Page 628 - Basic Electrical Engineering

Figure 7.17 Method of changing the direction of rotation of a dc motor: (a) polarities of both
armature and field positive, leading to clockwise rotation; (b) polarities of armature changed, leading to
anticlockwise rotation; (c) polarities of supply to field changed, leading to anticlockwise rotation; (d)
polarities of both armature and field reversed, no change in direction of rotation

V − E = I R a (i)
a
b
or, E = V − I R a (ii)
b
a

(7.3)
or,

2
and VI = E I + I R a (iii)
a
b a
a
From equation (iii) we can write for a dc motor,

Electrical Input power = Electrical equivalent of mechanical power
developed + Armature copper loss Electrical equivalent of mechanical power

developed – Rotationed losses = Mechanical power output

7.8 TORQUE EQUATION

Torque developed T, angular velocity ω, and mechanical power, P are related
as

P = Tω

where T is in Nm and ω is in rad/sec.
In a dc motor, electrical power converted into mechanical power can be

expressed as