Losses in D.C. Generators

Losses in D.C. Generators Points : Losses in D.C. Generators, The losses in d.c. machines may be divided into two groups:
(1) Variable losses, and
(2) Constant losses. 1. Variable losses Are proportional to the square of the load current in the armature and the series and interpole windings when employed. Due to the contact resistance between the brushes and the commutator, there is amore variable loss. 2. Constant losses Are assumed to be constant over the load range and comprise:
(a) Iron loss (due to hysteresis and eddy currents)
(b) Windage and friction loss, and
(c) Shunt field excitation loss.
The above losses can be calculated or obtained separately from test results. The eddy currents losses due to change in flux with load are additional to above losses and known as stray losses.
The above two groups of losses are discussed below: (1) Armature losses Thee losses are of two types:
a) I¹ 2 R loss in the windings of the armature; and
(b) Iron loss in the core of the armature, due to the hysteresis and eddy currents. Hysteresis loss depends upon the quality of iron. (2) Commutator losses Commutator losses occur due to the contact resistance and friction between the brushes and the commutator segments.
(a) Contact resistance losses are dependent upon the quality of the brushes.
(b) The frictional losses in the commutator segments and brushes depend upon the brush pressure on the segments, the coefficient of friction and the peripheral speed of the commutator. (3) Excitation losses These include the loss in i shunt circuit (if so used) which is equal to the product of the shunt current and the terminal voltage. In a shunt-wound generator, this loss varies between no-load and full-load, whereas in a level compound generator it is almost constant. Excitation losses also occur in commutating poles or interpoles series, and compensating windings (if used), and are proportional to the square of the armature current. (iv) Losses due to bearing friction and windage The losses due to bearing friction are roughly proportional to the speed. Windage loss is ordinarily very small unless the generator is fitted with a cooling fan device, and in that case, windage loss is proportional to the cube of the speed. (v) Losses due to stray load The effect of armature reaction to distort to flux. The flux densities in certain regions of the armature are increased and in certain other regions they are decreased. Hence- the iron loss depends upon the square of the flux density, Hence the iron loss is increased due to armature reaction. Such losses are known as stray losses.