Download ATE1120: Electrical Fundamental-II

Electrical Machines
Module 2: DC Machines
PREPARED BY
Academic Services Unit
April 2012
© Institute of Applied Technology, 2012
ATE1230: Electrical Machines
Module 2: DC Machines
Module Objectives
Upon successful completion of this module, students should be able to:
1. Describe the principle of operation of DC motors.
2. Describe the main differences and similarities between a dc
generator and a dc motor
3. List the advantages and disadvantages of the different types
of dc motors
4. State the principle by which generators convert mechanical
energy to electrical energy
5. State what component causes a generator to produce direct
current rather than alternating current
6. State the three classifications of dc generators
7. Build and test a simple DC motor.
Module Contents:
Topic
Page No.
2.1
Introduction to DC Machines
3
2.2
DC Motors
4
2.3
DC Generators
12
2.4
Lab Activity 1
19
2.5
Review Exercises
21
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2.1 Introduction to DC Machines
Converters that are used to continuously convert electrical input to
mechanical output or vice versa are called electrical machines. The
process
of
conversion
is
known
as
electromechanical
energy
conversion. An electrical machine is therefore a link between an
electrical system and a mechanical system. In these machines the
conversion is reversible. If the conversion is from mechanical to
electrical energy, the machine is said to act as a generator. If the
conversion is from electrical to mechanical energy, the machine is said
to act as a motor. These two effects are shown in Figure 2.1. In these
machines, conversion of energy from electrical to mechanical form or
vice versa results from the following two electromagnetic phenomena.
1. When a conductor moves in a magnetic field, voltage is induced
in the conductor. (Generator action)
2. When a current carrying conductor is placed in a magnetic field,
the conductor experiences a mechanical force. (Motor action)
Figure 2.1: The Energy directions in generator and motor actions
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Task 1:
Check the following and other websites for demonstrations of the
principle of operation of motors and generators
http://library.thinkquest.org/27948/motor.html
http://www.animations.physics.unsw.edu.au/jw/electricmotors.html
2.2 DC Motors
DC motors have been used in industrial applications for years. Coupled
with a DC drive, DC motors provide very precise control. DC motors can
be used with conveyors, elevators, marine applications, material
handling (paper, plastics, rubber, steel) and textile applications to
name a few.
2.2.1 CONSTRUCTION
DC motors as shown in Figure 2.2 consist of one set of coils, called
armature winding, inside another set of coils or a set of permanent
magnets, called the stator. Applying a voltage to the coils produces a
torque in the armature, resulting in motion. The main parts of the DC
motor are :
1. STATOR: The stator consists of either a permanent magnet or
electromagnetic windings. The stator generates a stationary
magnetic field around the rotor which occupies the central part of
the motor.
2. ARMATURE (Rotor): The armature shown in Figure 2.3 is made
up of one or more electric windings around armature arms. These
electric windings generate a magnetic field when energized by the
external current. The magnetic poles thus generated by this rotor
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field are attracted to the opposite poles generated by the stator
field and repelled by the similar poles, which causes the armature
to rotate.
3. COMMUTATOR: The DC motor doesn’t use an external current
switching device, instead it uses a mechanical connector called
the commutator which is a segmented sleeve usually made of
copper, mounted on the rotating shaft. The current +/- is
supplied to this commutator segments with the help of brushes.
4. BRUSHES: As the motor turns, the brushes slide over the
commutator segments hence creating the variable magnetic field
in different arms through the commutator segments attached to
the windings. Hence a dynamic magnetic field is generated in the
motor when a voltage is applied across the brushes. The brushes
mechanism holder is shown in Figure 2.4.
Figure 2.2: Permanent Magnet DC Motor
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Figure 2.3: Armature and Commutator
Figure 2.4: Brush holder mechanism
2.2.2 Basic DC Motor Operation
A. Magnetic Fields
You will recall that there are two electrical elements of a DC motor,
the field windings and the armature. The armature windings are
made up of current carrying conductors that terminate at a
commutator. DC voltage is applied to the armature windings through
carbon brushes which ride on the commutator.
In small DC motors, permanent magnets can be used for the stator.
However, in large motors used in industrial applications the stator is
an electromagnet. When voltage is applied to stator windings an
