21-06-2012, 05:44 PM
DC motor Losses
DC motor Losses.ppt (Size: 510 KB / Downloads: 60)
Losses in a D.C. Machine
The losses in a d.c. machine (generator or motor) may be divided into three classes:
All these losses appear as heat and thus raise the temperature of the machine.
They also lower the efficiency of the 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 following formula,
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.
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.
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.
Constant and Variable Losses
The losses in a d.c. motor may be sub-divided into
(i)constant losses
(ii) variable losses.
Constant lossesThose 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© shunt field losses