EET 306
ELECTRICAL MACHINES
Syafruddin Hasan
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DC machines
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DC Machines
The direct current (dc) machine can be used as
a motor or as a generator.
DC Machine is most often used for a motor.
The major advantages of dc machines are the
easy speed and torque regulation.
However, their application is limited to mills,
mines and trains. As examples, trolleys and
underground subway cars may use dc motors.
In the past, automobiles were equipped with
dc dynamos to charge their batteries.
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DC MACHINES
Even today the starter is a series dc motor
However, the recent development of power
electronics has reduced the use of dc motors and
generators.
The electronically controlled ac drives are gradually
replacing the dc motor drives in factories.
Nevertheless, a large number of dc motors are still
used by industry and several thousand are sold
annually.
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DC MACHINES: INTRODUCTION
DC machine=stator + rotor (armature)
– Stator: does not move, the outer frame of the machines is
made of ferromagnetic materials.
– Rotor (Armature): free to move, the inner part of the machine is
made of ferromagnetic materials.
– Field winding: is wound on the stator poles to produce magnetic
field (flux) in the air gap.
– Armature winding: is composed of coils placed in the armature
slots.
– Commutator: is composed of copper bars, insulated from each
other. The armature winding is connected to the
commutator.
– Brush: is placed against the commutator surface. Brush is used
to connect the armature winding to external circuit through
commutator.
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DC MACHINES: INTRODUCTION
In DC 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.
2. When a current carrying conductor is placed in magnetic field, the
conductor experiences a mechanical forces
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DC MACHINES: CONSTRUCTION
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DC MACHINES: CONSTRUCTION
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DC Machine Construction
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DC Machine Construction (cont..)
The stator of the dc motor has poles, which are excited by
dc current to produce magnetic fields.
In the neutral zone, in the middle between the poles,
commutating poles are placed to reduce sparking of the
commutator. The commutating poles are supplied by dc
current.
Compensating windings are mounted on the main poles.
These short-circuited windings damp rotor oscillations.
• The poles are mounted on an iron core that provides a closed
magnetic circuit.
• The motor housing supports the iron core, the brushes and
the bearings.
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DC Machine Construction (cont..)
The rotor has a ring-shaped laminated iron core with slots.
Coils with several turns are placed in the slots. The distance
between the two legs of the coil is about 180 electric degrees.
The coils are connected in series through the commutator
segments.
The ends of each coil are connected to a commutator segment.
The commutator consists of insulated copper segments mounted
on an insulated tube.
Two brushes are pressed to the commutator to permit current
flow.
The brushes are placed in the neutral zone, where the magnetic
field is close to zero, to reduce arcing.
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DC Machine Construction (cont..)
The rotor has a ring-shaped
laminated iron core with slots.
The
commutator consists of
insulated
copper
segments
mounted on an insulated tube.
Two brushes are pressed to the
commutator to permit current
flow.
The brushes are placed in the
neutral zone, where the magnetic
field is close to zero, to reduce
arcing.
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DC Machine Construction (cont..)
The commutator switches the
current from one rotor coil to the
adjacent coil,
The
switching requires the
interruption of the coil current.
The sudden interruption of an
inductive current generates high
voltages .
The
high voltage produces
flashover and arcing between the
commutator segment and the
brush.
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DC Machine Construction (cont..)
Rotation
I r_dc/2
Brush
Ir_dc/2
I r_dc
Shaft
Pole
winding
|
1
2
8
N
3
7
6
S
4
5
Insulation
Rotor
Winding
I r_dc
Copper
segment
Commutator with the rotor coils connections
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DC Machine Construction (cont..)
Details of the commutator of a dc motor.
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DC Machine Construction (cont..)
Rotor of a dc machine
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DC Machine Construction (cont..)
DC motor stator with poles visible.
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DC Machine Construction (cont..)
Cutaway view of a dc motor.
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Armature Windings
The armature windings are usually former-wound. This are first wound
in the form of flat rectangular coils and are then puller. Various
conductors of the coils are insulated each other. The conductors are
placed in the armature slots which are lined with tough insulating
material. This slot insulation is folded over above the armature
conductors placed in the slot and is secured in place by special hard
wooden or fibre wedges.
Lap and wave Windings
There are two types of windings mostly employed:
• Lap winding
• Wave winding
The difference between the two is merely due to the different arrangement of the end
connection at the front or commutator end of armature.
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Types of DC Machines
DC machines are usually classified according to the way
in which their fields are excited.
1. Separately-excited DC machine
2. Self-excited DC machine
(a) Shunt DC machine
(b) Series DC machine
(c) Compound DC machine
(i) Long shunt compound
(ii) Short shunt compound
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Generating an AC Voltage
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The voltage generated in any dc
generator inherently alternating
and only becomes dc after it
has been rectified by the
commutator.
