Table Of ContentSixth Edition, last update July 25, 2007
2
Lessons In Electric Circuits, Volume II – AC
By Tony R. Kuphaldt
Sixth Edition, last update July 25, 2007
i
c⃝2000-2013, Tony R. Kuphaldt
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• Second Edition: Printed in September of 2000. Illustrations reworked in standard graphic
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• Fifth Edition: Printed in November 2002. New sections added, and error corrections
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• Sixth Edition: Printed in June 2006. Added CH 13, sections added, and error corrections
made, figure numbering and captions added, since the fifth edition.
ii
Contents
1
BASIC AC THEORY
1
1.1
What is alternating current (AC)? . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1.2
AC waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
1.3
Measurements of AC magnitude . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
1.4
Simple AC circuit calculations
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
1.5
AC phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20
1.6
Principles of radio
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
1.7
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
2
COMPLEX NUMBERS
27
2.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
2.2
Vectors and AC waveforms
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
2.3
Simple vector addition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32
2.4
Complex vector addition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
2.5
Polar and rectangular notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
37
2.6
Complex number arithmetic
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
42
2.7
More on AC ”polarity” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
44
2.8
Some examples with AC circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . .
49
2.9
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
55
3
REACTANCE AND IMPEDANCE – INDUCTIVE
57
3.1
AC resistor circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
57
3.2
AC inductor circuits
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
59
3.3
Series resistor-inductor circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
64
3.4
Parallel resistor-inductor circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . .
71
3.5
Inductor quirks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
74
3.6
More on the “skin effect” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
77
3.7
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
79
4
REACTANCE AND IMPEDANCE – CAPACITIVE
81
4.1
AC resistor circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
81
4.2
AC capacitor circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
83
4.3
Series resistor-capacitor circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . .
87
4.4
Parallel resistor-capacitor circuits . . . . . . . . . . . . . . . . . . . . . . . . . . .
92
iii
iv
CONTENTS
4.5
Capacitor quirks
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
95
4.6
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
97
5
REACTANCE AND IMPEDANCE – R, L, AND C
99
5.1
Review of R, X, and Z . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
99
5.2
Series R, L, and C
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
101
5.3
Parallel R, L, and C
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
106
5.4
Series-parallel R, L, and C
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
110
5.5
Susceptance and Admittance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
119
5.6
Summary
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
120
5.7
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
120
6
RESONANCE
121
6.1
An electric pendulum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
121
6.2
Simple parallel (tank circuit) resonance . . . . . . . . . . . . . . . . . . . . . . . .
126
6.3
Simple series resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
131
6.4
Applications of resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
135
6.5
Resonance in series-parallel circuits . . . . . . . . . . . . . . . . . . . . . . . . . .
136
6.6
Q and bandwidth of a resonant circuit
. . . . . . . . . . . . . . . . . . . . . . . .
145
6.7
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
151
7
MIXED-FREQUENCY AC SIGNALS
153
7.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
153
7.2
Square wave signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
158
7.3
Other waveshapes
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
168
7.4
More on spectrum analysis
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
174
7.5
Circuit effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
185
7.6
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
188
8
FILTERS
189
8.1
What is a filter? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
189
8.2
Low-pass filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
190
8.3
High-pass filters
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
196
8.4
Band-pass filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
199
8.5
Band-stop filters
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
202
8.6
Resonant filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
204
8.7
Summary
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
215
8.8
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
215
9
TRANSFORMERS
217
9.1
Mutual inductance and basic operation . . . . . . . . . . . . . . . . . . . . . . . .
218
9.2
Step-up and step-down transformers
. . . . . . . . . . . . . . . . . . . . . . . . .
232
9.3
Electrical isolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
237
9.4
Phasing
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
239
9.5
Winding configurations
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
243
9.6
Voltage regulation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
248
CONTENTS
v
9.7
Special transformers and applications . . . . . . . . . . . . . . . . . . . . . . . . .
