10-12-2012, 12:46 PM
Speed Control of DC Motors
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Speed Control of Shunt motor:
(i) Variation of Flux or Flux Control Method: By decreasing the flux, the
speed can be increased and vice versa. The flux of a dc motor can be
changed by changing Ish with help of a shunt field rheostat. Since Ish is
relatively small, shunt field rheostat has to carry only a small current,
which means I2
shR loss is small, so that rheostat is small in size.
(ii) Armature or Rheostatic Control Method: This method is used when
speeds below the no-load speed are required. As the supply voltage is
normally constant, the voltage across the armature is varied by inserting a
variable rheostat in series with the armature circuit. As controller
resistance is increased, voltage across the armature is decreased, thereby
decreasing the armature speed. For a load constant torque, speed is
approximately proportional to the voltage across the armature.
(iii) Voltage Control Method:
(a) Multiple Voltage Control: In this method, the shunt field of the motor
is connected permanently to a fixed exciting voltage, but the armature is
supplied with different voltages by connecting it across one of the several
different voltages by means of suitable switchgear. The armature speed
will be approximately proportional to these different voltages. The
intermediate speeds can be obtained by adjusting the shunt field regulator.
(b) Ward-Leonard System: This system is used where an unusually wide
and very sensitive speed control is required as for colliery winders,
electric excavators, elevators and the main drives in steel mills and
blooming and paper mills. M1 is the main motor whose speed control is
required. The field of this motor is permanently connected across the dc
supply lines. By applying a variable voltage across its armature, any
desired speed can be obtained.
Speed Control of Series Motors:
1. Flux Control Method: Variations in the flux of a series motor can be
brought about in any one of the following ways:
(a) Field Diverters: The series winding are shunted by a variable
resistance known as field diverter. Any desired amount of current can be
passed through the diverter by adjusting its resistance. Hence the flux can
be decreased and consequently, the speed of the motor increased.
(b) Armature Diverter: A diverter across the armature can be used for
giving speeds lower than the normal speed. For a given constant load
torque, if Ia is reduced due to armature diverter, the must increase
(∵TaIa ) This results in an increase in current taken from the supply
(which increases the flux and a fall in speed (N I/ )). The variation in
speed can be controlled by varying the diverter resistance.