Page 643 - Basic Electrical Engineering
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alternation of flux, thickness of laminations used, and the volume of the core
material.
(c) Loss in the field windings
Losses take place in the field windings due to flow of current. This loss is
equal to (VI ) W where V is the applied voltage and I is the field current.
f
f
(d) Friction and windage losses
Due to rotation of the armature, air-friction loss which is also called windage
loss, takes place. Frictional loss occurs due to brush and commutator rubbing
and loss due to bearing friction.
7.13.2 Efficiency of DC Machine
The efficiency of a dc machine is expressed as
Where C is the sum of iron, friction, and windage losses.
7.13.3 Condition for Maximum Efficiency
To determine the condition for maximum efficiency we will differentiate the
expression for η w.r.t. I and equate to zero as
a
From which,