251
9.8
Practical considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
268
9.9
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
281
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
281
10 POLYPHASE AC CIRCUITS
283
10.1 Single-phase power systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
283
10.2 Three-phase power systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
289
10.3 Phase rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
296
10.4 Polyphase motor design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
300
10.5 Three-phase Y and Delta configurations . . . . . . . . . . . . . . . . . . . . . . . .
306
10.6 Three-phase transformer circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . .
313
10.7 Harmonics in polyphase power systems . . . . . . . . . . . . . . . . . . . . . . . .
318
10.8 Harmonic phase sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
343
10.9 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
345
11 POWER FACTOR
347
11.1 Power in resistive and reactive AC circuits . . . . . . . . . . . . . . . . . . . . . .
347
11.2 True, Reactive, and Apparent power . . . . . . . . . . . . . . . . . . . . . . . . . .
352
11.3 Calculating power factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
355
11.4 Practical power factor correction . . . . . . . . . . . . . . . . . . . . . . . . . . . .
360
11.5 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
365
12 AC METERING CIRCUITS
367
12.1 AC voltmeters and ammeters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
367
12.2 Frequency and phase measurement . . . . . . . . . . . . . . . . . . . . . . . . . .
374
12.3 Power measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
382
12.4 Power quality measurement
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
385
12.5 AC bridge circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
387
12.6 AC instrumentation transducers . . . . . . . . . . . . . . . . . . . . . . . . . . . .
396
12.7 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
406
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
406
13 AC MOTORS
407
13.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
408
13.2
Synchronous Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
412
13.3 Synchronous condenser
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
420
13.4 Reluctance motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
421
13.5 Stepper motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
426
13.6 Brushless DC motor
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
438
13.7 Tesla polyphase induction motors
. . . . . . . . . . . . . . . . . . . . . . . . . . .
442
13.8 Wound rotor induction motors
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
460
13.9 Single-phase induction motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
464
13.10 Other specialized motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
469
13.11 Selsyn (synchro) motors
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
471
13.12 AC commutator motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
479
vi
CONTENTS
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
482
14 TRANSMISSION LINES
483
14.1 A 50-ohm cable? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
483
14.2 Circuits and the speed of light
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
484
14.3 Characteristic impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
486
14.4 Finite-length transmission lines . . . . . . . . . . . . . . . . . . . . . . . . . . . .
493
14.5 “Long” and “short” transmission lines . . . . . . . . . . . . . . . . . . . . . . . . .
499
14.6 Standing waves and resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
502
14.7 Impedance transformation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
522
14.8 Waveguides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
529
A-1 ABOUT THIS BOOK
537
A-2 CONTRIBUTOR LIST
541
A-3 DESIGN SCIENCE LICENSE
549
INDEX
552
Chapter 1
BASIC AC THEORY
Contents
1.1
What is alternating current (AC)?
. . . . . . . . . . . . . . . . . . . . . . . .
1
1.2
AC waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
1.3
Measurements of AC magnitude
. . . . . . . . . . . . . . . . . . . . . . . . . 12
1.4
Simple AC circuit calculations
. . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.5
AC phase
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
1.6
Principles of radio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1.7
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
1.1
What is alternating current (AC)?
Most students of electricity begin their study with what is known as direct current (DC), which
is electricity flowing in a constant direction, and/or possessing a voltage with constant polarity.
DC is the kind of electricity made by a battery (with definite positive and negative terminals),
or the kind of charge generated by rubbing certain types of materials against each other.
As useful and as easy to understand as DC is, it is not the only “kind” of electricity in use.
Certain sources of electricity (most notably, rotary electro-mechanical generators) naturally
produce voltages alternating in polarity, reversing positive and negative over time. Either as
a voltage switching polarity or as a current switching direction back and forth, this “kind” of
electricity is known as Alternating Current (AC): Figure 1.1
Whereas the familiar battery symbol is used as a generic symbol for any DC voltage source,
the circle with the wavy line inside is the generic symbol for any AC voltage source.
One might wonder why anyone would bother with such a thing as AC. It is true that in
some cases AC holds no practical advantage over DC. In applications where electricity is used
to dissipate energy in the form of heat, the polarity or direction of current is irrelevant, so
long as there is enough voltage and current to the load to produce the desired heat (power
dissipation). However, with AC it is possible to build electric generators, motors and power
1
2
CHAPTER 1. BASIC AC THEORY
I
I
DIRECT CURRENT
(DC)
ALTERNATING CURRENT
(AC)
I
I
Figure 1.1: Direct vs alternating current
distribution systems that are far more efficient than DC, and so we find AC used predominately
across the world in high power applications. To explain the details of why this is so, a bit of
background knowledge about AC is necessary.
