These evaluations need to be able to account for an accurate circuit geometry, and the current flowing through coil connected via a commutator needs to be handled accurately, as well. This is why an electromagnetic field analysis using the finite element method (FEM) is necessary to account for everything.
This Application Note presents an analysis to obtain the speed versus torque and torque versus current for a motor that has 2 brushes, 6 poles, and 19 slots.
Speed-Torque Curve and Torque-Current Curve
Fig. 1 shows the speed versus torque curve and fig. 2 shows the current versus torque curve.
As shown in fig. 1, the torque in the DC brush motor decreases as the rotation speed increases. This is because the current flowing in the coil is reduced by the increase of reverse electromagnetic force produced as the rotation speed increases. As shown in fig. 2, the relationship between the current and torque is mostly linear, and the torque coefficient of this brush motor is 0.05 Nm/A.
Magnetic Flux Density Distribution
The magnetic flux density increases at lower a lower rotation speed, as indicated in the figure. This is because the current flowing in circuit is larger, as mentioned earlier. A motor and control design that does not allow magnetic saturation to occur at low rotation speeds is desirable because the performance of the motor worsens with magnetic saturation.