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Here is the "Electrician's Mate 3 - Navy Training Courses"
(NAVPERS 10548) in its entirety (or will be eventually). It should provide one of
the Internet's best resources for people seeking a basic electricity course - complete
with examples worked out. See
copyright.
See Table of Contents.
¶ U.S. GOVERNMENT PRINTING OFFICE;
1949
CHAPTER 4
INDUCTION
ELECTRICITY FROM MAGNETISM
(* VERY IMPORTANT
NOTE: Current flow here is defined as from negative to positive, which is
opposite of today's convention. Modern convention of positive-to-negative
current flow, with negative-to-positive electron flow requires a
"right-hand rule." See
Right-Hand Rule page on RF Cafe).
To produce an electric current a source of emf is necessary. The oldest and simplest known source of emf is
the piling up of a static charge; that is, generating electric charges by friction and storing up these charges in
a condenser. While it is possible to build up static charges to potentials of many thousand volts, these charges
cannot be used to provide power for electric motors and the like because very little energy is stored up in static
charges. Hence, they possess no reserve or energy to keep the electric current flowing, and once the spark has
jumped the gap the potential falls to almost zero and the flow of electrons practically stops.
As you
learned in Basic Electricity, static charges can also be created continuously by a number of different
electrostatic machines, but the rate of building is too slow to make these machines of practical value as a source
of emf.
For many years, primary cells were the only practical source of emf, but these were able to
produce only small currents-currents strong enough to power only low-current devices such as the telegraph and
telephone. The electric motor, heating coils, and the hundred-and-one other heavy current-consuming devices common
today were not possible since batteries could not furnish the required amount of current.
All the modern
electrical machines had to wait until a cheaper, more efficient, and larger source of emf was found. This came
about when the principle of INDUCED ELECTROMOTIVE FORCES, or ELECTROMAGNETIC INDUCTION was discovered by Michael
Faraday in 1831. Though the development of this principle of obtaining electricity from magnetism came about
little more than 100 years ago, such rapid progress has since been made in designing better and larger generators
based on this principle that now millions of kilowatts of electric power are delivered by these machines daily to
homes and industry. And in the Navy generators based on the induction principle discovered by Faraday have been so
far developed that those used in battleships and carriers could supply whole cities of moderate size with all the
electric power necessary to run their factories, operate their street cars, and meet all the other electric power
requirements of the community.
The story of how a generator does this starts with the following
illustration.
Figure 56. - An emf is produced when a conductor cuts a magnetic field.
In figure 56, a GALVANOMETER, an instrument that shows DIRECTION and AMOUNT of current, is connected to a
conductor. When the CONDUCTOR is moved DOWNWARD, into the field between the poles of a magnet, the galvanometer
NEEDLE is DEFLECTED, indicating a current to be flowing in the conductor. The current produced flows in such a
direction that the needle is deflected from the center of the scale TO THE RIGHT, as the figure shows. When the
CONDUCTOR is moved UPWARD, out of the field, the NEEDLE is DEFLECTED in the OPPOSITE DIRECTION, indicating that
the current has been reversed. When the CONDUCTOR is held MOTIONLESS, NO DEFLECTION is present; but MOVING the
conductor QUICKLY through the field produces a LARGE DEFLECTION of the needle, and a SLOW MOVEMENT produces a
SMALL DEFLECTION.
Here is what you have observed:
A downward movement of the conductor causes a deflection of the galvanometer
in one direction.
An upward movement causes a deflection in the opposite direction.
The faster the movement, the
greater the deflection. No movement, no deflection.
Figure 57. - Direction of induced emf depends on direction of cutting a magnetic
field.
You therefore conclude, as Faraday did when he discovered this phenomenon, that: MOVING A CONDUCTOR ACROSS A
MAGNETIC FIELD GENERATES AN EMF which produces a current in the conductor; THE FASTER THE CONDUCTOR MOVES, THE
GREATER THE EMF PRODUCED and therefore the greater the current; and REVERSING THE DIRECTION OF MOVEMENT OF THE
CONDUCTOR REVERSES THE EMF and therefore also reverses the current.
INDUCTION
The phenomenon of generating an emf and thus causing a current to flow in a conductor when it cuts across a
magnetic field, as illustrated in figures 56 and 57, is called INDUCTION. Although in these illustrations the
conductor moved and the field stood still, a VOLTAGE CAN ALSO BE INDUCED BY MOVING THE FIELD AND HOLDING THE
CONDUCTOR STATIONARY. Thus, INDUCTION will take place whenever you have a MAGNETIC FIELD, a CONDUCTOR and a
RELATIVE MOVEMENT exists between the two.
