The basic structure is the same as in a brush motor, but when the supplied power is alternating current, the output itself varies periodically. Further, they are used under difficult conditions such as when inrush current occurs when a commercial power supply is directly connected from a stopped state. Therefore, electromagnetic field analysis using the finite element method (FEM), which can account for nonlinear magnetization properties of the materials, is useful in order to evaluate magnetic saturation during overload.
This note presents how the characteristics of a universal motor can be obtained, including torque versus current (T-I), torque versus speed (T-N), and magnetic flux density distribution.
Magnetic Flux Density Distribution
More current flows in a universal motor during low rotation speeds than during high rotation speeds, so magnetic flux and torque are both higher. However, a magnetic circuit and motor design that does not allow magnetic saturation to occur at low rotation speeds is desirable because performance may be worse for a motor in which magnetic saturation occurs easily.
Speed-Torque and Torque-Current Curves
The speed-torque curve is shown in fig. 2, and the torque-current curve is shown in fig. 3.
As can be seen from the two graphs, in a universal motor, the torque gets smaller as rotation speed rises, and the torque gets larger as current increases.