Power tools are basic working tools used for production and manufacturing in the machinery and mechanical industries. The motor drive plays an important role in power tool applications. The performance of the motor drive will then directly or indirectly affect the quality and precision of the processing metal components. Most of the traditional motor drive control of a brushless direct current (BLDC) motor employs the Hall-effect position sensors to detect the rotor position. However, the installing sensors are prone to degrading the performance due to variations in temperature and the harsh environment. This disadvantage can be overcome with sensorless solutions. Among these sensorless solutions, the zero-crossing point detection of the back electromotive force (BEMF) is popular. Nevertheless, for the 180-degree conduction mode, it is impossible to directly detect the BEMF because of the three terminals of the motor which are conducted at any time for an electrical cycle. Therefore, a novel sensorless circuit approach based on the terminal line to line voltage is proposed in this paper. Moreover, an improved circuit scheme with a Schmitt trigger for sensing the BEMF is also proposed and implemented to obtain the precisely resembling Hall-effect signals. Finally, a prototype of a sensorless BLDC motor drive with a 180-degree conduction mode speed control for power tools is designed and implemented in this paper. The experimental results show that the proposed circuit works properly and validates the feasibility and fidelity of the motor drive system.

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