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electromagnet with north and south poles is established. The
resultant magnetic field is static (non-rotational). For simplicity of
explanation, the stator will be represented by permanent magnets in
the following illustration in Figure 2.5.
Figure 2.5: Simple DC motor
B. Generated Force
If a current carrying conductor is placed into the field of a
permanent magnet as shown in Figure 2.6(c) a force F will be
exerted on the conductor to push it out of the magnetic field.
To understand the force, let us consider each magnetic field acting
alone. Figure 2.6(a) shows the magnetic field due to the current
carrying conductor only. Figure 2.6(b) shows the magnetic field due
to the permanent magnet in which is placed the conductor carrying
no current. Figure 2.6(c) shows the effect of the combined magnetic
fields which are distorted and, because lines of magnetic flux never
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cross, but behave like stretched elastic bands, always trying to find
the shorter distance between a north and south pole, the force F is
exerted on the conductor, pushing it out of the permanent magnetic
field.
Figure 2.6: Force on a current carrying conductor
As can be seen from the below illustration in Figure 2.7, conductors
on the left side tend to be pushed up. Conductors on the right side
tend to be pushed down. This results in a motor that is rotating in a
clockwise direction.
This is the basic motor principle, and the force F is dependent upon
the strength of the magnetic field, the magnitude of the current
flowing in the conductor, and the length of conductor within the
magnetic field.
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Figure 2.7: Basic motor operation
2.2.3 Classifications of DC Motors
Practical motors are constructed as shown in Figure 2.8. All DC motors
contain a fffield winding wound on pole pieces attached to a steel yoke.
The armature winding rotates between the poles and is connected to
the commutator. Contact with the external circuit is made through
carbon brushes rubbing on the commutator segments. Direct Current
motors are classified by the way in which the field and armature
windings are connected, which may be in series or in parallel.
Figure 2.8: Industrial DC machine construction
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A. DC Series Motor
The field and armature windings are connected in series and
consequently share the same current. The series motor has the
characteristics of a high starting torque but a speed which varies
with load. Theoretically the motor would speed up to selfdestruction, limited only by the windage of the rotating armature
and friction, if the load were completely removed. Figure 2.9 shows
series motor connections and characteristics. For this reason the
motor is only suitable for direct coupling to a load, except in very
small motors, such as vacuum cleaners and hand drills, and is
ideally suited for applications where the machine must start on load,
such as electric trains, cranes and hoists.
Figure 2.9: Series motor connections and characteristics.
Applications :
1. electric drill motor,
2. vacuum cleaner motor,
3. electric train motor
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B. DC Shunt Motor
The field and armature windings are connected in parallel (Figure
2.10). Since the field winding is across the supply, the flux and
motor speed are considered constant under normal conditions. In
practice, however, as the load increases the field flux distorts and
there is a small drop in speed of about 5% at full load, as shown in
Figure 2.10. The machine has a low starting torque and it is
advisable to start with the load disconnected. The shunt motor is a
very
desirable
d.c.
motor
because
of
its
constant
speed
characteristics. It is used for driving power tools, such as lathes and
drills.
Figure 2.10: Shunt motor connections and characteristics.
C. Compound motor
The compound motor has two field windings, one in series with the
armature and the other in parallel. The arrangement (short shunt
and long shunt) of compound motor connections is given in Figure
2.11. The compound motor may be designed to possess the best
characteristics of both series and shunt motors, that is, good
starting torque together with almost constant speed. Typical
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applications are for electric motors in steel rolling mills, where a
constant speed is required under varying load conditions.
Figure 2.11: Compound motor connections
Conduct Lab Activity I on page 19.
2.3 DC Generators
2.3.1 Generator principle
An electrical generator is a machine which converts mechanical
energy (or power) into electrical energy (or power). Induced
voltage, known as electromotive force (e.m.f.) is produced in it
through electromagnetic induction. This e.m.f. causes a current to flow
if the conductor circuit is closed. Hence, two basic essential parts of an
electrical generator are:

