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¶ U.S. GOVERNMENT PRINTING OFFICE; 1945 - 618779
CHAPTER 14 GENERATORS ELECTRICAL PUMPS The modern fighting ship
consumes a tremendous amount of electrical energy. The electrical machinery
furnishes her men with food, water, and fresh air. Her nerves are electrical
wires coordinating all her activities, all her power, to make her a
To get the electrical energy necessary to
do her many jobs, the modern ship operates huge generators. The dynamo
room is the heart of her nerves and her muscles. It provides the ears
and the eyes for her guns, the muscles for her rudder, and make her
skipper's voice carry into every compartment. All this energy is derived
from oil by the simple process of A WIRE CUTTING A FIELD OF FLUX-INDUCTION.
Generators - the engines of induction - are electrical pumps. They
force electrical energy through the ship - from her stem to her stern.
Although generators are SIMPLE IN PRINCIPLE - mutual induction machines
- they are sometimes COMPLEX IN DESIGN. When the design of a generator
seems unnecessarily complicated, just remember that it was built to
do a job. And that job has certain requirements. If the only way to
meet the requirements is by complicating the machinery - then it's going
to be complicated. You don't handle a racing boat like you do a fishing
HOW A GENERATOR IS BUILT
Since a generator is a mutual induction job, its first requirement
is a magnetic field. The simplest generator field is built like the
drawing in figure 121. Two electromagnets are mounted in a circular
, iron frame called a YOKE. These electromagnets are wound so as to
produce opposite polarity. Notice how the magnetic circuit is entirely
in iron except . at the center, between the poles. This area - between
the pole pieces - is the only part of the field outside the iron.
Figure 121. - Generator-magnetic field.
The yoke, its pole pieces, windings, and the field produced are
the primary circuit. The secondary circuit is a coil wound on an iron
core. The coil and core, mounted on a shaft is the ARMATURE. Figure
122 shows a typical armature. To make the generator complete, the armature
of figure 122 fits into the area between the pole pieces of figure 121.
HOW IT WORKS
The frame of the generator stands still - the field of flux is steady
and stationary. But; the armature shaft is rotated by a source, of mechanical
power - the PRIME MOVER. And as the armature is rotated, the conductors
of the coil cut through the field flux. As in the simplest, or the most
complicated, system, conductors cutting flux produce an induced voltage.
Figure 122. - Generator-armature.
These are the elements of a generator -
field produced by electromagnets.
2. The prime
mover feeding mechanical energy into the generator by rotating
3. The armature carrying a coil of
wire through the field and producing an induced emf.
UNDERSTANDING THE ACTION
The easiest way to understand what happens in an armature, is to
lift ONE TURN of the coil off its iron core and study it alone. Figure
123 is a single turn rotating in the magnetic field. The coil is shown
in four positions which represent one complete revolution of the coil.
In A the coil is producing zero voltage - the galvanometer reads zero.
It's zero, because, in this position, the coil is cutting NO flux. How
can a coil move in a field and yet cut no flux? By moving parallel to
the lines of force. Notice in A that both sides of the coil are moving
in a straight line between the poles. When conductors are moving this
way they slip between the "rubbery" lines of force and do not break
Figure 123. - Armature coil revolving
in magnetic field.
In B the coil has moved to a position at right angles with A. Now
the black side of the coil is cutting DOWNWARD and inducing a voltage
OUT. And the white side is cutting UPWARD and inducing a voltage IN.
The galvanometer attached to the two terminals of the coil deflects.
Trace this circuit through. Notice that although the two induced voltages
are opposite - one in and the other out - the voltage for the TOTAL
coil is the ADDITION of these two. Trace the current through the coil
- you go WITH both voltage arrows. This means that both voltages add
force to the current.
In C, the coil has turned one half of a
complete revolution. C is like A except upside down. Again, the coil
sides are moving parallel to the lines of force. The induced voltage
In D the coil position is the reverse of position B.
The black side is now cutting UPWARD and has an induced voltage IN.
The white side is cutting DOWNWARD and has an induced voltage OUT. Notice
that the current direction in the coil is the exact reverse of position
B. This is not amazing - you know that reversing the direction of cutting
reverses the direction of the induced voltage. Use your generator hand
rule - it will prove the arrows are correct.
The fifth position
(if one were shown) would duplicate A. You have followed a coil through
one complete revolution. Two facts stand out. First, there are two positions
where the coil is moving parallel to the field-the induced voltage is
zero. These positions are called the NEUTRAL PLANE of the generator
- in this two pole job the neutral plane is midway between the poly
pieces. Second, during one half of the revolution, the coil's induced
voltage is in one direction (counterclockwise). During the other half
of the revolution, the coil's induced voltage is in the opposite direction
Half the time one way, and half the time, the other
way? Sounds familiar. It is - that's ALTERNATING CURRENT. A rotating
coil always produces alternating current.