If a machine is constructed to rotate a magnetic field around a set of stationary wire coils
with the turning of a shaft, AC voltage will be produced across the wire coils as that shaft
is rotated, in accordance with Faraday’s Law of electromagnetic induction. This is the basic
operating principle of an AC generator, also known as an alternator: Figure 1.2
N
S
+
-
Load
I
I
N
S
Load
no current!
no current!
Load
N
S
N
Load
S
-
+
I
I
Step #1
Step #2
Step #3
Step #4
Figure 1.2: Alternator operation
1.1.
WHAT IS ALTERNATING CURRENT (AC)?
3
Notice how the polarity of the voltage across the wire coils reverses as the opposite poles of
the rotating magnet pass by. Connected to a load, this reversing voltage polarity will create a
reversing current direction in the circuit. The faster the alternator’s shaft is turned, the faster
the magnet will spin, resulting in an alternating voltage and current that switches directions
more often in a given amount of time.
While DC generators work on the same general principle of electromagnetic induction, their
construction is not as simple as their AC counterparts. With a DC generator, the coil of wire
is mounted in the shaft where the magnet is on the AC alternator, and electrical connections
are made to this spinning coil via stationary carbon “brushes” contacting copper strips on the
rotating shaft. All this is necessary to switch the coil’s changing output polarity to the external
circuit so the external circuit sees a constant polarity: Figure 1.3
Load
N S
N S
-
+
+
-
I
N S
S
N
Load
Step #1
Step #2
N S
S
N
Load
N S
Load
S
N
-
-
I
+
+
Step #3
Step #4
Figure 1.3: DC generator operation
The generator shown above will produce two pulses of voltage per revolution of the shaft,
both pulses in the same direction (polarity). In order for a DC generator to produce constant
voltage, rather than brief pulses of voltage once every 1/2 revolution, there are multiple sets
of coils making intermittent contact with the brushes. The diagram shown above is a bit more
simplified than what you would see in real life.
The problems involved with making and breaking electrical contact with a moving coil
should be obvious (sparking and heat), especially if the shaft of the generator is revolving
at high speed. If the atmosphere surrounding the machine contains flammable or explosive
4
CHAPTER 1. BASIC AC THEORY
vapors, the practical problems of spark-producing brush contacts are even greater. An AC gen-
erator (alternator) does not require brushes and commutators to work, and so is immune to
these problems experienced by DC generators.
The benefits of AC over DC with regard to generator design is also reflected in electric
motors. While DC motors require the use of brushes to make electrical contact with moving
coils of wire, AC motors do not. In fact, AC and DC motor designs are very similar to their
generator counterparts (identical for the sake of this tutorial), the AC motor being dependent
upon the reversing magnetic field produced by alternating current through its stationary coils
of wire to rotate the rotating magnet around on its shaft, and the DC motor being dependent on
the brush contacts making and breaking connections to reverse current through the rotating
coil every 1/2 rotation (180 degrees).
So we know that AC generators and AC motors tend to be simpler than DC generators
and DC motors. This relative simplicity translates into greater reliability and lower cost of
manufacture. But what else is AC good for? Surely there must be more to it than design details
of generators and motors! Indeed there is. There is an effect of electromagnetism known as
mutual induction, whereby two or more coils of wire placed so that the changing magnetic field
created by one induces a voltage in the other. If we have two mutually inductive coils and we
energize one coil with AC, we will create an AC voltage in the other coil. When used as such,
this device is known as a transformer: Figure 1.4
Transformer
AC
voltage
source
Induced AC
voltage
Figure 1.4: Transformer “transforms” AC voltage and current.