WHICH WAY AND HOW MUCH
There are two points about the phenomenon of induction that are of considerable importance to an electrician's
mate. The first is, IN WHICH DIRECTION will the current flow? And the second is, HOW MUCH current will flow? The
first can be answered using a simple hand rule, but the second requires the use of delicate measuring instruments.
HAND RULE FOR GENERATORS
The direction of the induced current depends on the direction of the magnetic field, and the direction of
movement of the conductor relative to the field. Notice that this makes THREE "directions" involved in determining
the direction of an induced current. They are- The direction in which the CONDUCTOR is MOVING relative to the
flux.
The direction of the flux FIELD. The direction of the INDUCED EMF. All three "directions" are
inter-dependent, and are summed up in the GENERATOR HAND RULE. It states- Place the thumb, first, and middle
fingers of the left hand all at right angles to each other as in figure 58. Then the FIRST FINGER points in the
FLUX direction, the THUMB points in the direction of the MOTION of the conductor, and the MIDDLE FINGER points in
the direction of the INDUCED EMF.
Figure 58. - The generator hand rule.
Figure 59 illustrates three examples of changing one of the directions. Note that the direction of emf changes
every time either the direction of conductor motion, or direction of the magnetic field changes.
The
generator hand rule will tell you the third "direction" anytime you know the other two. Sometimes it will be
difficult to get your finger lined up with the known directions but just remember that it makes no difference
whether you face the conductor, stand to one side of the conductor, or turn your back to the conductor. As long as
your thumb points in the direction of motion, your first finger in the direction of the flux, then your middle
finger must point in the direction of the induced emf. Stand on your head if you must-but get those fingers lined
up! It might help you to construct a drawing like figure 60. Draw a circle for the cross-section of the conductor.
Then run arrows out in the direction of the flux and the motion. You can apply :he generator hand rule directly to
the diagram. YOUR middle finger tells you whether a dot (.) or a cross (+) goes ill the cross-section of the wire.
135. DIRECTION OF FLUX DIRECTION OF FLUX (C)
Figure 59. - Generator hand rule, examples.
The left hand rule is important because you will use it many times in your work on generators to determine the
direction of the current. It can also be used to determine the polarity of a generator field if you know the
direction of the current and direction of rotation of the armature.
Figure 60. - Model for the generator hand rule.
STRENGTH OF INDUCED EMF'S
You learned in an earlier paragraph that the faster the conductor moves across a magnetic field, the greater
the induced emf. In moving across a magnetic field the conductor is cutting lines of force. So another way to say
this is:
THE MORE FLUX LINES A CONDUCTOR CUTS EACH SECOND THE GREATER IS THE INDUCED EMF.
It has
been calculated that 100,000,000 LINES of flux must be CUT PER SECOND to PRODUCE ONE VOLT. Then 200,000,000 lines
cut per second would produce 2 volts, etc. This is a lot of flux to cut in one second, so to make generators which
produce appreciable voltages they must have powerful magnetic fields. The fields produced by permanent magnets are
relatively weak, but with electromagnets enormously powerful fields can be produced. That is why the field magnets
of all generators are electromagnets.
METHODS OF INCREASING INDUCED EMF'S
This principle, THE NUMBER OF FLUX LINES CUT PER SECOND, is the key to all the possible methods for increasing
the voltage output of a generator. There are three: 1) cut ;he lines faster, by increasing the speed of movement
of the conductor; 2) cut more lines with the same conductor, which means increasing the strength of the magnetic
field; and 3) cut the lines with more than one conductor, which means making the conductor into a coil so that the
field is cut by many lengths of the same conductor, instead of by a single length.
Method (1) is
accomplished by increasing the speed of rotation of the generator. This explains the increased 70ltage output of
an automobile or motor-launch generator when the engine is raced.
Method (2) is accomplished by increasing the
ampere-turns of the electromagnet producing the field. This can be done either by increasing the field current, or
by increasing the number of turns of the magnet coil.
Method (3) works because when a conductor is formed
into a coil, each turn is in series with the other turns. There£ore, the voltage generated in each turn is added
to the voltages generated in the other. Thus, suppose one conductor cutting a field produces 10 volts. This same
conductor, coiled into 5 turns, and cutting the same field, will produce 50 volts.
MUTUAL INDUCTION
"Mutual" means that something is shared. MUTUAL INDUCTION means that two circuits share the energy of one.
Strictly speaking, it means an electrical phenomenon in which, energy is transferred from one circuit to another.
