Dc motor

DC MOTOR
Electrical motor: It is a machine which
convert electrical energy into mechanical
energy.
AC Motor: motor that runs on alternating
current (AC) electricity.
DC Motor: motor that runs on direct current (DC)
electricity.

Principle of operation of DC Motor:
When current carrying conductor is placed in a magnetic
field it experience a force.

Function of each part of DC Motor:
Yoke:
• It is outer cover of dc motor also called as frame.
• It provides protection to the rotating and other part of
the machine from moisture, dust etc.
• Yoke is an iron body which provides the path for the flux
to complete the magnetic circuit.
• It provides the mechanical support for the poles.
• Material Used: low reluctance material such as cast
iron, silicon steel, rolled steel, cast steel etc.

Poles, and pole core:
• Poles are electromagnet, the
field winding is wound over it.
• It produces the magnetic flux
when the field winding is excited.
• The construction of pole is done using the lamination of
particular shape to reduce the power loss due to eddy
current.
pole shoe:
• Pole shoe is an extended part of a pole. Due to its typical
shape, it enlarges the area of the pole, so that more flux
can pass through the air gap to armature.
• Material Used: low reluctance magnetic material such
as cast steel or cast iron is used for construction of pole
and pole shoe.

Field winding: field coil wound on pole
• The coil wound on the pole core are called field coils.
• Field coils are connected in series to form field winding.
• Current is passed through the field winding in a specific
direction, to magnetize the poles and pole shoes. Thus
magnetic flux is produce in the air gap between the pole
shoe and armature.
• Field winding is also called as Exciting winding.
• Material Used for copper conductor is copper.
• Due to the current flowing through the field winding
alternate N and S poles are produced.

Armature core:
• Armature core is a cylindrical drum mounted on the
shaft.
• It is provided with large number of slots all over its
periphery and it is parallel to the shaft axis.
• Armature conductors are placed in these slots.
• Armature core provides low reluctance path to the flux
produced by the field winding.
• Material used: high permeability, low reluctance cast
steel or cast iron material is used.
• Laminated construction of iron core is used to minimize
the eddy current losses.

Armature winding:
• Armature conductor is placed in a armature
slots present on the periphery of armature core.
• Armature conductor are interconnected to form the
armature winding.
• When the armature winding is rotated using a prime
mover, it cuts the magnetic flux lines and voltage gets
induced in it.
• Armature winding is connected to the external circuit
(load) through the commutator and brushes.
• Material Used: Armature winding is suppose to carry
the entire load current hence it should be made up of
conducting material such as copper.

Commutator:
• It is a cylindrical drum mounted on the
shaft along with the armature core.
• It is made up of large number of wedge
shaped segments of hard-drawn copper.
• The segments are insulated from each
other by thin layer of mica.
• Armature winding are tapped at various points and these
tapping are successively connected to various segments of
the commutator.
Function of commutator:
• It converts the ac emf generated internally into dc
• It helps to produce unidirectional torque.
Material Used: it is made up of copper and insulating
material between the segments is mica.

Brushes:
• Current are conducted from the armature to the external
load by the carbon brushes which are held against the
surface of the commutator by springs.
• Function of brushes: To collect the current from the
commutator and apply it to the external load in
generator, and vice versa in motor.
• Material Used:
Brushes are made of carbon and they are rectangular
in shape.

Back emf:
• When the armature winding of dc motor is start rotating
in the magnetic flux produced by the field winding, it cuts
the lines of magnetic flux and induces the emf in the
armature winding.
• According to Lenz’s law (The law that whenever there is an induced
electromotive force (emf) in a conductor, it is always in such a direction that
the current it would produce would oppose the change which causes the
induced emf. ), this induced emf acts in the opposite
direction to the armature supply voltage. Hence this emf
is called as back emfs.
𝐸 𝑏 =
𝑁∅𝑍
60
𝑃
𝐴
Volts +
𝑁= speed in rpm
∅= flux per pole armature A1
𝑍= no of conductors supply voltage 𝐸 𝑏
𝑃=no of pole pairs A2
𝐴=area of cross section of conductor
𝐸 𝑏= back emf _

