Yes... Skogsgurra is correct. There are many types of DC motors, so you'll need to clarify what you have and what is the typical load scenario/application if you wish to gather answers which are useful and relevant. Sounds like you have a DCPM machine however.
Other effects you will have to consider is "brush drop" and environmental conditions such as temperature and humidity.
Brush drop is a non-linear voltage drop which is similar to a silicon diode drop in an electronic circuit. The appearance of brush drop is a consequence of the thin layer of cuprous oxide, an insulator that develops on the surface of the commutator. This is a result of a reaction between atmospheric oxygen and the commutator material facilitated by the heat generated by motor operation and arcing. Note conduction through the cuprous oxide layer occurs due to localized dielectric breakdown when current is passed through the brushes. The pros of this cuprous oxide film are that the voltage drop tends to reduce arcing effects, and serves to extend brush life. The con is that the voltage drop reduces the efficiency of lower voltage motors. Moreover the structural integrity of the film can change in very low humidity environments; consequently brush wear may be accelerated under these conditions.
Hope this helps...