How it is done is shown in figure 61. Coil A is the PRIMARY circuit and gets its energy from the battery. When the
switch is closed the current starts to flow and a magnetic field expands out of coil A. Coil A thus changes the
ELECTRICAL energy of the battery into the MAGNETIC energy of a magnetic field. When the field of coil A is
expanding it cuts across coil B (the SECONDARY circuit), inducing an emf in coil B. The galvanometer in circuit B
is then deflected, indicating the current produced by the induced emf.
Here is an interesting fact - the
induced voltage MIGHT have resulted from moving coil B through the flux of coil A. But the voltage was induced
WITHOUT moving coil B. When the switch to A was open, A had no current and no field. But as soon as the switch was
closed, current surged through the coil and a field blossomed out. This expanding field "moved" across the wires
of coil B-thus lines of force were cut and a voltage was induced, WITHOUT MOVING COIL B.
In a small
fraction of a second the field expands to its maximum strength and remains constant as long as the full current
flows. "Moving" of flux lines across coil B stops and induction ceases-as shown by galvanometer returning to zero.
If the switch is opened, the field collapses back to the wires of coil A. Again the changing flux "moves" across
the wires of coil B, but in the opposite direction. The galvanometer needle now deflects, but in the opposite
direction, indicating that a voltage has again been induced, but in the reverse direction. The important point for
you to see here is that INDUCTION OCCURS ONLY WHEN A FIELD IS CHANGING - either building up or collapsing, and
that a CHANGING FIELD PRODUCES INDUCED EMF'S EXACTLY AS A FIELD MOVING ACROSS A CONDUCTOR. This principle of
generating induced voltage by holding the coils steady and forcing the field to change is used in a vast number of
elec6cal devices. The spark coil in an automobile and the transformer are the most common.
Figure 61. - Mutual induction circuits.
You can always spot a MUTUAL INDUCTION set-up by its TWO circuits. One circuit - the primary - gets its energy
from a voltage source (generator or battery) and the other circuit - the secondary - gets its energy by induction
from the field of the primary.
In summary, there are then two methods of generating INDUCED emf's:
When lines of force are cut by a conductor, due to movement of the field or the conductor.
This is the
principle underlying all generators.
When the current in a primary circuit changes, and its flux lines
cut across the conductors
of the secondary. This principle underlies all transformers, induction coils, etc.
LENZ'S LAW
The principle that a conductor carrying a current is surrounded by a magnetic field holds true for currents
resulting from induced emf's as well as for currents from any other source. To illustrate this, look at the four
drawings in DOWNWARD PUSH N as A N figure 62. The first diagram, A, shows a conductor at rest in a stationary
magnetic field. Being stationary, the conductor has no induced emf and no current. The second diagram, B, shows
the conductor being pushed downward. Note that two items have been added-the downward push and the resulting
induced current in the conductor. Since a magnetic field surrounds every conductor carrying current, the conductor
will have a field of its own due to the induced current, and the generator hand rule tells you that this field
will be in a counter-clockwise direction, as shown in the third diagram, C. There are now TWO fields surrounding
the conductor-the one from the MAGNET and the one from the induced CURRENT in the conductor. The first is a
straight line field travelling from the North pole to the South pole. The second is a circular field surrounding
the conductor.
Figure 62. - Lenz's law.
Because magnetic lines never cross, the lines of these two fields must either CROWD TOGETHER producing a
STRONG resultant field, or they must CANCEL, producing a WEAK resultant field. A of figure 63 shows what happens
ABOVE the wire. The two magnetic fields meet head-on and CANCEL each other. The cancellation of flux lines results
in a WEAK field ABOVE the conductor.
Figure 63. - Lenz's law-action between conductor and magnetic fields.
B of figure 63 shows that BELOW the wire the two magnetic fields are in the same direction, so their fluxes
ADD. This addition of flux lines results in a STRONG field BELOW the conductor.
Thus as a result of the
induced current, the magnet field is distorted by the field around the conductor, resulting in a weak field above
and a strong field below the conductor. Now remember that flux lines tend to push each other apart.
Therefore, as you can see in figure 62D, the flux LINES BELOW the conductor, pushing each other apart, tend to
PUSH the conductor UP, while the LINES ABOVE the conductor tend to PUSH it DOWN. But there are more flux lines
below the conductor, so the upward push is greater, and the conductor has magnetic force tending to move it up.
All these conditions are summed up in D of figure 62. Better review them. Refering to figures 62 and 63, you
observe -
The DISTORTED FIELD resulting from the combination of the straight field of the poles
and the circular field of the induced current in the conductor.
The DOWNWARD FORCE applied by a push on
the conductor.
The UPWARD FORCE which results from the distorted field.