The fundamental significance of a transformer is its ability to step voltage up or down from
the powered coil to the unpowered coil. The AC voltage induced in the unpowered (“secondary”)
coil is equal to the AC voltage across the powered (“primary”) coil multiplied by the ratio of
secondary coil turns to primary coil turns. If the secondary coil is powering a load, the current
through the secondary coil is just the opposite: primary coil current multiplied by the ratio
of primary to secondary turns. This relationship has a very close mechanical analogy, using
torque and speed to represent voltage and current, respectively: Figure 1.5
If the winding ratio is reversed so that the primary coil has less turns than the secondary
coil, the transformer “steps up” the voltage from the source level to a higher level at the load:
Figure 1.6
The transformer’s ability to step AC voltage up or down with ease gives AC an advantage
unmatched by DC in the realm of power distribution in figure 1.7. When transmitting electrical
power over long distances, it is far more efficient to do so with stepped-up voltages and stepped-
down currents (smaller-diameter wire with less resistive power losses), then step the voltage
back down and the current back up for industry, business, or consumer use.
Transformer technology has made long-range electric power distribution practical. Without
1.1.
WHAT IS ALTERNATING CURRENT (AC)?
5
+
+
Large gear
Small gear
(many teeth)
(few teeth)
AC
voltage
source
Load
high voltage
low current
low voltage
high current
many
turns
few turns
Speed multiplication geartrain
"Step-down" transformer
high torque
low speed
low torque
high speed
Figure 1.5: Speed multiplication gear train steps torque down and speed up. Step-down trans-
former steps voltage down and current up.
+
+
Large gear
Small gear
(many teeth)
(few teeth)
AC
voltage
source
Load
low voltage
high current
high voltage
low current
few turns
many turns
Speed reduction geartrain
"Step-up" transformer
low torque
high speed
high torque
low speed
Figure 1.6: Speed reduction gear train steps torque up and speed down. Step-up transformer
steps voltage up and current down.
Step-up
Step-down
Power Plant
low voltage
high voltage
low voltage
. . . to other customers
Home or
Business
Figure 1.7: Transformers enable efficient long distance high voltage transmission of electric
energy.
6
CHAPTER 1. BASIC AC THEORY
the ability to efficiently step voltage up and down, it would be cost-prohibitive to construct
power systems for anything but close-range (within a few miles at most) use.
As useful as transformers are, they only work with AC, not DC. Because the phenomenon of
mutual inductance relies on changing magnetic fields, and direct current (DC) can only produce
steady magnetic fields, transformers simply will not work with direct current. Of course, direct
current may be interrupted (pulsed) through the primary winding of a transformer to create
a changing magnetic field (as is done in automotive ignition systems to produce high-voltage
spark plug power from a low-voltage DC battery), but pulsed DC is not that different from
AC. Perhaps more than any other reason, this is why AC finds such widespread application in
power systems.
• REVIEW:
• DC stands for “Direct Current,” meaning voltage or current that maintains constant po-
larity or direction, respectively, over time.
• AC stands for “Alternating Current,” meaning voltage or current that changes polarity or
direction, respectively, over time.
• AC electromechanical generators, known as alternators, are of simpler construction than
DC electromechanical generators.
• AC and DC motor design follows respective generator design principles very closely.
• A transformer is a pair of mutually-inductive coils used to convey AC power from one coil
to the other. Often, the number of turns in each coil is set to create a voltage increase or
decrease from the powered (primary) coil to the unpowered (secondary) coil.
• Secondary voltage = Primary voltage (secondary turns / primary turns)
• Secondary current = Primary current (primary turns / secondary turns)
1.2
AC waveforms
When an alternator produces AC voltage, the voltage switches polarity over time, but does
so in a very particular manner. When graphed over time, the “wave” traced by this voltage
of alternating polarity from an alternator takes on a distinct shape, known as a sine wave:
Figure 1.8
In the voltage plot from an electromechanical alternator, the change from one polarity to
the other is a smooth one, the voltage level changing most rapidly at the zero (“crossover”)
point and most slowly at its peak. If we were to graph the trigonometric function of “sine” over
a horizontal range of 0 to 360 degrees, we would find the exact same pattern as in Table 1.1.
The reason why an electromechanical alternator outputs sine-wave AC is due to the physics
of its operation. The voltage produced by the stationary coils by the motion of the rotating
magnet is proportional to the rate at which the magnetic flux is changing perpendicular to the
coils (Faraday’s Law of Electromagnetic Induction). That rate is greatest when the magnet
poles are closest to the coils, and least when the magnet poles are furthest away from the coils.