Voltage and Power equation of DC Motor:
𝑉 = 𝐸𝑏 + 𝐼𝑎𝑅𝑎
If we multiply the above equation by 𝐼𝑎, we will get
𝑉𝐼𝑎 = 𝐸𝑏𝐼𝑎 + 𝐼𝑎2 𝑅𝑎
𝑉𝐼𝑎 = 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑠𝑢𝑝𝑝𝑙𝑖𝑒𝑑 𝑡𝑜 𝑡𝑕𝑒 𝑚𝑜𝑡𝑜𝑟
𝐸𝑏𝐼𝑎 = 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝑒𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 𝑜𝑓 𝑡𝑕𝑒 𝑚𝑒𝑐𝑕𝑎𝑛𝑖𝑐𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 𝑏𝑦 𝑡𝑕𝑒 𝑚𝑜𝑡𝑜𝑟
𝐼𝑎2 𝑅𝑎 = 𝑝𝑜𝑤𝑒𝑟 𝑙𝑜𝑠𝑠 𝑡𝑎𝑘𝑖𝑛𝑔 𝑝𝑙𝑎𝑐𝑒 𝑖𝑛 𝑎𝑟𝑚𝑎𝑡𝑢𝑟𝑒 𝑤𝑖𝑛𝑑𝑖𝑛𝑔
Thus,
𝐸𝑏𝐼𝑎 = 𝑉𝐼𝑎 − 𝐼𝑎2
𝑅𝑎
=input power-power loss
thus, 𝐸𝑏𝐼𝑎= Gross mechanical power produce by the
motor
= Pm

Torque equation of DC Motor:
𝑚𝑒𝑐𝑕𝑎𝑛𝑖𝑐𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑡𝑜 𝑟𝑜𝑡𝑎𝑡𝑒 𝑡𝑕𝑒 𝑠𝑕𝑎𝑓𝑡 𝑜𝑛
𝑚𝑒𝑐𝑕𝑎𝑛𝑖𝑐𝑎𝑙 𝑠𝑖𝑑𝑒 = 𝑇𝜔………………………………………………
T =Torque in Newton-meter
𝜔= angular velocity in radian /second
𝑔𝑟𝑜𝑠𝑠 𝑚𝑒𝑐𝑕𝑎𝑛𝑖𝑐𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑝𝑟𝑜𝑑𝑢𝑐𝑒 𝑏𝑦 𝑡𝑕𝑒 𝑚𝑜𝑡𝑜𝑟 𝑜𝑛
𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝑠𝑖𝑑𝑒 = 𝐸𝑏𝐼𝑎…………………………………………………
Eb = back emf in volts
Ia = armature current in ampere
equating eqnuation and , we get
𝐸𝑏𝐼𝑎 = 𝑇𝜔……………………………………………………
2
1
3
1 2

𝜔 =
2𝜋𝑁
60
………………………………
2𝜋𝑁
60
= Speed in rpm
And 𝐸𝑏 =
𝑁∅𝑍𝑃
𝐴60
Thus, equation become
𝑁∅𝑍𝑃
𝐴60
𝐼𝑎 = 𝑇
2𝜋𝑁
60
𝑇 =
𝑃∅𝑍𝐼𝑎
2𝜋𝐴
=
0.159𝑃∅𝑍𝐼𝑎
𝐴
=
0.159𝑃𝑍
𝐴
∅𝐼𝑎
𝑃, 𝑍 𝑎𝑛𝑑 𝐴 are constant, hence we can say
𝑇 ∝ ∅ 𝐼𝑎
Thus torque produce by the DC Motor is proportional to the
main field flux ∅ and armature current 𝐼𝑎
3

Types of DC Motor:
• Classification of the d.c. motor depends on the way of
connecting the armature and field winding of a d.c.
motor:
1. DC Shunt Motor
2. DC Series Motor
3. DC Compound Motor
Short shunt compound long shunt compound
Cumulative Differential Cumulative Differential
compound compound compound compound
motor motor motor motor

DC Shunt Motor:
• In dc shunt motor the armature
and field winding are connected in
parallel across the supply voltage
• The resistance of the shunt
winding 𝑅 𝑠𝑕 is always higher than the
armature winding 𝑅 𝑎
• Since V and 𝑅 𝑠𝑕 both remains constant the 𝐼𝑠𝑕 remains
essentially constant, as field current is responsible for
generation of flux.
thus ∅ ∝ 𝐼𝑠𝑕
• So shunt motor is also called as constant flux motor.

Torque and Speed equation of DC Shunt Motor:
As we have seen for dc motor
𝑇 ∝ ∅ 𝐼𝑎
But for dc shunt motor :∅ ∝ 𝐼𝑠𝑕
And 𝐼𝑠𝑕 is constant , thus ∅ is also constant
So torque in dc shunt motor is
𝑇 ∝ 𝐼𝑎
For dc motor
𝐸𝑏 =
𝑁∅𝑍𝑃
𝐴60
𝑍 , 𝑃, 𝐴, ∅ 𝑎𝑛𝑑 60 are constants
Thus, 𝑁 ∝ 𝐸𝑏 ∝ (𝑉 − 𝐼𝑎𝑅𝑎)

Characteristics of DC Shunt Motor:
To study the performance of the DC shunt Motor various
types of characteristics are to be studied.
1. Torque Vs Armature current characteristics.
2. Speed Vs Armature current characteristics.
3. Speed Vs Torque characteristics.

