1.1 FARADAY’S LAW OF ELECTROMAGNETIC INDUCTION
In the manual ‘Fundamentals of
Electricity 1’ it is explained how Faraday was led to propound his ‘Law of
Electromagnetic Induction’.
FIGURE 1.1
FARADAY’S LAW OF
ELECTROMAGNETIC INDUCTION
This law states that, if a conductor
is moved in a magnetic field, then an ‘electromotive force’ (emf) - or, simply,
a voltage - is induced in that conductor, as shown in Figure 1.1. It follows that, if the ends of the conductor
are connected to an external load, then an electric current, driven by that
voltage, will flow from the conductor, through the load and back again. The set of conductors in which the voltage is
induced is called the ‘armature’.
Whereas Oersted showed that an electric
current in a wire gives rise to an artificial magnetic field, Faraday showed
the opposite - that if a wire moves in a magnetic field an artificial charge,
or voltage, will be created in that wire.
Faraday also showed that the magnitude of the voltage induced in the
moving conductor depends on the strength of the magnetic field and the speed of
movement, and on nothing else.
These two laws form the whole basis
of electrical power generation, both a.c. and d.c. Starting with a magnetic field, either a
natural magnet or an artificial electromagnet of Oersted’s type, a conductor or
a number of conductors are caused to move past it, from which the current is
extracted as they are moving. First
however look at one other rule which determines how this is achieved and the
directions of movement and induced voltage.
Figure 1.1 also shows ‘Fleming’s
Right-hand Rule for Generators’. If the
right hand is held with the thumb, forefinger and centre finger extended
mutually at right angles, then, with the magnetic field in the direction (North
to South) pointed by the forefinger and the motion of the conductor in the
direction indicated by the thumb, the centre finger will point in the direction
in which the emf (i.e. voltage) is induced in that conductor (and in which
current will flow when connected to a load).
The following
paragraphs show how this can be put into practice.
FIGURE 1.2
APPLICATION OF FARADAY
PRINCIPLE
1.2 A.C. GENERATORS
Consider the scene (Figure 1.2(a))
where two girls are swinging a skipping rope.
Suppose the ‘rope’ is a copper wire with its ends connected to a
voltmeter, and suppose the rope swings between the poles of a large magnet -
north pole overhead and south pole in the ground. There is then a downward magnetic field all
over the rope.
If the rope is swinging
anti-clockwise as seen from the left, as it passes the 12 o’clock position it
is moving at its fastest past the N magnetic pole, and, by Fleming’s Right-hand
Rule, a voltage will be induced from right to left. The voltmeter will swing one way - say to the
right.
As the rope moves on it moves less
quickly across the magnetic field until, by 9 o’clock, it is not crossing it at
all. No voltage will be induced, and the
voltmeter indication falls to zero.
After 9 o’clock as the rope
continues its swing it begins to move through the field in the other
direction. The right-hand rule says that
the voltage is induced the other way (left to right), and the voltmeter needle
swings to the other side. At 6 o’clock
the rope is moving at its fastest past the S-pole, and the voltmeter reaches
its maximum left swing.
So on, past 3 o’clock, where the
induced voltage is again zero, back to 12 o’clock where the rope is once more
moving at its fastest past the N-pole, and the voltmeter needle swings back to
its maximum reading to the right.
If the voltage indicated by the
voltmeter is plotted against the rope’s position (considered as 360° for one revolution) it takes a waveform (Figure 1.2(b)) - maximum
positive at 12 o’clock (0° and 360°), maximum negative at 6 o’clock (180°) and zero at 9 o’clock and 3 o’clock (90° and 270°). The shape of the curve is that of a
pure sine-wave (or more strictly in this case a pure cosine-wave).
FIGURE 1.3
A.C. GENERATION - FIXED
FIELD
Suppose, instead of the skipping
rope, there were a loop of stiff wire on a shaft which can be turned, as shown
in Figure 1.3. Suppose each end of the
wire is connected to a slipring, insulated from the shaft, upon which brushes
bear which are connected to a load or voltmeter as before.
As the shaft is turned, one bar
passes the N-pole as the other passes the S-pole. Voltage is induced one way in one of them and
the opposite way in the other. But as
they are in series the two voltages add up and appear as a double voltage at
the sliprings, and so at the voltmeter.
Faraday’s theory required only that
the conductor should be moving through a magnetic field - that is, that there
should be relative motion between conductor and field. It would work just as well if the magnetic
field moved past the conductor.
