Single-phase AC series motor, commonly known as series motor or universal motor (UniversalMotor is called abroad, named after AC and DC universal), the armature winding and the excitation winding work together in series. Single-phase series-excited motors are also called AC-DC dual-purpose series-excited motors, which can work with either AC or DC power. The advantages of the single-phase series motor are that it has high speed, large starting torque, small size, light weight, not easy to stall, and a wide range of applicable voltages. It can be adjusted by voltage regulation, which is simple and easy to implement. Therefore, it is widely used in power tools, such as angle grinders, hand drills, etc.