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Muhammad Umer Farooq
Email: f
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com

Motor
A machine that produce motion or power for doing work
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Motor consist of following parts
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Rotor: The rotor is inner part which rotates
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Armature winding
is through which voltage is applied or induced
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Windings are usually made-up of copper
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1) Dc Motor
2) Ac Motor

Dc Motor
A dc motor is any class of electrical machines that converts direct current electrical power
into mechanical power
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Dc motors consist of one set of coils, called armature winding inside another set of coils
or permanent magnets, called stator
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The Equivalent circuit of dc motor is

Types of Dc Motors
1)
2)
3)
4)
5)

Shunt Motor
Separately Excited Motor
Series Motor
Permanent Magnetic Dc Motor (PMDC)
Compounded Motor

Shunt Motor
The rotor and stator are connected in parallel combination
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This type of motor is a “shunt wound” Dc motor
and winding is known as shunt winding
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Construction
The construction of dc motor is same as any other dc motor
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Stator (Shunt winding): Input power is applied to the stationary element of motor i
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shunt
winding
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As the
turns are made up of thin wire the shunt winding (field winding) is quite small in size
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Rotor (Armature): The armature generally called “rotor” handles shaft load
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High current pass through the
armature during the motor start up or when the motor is running at lower speed
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Commutator: The commutator and brush arrangement provides current from static field
windings to the rotor
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Working Principle
When an electric voltage is applied to shunt dc motor, due to high resistance of shunt
winding, it draws very low current
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The armature draws high current, thus also generating
a high magnetic field
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As the magnetic field go stronger, rotational torque will increase, thus
resulting in an increase of rotational speed of the motor
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As armature rotates
in a magnetic field, it induces electricity
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So the current through the armature decreased and the
speed of the motor is self-regulated
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So shunt motor are used to handle small
load shaft load that only need low torque initially
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In series motor, the load is
inversely proportional to speed of armature
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If the load is less, armature speed will increase
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Unlike series motor, the speed of shunt motor is independent of shaft load
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Slowing down the
speed reduces back EMF, which in turn increases the current in armature branch
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On the other hand, if the load is decreased,
then motor speed will rise instantaneously
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Gradually the motor will reduces its speed
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Because of this feature, this motor is used for automotive and industrial purpose
where fine precision of motor speed is required

Motor speed control
One can control the speed of motor in two ways
1) By varying the current supplied to the rotor
2) By varying the current supplied to the stator

Graph:

No Load to Loaded Condition:

The increase in amount of torque increases the speed and thus compensating for
speed loss on loading
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A dc shunt motor has decreasing torque when its speed increases
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Characteristics of DC Shunt Motors
Torque vs
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Though at
heavy loads, Φ decreases in a small amount due to increased armature reaction
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Hence the Ta-Ia characteristic for a dc shunt motor will be a straight
line through origin
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Speed vs
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But, back EMF is also almost
constant, the speed remains constant
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But, the Eb decreases slightly more than Φ, and hence the speed
decreases slightly
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And hence, a shunt motor can be assumed as a constant speed motor
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Separately Excited Motor
The rotor and stator are connected from a different power supply
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The field winding is supplied from an
independent source at constant voltage
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The armature and field winding voltage loop equations are
VT = EA + IARA
VF = IFRF
The field circuit resistance RF is made up of the actual winding resistance, Rfield and a
variable resistance, Radj which can be used to control the field current
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The field current (and hence flux) can be
controlled by either adjusting the field circuit resistance or the field supply voltage
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Speed control
DC motors are in general much more adaptable to adjustable speed drives than ac motors which
are associated with a constant speed rotating fields
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It is thus
necessary to gather an idea about speed control methods along with their associated characteristics
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The above equation explains the concepts for different methods
of speed control
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The lowest speed corresponds to zero resistance in field rheostat
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The highest speed is limited by armature
reaction under weak field conditions, causing motor instability or poor commutation
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The torque on the other hand varies directly with flux
and therefore has its highest allowable value at the lowest speed
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Varying Armature Terminal Voltage (Vt)
A change of the armature terminal voltage results in a change in speed with constant
excitation
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In this mode the torque
developed by motor is almost equal to rated torque at any speed from zero to rated speed
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Application
Separately excited dc motors are very often used as actuators in industrial applications
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DC motors are preferred over ac motors because of their lower
manufacturing costs and, ease of controller implementations, since their mathematical
model is simpler
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This motor has high starting torque
and variation of speed with load
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Construction
The key components of a DC series motor are the armature (rotor), stator, commutator,
field windings, axle, and brushes
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The output power source is
connected to the armature windings through a brush arrangement connected to the
commutator
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The field winding
should be able to support high current because the greater the amount of current through
the winding, the greater will be the torque generated by the motor
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Heavy gauge wire does not allow a large
number of turns
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Working Principle
An external voltage source is applied across the series configuration of field winding and
armature
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Initially at the motor start up, with
the voltage source connected to the motor, it draws a huge amount of current because
both the winding and the armature of the motor, both made up of large conductors, offer
minimum resistance to the current path
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This strong magnetic field provides high torque to the armature shaft, thus invoking the
spinning action of the armature
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The rotating armature in the presence of the magnetic field results in
counter EMF, which limits the current build up in the series combination of armature and
winding
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Practically this is what required from the motors
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Subsequently lesser torque will
keep the load on the move
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However, the amount of torque generated by motor is directly proportional to the
winding current
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Motor Speed
In DC series motors, a linear relationship exists between the amount of torque produced
and the current flowing through the field windings
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To increase the speed of the motor, decrease the field current by placing a small
resistance in parallel to the winding and armature
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To decrease the speed, use an external series resistance along with the field winding and
armature
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Unlike DC shunt motor, series motor does not
operate at the constant speed
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Application
Series motors can produce large turning effect, or torque, from a stand still
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DC series motors are used mainly for industrial applications, e
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elevator and
pulley and winches systems for carrying heavy loads
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An automobile engine can be
started by this motor which draws around 500A of current
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Advantage
The construction, designing, and maintenance of these motors is very easy
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A final advantage of series motors is that they can be
used by providing either an Alternating Current (AC) or Direct Current (DC) power source
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As the armature speed increases, the current
through the winding decreases which further helps in reducing the counter EMF
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As this process continues, the motor speed
increases beyond the limit thus causing devastation to the motor
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The working principle of dc motor is same, i
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when a current carrying conductor is
placed in a magnetic field, it experiences a force
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Construction
In a PMDC motor, permanent magnets (located in stator) provide magnetic field, instead of
stator winding
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Permanent magnets
are usually made from rare earth materials or neodymium
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Rotor is made from layers of
laminated silicon steel to reduce eddy current losses
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Commutator is made from
copper and brushes are usually made from carbon or graphite
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The commutator is in segmented form to achieve unidirectional torque
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Characteristics
Characteristics of PMDC motors are similar to the characteristics of shunt motor in terms
of torque, speed and armature current
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Applications of Permanent Magnet DC Motors
Permanent magnet dc motors are extensively used where smaller power ratings are required,
e
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in toys, small robots, computer disc drives etc
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For smaller ratings, use of permanent magnets reduces manufacturing cost
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No need of field excitation winding, hence construction is simpler
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No need of electrical supply for field excitation, hence PMDC motor is relatively more
efficient
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Relatively smaller in size

5
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Since the stator in PMDC motor consists of permanent magnets, it is not possible to add
extra ampere-turns to reduce armature reaction
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2
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