DC MOTOR THEORY
DC Motors
(6-3)
Ia
Et
ECEMF
Ra
where
Ia
= armature current
Et
= terminal voltage
ECEMF
= counter EMF
Ra
= armature resistance
DC Motor Speed
The field of a DC motor is varied using external devices, usually field resistors. For a constant
applied voltage to the field (E), as the resistance of the field (Rf) is lowered, the amount of
current flow through the field (If) increases as shown by Ohm’s law in Equation (6-4).
(6-4)
If
E
Rf
An increase in field current will cause field flux (Ff) to increase. Conversely, if the resistance
of the field is increased, field flux will decrease. If the field flux of a DC motor is decreased,
the motor speed will increase. The reduction of field strength reduces the CEMF of the motor,
since fewer lines of flux are being cut by the armature conductors, as shown in Equation (6-5).
(6-5)
ECEMF
K
FF
N
A reduction of counter EMF allows an increase in armature current as shown in Equation (6-6).
(6-6)
Ia
Et
ECEMF
Ra
This increase in armature current causes a larger torque to be developed; the increase in armature
current more than offsets the decrease in field flux as shown in Equation (6-7).
(6-7)
T
K
FF
Ia
This increased torque causes the motor to increase in speed.
T N
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