Electrical Machines I

18EEC202T

UNIT - II
DC MOTORS

By
Er.V.JAYAKUMAR M.E.
AP/EEE.
Er.V.JayaKumar, AP/EEE 1
Er.V.JayaKumar, AP/EEE 2
❖ Introduction of DC motor
❖ Back EMF & torque equation
❖ Speed & torque relation
❖ Starting of DC motors
❖ Power relationship in motor
❖ Speed control methods for DC shunt &
series motors
❖ Applications Er.V.JayaKumar, AP/EEE 3
Er.V.JayaKumar, AP/EEE 4
• A Motor is a machine that converts electrical energy into
mechanical energy.
• Basically, there is a no constructional difference
between a DC motor and a DC generator. The same DC
machine can be run as a generator or motor.
• Its operation is based on the principle that “when a
current carrying conductor is placed in a magnetic
field, the conductor experiences a mechanical force”.
• The direction of this force is given by “Fleming’s Left Hand
Rule” and magnitude is given by
F = B I L Newton
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 CLASSIFICATION OF DC MOTOR

SERIES MOTOR

SHUNT MOTOR
SELF
EXCITED
DC CUMMULATIVE
MOTOR LONG
SHUNT
SEPARATELY DIFFERENTIAL
EXCITED COMPOUND
MOTOR
CUMMULATIVE
SHORT
SHUNT
Er.V.JayaKumar, AP/EEE
DIFFERENTIAL6
PRINCIPLE OF WORKING OF DC MOTOR

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The simplest DC machine
segments

brushes
CHANGE OF DIRECTION OF MOTOR
• The Direction Of Rotation(DOR)
of the motor could be changed
either by
• changing the direction of armature
terminals (or)
• the polarity of the field Excitation.
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 BACK EMF (OR) COUNTER EMF
• When the armature rotates, it’s conductors cut
the magnetic flux. Therefore, emf is induced in
them as per Faraday’s Law of electromagnetic
induction. This emf is known as Back emf Eb (or)
Counter emf.
• The back emf opposes the applied voltage.
• Since the back emf is induced due to generator action,
its magnitude is given by the same expression as that
for the generated emf in a DC generator.

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Back EMF.

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 SIGNIFICANCE OF BACK EMF
• The presence of back emf. makes the
DC motor as a self-regulating
machine.
• It makes the motor to draw as much
armature current as it is just
sufficient to develop the torque
required by the load.
• Armature current, Ia = ( V - Eb ) / Ra
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• A 220 V motor has an armature
circuit resistance of 0.5 ohms. If
the full load armature current is 20
Amperes, find the back emf
induced in the armature.

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• A 6 pole, 500 V shunt motor has 720
wave connected conductors in the
armature.
• The full-load armature current is 60 A,
and the flux per pole is 0.03 wb.
• The armature resistance is 0.2 ohms
and the contact drop is 1 V per brush.
• Calculate the full-load speed of the
motor. Er.V.JayaKumar, AP/EEE 19
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EQUIVALENT CIRCUIT OF DC MOTOR

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 POWER EQUATION
• The above voltage equation is multiplied by “Ia”, we
get
V Ia = E b Ia + I a 2 R a
• This is known as power equation.
V Ia=Electric power supplied to armature
• Eb Ia =Power developed by armature
• Ia2 Ra=Electric power wasted in armature
• A small portion is wasted as Ia2 Ra and the
remaining portion Eb Ia is converted into mechanical
power within armature.
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 TORQUE EQUATION
• The measure of causing the rotation of a wheel
(or) twisting moment of a force about the axis is
called the torque.
• Torque (T)=Force (F)  Radius (r)
• Consider a wheel of radius “r” meters acted by a
circumferential force ‘F’ newton as shown in
Fig.
• Let the force “F” causes the wheel to rotate at
‘n’ rps.
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• Determine the value of torque in kg-m
developed by the armature of a 6 pole
wave wound motor having 492
conductors, 35 mwb per pole when the
total armature current is 40 amperes.

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 A 500 V, DC series motor has 4 poles
wave wound armature with 948
conductors and total resistance of 2
ohm.
When the mechanical torque
developed is 45 kg-m,
the flux per pole is 0.2 weber.
Calculate the current taken by the
motor and the speed.
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SHAFT TORQUE

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• Determine the torque
developed and shaft torque of
220 V, 6 pole series motor with
1000 conductors wave
connected supplying a load of
8.5 kW by taking 47 A from the
mains. The flux/pole is 25 mwb
and its armature resistance is
0.6 ohm. Er.V.JayaKumar, AP/EEE 35
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Relationship Between Speed &
Back Emf., Flux

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• A 250 V shunt motor on no-load
runs at 1000 rpm and takes 5A.
• The total armature and shunt field
resistances are 0.2 ohm and 250 ohm
respectively.
• Calculate the speed when loaded and
taking current of 50 A

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• A 10 HP, 230 V, shunt motor has an
armature circuit resistance of 0.5 ohm and
a field resistance of 115 ohm.
• At no-load and rated voltage, the speed is
1200 RPM and the armature current is 2 A.
• If the load is applied, the speed drops to
1100 rpm.
• Determine the armature current, the line
current and torque.
Er.V.JayaKumar, AP/EEE 42
• ☺Solution: Back emf at no-load,
• Eb=V – Ia1 Ra = 230 – 2  0.5 = 229 V
• Back emf at loaded condition,

