Enhanced Speed Characteristics of High-Torque-Density BLDC Motor for Robot Applications Using Parallel Open-End Winding Configuration

High-torque-density motors are essential in humanoid, wearable, and rehabilitation robots due to their ability to minimize gear ratios, improve back-drivability, and support compact joint design. However, their inherently high back-EMF limits speed performance, and safety regulations often constrain supply voltages to below 50 V in human-interactive environments. To overcome these limitations, this study introduces a novel winding strategy called parallel open-end winding (POEW), which combines the benefits of two individual approaches: Parallel Connected Winding (PCW) and Open-End Winding (OEW). PCW reduces phase resistance and inductance, thereby mitigating voltage drop and back-EMF, while OEW eliminates the neutral point, allowing full-phase voltage utilization. Experimental results show that the POEW configuration achieves a 3.5-fold increase in maximum speed compared to the conventional Series-Connected Winding (SCW), without altering the rotor or stator structure. Torque constant measurements confirm that all proposed configurations maintain torque output with minimal variation. Although the motor constant slightly decreases due to the higher current in parallel paths, the significant speed enhancement under low-voltage conditions demonstrates the practicality and effectiveness of POEW for advanced robotic applications requiring both high torque and speed.