These facts tell you that whenever you
apply a push to shove a conductor across a magnetic field, there is induced a current which sets up a field that
tries to move the conductor back against the push. This is Lenz's law: IN ALL CASES OF ELECTROMAGNETIC INDUCTION,
THE DIRECTION OF THE INDUCED EMF IS SUCH THAT THE MAGNETIC FIELD SET UP BY THE RESULTING CURRENT TENDS TO STOP THE
MOTION WHICH IS PRODUCING THE EMF.
LENZ'S LAW AND THE OPERATION OF GENERATORS
Let's look into this action a little further. Suppose you try to push a conductor DOWNWARD through a magnetic
field. Immediately the induced current sets up a field that tries to push the conductor UPWARD (Fig. 62D). The
force YOU use in the DOWNWARD push is bucked by the magnetic UPWARD push. If you push harder, the conductor goes
faster, cuts magnetic lines faster, and so has a higher induced emf. But this only produces more induced current
and a stronger conductor field. Consequently, a stronger UPWARD force is automatically produced to buck the
stronger DOWNWARD force.
So, if you push a conductor faster across a magnetic field t0 produce more electrical output, you have to
push harder on the conductor. Practically this means that, to get more electrical output from a generator, you
have to use more power to drive the generator. For example, if the generator is driven by a steam engine, you must
increase the steam flow to increase the generator output.
Here's another example. Have you ever heard a
motor-driven welding generator? When the welding arc is struck the driving motor whines and labors. Lenz's law is
working. The arc increased the generator's current output and the driving motor is working against the increased
opposing force which was set up by this increased current. The motor must increase its input to balance the
increased output for the arc.
SELF INDUCTION
There are only three items required to generate an induced voltage-a conductor, a magnetic field, and moving
or changing flux lines of this field cutting across the conductor. These three items give you LINES OF FORCE CUT
BY A CONDUCTOR. Look at figure 64. Are these three items present in this circuit?
Conductors? - the coil
has plenty of them.
Magnetic field? - the coil sets it up whenever current flows.
Flux lines
cutting the conductor? - occurs only when the field is changing.
Figure 64. - Self-induction circuit.
To make the field change, all you have to do is open and close the switch. This kind of induction is called
SELF-INDUCTION, and here is how it works: At the instant the switch is closed the current starts and magnetic
lines expand from the center of each conductor. As these lines blossom outward, they cut across the other
conductors of the coil. An emf is induced in each conductor cut by the flux.
Figure 65 shows an
enlargement of only two turns of the coil in figure 64. Flux is pictured blossoming out from one of the turns.
Notice how t
hese lines cut the next turn.
Figure 65. - Self-induction in one turn.
Figure 66. - Self-induction fields around conductors.
Now, applying the generator hand rule, determine the direction of the induced voltage in the second turn. It's
easier to use the rule on a cross-section of the coil as in figure 66. Flux direction is down (first finger).
Motion is to the right thumb). (Attention - the flux is moving across the. conductor to the left - the effect is
the same as though the conductor were moving to the right.) Induced voltage (middle finger) is in a direction into
the paper. But the battery voltage is producing current flow out. Hence that is exactly what happens. THE INDUCED
VOLTAGE OPPOSES THE FLOW OF CURRENT.
What happens when the switch is opened? The field collapses and cuts
across the conductor in the opposite direction. Because the direction of motion has reversed, the induced emf is
now IN. Thus, IN A COLLAPSING FIELD, THE INDUCED EMF AIDS THE FLOW OF CURRENT.
These are the
characteristics of self-induction -
Any coil will INDUCE a VOLTAGE in itself WHENEVER its CURRENT value CHANGES.
When the CURRENT is
INCREASING (field expanding), the INDUCED EMF OPPOSES
CURRENT FLOW.
When the CURRENT IS
DECREASING (field contracting), the INDUCED EMF AIDS
the CURRENT flow.
This is in effect, just another case of Lenz's law. Only in place of the force moving the conductor you have
the applied voltage from the battery, and in place of the bucking force you have the self-induced voltage. Hence
for self-induction, Lenz's law states: the induced voltage OPPOSES the applied voltage when the current is
INCREASING and AIDS the applied voltage when the current is DECREASING. Or, in self-induction the INDUCED VOLTAGE
OPPOSES ANY CHANGES IN THE CURRENT value.
