LAWS and PRINCIPLE of DC MOTOR

LAWS AND PRINCIPLES OF ELECTROMECHANICAL ENERGY CONVERSION: DC MOTOR

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

(i) Mechanical efficiency

(ii) Electrical efficiency

(iii) 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 i.e.

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.