Torque Vs Armature current
characteristics of DC Shunt motor
This characteristic gives us information that, how torque of
machine will vary with armature current, which depends
upon load on the motor.
𝑇 ∝ 𝐼𝑎
Thus,

Speed Vs Armature current
characteristics of DC Shunt Motor
The back emf of dc motor is 𝐸𝑏 =
𝑁∅𝑍𝑃
𝐴60
= 𝑉 − 𝐼𝑎𝑅𝑎
• Therefore shunt motor is considered as constant speed motor.

Speed Vs Torque
characteristics of DC Shunt motor
• From the above two characteristics of
dc shunt motor, the torque developed
and speed at various armature
currents of dc shunt motor may be
noted.
• If these values are plotted, the graph
representing the variation of speed
with torque developed is obtained.
• This curve resembles the speed Vs
current characteristics as the torque is
directly proportional to the armature
current.

Applications of DC shunt Motor:
These motors are constant speed motors, hence used in
applications requiring constant speed.
Like:
1) Lathe machine
2) Drilling machine
3) Grinders
4) Blowers
5) Compressors

DC Series Motor:
• In this type of DC motor the armature and field windings
are connected in series.
• the resistance of the series field
winding Rs is much smaller than
the armature resistance Ra
• The flux produced is proportional
to the field current but in this
𝐼𝑓= 𝐼 𝑎 thus ∅ ∝ 𝐼𝑎
• Thus flux can never become constant in dc series motor
as load changes If and Ia also gets changed
• Thus dc series motor is not a constant flux motor.

Torque and Speed equation of DC Series Motor:
As we have seen for dc motor
𝑇 ∝ ∅ 𝐼𝑎
But for dc series motor as 𝐼𝑓= 𝐼 𝑎 𝑡𝑕𝑢𝑠 ∅ ∝ 𝐼𝑎
So torque in dc series motor is
𝑇 ∝ 𝐼𝑎2
For dc motor
𝐸𝑏 =
𝑁∅𝑍𝑃
𝐴60
𝑍 , 𝑃, 𝐴 𝑎𝑛𝑑 60 are constants
Thus,𝑁 ∝
𝐸𝑏
∅
∝
𝑉−𝐼𝑎𝑅𝑎 −𝐼𝑠 𝑅 𝑠
∅
=
𝑉−𝐼𝑎(𝑅𝑎+𝑅 𝑠)
∅
....... as 𝐼𝑎=𝐼𝑠
for dc series motor

Characteristics of DC Series Motor:
To study the performance of the DC series Motor various
types of characteristics are to be studied.
3. Speed Vs Torque characteristics

Torque Vs Armature current
characteristics of DC Series motor
• Torque developed in any dc motor is
𝑇 ∝ ∅𝐼𝑎
• In case of a D.C. series motor, as field current is equal to
armature current, and for small value of 𝐼 𝑎
∅ ∝ 𝐼 𝑎
• Therefore the torque in the dc series motor for small
value of 𝐼 𝑎
𝑇 ∝ 𝐼𝑎2
• When 𝐼 𝑎 is large the ∅ remains the constant due to
saturation, thus torque is directly proportional to
armature current for large value of 𝐼 𝑎
𝑇 ∝ 𝐼 𝑎

• Thus Torque Vs Armature current characteristics begin to
raise parabolically at low value of armature current and
when saturation is reached it become a straight line as
shown below.

Speed Vs Armature current
characteristics of DC Series Motor
Consider the following equation:
𝑁 =
𝐾(𝑉 − 𝐼𝑎𝑅𝑎)
∅
When supply voltage V is kept constant, speed of the motor
will be inversely proportional to flux. In dc series motor
field exciting current is equal to armature current which is
nothing but a load current. Therefore at light load when
saturation is not attained, flux will be proportional to the
armature current and hence speed will be inversely
proportional to armature current. Hence speed and
armature current characteristics is hyperbolic curve upto
saturation.

• As the load increases the armature current increases and
field gets saturated, once the field gets saturated flux will
become constant irrespective of increases in the
armature current. Therefore at heavy load the speed of
the dc series motor remains constant.
• This type of dc series motor has high starting torque.

