A universal motor is a motor that rotates on both direct and alternating currents. A universal motor is used in home appliances and industrial machines because these motors are robust, compact, light, and fast with a simple construction, and have a large starting torque. However, problems such as vibration and a reduction in starting torque caused by the cogging torque occur as the size of the motor becomes smaller. Evaluating the starting performance of a universal motor at the design stage is necessary to resolve these problems.
This document introduces the use of a magnetic field analysis to obtain the speed versus time graph, the current waveform, and the torque versus time graph for a universal motor.
The speed versus time graph from when the motor starts to 0.4 seconds is indicated in Fig. 1, the current waveform of the stator coil is indicated in Fig. 2, and the torque versus time graph is indicated in Fig. 3.
The universal motor reaches a rotation speed of 6,000 rpm at approximately 0.35 seconds as indicated in Fig. 1. Furthermore, the larger the current flowing through the stator coil is, the larger the torque becomes when the motor is started. As the rotation speed increases, the current and torque decrease.