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 DC MOTOR CHARACTERISTICS
• The performance of a DC motor can be determined
from its characteristics curves known as motor
characteristics.
(i) Torque versus Armature current characteristics
(T vs Ia)  Electrical characteristics.
(ii)Speed versus Armature current characteristics
(N vs Ia)
(iii)Speed versus Torque characteristics
(N vs T)  Mechanical characteristics.
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Speed Versus Armature Current (N vs Ia)

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Speed Versus Torque Characteristics

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Dc series motor characteristics

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Speed VS armature current

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DC COMPOUND MOTORS

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 Starters

• DC motors are self-starting motors, whenever the

armature and field winding of a DC motor receives
supply, motoring action takes place.
• So, DC motors do not require any additional device to
start it.

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 Necessity of Starter
• When a dc motor is started with full voltage
applied across its armature terminals during the
starting period, it will draw more current than its
rated current.
• This excessive current will overheat the armature
winding and may even damage the winding.
• During starting period, a variable resistance called
a starter is connected in series with the armature circuit to
limit the starting current.

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 TYPES OF STARTERS
There are three types of starters.
1. Two point starter
2. Three point starter
3. Four point starter

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 Terminals of the
Sl. No. Name of the Starter Motor
Starter

1. Two Point L = Line

F = Field DC Series Motor

2.

Three Point L = Line DC Shunt Motor

F = Field
A = Armature

3.

L+ = Line (+ve) Shunt and Compound Motor

Four Point L– = Line (–ve)
F = Field
A = Armature

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Two point starter

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THREE POINT STARTER

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 NO VOLT COIL
• If the supply voltage is suddenly
interrupted (or) If the field excitation is
accidentally out, the No-Volt Release Coil is
demagnetized and the handle goes back to
the OFF position under the pull of the
spring.
• If the NVR coil were not used, then in case
of failure of supply, the handle would
remain on the final stud.
• If the supply is restored, the motor will be
directly connected across the supply.
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 over-load release coil
• If the motor is over-loaded (or) a fault occurs,
it will draw excessive current from the supply.
• This current will increase the ampere turns of
the over-load release coil and pull the soft iron
keeper, thus short-circuiting the no-volt release
coil.
• The no-volt release coil is demagnetized and the
handle is pulled to the OFF position by the
spring.
• Thus, the motor is automatically disconnected
from the supply. Er.V.JayaKumar, AP/EEE 60
 DEMERITS
• In a three-point starter, the no-volt release coil is
connected in series with the shunt field circuit. So
that, it carries the shunt field current.
• A motor can be run for a higher speed than normal
speed by reducing the field current.
• The field current may be weakened to such an extent
that the no-volt release coil may not be able to keep
the starter arm in the ON position.
• This may disconnect the motor from the supply
when it is not desired. This drawback can be
overcome by four point starter
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FOUR POINT STARTER

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•The drawback of a 3 point starter
as mentioned above is overcome in
a 4 point starter by connecting the
NVR coil across the line instead
of connecting it in series with
the shunt field circuit.
•This makes a wide range of field
adjustments possible.
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• When the arm touches stud no.1, line
current divides into three path.
• One part passes through starting
resistance armature and series field.
• Second part passes through the shunt
field winding.
• The third part passes through no-volt
release coil and protective resistance.
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 Methods of Speed Control of DC Motor
1.By varying the field flux (). This is
known as flux control method.
2.By varying the resistance in the
armature circuit. This is known as
armature control method.
3.By varying the applied voltage. This is
known as voltage control method.
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 FIELD CONTROL METHOD

• By this method, the speed control above rated

value is possible

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 Armature Control Method
• The speed is directly proportional to the voltage
applied across the armature.
• As the supply voltage is normally constant, the
voltage across the armature can be controlled by
adding a variable resistance in series with the
armature.
• The field winding is excited by the normal voltage.
Hence Ish is maintained at its rated value for this
method. Er.V.JayaKumar, AP/EEE 68
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This method can only provide speeds
below the normal speed.

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Ward-Leonard System of Speed Control

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• This is a practical application of the method of speed
control by voltage control.
• The greatest advantage of this method is that the motor
speed can be varied in either direction over a wide range.

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 Speed Control of DC Series Motor

• Like DC shunt motors, the speed of DC

series motor can also be controlled by
different methods.
• The most important methods are:
• (i) Armature Control Method
• (ii) Flux Control Method
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 Armature Control Method
• In this method of speed control, the voltage applied
to the armature is varied by means of a series
rheostat.

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• When the Rheostat is in Maximum Position
➢speed is minimum.
• When the Rheostat is in Minimum Position
➢ speed is maximum .

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 Flux Control Method
• In this method of speed control the flux established
by the excitation current is suitably varied according
to the speed requirement.

❖(a) Field Diverter Method

❖(b) Armature Diverter Method
❖(c) Tapped Field Control Method.
❖(d) Paralleling Field Coils

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(a) Field Diverter Method

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(b) Armature Diverter Method

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(c) Tapped Field Control Method.

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Paralleling Field Coils

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