Magnetic field.

Conductor or conductors which can move as to cut the flux.
Generators are driven by a source of mechanical power, which is
usually called the prime mover of the generator(steam turbine,
diesel engine, or even an electric motor).
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Simple loop generator
Figure 2.12 shows a single turn rectangular copper coil (AA′BB′ )
rotating about its own axis in a magnetic field provided by either
permanent magnets or electromagnets. The two end of the coil are
joined to two slip-rings which are insulated from each other and from
the central shaft. Two collecting brushes (carbon or copper) press
against the slip-rings. The rotating coil may be called armature and
the magnets as field magnets.
One way to generate an AC voltage is to rotate a coil of wire at
constant angular velocity in a fixed magnetic field, as in Figure 2.12,
(slip rings and brushes connect the coil to the load). The magnitude of
the resulting voltage is proportional to the rate at which flux lines are
cut, and its polarity is dependent on the direction the coil sides move
through the field.
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Figure 2.12(a):
0 Position: Coil sides move parallel
to flux lines. Since no flux is being
cut, induce voltage is zero.
Figure 2.12(b):
90 Position: Coil end A is positive
with respect to B. Current direction is
out of slip ring A.
Figure 2.12(c):
180 Position: Coil again cutting no
flux. Induced voltage is zero.
Figure 2.12(d):
270 Position: Voltage polarity has
reversed, therefore, current direction
reverses.
Figure 2.13: Generated waveform
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2.3.2 Construction of DC Generators
The parts of a simple DC generator are shown in Figure 2.14. A rotating
armature coil passes through a magnetic field that develops between
the
north
and
south
polarities
of
permanent
magnets
or
electromagnets. As the coil rotates, electromagnetic induction causes
current to be induced into the coil. The current produced is an
alternating current. However, it is possible to convert the alternating
current that is induced into the armature into a form of direct current.
This conversion of AC into DC is accomplished through the use of a
commutator. The conductors of the armature of a DC generator are
connected to commutator segments.
The commutator shown in Figure 2.14 has two segments, which are
insulated from one an other and from the shaft of the machine on
which it rotates. An end of each armature conductor is connected to
each commutator segment. The purpose of the commutator is to
reverse the armature coil connection to the external load circuit at the
same time that the current induced in the armature coil reverses. This
causes DC at the correct polarity to be applied to the load at all times
as shown in Figure 2.15.
Figure 2.14: Simple diagram of a generator
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(a)
(b)
Figure 2.15(a): Pulsating DC developed by a simple single coil
generator.
Figure 2.15(b): Pure DC developed by a more complex generator using
many turns of wire and many commutator segments.
2.16: A practical DC generator
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2.3.3 Types of D.C Generators:
D.C Generators are classified according to the way in which a magnetic
field is developed in the stator of the machine. Thus, there are three
basic classification DC generators
(1) permanent-magnet field.
(2) separately-excited field.
(3) self-excited field.
1) Permanent-Magnet Field
permanent-magnet DC machines are widely found in a wide variety of
low-power applications. The field winding is replaced by a permanent
magnet, resulting in simpler construction. Notice that the rotor of this
machines consists of a conventional DC armature with commutator
segments and brushes.
2) Separately-Excited Field
Separately-excited generators are those whose field magnets are
energized from an independent external source of DC current. It is
shown diagrammatically in Figure 2.17.
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Figure 2.17: Simplified illustration of a separately excited DC generator.
3) Self-Excited Field
Self-excited generators are those whose field magnets are energized
by the current produced by the generators themselves. Due to
residual magnetism, there is always some flux present in the poles.
When the armature is rotated, some e.m.f and hence some induced
current is produced which is partly or fully passed through the field coils
thereby strengthening the residual pole flux.
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2.4 Lab Activity 1
Objective:
Motors are the fundamental driving force of the modern world. It is a very
rare occasion when you do not see or use the action of a motor during your
daily life. So how do they work? With this activity, you will build your own
simple DC motor.
Components used:
a) magnet, ceramic disc
b) Copper enamled wire, 20–22 gauge, 60 cm
c) wire pieces, 16–18 gauge, 7–8 cm, 2Nos
d) Battery clips with alligator clip leads, 9-V
e) DC Power supply.
f) Pliers, needle-nose with wire cutters
g) Polystyrene foam or cardboard piece, 8 cm × 8 cm × 2.5 cm
h) Tube or rod, approximately 2 cm in diameter
Procedure:
Visit the websites below and learn how to build your own DC motor
http://www.youtube.com/watch?v=9Wby4aHyXJQ&feature=related
www.youtube.com/watch?v=pDQUmJVMhcs
Use the following hints to help you build and test the motor:
1. Set the DC power supply to give an initial output of 3V.
2. Wrap the copper wire around the tube few times and tie the two ends
as seen in the video. Remove the insulation completely from one end
and only on one side from the other end.
3. Increase/decrease the voltage of the power supply and observe the
effect on the speed of rotation.
4. Reduce the number of turns of the coil and observe the effect on the
speed of rotation.
5. Add another magnet and observe the effect on the speed of rotation.
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Write down you conclusions here and comment on the performance of the
motor with respect to the following:

The strength of the magnetic field

The amount of current supplied to the motor

The number of turns of the rotor coil
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2.5 Review exercise
Module 2 WORKSHEET DC MACHINES
1. The series motor used in many applications such as:
a)
constant speed steel rolling mills
b)
electric hand drills
c)
electric trains and trams
d)
vacuum cleaners .
2. The compound motor used in many applications such as:
a)
constant speed steel rolling mills
b)
electric hand drills
c)
electric trains and trams
d)
vacuum cleaners .
3. The advantage of a d.c. machine is:
a)
it is easy to control the speed
b)
it runs at an almost constant speed
c)
it requires little maintenance
d)
it is practically indestructible .
4. What factors determine the direction of rotation in a dc motor?
5. What is the main disadvantage of a series motor?
6. What is the main advantage of a series motor?
7. What advantage does a shunt motor have over a series motor?
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8. Whenever ____________ flows through a conductor a magnetic
field is generated around the conductor.
9.
Identify the following motor types
10. What component causes a generator to produce dc voltage
rather than ac voltage at its output terminals?
11. An elementary, single coil, dc generator will have an output
voltage with how many pulsations per revolution?
12. Generators convert mechanical motion to electrical energy using
what principle?
13. What is the purpose of the slip rings?
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Refrences:
Advanced Electrical Installation Work_ 5th Edition
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