FIGURE 1.4
A.C. GENERATION - ROTATING FIELD (PERMANENT MAGNET)
In the arrangement shown in Figure
1.4 this is just what is happening. The
stiff wire loop is fixed, and the permanent magnet is rotated past it and
inside it. As a pole passes a fixed
conductor a maximum voltage is induced in it, opposite voltages on opposite
sides, and they add up to give a double voltage at the terminals or at the
voltmeter. Only in this case no
sliprings or brushes are needed - a great advantage for many reasons, not least
that it eases maintenance.
So far we have only considered a
permanent magnet as producing the magnetic field. But far better results can be achieved by
using an electromagnet, as in Figure 1.5, which can produce much stronger
fields and therefore much higher induced voltages. In that case however d.c. power must be
provided to the coil which magnetises it.
This can come from a battery or other d.c. source, but a pair of slip-rings must be reintroduced to bring the battery current to the moving
magnetising coil - called the ‘field coil’.
This coil is said to ‘excite’ the field, and the whole process is called
‘excitation’.
FIGURE 1.5
A.C. GENERATION - ROTATING
FIELD (ELECTROMAGNET)
Because the field magnet is not
permanent but is an electromagnet, it is possible to vary the coil current by a
resistance and so to vary the strength of the magnetic field itself. This in turn will vary the amount of the
induced voltage. Here then is a simple
method of controlling the main machine’s voltage by varying the excitation.
FIGURE 1.6
A.C. GENERATION - VOLTAGE
CONTROL
Figure 1.6 shows in principle how
this is done. Top left is the generator,
and on its right are the sliprings carrying the exciting current to the field
coils. Connected to the generator’s
output lines is a voltmeter, which can be seen by the operator. He knows what voltage he wants to see. If it is not right, he adjusts the resistance
controlling the level of field current from the battery. As he adjusts this current up or down, so the
voltmeter will indicate a rise or fall of output voltage. He continues until
the voltmeter reads the voltage desired.
FIGURE 1.7
A.C. GENERATION - AUTOMATIC
VOLTAGE CONTROL
Figure 1.6 showed an operator
intervening between the voltage output and the adjustment needed to correct
it. The next stage is to make it
automatic. An electronic device called
an ‘Automatic Voltage Regulator’ (AVR for short) senses the output voltage and
compares it with a datum which has previously been set on by hand. It decides whether the output voltage is
correct, too high or too low. This is shown
in Figure 1.7.
In this case the battery which has
hitherto been providing the d.c. power for the field coils is replaced by a
d.c. generator, called an ‘Exciter’, driven mechanically by the main generator
shaft. If the AVR has decided that the
generator’s output voltage is too high or too low, it signals the exciter to
decrease or increase its current to the main generator’s field coils, so
bringing the main generator’s output voltage down, or up, until it is
correct. All platform and onshore
generators work on this principle.
The AVR acts on the machine’s
voltage much as a governor acts on its speed.
Both can have their datums set on by hand, after which the voltage (or
speed) should be held automatically between close limits, no matter how the
loading varies. If either device fails,
it can be put into manual control.
The final stage in the development
of an a.c. generator is shown in Figure 1.8.
The exciter, instead of being belt- or gear-driven by the main shaft and
requiring sliprings to bring the field coil current into the generator, is now
moved up to the main machine itself and shares.
FIGURE 1.8
A.C. GENERATION - BRUSH
LESS EXCITATION
a common shaft. A small second, or ‘pilot’, exciter is
sometimes added to excite the main exciter.
The AVR, as before, signals the main exciter whether to raise or lower
its output to the field coils, to which it is connected by cables through the
hollow shaft.
For mechanical reasons it is not
possible with this arrangement to use a d.c. generator as an exciter, as the
brushgear would have to rotate. Instead
the exciter is another, but smaller, a.c, generator working on the principle of
Figure 1.3 - that is, with stationary field and rotating armature. The a.c. output from this armature is taken
through static ‘rectifiers’ rotating with the shaft; they convert the a.c. into
d.c. and pass it on to the rotating field winding of the main generator.
This arrangement gives better and
tighter control over the voltage, but its prime advantage is that it totally
dispenses with the sliprings and brushes.
It is known as ‘Brushless Excitation’ and is now used almost exclusively
on all platforms and many shore-side generators.
The actual construction of an a.c.
generator and of its driving engine is described in the manual ‘Electrical
Generation Equipment’.
1.3 3-PHASE A.C. GENERATORS
Figure 1.4 showed how the generator
developed into a magnet or ‘field’ rotating inside a fixed loop. As the poles passed each side of the loop, an
alternating voltage was induced in it which was conveyed to the terminals, and
thence to any connected load.
