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Contents. • U.S. Government Printing Office; 1945 - 618779
In keeping with a very common practice of using
water flowing through a garden hose as a teaching aid analogy for newcomers, the
title of this chapter of the NAVPER 10622 Basic Navy Training Courses, "Generators
- Electrical Pumps," is likening electrical generators to water pumps. It is an
apt analogy, but whereas a water pump can cause water pressure and flow of a physical
substance already on-hand, electrical "pumps," aka generators, literally creates
its "flow" from thin air (even a vacuum with no air). An electrical generator exploits
the phenomenon discovered by (or at least credited to)
Michael Faraday whereby a conductor moving through a magnetic
field - or a magnetic field moving past a conductor - induces within that conductor
a current whose direction and intensity (where the symbol for current, "I" comes
from) is governed by
Faraday's law of induction (as distinct from
Faraday's law of electrolysis).
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 fighting machine.
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 tug.
Figure 121 - Generator-magnetic field.
Figure 122 - Generator-armature.
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. 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
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. These are the elements of a generator -
1. The field produced by electromagnets.
2. The prime mover feeding mechanical energy into the generator
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 them.
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 is zero.
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
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.
But how 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.
Figure 124 - Slip ring coil revolving in magnetic field.
Figure 125 - Graph of alternating emf.
Figure 126 - Graph of direct emf.
Figure 127 - Two coil armature.
Figure 128 - Two coil voltage.
Figure 129 - Four coil armature.
Figure 130 - Four coil voltage.
Figure 131 - Gramme ring armature.
Figure 132 - Drum wound armature.
Figure 133 - Modern drum wound armature.
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.
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.
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
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 process.
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.
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 circuit. 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."
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.
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.
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 other.
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
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.
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 commutator.
Figure 133 shows a modern drum wound armature. Notice the great number of coils
and commutator segments to give this job a high and steady voltage.
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.
Figure 134 - Parts of a generator.
It's perfectly 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.
Generally 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
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 pole alternator.
Figure 135 - Four pole alternator.
Figure 136 - Schematic of stator and load.
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.
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
Posted August 13, 2019