354 Electrical Power Systems Technology
decrease. As speed decreases, the voltage induced into the conductors of
the motor through generator action (cemf) decreases. The generated volt-
age or counterelectromotive force depends upon the number of rotating
conductors and the speed of rotation. Therefore, as speed of rotation de-
creases, so does the cemf.
Figure 14-3. Operational characteristics of DC motors
The counterelectromotive force generated by a DC motor is in oppo-
sition to the supply voltage. Therefore, the actual working voltage of a DC
motor may be expressed as:
VT = VC + IA RA
where:
VT = the terminal voltage of the motor in volts,
VC = the cemf generated by the motor in volts, and
IA RA = the voltage drop across the armature of the motor in volts.
Since the cemf is in opposition to the supply voltage, the actual work-
ing voltage of a motor will increase as the cemf decreases. As the result of
an increase in working voltage, more current will flow through the arma-
ture conductors that are connected to the DC power supply. Since torque
is directly proportional to armature current, the torque will increase as the
armature current increases.
To briefly discuss the opposite situation, if the mechanical load con-
nected to the shaft of a motor decreases, the speed of the motor will tend
to increase. An increase in speed causes an increase in generated voltage.
Since cemf is in opposition to the supply voltage, as cemf increases, the ar-
mature current decreases. A decrease in armature current causes a decrease
in torque. We can see that torque varies with changes in load, but we need