It's proved that rotating coils produce alternating current. But
- go back to figure 123 and check up on those galvanometer readings.
How about it? In both Band D the deflection is toward the right. This
indicates that DIRECT CURRENT - is flowing OUTSIDE the coil. How come
- A.C. inside the coil and D.C. outside? The a.c. has been RECTIFIED
- that is, changed from alternating to direct current. The COMMUTATOR
did the job.
Examine the terminal ends of the coil in figure
123. Each end is connected to a one half of a copper ring. These two
halves of a copper ring, taken together are the COMMUTATOR. Now notice
how the commutator is connected to the outside circuit (the galvanometer).
On each half of the commutator (the halves are called SEGMENTS) rides
a block of carbon called a BRUSH. The brush and commutator connect the
ROTATING coil and the STATIONARY galvanometer. Without brushes and commutators,
the leads from a coil would be twisted off after only a few revolutions.
That's one purpose of a commutator-brush system - it provides a SLIPPING
CONTACT between rotating armature and stationary load.
does the commutator rectify the current? Let the brush where current
comes OUT of the coil be called NEGATIVE, and the brush where current
goes IN the coil be called POSITIVE. Now follow the coil through A,
B, C, and D of figure 123. The commutator segment attached to the side
of the coil having current OUT is always in contact with the NEGATIVE
brush. And the segment attached to the side having current IN is always
in contact with the POSITIVE brush. Another way of, saying the same
thing - the rotating coil with its reversing current carries its segments
around with it. At the instant the coil goes through the neutral plane
the current reverses AND AT THE SAME INSTANT the segments switch brush
connections. This is the other important purpose of the commutator -
it rectifies the generated A.C., delivering D.C. to the external circuit.
SLIP RINGS - A.C.
Instead of connecting a rotating coil to a commutator, connect each
terminal of the coil to a SLIP RING. Slip rings are simply smooth rings
of good conductor material. Now brushes riding on these slip rings will
pick up a.c. and deliver it to the external circuit. Figure 124 shows
a rotating coil with slip rings attached.
Figure 124. - Slip ring coil revolving
in magnetic field.
Starting with A, the coil is in the neutral plane - no induced voltage.
In B, the coil is at right angles to the flux. The induced voltage in
the BLACK side of the coil is OUT. In the WHITE side, it is IN. SO you
call the white ring POSITIVE and the black ring NEGATIVE. In C, the
coil is again in the neutral plane. In D, the coil is once more cutting
flux at right angles. But, now the induced voltage in the BLACK side
is IN. And in the WHITE side, it is OUT. Now, you call the white ring
NEGATIVE and the black ring POSITIVE. This means that through one half
of the revolution the white ring is positive and through the other half
it is negative. The same is true of the black ring. Consequently the
current in the external circuit reverses itself every time the coil
current reverses. And the reverses occur every time the coil passes
through the neutral plane.
Figure 125.-Graph of alternating emf.
SUMMARY OF D.C. AND A.C.
All coils, rotating in a magnetic field, have a-c voltage induced.
This a.c. can be connected directly to an external circuit by means
of slip rings. Or, it can be rectified by means of a commutator in order
to deliver d. c. to the external circuit.
Figure 125 is a graph
of a-c voltage. Notice the small coils above the graph. Each coil is
in the proper position to produce the emf indicated on the graph.
Figure 126 is a graph of d-c voltage. Again the small coil's position,
corresponds to the voltage indicated.
You are probably wondering
what happens when the coils are somewhere in between zero (neutral plane)
positions and maximum (right angle) positions. The coil is cutting flux
all right, but not as many lines per second as at the maximum. Actually,
the conductors are cutting through the flux field at an angle. The closer
this angle comes to 90° with the flux, the more lines the conductor
cuts. The closer this angle comes to 0° with the flux, the fewer lines
the conductor cuts. The result is that the voltage builds up in a smooth
upward sweep - from a zero value at the neutral plane, to a maximum
value at 90° from the neutral plane. The opposite is true when the coil
sides are going from a maximum point to a zero point. The voltage decreases
in a smooth downward sweep. The build-up and build-down is a SMOOTH
Figure 126.-Graph of direct emf.
You should recognize the two graphs of figures 125 and 126 as typical
graphs of alternating current and pulsating direct current. Graphs of
these two types of current always have the general shapes of these figures.
