A new, simple partial discharge (PD) detector for low-voltage rotating electrical machines using acoustical emission (AE) technique is dseveloped in this study. Common electric components were used in the detector, which reduces the cost of inspection, comparing with the traditional PD detection methods that use expensive equipment costing from tens of thousands to several millions dollars. Experimental results by resonant type AE sensors (150 kHz) which utilize power line-cycle in a microcontroller unit (MCU) as reference to measure the PDs generated in a low-voltage motor are presented. The AE signals are then amplified by a pre-amplifier (30 kHz ∼ 300 kHz, 34 dB). Since the resonant frequency of the sensor is much lower than that of the electromagnetic (EM) interferences around the motor, the effects of noise is substantially reduced by this method. In the mean time, the use of 150-kHz resonant type AE sensor also avoids the disturbance of mechanical vibration noise. According to the experiment results, the measurement system developed in this study can be used to detect the PDs' AE signals correctly. The AE measurement scheme proposed in this study provides an effective, low-cost method for PD measurements.