Speed Vs Torque
characteristics of DC Series motor
• The Speed Vs Torque characteristics of dc series motor
will be similar to the Speed Vs Armature current
characteristics it will be rectangular hyperbola, as shown
in the fig.

Applications of DC series Motor-
These motors are useful in applications where starting
torque required is high and quick acceleration. Like:
1) Traction
2) Hoists and Lifts
3) Crane
4) Rolling mills
5) Conveyors

DC Compound Motor:
• The DC compound motor is a combination of the series
motor and the shunt motor. It has a series field winding
that is connected in series with the armature and a shunt
field that is in parallel with the armature. The
combination of series and shunt winding allows the
motor to have the torque characteristics of the series
motor and the regulated speed characteristics of the
shunt motor. Several versions of the compound motor
are:
• Short shunt Compound Motors
• Long shunt Compound Motors

Short shunt compound motor:
• When shunt field winding is connected
in parallel with armature like dc shunt
motor and this assembly is connected in
series with the series field winding then
this type of motor is called as short shunt
compound motor.
• Depending on the polarity of the connection short shunt
motor is classified as:
1. Cumulative compound motor.
2. Differential compound motor.

Cumulative compound motor (short shunt):
• Figure shows a diagram of the cumulative compound
motor. It is so called because the shunt field is connected
so that its coils are aiding the magnetic fields of the
series field and armature.
• In this figure that the top of the shunt field is positive
polarity and that it is connected to the positive terminal
of the armature.

• The cumulative compound motor is one of the most
common DC motors because it provides high starting
torque and good speed regulation at high speeds. Since
the shunt field is wired with similar polarity in parallel
with the magnetic field aiding the series field and
armature field, it is called cumulative. When the motor is
connected this way, it can start even with a large load
and then operate smoothly when the load varies slightly.
• You should recall that the shunt motor can provide
smooth operation at full speed, but it cannot start with a
large load attached, and the series motor can start with a
heavy load, but its speed cannot be controlled. The
cumulative compound motor takes the best
characteristics of both the series motor and shunt motor,
which makes it acceptable for most applications.

Differential Compound Motor (short shunt):
Differential compound motors use the
same motor and windings as the
cumulative compound motor, but they
are connected in a slightly different
manner to provide slightly different
operating speed and torque characteristics.
Figure shows the diagram for a differential
compound motor with the shunt field
connected so its polarity is reversed to the
polarity of the armature. Since the shunt field is still
connected in parallel with only the armature, it is
considered a short shunt.

In the above diagram you should notice that Fl and F2 are
connected in reverse polarity to the armature. In the
differential compound motor the shunt field is connected
so that its magnetic field opposes the magnetic fields in the
armature and series field. When the shunt field's polarity is
reversed like this, its field will oppose the other fields and
the characteristics of the shunt motor are not as
pronounced in this motor. This means that the motor will
tend to overspeed when the load is reduced just like a
series motor. Its speed will also drop more than the
cumulative compound motor when the load increases at
full rpm. These two characteristics make the differential
motor less desirable than the cumulative motor for most
applications.

Long shunt compound motor:
• when the shunt field is connected
in parallel with both the series field
and the armature then this type of
motor is called as long shunt
compound motor.
• Depending on the polarity of connection
of shunt field winding, series field winding
and armature, long shunt motor is classified as:
1. Cumulative Compound Motor.
2. Differential Compound Motor.

Cumulative Compound Motor (long shunt):

Characteristics of DC compound Motor:
To study the performance of the DC compound Motor
various types of characteristics are to be studied.
3. Speed Vs Torque characteristics

• In dc compound motors both shunt and series field
acting simultaneously.
• In cumulative compound motor series field assist the
shunt field.
• In such motors when armature current increases the field
flux increases.
• So for given armature current the torque developed will
be greater and speed lower when compared to a dc shun
motor.
• In differential compound motor series field opposes the
shunt field, therefore when armature current decreases
the field flux decreases, so for given armature current the
torque developed will be lower and speed greater when
compare to the dc shunt motor.

Torque Vs Armature current and Speed Vs Armature current
characteristics of dc compound motors
Speed Vs Torque characteristics are compared with that of shunt motor.

Applications of DC Compound Motor:
Cumulative Compound Motor:
• These motors have high starting torque.
• They can be operated even at no loads as they run at
a moderately high speed at no load.
• Hence cumulative compound motors are used for
the following applications.
1. Elevators
2. Rolling mills
3. Punches
4. Shears
5. planers

Applications of DC Compound Motor:
Differential Compound Motor:
• The speed of these motors increases with increases in
the load which leads to an unstable operation.
• Therefore we can not use this motor for any practical
applications.