A single coil rotating in a magnetic field is like an 8-cylinder
job hitting on only one. The output power is weak and fluctuating. Fluctuation
is a characteristic of a.c. And adding more coils does not eliminate
the regular rise and fall of a-c voltage. But adding more coils to a
d-c job smooths out the fluctuation and changes the direct current from
pulsating to regular d.c.
Figure 127.-Two coil armature.
Here is how it works. In building up an armature from one to many
coils, first add one more coil at right angles to the first. Figure
127 shows the two coils arranged on an armature at right angles to each
other. When this armature is rotated, the black coil is going to be
one-quarter of a revolution behind the white coil. Which means that
the induced voltage of the black is at zero value when the white is
at maximum value. Notice that a four segment commutator is required
for the terminals of the two coils. Brushes riding on this commutator
contact ONLY the coil producing the BEST voltage. Figure 128 is a graph
of the volt-ages produced by both coils. The heavy part of the graph
is the voltage picked up by the brushes. This is the voltage delivered
to the external ciruit. Notice that the voltage is more level than it
was with one coil. True, it still is a pulsating voltage - but now it
doesn't go all the way down to zero. Adding the extra coil has taken
out some of the "bumps."
Figure 128. - Two coil voltage.
Add two more coils, placing them midway between the original coils
on the armature. Now you have a generator like figure 129. Figure 130
shows the voltage produced - by this four coil job.
NOTE - it's
now an eight segment commutator and the brushes are catching only the
very peaks of each coil voltage. Yes-it's still pulsating d.c. But a
mild type - the rise and fall is short.
From this four coil job
to the simplest commercial generator is only a short step. Figure 131
shows a GRAMME RING ARMATURE, one of the first practical armatures.
Figure 129. - Four coil armature.
Figure 130. - Four coil voltage.
The gramme ring armature does a whale of a lot that the one, two,
or four coil jobs did not do. FIRST - the coil is wound on iron. This
reduces the reluctance of the magnetic circuit by eliminating almost
all the air gap. Consequently, a stronger field and a higher induced
voltage in the armature. SECOND - the windings are in series - the individual
voltages of the 'turns add together. Consequently, a higher voltage
at the terminals of the generator. THIRD - the coils form TWO paths
between the brushes - one path up either side of the ring. Therefore,
this armature can carry more current without overheating.
Figure 131. - Gramme ring armature.
Suppose you follow one ampere of current through this generator.
Entering the commutator at the positive brush, the current can only
go into ONE segment because all the segments are insulated from each
From the segments, the current goes out to the winding
on the ring via the ARMATURE LEAD marked A. At the winding, the current
splits - half going up the right side and half going up the left side.
And why does current go UP these windings? Use your generator hand rule
- it will tell you that as the current goes through each successive
turn of wire, the induced voltage gives it a "kick" upward. The first
set of turns give it a kick of 20 volts. The second and third sets each
provide a kick of 40 volts. And the fourth set, like the first, provides
20 volts. Adding these induced voltages - they're in series-the current
has a total potential of 120 volts. The currents from each side of the
ring meet at lead B - both backed by 120 volts 9f potential. The lead
provides a path to the commutator segment for both currents. The brush
picks up the current from the segment and delivers it via a BRUSH LEAD
to the load. At the load the current loses its voltage - yes, all the
120 volts - doing the work of the load. Then, at zero voltage, the current
reenters the armature and gets kicked again by induced voltage until
it has a potential of 120 volts - it's again ready for another circuit
through the load doing the load's work.
Now, how come the second
and third sets of turns provided 40 volts, whereas, the first and fourth
sets only furnished 20 volts? It's simple - the second and third sets
are moving almost at right angles to the flux. They're cutting lines
of force at a high rate. The first and fourth sets are cutting at a
wide angle and consequently only break about half as many lines as the
second and third sets. The Gramme ring armature was designed to do this
job - provide a HIGH voltage and a STEADY voltage. It does both by means
of series connections. Notice that, as the armature is turning, one
set of turns after the other moves into the flux field to provide a
high and steady voltage.
The modern armature, makes use of the
DRUM WINDING, shown in figure 132.
Figure 132. - Drum wound armature.
Again, series connections and many coils. The principal advantages
of the drum winding lie in (1) the saving of wire, (2) the reduction
of reluctance, (3) the ease of repair. In the Gramme ring, half of the
windings do not cut flux. They're on the INSIDE SURFACE of the iron
ring, while the flux is traveling WITHIN the ring. In the drum armature,
all the windings are placed on the OUTSIDE SURFACE of the iron core.
The flux is cut by EVERY conductor as the lines jump from the iron pole
piece to the iron core of the armature. The drum armature core is iron
all the way through as contrasted to the air center of the Gramme ring.
Air increases reluctance - therefore, the drum armature has less reluctance.