DC machines
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Direct Current Generator
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Different Between AC and DC Generators
AC
DC
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AC or DC
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Generated emf or back emf or emf Equation of a DC Generator
General form of generated emf,
Φ = flux/pole (Wb)
Z = total number of armature conductors
= number of slots x number of
conductor/slot
P = number of poles
A = number of parallel paths in armature
N = armature rotation (rpm)
E = emf induced in any parallel path in
armature
or
E=CNΦ
where
Note:
A = 2
(for wave winding)
A = P
(for lap winding)
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Types of Generators
• Separately-excited generators are those whose field magnetics are energized from an
independent external source of dc current.
Equivalent circuit of separately excited dc gen.
Separately excited 2-pole generator
V = terminal (load) voltage
E = emf induced
Ia = armature current
IL = load current
V = E − Ia.Ra
Ra = armature circuit resistance
Vf = field circuit voltage
Rf = field circuit resistance
If = field current
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Ia = IL
Vf = I f .Rf
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(b) Self excited generators are those whose field magnets are energized by
the current produced by the generators them selves.
(1) Shunt Generator. The field windings are connected across or parallel with the
armature conductors and the full voltage of the generator across them.
Equivalent circuit of dc shunt generator
DC shunt generator
V =E−Ia.Ra
Vf =V =Ish.Rsh
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where Ish = shunt field current
Rsh = shunt field resistance
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(2) Series Generator. In this case, the field windings are joined in series
with the armature conductors. As they carry full load current, they
consist of relatively few turns of thick wire or strip. Such
generators are rarely used.
V = E − Ia (Ra + Rse)
where Rse = series field resistance
Ise = series field current
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(3) Compound Generator. It is a combination of a few series and a few
shunt windings and can be either short or long-shunt.
short-shunt compound dc generator
or
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Long-shunt compound dc generator
Note : Compound generator also refer as:
1. Cumulatively compounded generator, where both a shunt and a
series field are present, and their effects are additive
2. Differentially compounded generator, where both a shunt and a
series field are present, and their effects are subtractive.
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An 8-pole dc shunt generator with 778 wave connected armature
conductors and running at 500 rpm, supplies a load of 12.5 Ω resistance
at a terminal voltage of 250 V. The armature resistance is 0.24 Ω and the
field resistance is 250 Ω. Find the armature current, the induced emf and
the flux per pole.
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Total Loss in a DC Generator
a). Magnetic losses (also known as iron or core losses)
Due to the rotation of the iron core of the armature in the magnetic flux
of the field poles, there are some losses taking place continuously in the
core and are known as iron or core losses. Iron losses consist of
(i) Hysteresis loss
(ii) Eddy current losses.
b) Copper losses
(i) Armature copper loss = Ia2 Ra
(not E Ia)
(ii) Field copper loss
In the case of shunt generator it is practically constant
Psh = Ish2 Rsh = V Ish
In the case of series generator
Pse = Ise2 Rse
(iii) The loss due to brush contact resistance. This is usually included in the
armature copper loss.
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Total Loss in a DC Generator (cont..)
c) Mechanical losses
These consist of
(i) Friction loss at bearing and commutator
(ii) Air-fraction or windage loss for rotating armature
Note: usually, magnetic and mechanical losses are collectively known
as stray losses.
Constant or Standing Losses
For shunt and compound generator, field Cu loss is constant. Hence,
stray losses and shunt Cu losses are constant in their case. These losses
are together known as standing or constant losses
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Power-Flow (power stages) Diagram
•Mechanical Efficiency
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Electrical Efficiency
or
Overall or Commercial Efficiency
or
or
Note: Useless specified otherwise, commercial efficiency is always to be understood.
Condition for Maximum Efficiency
Efficiency is maximum when
Variable loss = Constant loss
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Problems with Commutation in Real Machine
In practice, there are two major effects that disturb the commutation process:
1.Armature Reaction
2. L di/dt voltages
If the magnetic field windings of a dc machine are connected to a power supply
and the rotor of the machine is turned by an external source of mechanical
power, then a voltage will be induced in the conductors of the rotor. This
voltage will be rectified into dc output by the action of the machine’s
commutator
Now, connect a load to the terminals of the machine, and a current will flow in
its armature windings. This current flow will produce a magnetic field of its own,
which will distort the original magnetic field from the machine’s poles. This
distortion of the flux in a machine as the load is increased is called armature
reaction
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Armature reaction causes 2 serious problems in real dc machine.
Problem 1 : Neutral-Plane Shift
Problem 2 : Flux Weakening
Solutions to the Problems with Commutation
Three approaches have been developed to partially or completely correct
the problems of armature reaction and L di/dt voltages:
1. Brush Shifting
2. Commutation poles or interpoles
3. Compensating windings
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Brush Shifting
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Commutation Poles or Interpoles
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