Speed Control of DC Motor:
• The speed equation of dc motor is
𝑁 ∝
𝐸𝑏
∅
∝
(𝑉−𝐼𝑎𝑅𝑎)
∅
• But the resistance of armature winding or series field winding
in dc series motor are small.
• Therefore the voltage drop 𝐼𝑎𝑅𝑎 or 𝐼𝑎(𝑅𝑎 + 𝑅 𝑠) across them
will be negligible as compare to the external supply voltage V
in above equation.
• Therefore 𝑁 ∝
𝑉
∅
, since V>>>> 𝐼𝑎𝑅𝑎
• Thus we can say
1. Speed is inversely proportional to flux ∅.
2. Speed is directly proportional to armature voltage.
3. Speed is directly proportional to applied voltage V.
So by varying one of these parameters, it is possible to change
the speed of a dc motor

Armature voltage control method:

Reversal of Direction of Rotation:
• The direction of the magnetic flux in the air gap depends
on the direction of the field current.
• And the direction of the force exerted on the armature
winding depends on the direction of flux and the
direction of armature current.
• Thus in order to reverse the direction of dc motor, we
have to reverse the direction of force.
• This can be achieved either by changing the terminals of
the armature or the terminals of the field winding.

Need of Starter:
We know that,V = 𝐸 𝑏 + 𝐼 𝑎 𝑅 𝑎.............for a dc shunt motor
and V = 𝐸 𝑏 + 𝐼 𝑎(𝑅 𝑎 + 𝑅 𝑠)….for a dc series motor
Hence the expression for 𝐼 𝑎 are as follows:
𝐼 𝑎 =
𝑉−𝐸 𝑏
𝑅 𝑎
…………………… for dc shunt motor
𝐼 𝑎 =
𝑉−𝐸 𝑏
(𝑅 𝑎+𝑅 𝑠)
………………..for dc series motor
At the time of starting the motor, speed N=0 and hence the
back emf 𝐸 𝑏=0. Hence the armature current at the time of
starting is given by,
𝐼 𝑎(𝑠𝑡𝑎𝑟𝑡𝑖𝑛𝑔) =
𝑉
𝑅 𝑎
………….for dc shunt motor
𝐼 𝑎(𝑠𝑡𝑎𝑟𝑡𝑖𝑛𝑔) =
𝑉
(𝑅 𝑎+𝑅 𝑠)
……for dc series motor

• Since the the values of 𝑅 𝑎 𝑎𝑛𝑑 𝑅 𝑠are small, the starting
currents will be tremendously large if the rated voltage is
applied at the time of starting.
• The starting current of the motor can be 15 to 20 times higher
than the full load current.
• Due to high starting current the supply voltage will fluctuate.
• Due to excessive current, the insulation of the armature
winding may burn.
• The fuses will blow and circuit breakers will trip.
• For dc series motors the torque T ∝ 𝐼 𝑎
2
. So an excessive large
starting torque is produced. This can put a heavy mechanical
stress on the winding and shaft of the motor resulting in the
mechanical damage to the motor.
• So to avoid all these effects we have to keep the starting
current of motor below safe limit. This is achieved by using
starter.

Principle of starter:
• Starter is basically a resistance which is connected in
series with the armature winding only at the time of
starting the motor to limit the starting current.
• The starter of starter resistance will remain in the circuit
only at the time of starting and will go out of the circuit
or become ineffective when the motor speed upto a
desire speed.

• At the time of starting, the starter is in the start position
as shown in fig. so the full starter resistance appears in
series with the armature. This will reduce the starting
current.
• The starter resistance is then gradually cut off. The motor
will speed up, back emf will be developed and it will
regulate the armature current. The starter is not
necessary then.
• Thus starter is pushed to the Run position as shown in fig
under the normal operating condition. The value of
starter resistance is zero in this position and it does not
affect the normal operation.
Types of starter:
1. Three point starter
2. Four point starter

Classes of Insulation:
Class Material Temperature
(max value )
Y Cotton fabric and silk but not impregnated in dielectric 90◦C
A Same as above but it is impregnated in some dielectric 105◦C
E Synthetic organic films 120◦C
B Glass fibres, Mica, Asbestos fibres bound together 130◦C
F Above material but with impregnations 155◦C
H Above material combined with silicon binding and
impregnation
180◦C
C Above material with ceramic or quartz wth or without
binding agent
More than
180◦C

Force acting on the armature conductor(Lorentz
force):

Dc motor

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