Download - part 12 emachines EEP 3243
-
8/9/2019 part 12 emachines EEP 3243
1/64
ELECTRICAL MACHINE
EEP 3243
Lt Cdr Ong Khye Liat RMN
1
-
8/9/2019 part 12 emachines EEP 3243
2/64
DC MACHINES
-
8/9/2019 part 12 emachines EEP 3243
3/64
Overview The steam age signaled the beginning of an industrial revolution.
The advantages of machines and gadgets in helping mass
production.
Thus a search for new sources of energy and novel gadgets
received great attention.
By the end of the 18th century the research on electric charges
received a great boost with the invention of storage batteries.
The moving charges or currents was discovered also associated
with magnetic field like a lodestone.
This led to the invention of an electromagnet and later the forceexerted on a current carrying conductor placed in the magnetic
field was invented.
3
-
8/9/2019 part 12 emachines EEP 3243
4/64
Cont. This can be termed as the birth of a motor.
Parallel research was contemporarily being done to invent a source
of energy to recharge the batteries in the form of a d.c. source of
constant amplitude (or d.c. generator).
The research on d.c. motors and d.c. generators proceeded onindependent paths.
The invention of a commutator paved the way for the birth of d.c.
generators and motors.
4
-
8/9/2019 part 12 emachines EEP 3243
5/64
Cont. The limitations of the d.c. system however became more and more
apparent as the power demand increased.
The invention of induction machines in the 1880s tilted the scale
in favor of a.c. systems mainly due to the advantage offered by
transformers The d.c. system, however could not be obliterated due to the able
support of batteries. Further, d.c. motors have excellent control
characteristics.
5
-
8/9/2019 part 12 emachines EEP 3243
6/64
Lodestone
6
Alodestone or
loadstone
is a naturally magnetized piece
ofthe mine
ral
magnetite. They are naturally occurring magnets, that attract pieces of
iron.
-
8/9/2019 part 12 emachines EEP 3243
7/64
AC Generator
7
-
8/9/2019 part 12 emachines EEP 3243
8/64
-
8/9/2019 part 12 emachines EEP 3243
9/64
Cont.
Because of their similar construction, the
fundamental properties of generators and motors
are identical.
If the brushes in AC generator could be switchedfrom one slip ring to the other every time the
polarity was about to change, we would obtain a
constant polarity voltage across the load.
9
-
8/9/2019 part 12 emachines EEP 3243
10/64
Cont.
A commutator in its simplest form is composed of a
slip ring that is cut in half, with each segment
insulated from the other as well as from the shaft.
The commutator revolves with the coil and thevoltage between the segments is picked up by 2
stationary brushes.
10
-
8/9/2019 part 12 emachines EEP 3243
11/64
Cont.
11
-
8/9/2019 part 12 emachines EEP 3243
12/64
Difference Between
AC and DC Generator
12
-
8/9/2019 part 12 emachines EEP 3243
13/64
Improving the Waveshape By increasing the number of coils and segments,
we can obtain a DC voltage that is very smooth.
The coils are lodged in a slots of a laminated iron
cylinder, that both constitute the armature.
13
-
8/9/2019 part 12 emachines EEP 3243
14/64
Cont. The % ripple is the ratio of RMS value of the AC
component of voltage to the DC component.
(Modern DC generator produce voltage having a
ripple of less than 5%.
14
-
8/9/2019 part 12 emachines EEP 3243
15/64
-
8/9/2019 part 12 emachines EEP 3243
16/64
Cont. Actual physical
construction
For reason of symmetry,
the coils are wound so
that 1 coil side is at thebottom of a slot and the
other is at the top.
This armature winding is
called a lap winding.
16
-
8/9/2019 part 12 emachines EEP 3243
17/64
Cont. The voltage ea induced in coil
A is exactly the same as thevoltage ec induced in coil C.However , the coil A is movingdownward and coil C ismoving upward. The polarities
of ea and ec, eb and ed areopposite.
This means that ea + eb + ec +ed = 0 at all times
The voltage between the
brushes is equal to (ea + ed) or(eb + ec)
17
-
8/9/2019 part 12 emachines EEP 3243
18/64
Induced Voltage
When the armature rotates, the voltage E induced ineach conductor depends upon the flux density
which it cuts.
18
-
8/9/2019 part 12 emachines EEP 3243
19/64
Cont. The conductors in slots 1 and 7 are exactly between the
poles, where the flux density is zero so the voltage induced
is zero. On the other hand, the conductors in slots 4 and 10
are directly under the centre of the poles, where the flux
density is greatest.
