DC GENERATOR POWER LOSSES and EFFICIENCY
The various power stages in a d.c. generator are represented diagrammatically in Fig. (1.39).
A – B = Iron and friction losses
B – C = Copper losses
Mechanical efficiency
Electrical efficiency
Commercial or overall efficiency
Unless otherwise stated, commercial efficiency is always understood.
Now, commercial efficiency
,
Condition for Maximum Efficiency
The efficiency of a d.c. generator is not constant but varies with load. Consider a shunt generator delivering a load current IL at a terminal voltage V.
The shunt field current Ish is generally small as compared to IL and, therefore, can be neglected.
The efficiency will be maximum when the denominator of Eq.(i) is minimum
The load current corresponding to maximum efficiency is given by;
Hence, the efficiency of a d.c. generator will be maximum when the load current is such that variable loss is equal to the constant loss. Fig (1.40) shows the variation of η with load current.
POWER LOSSES
The losses in a d.c. machine (generator or motor) may be divided into three classes viz (i) copper losses (ii) iron or core losses and (iii) mechanical losses. All these losses appear as heat and thus raise the temperature of the machine. They also lower the efficiency of the machine.
1. Copper losses
These losses occur due to currents in the various windings of the machine
Note. There is also brush contact loss due to brush contact resistance (i.e., resistance between the surface of brush and surface of commutator). This loss is generally included in armature copper loss.
2. Iron or Core losses
These losses occur in the armature of a d.c. machine and are due to the rotation of armature in the magnetic field of the poles. They are of two types viz., (i) hysteresis loss (ii) eddy current loss.
A. Hysteresis loss
Hysteresis loss occurs in the armature of the d.c. machine since any given part of the armature is subjected to magnetic field reversals as it passes under successive poles. Fig. (1.36) shows an armature rotating in two-pole machine. Consider a small piece ab of the armature. When the piece ab is under N-pole, the magnetic lines pass from a to b. Half a revolution later, the same piece of iron is under S-pole and magnetic lines pass from b to a so that magnetism in the iron is reversed. In order to reverse continuously the molecular magnets in the armature core, some amount of power has to be spent which is called hysteresis loss. It is given by Steinmetz formula. This formula is
In order to reduce this loss in a d.c. machine, armature core is made of such materials which have a low value of Steinmetz hysteresis co-efficient e.g., silicon steel.
b. Eddy current loss
In addition to the voltages induced in the armature conductors, there are also voltages induced in the armature core. These voltages produce circulating currents in the armature core as shown in Fig. (1.37). These are called eddy currents and power loss due to their flow is called eddy current loss. The eddy current loss appears as heat which raises the temperature of the machine and lowers its efficiency.
core resistance can be greatly increased by constructing the core of thin, roundIf a continuous solid iron core is used, the resistance to eddy current path will be small due to large cross-sectional area of the core. Consequently, the magnitude of eddy current and hence eddy current loss will be large. The magnitude of eddy current can be reduced by making core resistance as high as practical. The iron sheets called laminations [See Fig. 1.38]. The laminations are insulated from each other with a coating of varnish. The insulating coating has a high resistance, so very little current flows from one lamination to the other. Also,
because each lamination is very thin, the resistance to current flowing through the width of a lamination is also quite large. Thus laminating a core increases the core resistance which decreases the eddy current and hence the eddy current loss.
It may be noted that eddy current loss depends upon the square of lamination thickness. For this reason, lamination thickness should be kept as small as possible.
3. Mechanical losses
These losses are due to friction and windage.
(i) friction loss e.g., bearing friction, brush friction etc.
(ii) windage loss i.e., air friction of rotating armature.
These losses depend upon the speed of the machine. But for a given speed, they are practically constant.
Note. Iron losses and mechanical losses together are called stray losses.
Constant and Variable Losses
The losses in a d.c. generator (or d.c. motor) may be sub-divided into
(i)constant losses (ii) variable losses.
(i) Constant losses
Those losses in a d.c. generator which remain constant at all loads are known as
constant losses. The constant losses in a d.c. generator are:
(a) iron losses
(b) mechanical losses
(c) shunt field losses
(ii) Variable losses
Those losses in a d.c. generator which vary with load are called variable losses.
The variable losses in a d.c. generator are:
Total losses = Constant losses + Variable losses
Note. Field Cu loss is constant for shunt and compound generators.
No comments:
Post a Comment