It's hard to repair a ring armature-damaged sets of turns must be replaced
by hand and spliced to the undamaged portion of the winding. In the
drum winding, any damaged coil can be lifted individually, repaired,
replaced, and re-connected by soldering to the proper segments of the
Figure 133. - Modern drum wound armature.
Figure 133 shows a modern drum wound armature. Notice the great
number of coils and commutator segments to give this job a high and
CALLING IT BY NAME
"That thing," "jigger," "it," "thing-a-ma-bob," and "gadget," may
be okay on the beach. But in your Navy you're supposed to know what
you are talking about. In fact, you've got to be able to make OTHERS
know what YOU'RE talking about. The parts of generators have accurate
names - USE THEM. Figure 134 shows the four main parts of a generator-the
FRAME, the ARMATURE, the COMMUTATOR, and the BRUSH RIGGING. Each part
is labeled with its correct name. LEARN "EM"! You'll sound a lot more
savvy on your job.
ALTERNATORS - A-C GENERATORS
It would be simple if alternators followed the generator pattern
in their development-one -coil, two coils, many coils .. It would be
nice and simple -BUT they just aren't built that way! Alternators have
a special design that's MECHANICALLY OPPOSITE to the generator. Alternators
ROTATE the FIELD and hold the ARMATURE STATIONARY.
true that alternators COULD be built by increasing the number of turns
of the coil and taking the a.c. off through slip rings. A coil of many
turns would step up the voltage to a usable value. And a very few alternators
are built this way. They work just like the d-c generator except that
the commutator is replaced by a set of slip rings.
speaking, the alternator is designed to produce a much higher voltage
than the d-c generator. In a.c., transformers can reduce this high generated
voltage to a safe value for use. This is impossible in d.c. - transformers
do not work on d.c.
Figure 134. - Parts of a generator.
Generating high voltages (as high as 25,000 volts) makes the use
of slip rings impossible. Any voltage above 1,000 volts cannot be handled
on slipping contacts - either commutator or slip rings - because of
arcing. Even as much as 700 or 800 volts arcs dangerously. These arcs
are like miniature bolts of lightning - jumping from brush to slip ring.
Each arc digs a pit into the slip ring, soon wearing it out. When the
voltage is in the thousands, arcs may jump from ring to ring, brush
to brush, or brush to frame. It's obvious, then, that slip rings cannot
be used to take high voltage a.c. off an alternator.
ALTERNATORS-ROTOR AND STATOR
To eliminate the dangers of arcing - and still generate high voltages
- conductors are held stationary and the flux field is moved across
them. The alternator does just that. The field poles are mounted on
a shaft and rotated. This is the ROTOR of an alternator. The energizing
current for the field poles - d.c. - must be fed into the rotor by slip
rings. This energizing d.c. is at a low voltage, usually 110 or 220
volts, so it is safe to use slip rings. The armature is wound as a many-turn
coil inside a slotted frame. This frame is stationary - it is called
the STATOR of an alternator. Figure 135 is a cross section of a four
Figure 135. - Four pole alternator.
It works this way - the rotor with its magnetic, field sweeps across
the stator windings. As the lines of force are cut by the stator windings,
a voltage is induced. Imagine the rotor of figure 135 turning. First
the N pole flux cuts the left side of the stator. After one quarter
of a revolution, the situation is reversed. The S pole flux cuts the
left side, and the flux has reversed for all windings on the stator.
The effect is that of reversing field direction. And, when field direction
is reversed, so is the direction of induced emf. Alternating current
is the product.
Figure 136. - Schematic of stator and
The alternator builds up a strong voltage by having many turns on
both rotor and stator. The multi-turn rotor produces a strong flux (NI).
And the multi-turn stator is connected in series so that voltage adds.
The combination of MANY conductors cutting a STRONG field generates
high voltages. If you trace current through an alternator as you did
through a Gramme ring, you will find the principles the same. Follow
the current through figure 136. Current enters the positive lead of
the stator and travels through the windings. It picks UP the induced
voltage, and leaves on the negative stator lead. Then it goes through
the load, where it loses its voltage doing work, and returns to the
stator to repeat the process. The fact that a.c. reverses direction
periodically does not alter this process. Regardless of whether the
current is positive or negative, it picks up voltage in the alternator
and spends voltages in the load.
MORE ON DESIGN
Different electrical loads require the employment of generators
of varying design. An a-c load requires an alternator - a d-c load requires
a generator. More specific requirements are met by utilizing various
connection patterns within the generator. These more complex jobs are
too advanced for this basic book. You have the PRINCIPLES -you'll get
the details in the book for YOUR rating.
Chapter 14 Quiz