Dc machines 3

dc machine

1. Loss in a DC machineLoss in a DC machine
(ii) Eddy current loss(ii) Eddy current loss
In addition to the voltages induced in the armature conductors, In addition to the voltages induced in the armature conductors,
there are also voltages induced in the armature core. These there are also voltages induced in the armature core. These
voltages produce circulating currents in the armature core as voltages produce circulating currents in the armature core as
shown in Fig. (1.37). These are called eddy currents and power shown in Fig. (1.37). These are called eddy currents and power
loss due to their flow is called eddy current loss. The eddy loss due to their flow is called eddy current loss. The eddy
current loss appears as heat which raises the temperature of the current loss appears as heat which raises the temperature of the
machine and lowers its efficiency.machine and lowers its efficiency.

2. Loss in a DC machineLoss in a DC machine
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. Loss in a DC machineLoss in a DC machine

3. Mechanical losses3. Mechanical losses

These losses are due to friction and windage.These losses are due to friction and windage.
(i) friction loss e.g., bearing friction, brush friction (i) friction loss e.g., bearing friction, brush friction
etc.etc.
(ii) windage loss i.e., air friction of rotating armature.(ii) windage loss i.e., air friction of rotating armature.
These losses depend upon the speed of the machine. These losses depend upon the speed of the machine.
But for a given speed, they are practically constant.But for a given speed, they are practically constant.
Note. Iron losses and mechanical losses together are Note. Iron losses and mechanical losses together are
called stray losses.called stray losses.

4. Loss in a DC machineLoss in a DC machine

Constant and Variable LossesConstant and Variable Losses
The losses in a d.c. generator (or d.c. motor) may be sub-divided intoThe losses in a d.c. generator (or d.c. motor) may be sub-divided into

(i)constant losses (ii) variable losses.(i)constant losses (ii) variable losses.
(i) Constant losses(i) Constant losses
Those losses in a d.c. generator which remain constant at all loads are Those losses in a d.c. generator which remain constant at all loads are
known asknown as
constant losses. The constant losses in a d.c. generator are:constant losses. The constant losses in a d.c. generator are:
(a) iron losses(a) iron losses
(b) mechanical losses(b) mechanical losses
(c) shunt field losses(c) shunt field losses
(ii) Variable losses(ii) Variable losses
Those losses in a d.c. generator which vary with load are called variable Those losses in a d.c. generator which vary with load are called variable
losses.losses.
The variable losses in a d.c. generator are:The variable losses in a d.c. generator are:


Total losses = Constant losses + Variable lossesTotal losses = Constant losses + Variable losses
Note. Field Cu loss is constant for shunt and compound generators.Note. Field Cu loss is constant for shunt and compound generators.

5. The power-flow diagramThe power-flow diagram
On of the most convenient technique to account for power losses in a
machine is the poFor a DC motor:
wer-flow diagram.
Electrical power is input to the machine, and the electrical and brush
losses must be subtracted. The remaining power is ideally converted
from electrical to mechanical form at the point labeled as Pconv.

6. Generator Power StagesGenerator Power Stages
MechanicalMechanical
Power Power
inputinput
Electrical Power Electrical Power
developed in developed in
armature.armature.
EIa, WattEIa, Watt
Iron andIron and
FrictionFriction
LossesLosses
Copper Copper
LossesLosses
Electrical Electrical
PowerPower
OutputOutput
VI, WattsVI, Watts
Total Losses = Constant Losses + Variable Losses

7. Efficiency Of DC machineEfficiency Of DC machine
MechanicalMechanical
Power InputPower Input
(A)(A)
Iron andIron and
FrictionFriction
LossesLosses
Electrical Electrical
PowerPower
developed indeveloped in
armature.armature.
EIa, Watt (B)EIa, Watt (B)
Copper Copper
LossesLosses
Electrical Electrical
PowerPower
OutputOutput
VI, Watts VI, Watts
(C)(C)
Mechanical Efficiency = B / A
Electrical Efficiency = C / B
Overall/Commercial Efficiency = C / A