When considering the main field axis orientation of DC machines, it is generally preferable to purchase a machine where the main field axis is horizontal. This orientation offers advantages in terms of ease of maintenance, access to components, and efficient cooling.
In machine A, with a horizontal main field axis, the overall construction and maintenance of the machine can be more convenient. It allows for easier access to the components, such as the armature, brushes, and commutator, simplifying inspection, cleaning, and repairs.
On the other hand, machine B, with a vertical main field axis, may present challenges in terms of access and maintenance. The vertical orientation can make it more difficult to reach certain components, requiring additional effort and possibly specialized tools or equipment.
Therefore, given the choice between the two machines, machine A with a horizontal main field axis is generally the preferable option.