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Speed control methods of a DC motor can be varied by varying flux, armature resistance
or applied voltage. Different speed control methods for different DC shunt and series
methods are there. We also know, DC motor is an electrical machine
that converts direct current electrical energy into mechanical energy. When the current carrying
armature is connected to the supply end though commentator segment, brushes are placed within
the North South Poles of permanent or electromagnets. By using these electromagnets operating principle
is depends on the Fleming's left hand rule to determine the direction of the force acting
on the armature conductors of the DC motor. In this video we will go to learn different
speed control methods of dc motor.
Speed of DC shunt motor is controlled methods by the stated below.
1. Flux control method. 2. Armature and Rheostatic control method.
3. Voltage control method. Voltage control methods are two types.
a. Multiple voltage control. b. Ward Leonard system.
Flux Control Method. In this method, speed of a dc motor is inversely
proportional to the flux per pole. Thus, by decreasing flux and speed can be increased
vice versa. To control the flux , he rheostat is added in series with the field winding
will increase the speed , because of this flux will decrease. So, the field current
is relatively small and hence copper loss is decreased. This method is quite efficient.
Armature and Rheostatic control method.
Speed of a dc motor is directly proportional to the back E M F. That means, when the supply
voltage V and the armature resistance Ra are kept constant, speed is directly proportional
to the armature current Ia. Thus, if we add a resistance in series with the armature,
Ia decreases and, hence, the speed also decreases. Greater the resistance in series with the
armature, greater the decrease in speed.
Multiple voltage control.
In this method, the shunt field is connected to a fixed exciting voltage and armature is
supplied with different voltages. Voltage across armature is changed with the help of
a suitable switchgear. The speed is approximately proportional to the voltage across the armature.
Ward Leonard system.
This Ward - leonard system is used where very sensitive speed control of the motor is required
(e.g electric excavators, elevators, etc.). The arrangement of this system is as required
in this figure.
In this method, the output from the generator G is fed to the armature of the motor M2 whose
speed is to be controlled. The output voltage of the generator G can be varied from zero
to its maximum value by means of its field regulator and, hence, the armature voltage
of the motor M2 is varied very smoothly. Hence, very smooth speed control of the dc motor
can be obtained by this method.
Speed of DC Series motor is controlled methods by the stated below.
a. Flux control method. Flux control methods are four types.
1. Field diverter. 2. Armature diverter.
3. Trapped field control. 4. Paralleling field coils.
b. Variable Resistance in series with armature. c. Series -parallel control method.
Field diverter. A veritable resistance is connected parallel
to the series field. This variable resistor is called as diverter, as the desired amount
of current can be diverted through this resistor and hence current through field coil can be
decreased. Hence, flux can be decreased to the desired amount and speed can be increased.
Armature diverter.
Diverter or Rheostat is connected across the armature.
For a given constant load torque, if armature current is reduced then flux must increase.
This will result in an increase in current taken from the supply and hence flux will
increase and subsequently speed of the motor will decrease.
Tapped field control.
As shown in figure, the field coil is tapped dividing number of turns. Thus we can select
different value of flux by selecting different number of turns.
Paralleling field coils. In this method, several speeds can be obtained
by regrouping coils as shown figure is below. .
Variable Resistance In Series With armature. By introducing a resistance in series with
the armature, voltage across the armature can be reduced. And, hence, speed reduces
in proportion with it.
Series Parallel Control. This system is widely used in electric traction,
where two or more mechanically coupled series motors are employed. For low speeds, the motors
are connected in series, and for higher speeds the motors are connected in parallel.
When in series, the motors have the same current passing through them, although voltage across
each motor is divided. When in parallel, the voltage across each motor is same although
the current gets divided.
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