Rotating machines part 3

027 Electrical & Electronics Principles
© Seychelles Institutes of Technology Prepared by Mr. Dinesh
Pg 25
6.0C ROTATING MACHINES
6.C.1 Direct current (DC) motor and generator
A DC motor consists of the same basic elements as a DC electric generator: i.e., is a multi-turn coil
which is free to rotate in a constant magnetic field.
Furthermore, the rotating coil is connected to the external circuit in just the same manner as in a DC
generator: i.e., via a split-ring commutator which reverses the polarity of the coil with respect to the
external circuit whenever the coil passes through the plane perpendicular to the direction of the magnetic
field.
Suppose that an external DC voltage source (e.g., a battery, or a DC generator) of emf V is connected
across the motor. The voltage source drives a steady current I around the external circuit, and through
the motor. As the current flows around the coil, the magnetic field exerts a torque on the coil, which
causes it to rotate.
Let us suppose that the motor eventually attains a steady-state rotation frequency f. The rotating coil
generates a back-emf Ԑ whose magnitude is directly proportional to the frequency of rotation.
6.C.2 Circuit diagram of a dc motor connected to an external dc emf source
Fig 11: Circuit diagram of a dc motor connected to an external dc emf source

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Fig 11 shows the circuit in question.
Where, R : internal resistance of the motor
V : external emf
Ԑ : back emf
Application of Ohm's law around the circuit gives
V = IR + Ԑ
I = 𝑉
𝑅 + Ԑ
The rate at which the motor performs mechanical work is
P = Ԑ I
𝐏 = Ԑ(
𝐕
𝐑 + Ԑ
)
6.C.3 DC motor works as generator
All types of DC motors can be used as generators, but most types have limitations.
A permanent magnet DC motor with a commutator only needs external force to turn its shaft in order
to function as a generator.
A permanent magnet DC motor without a commutator is a brushless DC motor. It cannot work as
generator.
A series wound, shunt wound and compound wound DC motors use a electromagnet to generate the
magnetic field and therefore won't work as generators.
A DC motor without a permanent magnet requires current to be supplied to the field windings in order
to function as a generator.

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6.0 ROTATING MACHINES
6.C.4 Brushed dc motor
This type of motor produces a magnetic field in a wound rotor (the part that rotates) by passing an
electrical current through a commutator and carbon brush assembly, hence the term “Brushed”. The
stators (the stationary part) magnetic field is produced by using either a wound stator field winding or by
permanent magnets. Generally brushed DC motors are cheap, small and easily controlled.
6.C.5 Parts of a simple dc motor
6.C.6 Parts of a simple dc motor

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6.C.7 Circuit diagram of a simple dc Motor
6.C.8 Circuit diagram of a simple dc Motor
Working Principle of a simple Brushed DC Motor
When electric current passes through a coil in a magnetic field, the magnetic force produces a torque
which turns the DC motor. The magnetic force acts perpendicular to both wire and the magnetic field.
Commutator
The commutator reverses the current each half revolution to keep the coil turning in the same direction.
Brushes
Made of carbon strips. It carry current to coil via commutator. Its purpose of is to reduce wear on the
commutator.
Armature
The part of a motor in which a current is induced by a magnetic field. The armature consists of a series
of coils mounted on a shaft and rotates through the magnetic field.
Field winding
The conducting wire connected to the armature that energizes the pole pieces. Field windings are
connected in series or parallel.

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6.C.9 Types of dc motor / generator
1. Separately excited DC Motor
2. Self excited DC Motor
 Shunt connection
 Series connection
 Compound connection
3. Permanent magnet DC Motor
6.C.10 Separately excited dc generator / motor
Circuit diagram of a separately excited dc generator
Circuit diagram of a separately excited dc motor

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6.C.11 Description of the essential features of a separately excited dc generator / motor
In a separately excited motor or generator,
 Separate supply sources are provided for excitation of both field coil and armature coil.
 The field coil is energized from a separate DC voltage source and the armature coil is also
energized from another source.
 Armature voltage source may be variable but, independent constant DC voltage is used for
energizing the field coil. So, those coils are electrically isolated from each other.
Where IA = Armature current and IF = Field current

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6.C.12 Load characteristics of a separately excited dc generator
Terminal Voltage ─ Load Current characteristic
Generated Voltage ─ Field Current characteristics
Terminal
Voltage, V
Load current, Ia
 The terminal voltage falls off slightly as load
increases.
 Use
1. Testing purpose in the laboratories.
2. Controlling speed for various applications.
 Disadvantage
1. A separate dc source is required.
Generated
Voltage, E
Field current, IF
 This curve is known as no load saturation
characteristic curve of dc generator.

