Losses in D.C. Motors

Losses in D.C. Motors Points : Losses in D.C. Motors Like generators, motor losses can be divided roughly into two main classes:
(a) Constant losses (independent of load), which are mainly the mechanical losses as well as the iron and core losses; and
(b) Variable losses (dependent on load), and these mainly consist of the copper losses. Constant Losses (a) Mechanical losses. They are common to all types of machines. These losses are due to:
(1) Brush-friction loss,
(2) Bearing-friction loss
(3) Windage loss.

The brush-friction loss varies with speed, and changes a little with load. The bearing-friction and windage losses also vary with the speed, and are little affected by load. In ball-bearing machines the bearing-friction is very small as compared with windage and brush friction, particularly in high-speed heavy current motors. With machines having a fan for cooling, the windage loss increases. But for series motors where the speed varies considerably the bearing-friction and windage losses may be regarded as constant. These losses are little affected by load, because the speed does not change appreciably.