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ELECTRICAL MACHINES (DC Motors)
Contents
Introduction to Electrical & Direct Current Machines
Constructional Details of DC Machines Principle of Operation of DC Machines Types of DC Motor Power Flow Diagram Characteristics of DC Motors Armature Reaction Speed Control of DC Motors Applications
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Learning Outcomes
Upon completion of chapter-2 you should be able to:
State the principle by which machines convertmechanical energy to electrical energy and vice versa.
Discuss the operating differences between differenttypes of dc machines.
Explain the characteristics of dc machines.
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Introduction
Energy Conversion
Energy exists in many forms.
One form of energy can be obtained from the other
form with the help of converters.
Light bulbs and heaters require energy in electrical
form.
Electrical Machines:
Converters that translate an electrical input to a
mechanical output or vice versa are called the electric
machines.This process of translation is electromechanical energy
conversion.
The magnetic system acts as the link between the
electrical and mechanical systems
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Introduction . . .
Electromagnetic phenomena:Electrical machines use the following electromagnetic
phenomena for their electromechanical energy
conversion:
Whenever the field in the vicinity of a conductor changes
(or) flux linking a conductor changes, an emf is induced inthat conductor (Faradays Law).
Whenever a current-carrying conductor is placed in a
magnetic field, the conductor experiences a mechanical
force.
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Motor action
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Introduction . . . .
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Energy flow diagram
Gen Transformer
(step-up)Transmission
LineTransformer(step-down)
Distribution /Utilization: Loads could beMotors,Lighting,
Heaters, Coolers,
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Over view of DC Machines
Direct-current (DC) machines are divided into dc
generators and dc motors.DC Generator
A dc generator is a machine that converts mechanical
energy into electrical energy (dc voltage & current) by
using the principle of magnetic induction.
DC generators are not as common as they used to be,
because dc, when required, is mainly produced by
electronic rectifiers.
1010
A dc motor is a machine that converts electrical energy
into mechanical energy by supplying a dc power
(voltage and current).
DC motors are widely used in many applications.
DC motors are everywhere! (In house, office, )
DC Motor
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Over view of DC Machines . . .
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CONSTRUCTIONAL DETAILS OF DC MACHINES
Stator: Field Poles (minimum 2) - magnetic flux Rotor : Armature - carries armature winding
Air gap (between poles and armature)
Commutator (ac to dc)
Carbon brushes (collects and carry current from
the commutator)
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Rotor of a DC machine
Tooth
Slot
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http://www.youtube.com/watch?v=Q4FlUP-kJe8
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Constructional details of dc machines . . .
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The ends of the windings are connected to the
commutator segments (built in copper and are very goodconductors).
Carbon brushes are placed over
commutator segments and serve as
leads for the electric connection.
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ARMATURE (Rotor)
Constructional details of dc machines . . .
The entire assembly of iron core, commutator, andwindings is called the armature.
The commutator is connected to the slotted iron core.
The windings of armatures are connected in differentways depending on the requirements of the machine.
More turns of conductor = higher rectified voltage 21
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There are two types of armature winding: Lap windingand Wave winding.
Lap Wound Armatures
are used in machines designed for low voltage and highcurrent
armatures are constructed with large wire because ofhigh current
Number of parallel paths = number of poles
ARMATURE . . .
Constructional details of dc machines . . .
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Wave Wound Armatures
are used in machines designed for high voltage and lowcurrent
are used in the small generator
No of parallel paths = 2
ARMATURE . . .
Constructional details of dc machines . . .
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Field winding
Constructional details of dc machines . . .
Most DC machines use electromagnets to provide themagnetic field.
Two types of field windings are used :
series field winding
shunt field winding
Series field windings
are so named because they are connected in series withthe armatureare made with relatively few turns of very large wire(sufficiently large to carry the current) and have a verylow resistance.
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Constructional details of dc machines . . .
Shunt field windingsHave relatively many turns of small wire, thus, it has amuch higher resistance than the series field.
is intended to be connected in parallel with, or shunt,the armature.
high resistance is used to limit current flow through thefield.
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When a DC machine uses both series and shunt fields,each pole piece will contain both windings.
The windings are wound on the pole pieces in such amanner that when current flows through the winding itwill produce alternate magnetic polarities.
