The three main methods of speed
controlling in D.C motors are flux control method, armature voltage control
method and applied voltage control method. In flux control method, by varying
the flux, speed is controlled. Decrease of flux increases the speed and
increase of flux decreases the speed. This method is also known as field
control method. A field rheostat which changes the field current is employed
here for changing flux. In shunt motors, shunt field rheostats are used. In
series motor, field diverters, armature diverters, tapped field control or
paralleling field coils are used. Field control method is more efficient and
economical. In armature voltage control method, a variable rheostat is
connected in series with the armature circuit. This rheostat is known as
controller resistance which can tune voltage across the armature. This method
is widely used for achieving low speeds. In applied voltage control method,
shunt field of the motor is permanently connected to a fixed exciting voltage.
Here switchgears are used to supply different voltages to armature.
