In dc generators, as in most electrical devices, certain forces act to decrease the efficiency. These forces, as they affect the armature, are considered as losses and may be defined as follows:
1. Copper loss in the winding 2. Magnetic Losses 3. Mechanical Losses
Copper lossThe power lost in the form of heat in the armature winding of a generator is known as Copper loss. Heat is generated any time current flows in a conductor.
loss is the Copper loss, which increases as current increases. The amount of heat generated is also proportional to the resistance of the conductor. The resistance of the conductor varies directly with its length and inversely with its cross- sectional area. Copper loss is minimized in armature windings by using large diameter wire.Copper loss is again divided as(i) Armature copper loss
= Armature copper loss. Where Ra =resistance of armature and interpoles and series field winding etc. This loss is about 30 to 40% of full -load losses.(ii) Field copper loss : It is the loss in series or shunt field of generator.
is the field copper loss in case of series generators, where Rse is the resistance of the series field widing.
is the field copper loss in case of shunt generators.This loss is about 20 to 30% of F.L losses.
(iii) The loss due to brsh contact resistance.It is usually inluded in the armture copper loss.
Magnetic Losses (also known as iron or core losses)(i) Hysteresis loss (Wh)Hysteresis loss is a heat loss caused by the magnetic properties of the armature. When an armature core is in a magnetic field, the magnetic particles of the core tend to line up with the magnetic field. When the armature core is rotating, its magnetic field keeps changing direction. The continuous movement of the magnetic particles, as they try to align themselves with the magnetic field, produces molecular friction. This, in turn, produces heat. This heat is transmitted to the armature windings. The heat causes armature resistances to increase. To compensate for hysteresis losses, heat-treated silicon steel laminations are used in most dc generator armatures. After the steel has been formed to the proper shape, the laminations are heated and allowed to cool. This annealing process reduces the hysteresis loss to a low value.
(ii) Eddy Current Loss (We)The core of a generator armature is made from soft iron, which is a conducting material with desirable magnetic characteristics. Any conductor will have currents induced in it when it is rotated in a magnetic field. These currents that are induced in the generator armature core are called EDDY CURRENTS. The power dissipated in the form of heat, as a result of the eddy currents, is considered a loss.
Eddy currents, just like any other electrical currents, are affected by the resistance of the material in which the currents flow. The resistance of any material is inversely proportional to its cross-sectional area. Figure, view A, shows the eddy currents induced in an armature core that is a solid piece of soft iron. Figure, view B, shows a soft iron core of the same size, but made up of several small pieces insulated from each other. This process is called lamination. The currents in each piece of the laminated core are considerably less than in the solid core because the resistance of the pieces is much higher. (Resistance is inversely proportional to cross-sectional area.) The currents in the individual pieces of the laminated core are so small that the sum of the individual currents is much less than the total of eddy currents in the solid iron core.
As you can see, eddy current losses are kept low when the core material is made up of many thin sheets of metal. Laminations in a small generator armature may be as thin as 1/64 inch. The laminations are insulated from each other by a thin coat of lacquer or, in some instances, simply by the oxidation of the surfaces. Oxidation is caused by contact with the air while the laminations are being annealed. The insulation value need not be high because the voltages induced are very small.Most generators use armatures with laminated cores to reduce eddy current losses.
These magnetic losses are practically constant for shunt and compound-wound generators, because in their case, field current is constant.
Mechanical or Rotational Losses
These consist of
(i) friction loss at bearings and comutator.
(ii) air-friction or windage loss of rotating armature
These are about 10 to 20% of F.L losses.
Careful maintenance can be instrumental in keeping bearing friction to a minimum. Clean bearings and proper lubrication are essential to the reduction of bearing friction.Brush friction is reduced by assuring proper brush seating, using proper brushes, and maintaining proper brush tension. A smooth and clean commutator also aids in the reduction of brush friction.
Usually, magnetic and mechanical losses are collectively known as Stray Losses. These are also known as rotational losses for obvious reasons.
As said above, field Cu loss is constant for shunt and compound generators.Hence, stray losses and shunt Cu loss are constant in their case.These losses are together known as standing or constant losses Wc.
Hence, for shunt and compound generators,
Total loss = armature copper loss + Wc
Armature Cu loss is known as variable loss because it varies with the load current.
Total loss = Variable loss + constant losses Wc