19
-
8/9/2019 part 12 emachines EEP 3243
20/64
Cont. The induced voltage remains essentially constant
as the armature rotates, because the number ofcoils between the brushes is always the same,irrespective of armature position.
Note that the brushes short circuit the coils inwhich the voltage is momentarily zero. They aresaid to be in the neutral position when they are;positioned on the commutator so as to short
circuit those coils in which the induced voltage ismomentary zero.
20
-
8/9/2019 part 12 emachines EEP 3243
21/64
Cont.
By shifting the brushes the output voltagedecrease. And in this position, the brushes
continually short circuit generated coils and cause
sparking (poor commutation).
21
-
8/9/2019 part 12 emachines EEP 3243
22/64
Neutral Zones
Neutral zones are those places on the surface of
the armature where the flux density is zero.
When the generator operates at no load, the neutral
zones are located exactly between the poles.
22
-
8/9/2019 part 12 emachines EEP 3243
23/64
Value of theInduced Voltage
Generated voltage is directly proportional to theflux/pole and the speed of rotation.
Only true if the brushes are on the neutral position.
Equation
23
-
8/9/2019 part 12 emachines EEP 3243
24/64
PROBLEM 1
24
-
8/9/2019 part 12 emachines EEP 3243
25/64
GeneratorUnderLoad: TheEnergy
ConversionProcess Because the conductors lie in a magnetic field, they
are subjected to a force according to Lorentz's Law.
The individual forces F on the conductors produce
a torque that acts opposite to the direction in whichthe generator is being drive.
25
-
8/9/2019 part 12 emachines EEP 3243
26/64
Cont.
To turn the generator, wemust exert a torque on the
shaft to overcome this
opposing electromagnetic
torque which resultingmechanical power is
converted into electrical
power.
26
-
8/9/2019 part 12 emachines EEP 3243
27/64
ArmatureReaction Drop The current flowing in the armature
coils also creates a magnetomotiveforce that distorts and weakens the
flux coming from the poles.
The effect of a magnetic field
produced by the armature mmf iscalled armature reaction drop.
The intensity of the armature flux
depends upon its mmf, which in turn
depends upon the current carried by
the armature.
27
-
8/9/2019 part 12 emachines EEP 3243
28/64
Cont. The flux in the neutral zone no
longer zero and will induces avoltage in the coils are short
circuited by the brushes.
Sparking may occur and its intensity
depend upon the armature flux andthe load current.
28
-
8/9/2019 part 12 emachines EEP 3243
29/64
Cont.
Armature mmf also distortsthe flux produced by the
poles and cause the neutral
zones have shifted in the
direction of rotation. This cause a reduction in the
induced voltage.
29
-
8/9/2019 part 12 emachines EEP 3243
30/64
Shifting the Brushes to
Improve Commutation
Due to the shift in the neutral zone, we could movethe brushes to reduce the sparking.
For generators, the brushes are shifted in the
direction of rotation and brushes motors are shiftedagainst the direction of rotation.
After the brushes are shifted the commutation
improves (less sparking).
30
-
8/9/2019 part 12 emachines EEP 3243
31/64
Cont.
This procedure is only practical to resolve small DCmachines commutation problem (if the load current
fluctuates).
To counter the effect of armature reaction inmedium and large power DC machines, a set of
commutating poles a placed between the main
poles.
31
-
8/9/2019 part 12 emachines EEP 3243
32/64
Cont.
The number of turns on thewindings is designed so that
the poles develop a mmf
equal and opposite to the
mmf of the armature. By nullifying the armature
mmf the flux in the space
between the main poles is
always zero.
32
-
8/9/2019 part 12 emachines EEP 3243
33/64
Types ofDC Generator
Separately Excited Generator. A pair of electromagnets (filed
poles) use to replace permanent
magnets.
DC field current is supplied by anindependent source (exciter)
As we increase the load, the
terminal voltage diminished
progressively (10%) because of
the voltage drop across armatureresistance, Ro.
33
-
8/9/2019 part 12 emachines EEP 3243
34/64
Cont.
ShuntGenerator (Self exited). Shunt-field winding is connected
in parallel with the armature
terminal, so that the generator
can self excited.
The progressive buildup
continues until Eo reaches a
maximum value determined by
the field resistance and the
degree of saturation.
34
-
8/9/2019 part 12 emachines EEP 3243
35/64
Cont. ShuntGenerator (Self exited).
We can determine the no-load value of
Eo if we know the saturation curve of
the generator and the total resistance
Rt of the shunt field circuit.
Critical resistance will be reachedwhere the slope of resistance line is
equal to that of the saturation curve in
its unsaturated region.
