CHAPTER 1 PRINCIPLES OF A.C. GENERATION

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.

D.C. generators are not much used today as power sources except for special applications, principally because the commutator limits the voltage which can be generated, and it also poses maintenance problems.