FIGURE
1.9
3-PHASE GENERATION -
WINDINGS
In practice such a system, though
used in small installations, is not very economical, and with most large
installations three separate loops are provided, equally spaced at 120° around the shaft as shown at the top of Figure 1.9 where they are
distinguished by different colours. The
same field rotates inside all three loops, so the same voltage is generated in
each, and at the same frequency, but, because of the 120° spacing, the voltage in each loop rises to its peak one-third of a
cycle, or 120°, later than the one before it.
There are in fact three generators in a single machine. The voltage from each of the three loops is
brought out to separate pairs of terminals A, A’; B, B’; and C,C’.
At first sight it would appear that
six wires would be needed to remove the current from the three loops, but
actually each loop shares a wire with its neighbour, as shown diagrammatically
at the bottom of the figure. Thus the
red and yellow loops share wire R; yellow and blue loops share wire Y, and blue
and red loops (if examined closely) share wire B. So only three wires are actually needed to
convey the power away from the generator.
They are variously identified as A, B, C or L1, L2, L3 or U, V, W (in
Continental machines) or are simply coloured red, yellow and blue.
Such a machine is called a ‘3-phase’
generator, and if you look at the overhead pylons all over the country you will
see that each carries one, or sometimes two, sets of three wires.
FIGURE 1.10
3-PHASE GENERATION -
WAVEFORMS
The voltage waveform of a 3-phase
system is simply three sine-waves similar to those of Figure 1.2, but each
displaced one-third of a cycle, or 120° behind the
other. This is shown in Figure 1.10.
The advantages of using a 3-phase
alternating current may be compared with those of using an engine with many
cylinders. With a single cylinder power
comes from the engine in spurts, but with the addition of more cylinders the
spurts of power overlap, producing a steadier level of power. In the same way the overlapping phases of a
3-phase circuit maintain a smoother and higher level of power. Power in large installations - and certainly
on all platforms and shore networks - is always produced and transmitted in
3-phase systems because the generators and power lines are less expensive than
those for a single-phase system carrying the same power, and also because they
are best adapted to motors (as is shown in the manual ‘Electric Motors’).
1.4 D.C. GENERATORS
Although the subject of ‘D.C.
Generators’ belongs more strictly to the manual ‘Fundamentals of Electricity 1’
than ‘Fundamentals of Electricity 2’, it follows so logically from the above
description of the generation that it is included here.
Reverting to Figure 1.3, a stiff wire
loop is rotating between the poles of a permanent magnet, and with its two ends
connected each to a separate slipring on which brushes bear. As the loop rotates, each side has induced in
it a voltage which changes direction at every half rotation - that is, as that
side passes a N- or a S-pole. Each
slipring, and so each brush, receives alternately a forward and a reversed
voltage, so that the output from the brushes to the load is ‘alternating’.
FIGURE 1.11
DIRECT CURRENT GENERATOR
Suppose now, instead of the two
sliprings, there had been a single slipring split into two and with a
conductor-end connected to each half, but still with two brushes, which will be
called ‘A’ and ‘B’. This is shown in
Figure 1.11.
As the side ‘X’ of the loop is
passing the N-pole, by Fleming’s Rule the voltage (V) induced in that loop is
towards the brushes, and in loop ‘Y’, which is passing the S-pole, it is away
from the brushes. As they are in series
a voltage (2V) appears across the two halves of the slipring, that half
connected to loop ‘X’ being positive and that to loop ‘Y’ negative, and so
brush ‘A’ also is positive and brush ‘B’ negative.
Half a revolution later the voltages
in loops ‘X’ and ‘Y’ are reversed, and the half slipring connected to loop ‘X’
is negative and to loop ‘Y’ positive.
But the split slipring itself has
now turned half a revolution, so loop ‘Y’ (positive) is now with brush ‘A’, and
loop ‘X’ (negative) with brush ‘B’. So
brush ‘A’ is still positive and brush ‘B’ is still negative, that is to say
there has been no change from half a revolution before. Brush ‘A’ remains positive and brush ‘B’
negative throughout. The split slipring
acts as a reversing switch, and the voltage does not alternate but remains
unidirectional at all times.
Such a machine is a ‘Direct Current
Generator’ (in the old days called a ‘Dynamo’) and is identified by having a
split slipring, the two parts being insulated from each other but the brushes
passing over both.
In practice of course such a
generator would have not one loop but many, so the slipring would be split not
into two parts but into several. It
would appear something like that shown on the top-left of Figure 1.11 and is
called a ‘Commutator’. It is the unique
indication of all d.c. machines.
No comments:
Post a Comment