35
-
8/9/2019 part 12 emachines EEP 3243
36/64
Cont. ShuntGenerator (Self exited).
It is easy to control the induced voltage of a
shunt excited generator by vary the exciting
current.
The terminal voltage of a shunt generator
falls off more sharply (15%) with increasingload than a separately excited generator
because its exciting current falls as the
terminal voltage drops.
36
-
8/9/2019 part 12 emachines EEP 3243
37/64
Cont. CompoundGenerator.
This generator was developed to
prevent the terminal voltage of a DC
generator from decreasing with
increasing load.
It is similar to a shunt generator,except that it has additional field coils
connected in series with the armature.
37
-
8/9/2019 part 12 emachines EEP 3243
38/64
Cont. Compound Generator.
When no load, the current in the seriescoils is zero and shunt coils carry exciting
current which produces the field flux (just
like standard self excited shunt generator)
When loaded, load current flows through
the series field coils and a mmf developedby these coils acts in the same direction
as the shunt field mmf. So the field flux
under load rises above its original no load
value which raises the value of Eo.
38
-
8/9/2019 part 12 emachines EEP 3243
39/64
Cont.
Differential Compound Generator.
The mmf of the series field acts opposite to the shunt
field. So the terminal voltage falls drastically with
increasing load.
Used in DC arc welders, because they tended to limit
the short circuit current and to stabilize the arc during
the welding process.
39
-
8/9/2019 part 12 emachines EEP 3243
40/64
Construction ofDC Generator
To appreciate the working and the
characteristics of these machines, it is
necessary to know about the different parts of
the machine - both electrical and non-electrical.
40
-
8/9/2019 part 12 emachines EEP 3243
41/64
Cont. The major parts can be
identified as, Body
Poles
Armature
Commutator and brushgear
Commutating poles
Compensating winding
Other mechanical parts
41
-
8/9/2019 part 12 emachines EEP 3243
42/64
Body The body constitutes the outer shell within which all
the other parts are housed. This will be closed at both the ends by two end covers
which also support the bearings required to facilitatethe rotation of the rotor and the shaft.
Even though for the generation of an emf in aconductor a relative movement between the field andthe conductor would be enough, due to practicalconsiderations of commutation, a rotating conductorconfiguration is selected for DC. machines. Hence the
shell or frame supports the poles and yoke of themagnetic system.
42
-
8/9/2019 part 12 emachines EEP 3243
43/64
Main Poles Solid poles of fabricated steel with separate/integral
pole shoes are fastened to the frame by means of
bolts.
Pole shoes are generally laminated. Sometimes
pole body and pole shoe are formed from the samelaminations.
Riveted through bolts hold the assembly together.
The pole shoes are shaped so as to have a slightly
increased air gap at the tips.
43
-
8/9/2019 part 12 emachines EEP 3243
44/64
Inter-poles
These are small additional poles located in betweenthe main poles.
These are also fastened to the yoke by bolts.
These are also called as commutating poles orcompoles.
44
-
8/9/2019 part 12 emachines EEP 3243
45/64
Armature The armature is where the moving conductors are
located. The armature is constructed by stacking laminated
sheets of silicon steel. Thickness of theselamination is kept low to reduce eddy current
losses. The core is divided into packets to facilitate
ventilation.
A
rmature construction process must ensureprovision of sufficient axial and radial ducts tofacilitate easy removal of heat from the armaturewinding.
45
-
8/9/2019 part 12 emachines EEP 3243
46/64
Field Winding As against permanent magnet excited machines the field winding
takes the form of a concentric coil wound around the main poles. These carry the excitation current and produce the main field in
the machine.
The resistance of such winding would be an order of magnitude
larger than the armature winding resistance. The total mmf required is divided equally between north and south
poles as the poles are produced in pairs. The mmf required to be
shared between shunt and series windings are apportioned as per
the design requirements.
46
-
8/9/2019 part 12 emachines EEP 3243
47/64
Armature Winding
The armature windings are in general pre-formed, taped andlowered into the open slots on the armature.
In the case of small machines, they can be hand wound. The coilsare prevented from flying out due to the centrifugal forces bymeans of bands of steel wire on the surface of the rotor in smallgroves cut into it.
In the case of large machines slot wedges are additionally used torestrain the coils from flying away.
The end portion of the windings are taped at the free end andbound to the winding carrier ring of the armature at thecommutator end.
The armature must be dynamically balanced to reduce thecentrifugal forces at the operating speeds.
47
-
8/9/2019 part 12 emachines EEP 3243
48/64
Compensating Winding
One may find a bar winding housed in the slots
on the pole shoes.
T
his is mostly found in d.c. machines of verylarge rating.