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6.C.13 Load characteristics of a separately excited dc motor
Speed – armature current characteristics
Torque – armature current characteristics
6.C.14 Application of separately excited dc motor
 Actuators
 Trains
 Automatic traction
Speed, N
Armature Current, IA
Torque, τ
Armature Current
 Torque is proportional to the flux and armature current.
 Flux is constant.
 Torque is proportional to the armature current.
 Huge current is needed to start heavy loads.
 So this type of motor do not starts on heavy loads.
 Speed of dc motor is proportional to back emf / flux
 When load is increased back emf and flux decrease due
to armature resistance drop and the armature reaction.
 However back emf decreases more than flux so that the
speed of the motor slightly decreases with load.

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6.C.15 Self excited shunt ─ wound dc generator / motor
The circuit diagram of a self excited shunt ─ wound dc Generator
The circuit diagram of a self excited shunt ─ wound dc motor

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6.C.16 Description of the essential features of a self excited shunt ─ wound dc generator / motor
In a self excited shunt ─ wound dc motor or generator,
 The field coils are connected in parallel with the armature winding.
 The shunt or field winding consists of fine copper coils of large number of turns.
 The armature current IA divides in two parts. One part is the shunt field current IF, flows through
shunt field winding and the other part is the load current I goes through the external load.
 IA = I + IF (generator)
 I = I A+ IF (motor)
Where I = the current to (or from) the supply, IA = Armature current and IF = Field current

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6.C.17 The characteristics of a self excited shunt ─ wound generator
Terminal voltage ─ Load current characteristic
Generated voltage ─ field current characteristic
 The generated emf is proportional to flux.
 The field current is also proportional to field
current.
 As the speed increases the value of armature current
also increases, hence the voltage drop increases.
Use
1. Lathe machine
2. Mill machine
3. Conveyors
 Load characteristic is similar to separately
excited machine.
 But falls at higher load currents.
 This is because the field is connected across the
armature, so that reducing terminal voltage
gives a reduced field current.
 Reducing the magnetic flux and induced EMF.
Use
1. Where load resistance is constant, so that
terminal voltage also remains constant.
Terminal
Voltage, V
Load current, I
Generated
Voltage, E
Field current, IF

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6.C.18 The characteristics of a self excited shunt ─ wound motor
Speed, N
Torque, τ
 For a shunt wound motor, the field winding is
connected in parallel with the armature circuit.
 The torque is directly proportional to the product
armature current. Since flux is constant.
 Speed is directly proportional to back emf and
inversely proportional to the flux.
On increasing the armature current speed drops
slightly owing to rise in voltage drop in armature.
 In dc shunt motor speed drop from no load to full
load is very small.
 The decrease in speed due to armature voltage drop
is partly compensated by reducing flux on account
of demagnetizing effect of armature reaction.
 So we consider DC shunt motor as constant speed
motor for all practical purposes.

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6.C.19 Self excited series ─ wound generator / motor
The circuit diagram of a series ─ wound dc generator
The circuit diagram of a series ─ wound dc motor

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6.C.20
Description of the essential features of a self excited series shunt − wound dc generator /
motor
In a series - wound generator the field winding is connected in series with the armature. The armature
current passes through the field, and since this current is often large, the conductors of field winding
have a large cross – sectional area. The magnetic field depends on the ampere – turns, so heavy current
results in few turns for series field.
V = E – IA RA – IARF (for series generator)
V = E + IA RA + IARF (for series motor)
I = IA = IF (generator and motor)
 I = Total current,
 IF = series field current and
 IA = armature current.
 V, E is the back emf
 RF = series field resistance
 RA = armature resistance

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6.C.21 The characteristics of a self excited series ─ wound generator
Terminal voltage ─ load current characteristic
 The Terminal voltage is proportional to Load
current.
Use
4. Booster on dc transmission lines
Terminal
Voltage, E
Load current, I

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6.C.22 The characteristics of a dc series shunt motor
Speed, N
Torque, τ
 The torque of dc series motor is proportional to
magnetic flux and armature current.
 Large torque produces large current.
Use
1. Traction
 Trains
 Milk delivery vehicles
2. Driving fans
3. Cranes
4. Hoists
 A large initial torque is required.
 So, in dc series motor speed is inversely
proportional to the armature current.
 In the graph you can see that, for smaller load
current the torque of dc series motor is quite
high but as the load current increases, the torque
get reduced.