Factors affecting the machine output
Speed
Field strength
No. of turns in the windings
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http://www.youtube.com/
watch?v=Ue6S8L4On-Y
http://www.youtube.com/wat
ch?v=0ajvcdfC65w
http://www.youtube.com/watch?v=M
FGqf6AfDB0
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DC motor principles . . .
The Advantages
The greatest advantage of DC motors may be speed
control.
Today, adjustable frequency drives can provideprecise speed control for AC motors, but they do so at
the expense of power quality, as the solid-state
switching devices in the drives produce a rich
harmonic spectrum. The DC motor has no adverse
effects on power quality.
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Power supply, initial cost, and maintenancerequirements are the drawbacks associated with DCmotors
Rectification must be provided for any DC motorssupplied from the grid. It can also cause power qualityproblems.
The construction of a DC motor is considerably morecomplicated and expensive than that of an AC motor,primarily due to the commutator, brushes, andarmature windings.
The drawbacks
DC motor principles . . .
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DC motor principle of operation . . .
All motors rely upon the force exerted by a magnetic fieldon a current-carrying conductor.
If a straight current carrying conductor is placed at rightangles to the uniform magnetic field existing between theNorth and south poles of a permanent magnet, the result
is shown in Fig.a. Two fields are present: the uniform field due to the
magnet with lines of force that are straight and parallel,and the circular field around the current-carryingconductor, shown dotted.
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DC motor principle of operation . . .
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DC motor principle of operation . . .
As the lines of force above the conductor in Fig.a point in
the same direction, they add together, and as the lines offorce below the conductor oppose each other, theysubtract. The resultant magnetic field is shown in fig. b.
Because the field is strong above the conductor andweak below the conductor, the distorted lines of forcetend to straighten like stretched elastic bands.
A force is thus exerted on the conductor, tending tomove it down, as indicated by the arrow.
If it were free to move, the conductor would leave themagnetic field.
If the current is reversed through the conductor, thecircular field around the conductor will also reverse.Hence, the conductor will tend to move in the oppositedirection, i.e., upwards.
Similarly, if the polarity of the main magnetic field is
reversed, the direction of conductor motion will change. 34
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N
S
Into the plane is denoted by a cross (X)and out of the plane is denoted by a dot (.)
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Torque
It is the turning or twisting force about an axis.
36
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sin
Force
ont
h
e
conductor
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Counter emf(Back emf) in dc motors
0 60E Zn P A
When a dc supply is connected to the dc motor, a
large current will flow through the armature conductors
because its resistance is very low.
Each current carrying conductor experiences a force
(because they are immersed in the magnetic field).
These forces add up to produce a powerful torque,causing the armature to rotate.
As soon as the armature begins to turn, a 2nd
phenomenon takes place: the generator effect. With
the armature rotating in the magnetic field, the
armature conductors generate an emf. This generated (induced) emf is proportional to the
speed of rotation of the motor and the flux per pole,
and is as follows:
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0SI E E R
Counter emf(Back emf) in dc motors . . .
Where, Z = total number of armature conductors
= effective flux per pole (Wb)n = speed of rotation (rpm)
P = no. of poles
A = no. of parallel paths
The generated voltage opposes the supply voltage, thuslimiting the armature current.
In case of a motor, the induced voltage, E0 is called
counter emf because it opposes the source voltage.
The armature current is given by
where ES = line supply voltage & E0 = counter
(generated) emf.
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Counter emf(Back emf) in dc motors . . .
When the motor is at rest, the counter emf (cemf) is zero
and so the starting current is given by: I = (ES-0) / R.
As the speed increases, the cemf increases, with the
result that the value of armature current diminishes.
When a motor runs at no-load, the counter-emf must be
slightly less than ES, so as to enable a small current to
flow, sufficient to produce the required torque.
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Mechanical power and torque
The electrical power supplied to the armature, which is
converted to mechanical power (mechanical powerdeveloped), is
where, P =mechanical power developed by the motor (W)
E0= induced voltage in the armature (cemf) (V)
I = total current supplied to the armature (A)
The mechanical power P is also given by the expression,
where n is the speed of rotation.
Combining the above two equations for P,
0P E I
9.55P T nT
09.55 6.28PZ InT E I T A
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where, T = torque developed (N-m)
Z = total number of armature conductors
F = effective flux per pole (Wb)
I = armature current (A)
6.28 = constant, to take care of units (=2)
Mechanical power and torque . . .
09.55 6.28P
Z InT E I T A
Speed of rotation
0 60E Zn P A
0, 60 ( )Speed n E A Z P