SELF-INDUCTION - THE HARM IN IT
The voltage of self induction can be very troublesome. Imagine that you are operating the switch controlling
the field coils on a large motor. These coils have thousands of turns. When the switch is closed, the voltage of
self induction does little damage. It opposes the increase of current flow for an instant (perhaps 0.1 second),
but as soon as the field is built up and stationary, the induced voltage ceases. On the other hand, when the
switch is opened, the field rapidly contracts. The induced voltage on collapse may be hundreds of times as strong
as the applied voltage. This tremendous induced voltage drives current across the opening switch terminals in the
form of an arc - it CAN burn both the operator and the switch very badly. All switches subject -- high induced
voltages are protected by discharge rheostat to absorb and dissipate the induced voltage, which might otherwise
cause dangerous arcs.
SELF-INDUCTION - THE GOOD IN IT
The voltage of self-induction can also be very useful. Imagine that you close the switch to energize the
sucking coil of a relay. The instant the switch is closed the current starts to rush in, but this produces a large
bucking voltage of self-induction which largely cancels the applied voltage, E. Very little voltage is therefore
left to force current through the circuit, and so only a small current flows through the relay a immediately after
the switch is closed.
As the voltage of self-induction dies down, the voltage applied to the coil circuit
comes up to the full applied voltage, E, and the current builds up GRADUALLY to the full value given by I=E/R.
Since the sucking coil will not pull in the iron core until nearly the full I=E/R current is flowing, there is a
TIME DELAY between the closing of the circuit of the relay coil and the pull-in of the relay core. The greater the
coil's inductance, the greater this time delay.
Time delays are necessary between the opening and closing
of different circuits for motor starting, for motor stopping, for the safe operation of much other electrical
equipment aboard ship. You will therefore run across time delay relays in nearly all the electrical equipment you
work on. Remember, the time delay depends on the inductance of the relay coil; so if you have to replace the coil
of one of these
BE SURE THE REPLACEMENT COIL HAS THE RIGHT INDUCTANCE to produce the required time delay.
NONINDUCTIVE COILS
Some electrical circuits require NONINDUCTIVE coils for operation. You say how come? How is it possible to
have a noninductive coil since all conductors carrying a current are surrounded by magnetic fields? Look at figure
67. The wire used to wind the coil is DOUBLED BACK, as illustrated in diagram A, so the starting end of the
conductor is next to the finishing end. The current flows from left to right in the upper wire but returns in the
OPPOSITE direction in the lower wire, therefore the MAGNETIC FIELDS in the two wires will be OPPOSITE and CANCEL
each other.
Figure 67. - Noninductive coil.
When the DOUBLED conductor is wound about a core as illustrated in figure 67B, the magnetic fields cancel each
other, resulting in a noninductive coil.
EDDY CURRENTS
A solid piece of metal usually is not thought of as a conductor in the same sense as the word "conductor" is
used with electrical circuits, but if a solid piece of metal moves through a magnetic field it too will have an
emf induced in it.
Figure 68. - Eddy currents in a disk.
In figure 68 the edge of a metal disk is placed between the poles of a horseshoe magnet. As long as the disk
is motionless, emf will be induced, but any motion by either the magnet he disk will result in flux lines being
cut by a conductor so will cause a small current to circulate within the metal. Since these currents have random
movements and follow irregular paths they are called EDDY CURRENTS.
Eddy currents obey all the usual laws
of induction. The most evident is the OPPOSITION TO MOVEMENT according to Lentz's law. As an example, if the disc
in figure 68 is being continuously rotated, the eddy currents will cause a DRAG d tend to slow down the rotation
of the disc.
Large, thick pieces of metals have stronger eddy currents than thin, small sections, because the larger cross
sections offer less resistance to the induced emf. Electrical devices like generators and motors where large
quantities of iron rotate in magnetic fields have their iron cores made of many small thin sections or LAMINATIONS
bolted together, rather than one large solid casting. The laminations do not PREVENT eddy currents, they merely
REDUCE them, because they increase the resistance of the eddy current path. The effect of lamination is
illustrated in figure 69. Notice the difference in the size of eddy currents in the two diagrams.
Figure 69. - Effect of laminations on eddy currents.
Eddy currents, like all other currents, produce heating. Heating of the iron cores of electrical motors and
generators only wastes energy and damages the insulation of the winding. It is therefore very important that the
cores of electrical machines be laminated, to reduce the eddy currents and therefore their heating effects.
Some electrical instruments such as sensitive voltmeters and ammeters use solid iron cores to INCREASE the effect
of eddy currents. This is done to produce a DAMPING EFFECT or drag, that causes the needle to come to rest QUICKLY
after energy has been applied to the meter. Without DAMPING action, the meter needle would swing back forth for
many minutes thus making the task of obtain meter readings slow and tedious.
Chapter 4 Quiz
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