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6.C.23 Self excited compound ─ wound dc generator / motor with short shunt
The circuit diagram of a Compound Wound DC Generator with short shunt connection
The circuit diagram of a Compound Wound DC Generator with short shunt connection

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6.C.24 Self excited compound ─ wound dc generator / motor with long shunt
The circuit diagram of a Compound Wound DC Generator with long shunt connection
The circuit diagram of a Compound Wound DC Motor with long shunt connection

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6.C.25
Description of the essential features of a self excited series shunt − wound dc generator /
motor
A compound wound has both a series and a shunt field winding.
There are two types of compound wound motor:
1. Cumulative compound
 Series winding is so connected that the field due to its assist that due to shunt winding.
2. Differential compound
 Series winding is so connected that the field due to it opposes that due to the shunt winding.
 I = Total current,
 IF = series field current and
 IA = armature current.
 V, E is the back emf
 Rsh = shunt field resistance
 Rse = series resistance

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6.C.26 The characteristics of a self excited shunt ─ wound generator
Terminal voltage ─ load current characteristic
Generated voltage ─ field current characteristic
Torque, τ
Armature current, IA
Speed , n
Armature current, IA
Differential
Compound
Comulative
Compound

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6.C.27 Solved questions on direct current motor / generator
Worked example 1
Explain essential parts of dc machine?
Solution
Essential parts of the dc machine are:
1. Field System
2. Armature
3. Commutator
4. Brushes
5. Bearings and Shaft
Worked example 2
Describe the main differences and similarities between a DC generator and a DC motor.
Solution
DC motor rotates as a result of two magnetic fields interacting with each other. The armature of dc
motor acts like an electromagnet when current flows through its coils. Since the armature is located
within the magnetic field of the field poles, these two magnetic fields interact. Like magnetic poles repel
each other, and unlike magnetic poles attract each other.
As in the dc generator, the dc motor has field poles that are stationary and an armature that turns on
bearings in the space between the field poles. The armature of a dc motor has windings on it just like the
armature of a dc generator. These windings are also connected to commutator segments. A dc motor
consists of the same components as a dc generator.
In fact, most dc generators can be made to act as motors, and vice versa.
Worked example 3
What are the types of dc generator?
Solution
DC Generators are classified into two types
1) separately excited DC generator
2) self excited DC generator, which is further classified into;
I. series
II. shunt and
III. compound (which is further classified into cumulative and differential).

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Worked example 4
Explain what is meant excitation?
Solution
The process of generating a magnetic field by means of an electric current is called excitation.
Worked example 5
Distinguish between self excited and separately excited generator?
Solution
Self excitation of generators or motors the magnetic field of the main poles is excited by a current
supplied to the windings of the main poles from the armature (rotor) winding.
Self-excited generators or motors, where the field winding receives its supply from the armature of its
own machine, and which are sub-divided into (a) shunt, (b) series and (c) compound wound generators.
Worked example 6
Describe the voltage output characteristics of the series-, shunt-, and compound-wound dc generators.
Solution
Shunt wound dc generator Voltage characteristics
 The output voltage in a dc shunt-wound generator varies inversely
as load current varies.
 The output voltage decreases as load current increases because the
voltage drop across the armature resistance increases (E = IR).
Series wound dc generator Voltage characteristics
 The voltage output increases as the load circuit starts drawing more
current.
 Under low-load current conditions, the current that flows in the
load and through the generator is small.

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Compound wound dc generator Voltage characteristics
 When load current increases,the armature voltage decreases just as
in the shunt-wound generator.
 This causes the voltage applied to the shunt-field winding to
decrease.
 This same increase in load current, since it flows through the series
winding.
Worked example 7
What does each dc connection represents?
A C
B D
E
Solution
A - Externally excited dc motor
B - Shunt dc motor
C - Series dc motor
D - Comulatively compounded dc motor
E - Differentially compounded dc motor

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Worked example 8
Describe the characteristics of the series-, shunt-, and compound-wound dc motor.
Solution
Characteristics of series-wound dc motor
Torque vs armature current (Ta-Ia) Speed vs armature current (N-Ia)
Speed vs torque (N-Ta)

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Characteristics of shunt-wound dc motor
Speed vs torque (N-Ta)

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Characteristics of compound-wound dc motor

Rotating machines part 3

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Rotating machines part 3