In smaller machines, they may be absent.
48
-
8/9/2019 part 12 emachines EEP 3243
49/64
Commutator It consists of copper segments tightly fastened
together with mica/micanite insulating separators on
an insulated base.
The whole commutator forms a rigid and solid
assembly of insulated copper strips and can rotate athigh speeds.
The surface of the commutator is machined and
surface is made concentric with the shaft and thecurrent collecting brushes rest on the same.
49
-
8/9/2019 part 12 emachines EEP 3243
50/64
Brush and Brush Holders Brushes rest on the surface of the commutator.
Normally electro-graphite is used as brush material. The actualcomposition of the brush depends on the peripheral speed ofthe commutator and the working voltage. The hardness of thegraphite brush is selected to be lower than that of the
commutator. The brush holders provide slots for the brushes to be placed.The connection from the brush is taken out by means of flexiblepigtail.
T
he brushes are kept pressed on the commutator with the helpof springs. This is to ensure proper contact between thebrushes and the commutator even under high speeds ofoperation.
50
-
8/9/2019 part 12 emachines EEP 3243
51/64
Cont. Jumping of brushes must be avoided to ensure arc
free current collection and to keep the brush contactdrop low.
Radial positioning of the brushes helps in providingsimilar current collection conditions for both directionof rotation.
For unidirectional drives trailing brush arrangementor reaction arrangement may be used.
Reaction arrangement is preferred as it results in zeroside thrust on brush box and the brush can slidedown or up freely.
51
-
8/9/2019 part 12 emachines EEP 3243
52/64
OtherMechanical Parts End covers, fan and shaft bearings form other
important mechanical parts.
End covers are completely solid or have opening
for ventilation. They support the bearings which are
on the shaft. Fans can be external or internal. In most machines
the fan is on the non-commutator end sucking the
air from the commutator end and throwing the same
out.
52
-
8/9/2019 part 12 emachines EEP 3243
53/64
Bearings Small machines employ ball bearings at both ends.
For larger machines roller bearings are used
especially at the driving end.
The bearings must be kept in closed housing with
suitable lubricant keeping dust and other foreignmaterials away.
Care must be taken to see that there are no bearing
currents or axial forces on the shaft both of whichdestroy the bearings.
53
-
8/9/2019 part 12 emachines EEP 3243
54/64
-
8/9/2019 part 12 emachines EEP 3243
55/64
Cont.
Consequently a four-pole generator could output twice thevoltage of a two-pole generator, a six-pole generator couldoutput three times the voltage of a two-pole. This allows outputvoltage to increase without also increasing the rotational rate.
In a multipole generator, the armature and field magnets aresurrounded by a circular frame or "ring yoke" to which the fieldmagnets are attached. This has the advantages of strength,simplicity, symmetrical appearance, and minimum magneticleakage, since the pole pieces have the least possible surface
and the path of the magnetic flux is shorter than in a two-poledesign.
55
-
8/9/2019 part 12 emachines EEP 3243
56/64
12-pole, 72-coil DC Generator
CoilA and C are
momentarily in neutral
zone, B is cutting the
flux coming from thecenter of the poles.
56
-
8/9/2019 part 12 emachines EEP 3243
57/64
Cont.
Coil width (coil pitch) is
the coil sides cut the flux
coming from the
adjacent N & S poles.
CoilA sides in slots 1
and 7 and they are in the
neutral zone. Coil B
sides in slot
57
-
8/9/2019 part 12 emachines EEP 3243
58/64
Cont. The voltage generated
between brushes x and y isequal to the sum of thevoltages generated by thecoils connected to
commutator segments.
The +ve brush sets areconnected together to formthe +ve terminal and so with
-ve brush sets.
58
-
8/9/2019 part 12 emachines EEP 3243
59/64
Torque
Torque is produced when a force exerts a
twisting action on a body, tending to make it
rotate.
Torque is equal to the product of the force times
the perpendicular distance between the axis of
rotation and the point of application of the force.
59
-
8/9/2019 part 12 emachines EEP 3243
60/64
Cont. The torque exerted on the pulley by
the tangential force is given by,
60
-
8/9/2019 part 12 emachines EEP 3243
61/64
PowerofaMachine
The mechanical power output of a machine dependsupon its rotational speed and the torque it develops.
The power is given by,
61
-
8/9/2019 part 12 emachines EEP 3243
62/64
Cont.We can measure the power output of
a
motor by means of a prony brake.
The torque is, T
The power is, P
62
-
8/9/2019 part 12 emachines EEP 3243
63/64
PROBLEM 2
63
-
8/9/2019 part 12 emachines EEP 3243
64/64
THE END
64