Electrical machines
Electrical machines-chapter 2-Dc motor1Learning Objectives:
Understand the construction and principle operation of a DC
motorApply the principle operation of DC motor.22Electric MotorThe
input is electrical energy (from the supply source), and the output
is mechanical energy (to the load).3
ConstructionDC motors consist of one set of coils, called
armature winding, inside another set of coils or a set ofpermanent
magnets, called the stator. Applying a voltage to the coils
produces a torque in the armature, resulting in motion.4NSStator
with with polesBrushRotorFieldStator: Stationary part of the
machine.The stator carries a field winding that is used to produce
the required magneticfield by DC excitation. Often know as the
field.Rotor: The rotor is the rotating part of the machine. The
rotor carries a distributed winding, and is the winding where the
emf is induced. Also known as the armature.ConstructionStator The
stator is the stationary outside part of a motor. The stator of a
permanent magnet dc motor is composed of two or more permanent
magnet pole pieces. The magnetic field can alternatively be created
by an electromagnet. In this case, a DC coil (fieldwinding) is
wound around a magnetic material that forms part of the
stator.5ConstructionRotor The rotor is the inner part which
rotates. The rotor is composed of windings (called armature
windings) which are connected to the externalcircuit through a
mechanical commutator. Both stator and rotor are made of
ferromagnetic materials. The two are separated by
air-gap.6ConstructionA winding is made up of series or parallel
connection of coils. Armature winding - The winding through which
the voltage is applied or induced. Field winding - The winding
through which a current is passed to produce flux (for the
electromagnet) Windings are usually made of
copper.7Construction8The rotor iron core is mounted on the
shaft.
Coils are placed in the slots.
The end of the coils are bent and tiedtogether to assure
mechanical strength.
Note the commutator mounted on theshaft. It consists of several
coppersegments, separated by insulation.CommutatorPoles
BrushesRotor windingFanBearingDC Motor Basic PrinciplesEnergy
ConversionIf electrical energy is supplied to a conductor lying
perpendicular to a magnetic field, the interaction ofcurrent
flowing in the conductor and the magnetic field will produce
mechanical force (and therefore,mechanical energy).9DC Motor Basic
PrinciplesValue of Mechanical Force
There are two conditions which are necessary to produce a force
on the conductor. The conductor mustbe carrying current, and must
be within a magnetic field. When these two conditions exist, a
force will beapplied to the conductor, which will attempt to move
the conductor in a direction perpendicular to the magneticfield.
This is the basic theory by which all DC motors operate.10DC Motor
Basic PrinciplesThe force exerted upon the conductor can be
expressed as follows : F = B x i x l (unit Newton)where B is the
density of the magnetic field, l is the length of conductor, and i
the value of current flowing in the conductor. The direction of
motion can be found using Flemings Left Hand Rule.11
Flemings Left Hand RuleThe first finger points in the direction
of the magnetic field (first - field), which goes from the North
pole to the South pole. The second finger points in the direction
of the current in the wire (second - current). The thumb then
points in the direction the wire is thrust or pushed while in the
magnetic field (thumb - torque or thrust).DC Motor Basic
Principles12
DC Motor Basic PrinciplesPrinciple of operationConsider a coil
in a magnetic field of flux density B. When the two ends of the
coil are connected across a DC voltage source, current I flows
through it. A force is exerted on the coil as a result of the
interaction of magnetic field and electric current. The force on
the two sides of the coil is such that the coil starts to move in
the direction of force.13
DC Motor Basic Principles14DC Motor Basic Principles15
Induced voltage in the armature winding of DC motorDC Motor
Basic PrinciplesThe value of current flowing through the armature
is dependent upon the difference between the applied voltage and
this counter-voltage. The current due to this counter-voltage tends
to oppose the very cause for its production according to Lenzs law.
It results in the rotor slowing down. Eventually, the rotor slows
just enough so that the force created by the magnetic field (F =
Bil) equals the load force applied on the shaft. Then the system
moves at constant velocity.16DC Motor Basic Principles17DC Motor
Basic PrinciplesNote that equation (induced voltage) gives the emf
induced in one coil. As there are several coils wound all around
the rotor, each with a different emf depending on the amount of
flux change through it, the total emf can be obtained by summing up
the individual emfs.The total emf induced in the motor by several
such coils wound on the rotor can be obtained by integrating
equation, and expressed as:where K is an armature constant, and is
related to the geometry and magnetic properties of the motor, and m
is the speed of rotation. The electrical power generated by the
machine is given by:18
DC Motor Basic Principles19Generated EMF in a Real DC
MachineWhereZ = total number of conductors, P = total number of
polesa = P for lap winding, a = 2 for wave winding, = flux, = speed
in rad/s and n = speed in rpm.
DC Motor Basic Principles20Counter EMF:When the motor is
running, internally generated emf, (EG = EC) opposes the applied
voltage, thus:I=V ERA T C AWhere: VT = terminal voltage, Ec =
counter EMF, RA is the armature resistance and IA is the armature
currentDC Motor Basic PrinciplesThe Relationship Between The
Induced Emf And Torque21
DC Motor Basic PrinciplesExample 122
DC Motor Basic PrinciplesExample 223
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DC Motor Basic PrinciplesEquivalent circuitThe equivalent
circuit of DC Motors (and Generators) has two components:Armature
circuit: It can be represented by a voltage source and a resistance
connected in series (the armature resistance). The armature winding
has a resistance, Ra.
The field circuit: It is represented by a winding that generates
the magnetic field and a resistance connected in series. The field
winding has resistance Rf.
25DC Motor Basic PrinciplesShunt Motors-Field and armature
windings are connected in parallel.
Series Motors-Field and armature windings are connected in
series.
Compound Motors-Has both shunt and series field so it combines
features of series and shunt motors.26DC Motor Basic
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DC Motor Basic PrinciplesExample 1
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DC Motor Basic PrinciplesExample 234
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DC Motor Basic PrinciplesShunt Motor: Speed Control44
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46DC Motor Basic PrinciplesShunt Motor : The effect of an open
field circuit47
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70Compound Motor: Speed ControlDC Motor Basic
PrinciplesTorque-Speed Characteristics:In order to effectively use
a D.C. motor for an application, it is necessary to understand its
characteristic curves. For every motor, there is a specific
Torque/Speed curve and Power curve. The relation between torque and
speed is important in choosing a DC motor for a particular
application.71DC Motor Basic Principles72
DC Motor Basic PrinciplesDC Motor Speed Control
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DC Motor Basic PrinciplesDC Motor Starting
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DC Motor Basic PrinciplesDC Motor Efficiency83
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86DC Motor Basic PrinciplesIt is important to mention that the
total input and output power can be calculated in many different
ways using the power flow diagram, depending on the information
given. Also note that the torque developed inside the rotor is
different from the final (output) torque supplied to the load due
to rotational losses.87
DC Motor Basic PrinciplesExample 188
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DC Motor Basic PrinciplesExample 290
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DC Motor Basic PrinciplesSelf Test 192
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