DC motors are typically designed starting with the armature, and the steps and general requirements are largely similar to those of AC motors, except that the armature windings are on the rotor, the magnetic poles are on the stator, and there is a commutator, which makes them slightly different.
1. Assembly drawing
When drawing the assembly drawing, first draw the core, pressure rings (or winding supports), and the ends of the armature winding, then draw the commutator, brushes, and stator components. Since the armature winding ends are part of the ventilation system, care should be taken when binding the ends to avoid excessively narrow spacing between adjacent conductors. In most cases, the axial dimension of the excitation winding should also be considered. To ensure proper brush and brush holder operation, the commutator should be located further away from the armature core than would be determined solely based on the armature winding ends. This is particularly important in low-power motors, where the commutator diameter and armature diameter are relatively close, while the axial dimension of the excitation winding is relatively large.
2. Commutator position determination
In medium and large DC motors, when determining the armature winding end extension dimensions or the commutator position, the axial extension of the compensation winding should also be considered. Because the ends of the compensation winding are often very long, if the rotor axial dimension is not increased accordingly, the determined commutator position will be inappropriate.
When selecting the commutator diameter, it's generally advisable to choose the smallest possible size. This is more economical and reduces brush friction losses. However, with a smaller diameter, the width of both the commutator segments and the brushes will decrease for a given brush width-to-segment width ratio. If the diameter is too small, the strength will be insufficient. Mechanical calculations should generally be performed for all commutator components.
When using end-cap bearings, the commutator sleeve cavity should have a sufficiently large air passage to facilitate ventilation. In large motors using pedestal bearings, the motor should be lowered slightly in the foundation for easier maintenance of the brush assembly.
3. Base Design and Control
In motors using pedestal bearings, the frame is symmetrical about the centerline of the armature core. The determination of the frame's cross-sectional dimensions mainly depends on magnetic circuit calculations and stiffness requirements; generally, if the former is met, the latter is also usually met. We will discuss the stiffness requirements for DC motor frames separately. For large motors performing important tasks, it is sometimes necessary to increase the axial length between the commutator-side front cover and the front bearing so that if the motor malfunctions, the frame can be moved towards the commutator for repair.
Regardless of whether end-cap bearings or pedestal bearings are used, the height of the mounting feet should be selected to allow for the installation and removal of the pole fixing screws. If a split mounting base is used, the position of the split should be considered in conjunction with the position of the pole fixing screws. For stability, for motors with foot mounting, the vertical line passing through the center of mass of the mounting base, or, when using end-cap bearings, the vertical line passing through the entire center of mass of the motor, must be within the outer perimeter of the motor mounting feet.
