Dc motor converts electrical energy into
Types of Dc Motor:
Dc shunt motor
Dc series motor
Dc compound motor
Dc separately excited motor
Permanent-magnet dc motor
Dc Shunt Motor:
In dc shunt motor the armature
and field winding are connected in parallel across the supply voltage, the
voltage same on field and armature winding. The resistance of the field winding
is higher than the armature winding. When electric voltage supplied to the shunt dc motor so due to high resistance
of the field winding it draws very low current. The armature draws high
current. Then it generated high magnetic field. After that motor starts
rotating when the magnetic field of the armature and shunt winding interact.
Shunt dc motor operation is same
as other dc motors. It has rotor (armature) , stator (field winding) and
commutator for the operation of motor. In a shunt dc motor a rotational torque
is produced in result of the cooperation between the magnetic field produced in the current
carrying armature and the magnetic field produced around the stator windings.
Current is supplied by the stator to the rotator through with commutator and
brushes arrangement. The stator is stationary thus power is applied directly to
In shunt dc motor the field windings of stator
are connected in parallel with the armature. The field windings of shunt motor
are made of fine coil of wire with large number of turns because small wire
cannot handle heavy currents therefore large number of turns require to produce
strong magnetic field. Shunt dc motor cannot carry high current because the
resistance of field windings in it is very high so it is unsuitable for
application requiring a high starting torque therefore dc shunt motor shaft
load to be small to start function.
The resistance of field winding
is very high therefore when electric field is supplied to shunt dc motor so
very low amount of current flows through the shunt coil. Armature draws enough
current to produce a strong magnetic field. Due to the interaction of magnetic
field around armature and field produced around the stator field the motor
starts to rotate. When the armature starts turning, it produces a back
EMF. When armature rotates in magnetic
field then electricity is induced in it. The polarity of this back EMF opposes
the armature current. When motor turns then armature current is controlled by
the back EMF and is kept low.
Diagram of DC Shunt Motor:
of DC Shunt Motor:
Dc shunt motor is similar to the other dc motors. It contain all major
parts which include a stator (field windings), a rotor (armature), commutator
and brushes. The shunt field winding is made of several turns on the coil of
wire. Turns are made with thin wire therefore shunt winding of motor is quite
small. Armature of shunt motor handles the shaft load therefore it has heavier
winding armature core to support higher current that passes through the
armature during the motor start or when motor is running at lower speed. When
motor speed increases then armature create counter electromagnetic force to o oppose
Brushes are the small parts of motor that conducts electrical current
between the stationary wires and the rotating wires of a motor. Every motor
contains more than one brush to conduct electrical current brushes are
generally made with carbon they also provide the safety to the spark and
bearing reduce the friction during operation. Commutator work on the same
principle of brushes. They also provide
current from the field winding to rotor.
The output characteristics of
a machine is play a most important role for observing machine internal
mechanism. In dc shunt motor the output characteristics is based on a relation between
torque and speed. The characteristics of motor is a plot of its output torque
When the load on the shaft of a shunt motor is
increased then the load torque will exceed the induced torque in the machine and
motor will start to slow down. When motor slows down then its internal
generated voltage E(a) drops so the armature current of the motor I(a)
increases. As the armature current increases then the induced torque of the
motor increases T (ind) and finally the induced torque and load torque are
equal at lower mechanical speed of rotation.
The output characteristics of
shunt dc motor can be obtained by the induced voltage and torque equations of
motor using kirchhoff’s voltage law.
V(t) = E(a) + I(a)R(a)
E(a) = [email protected](m)
= K @
This equation is a straight line with negative slope by this
graph it is realize that the speed of
motor increase with induced torque. The terminal voltage supplied by the
dc source is assumed to be constant. If terminal voltage not constant then the
voltage variation will affect the shape of curve.
Armature Current Vs
Another effect of the motor that can also affect the shape
of the curve of torque—speed is armature reaction. If a motor has armature
reaction then its load increases so the flux weakening effects reduce its flux.
If flux ? is assumed to be constant, so the
speed and back EMF has direct relation. Back EMF is also constant therefore speed
should remain constant. Practically ? and E(b) decreases with increase in load.
Back EMF decreases slightly more than ?, therefore the speed decreases
slightly. In General, the speed decreases only 5 to 15% of full load speed.
Therefore, a shunt motor
can assumed as a constant speed motor. In speed vs. armature current characteristic the straight line
shows the ideal characteristic and actual characteristic show with dotted line.
Uses of DC
1. Dc shunt motors are used where the
speed is constant under varying load condition is requirement .
2. These motors are used in lathes,
3. These motors are used in fans and
4. It also uses as a wood working
5. It also uses as a weaving machines.