD or LD faults is sufficiently small, the motor phase PWM voltage is measured for the flux estimation independent of the inverter nonlinearity. Three interior PM (IPM) motors with different magnet fault conditions are customized for experimental verification. By using the proposed magnet flux estimation, both magnet faults are detected under full operation region.