DC Motors
DC MOTOR THEORY
T = KFIa
(6-1)
where
T = torque, lb-ft
K = a constant depending on physical size of motor
F = field flux, number of lines of force per pole
Ia = armature current
Generator Action in a Motor
A generator action is developed in every
Figure 6 Counterelectromotive Force (CEMF)
motor. When a conductor cuts lines of force,
an EMF is induced in that conductor.
Current to start the armature turning will flow
in the direction determined by the applied DC
power source.
After rotation starts, the
conductor cuts lines of force. By applying
the left-hand rule for generators, the EMF that
is induced in the armature will produce a
current in the opposite direction. The induced
EMF, as a result of motor operation, is called
counterelectromotive force, or CEMF, as
illustrated in Figure 6.
Since the CEMF is generated by the action of
the armature cutting lines of force, the value of CEMF will depend on field strength and armature
speed, as shown in Equation (6-2).
ECEMF = KFN
(6-2)
where
ECEMF
= counter EMF
K
= constant
F
= field flux strength
N
= speed of the armature
The CEMF opposes the applied voltage and functions to lower armature current. The effective
voltage acting in the armature of a motor is the applied voltage, minus the counter EMF.
Armature current can be found by using Ohm’s law, as shown in Equation (6-3).
Rev. 0
Page 5
ES-06
