Alternating Current Fundamentals
Transcript of Alternating Current Fundamentals
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T&D PowerSkillsGeneral Guidelines for Students
This training unit is composed of a DVD and associated Student Manual. The DVD
contains one Course. The course is divided into Lessons, where each Lesson consists ofa number ofTopics. The number of Lessons and Topics will vary with each course.
Recommended Sequence of Instruction
1. After the instructors introductory remarks, read the segment objectives found in theblock at the beginning of the first segment.
2. Briefly discuss the segment objectives with the instructor and other class members.3. View the first segment of the DVD.4. Read the text segment that corresponds to the first segment of the DVD.5. Answer the questions at the end of the text segment. Check your answers with the
correct answers provided by the instructor.
6. Participate in a class discussion of the material just covered. Ask any questions you
might have concerning the material in the DVD and the text, and note any additionalinformation given by the instructor.
7. Before proceeding, be sure you understand the concepts presented in this segment.8. Work through all segments in this manner.9. A Course Test covering all the material will be administered by the instructor upon
completion of the unit.
10.Additional instruction and testing may be provided, at the instructors discretion.
This T&D PowerSkills workbook is designed to be
used in conjunction with the associated training DVD/video.
OSHA Regulations Snap-Shot
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OSHA Regulations, primarily in 1926.955, 1910.269 and 1910.268 will be used in
conjunction with this training unit. Where applicable, regulations will be highlighted and
placed in a box like this. Instructors and students are expected to review the currentOSHA Regulations to familiarize the student with the safety requirements expected by
USDOL OSHA, specifically as they relate to the topic being discussed. This information
is an important part of this training unit.
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Field Perform anc e Require m ent s (FPR)
NAME: _____________________________ #___________Complete
Incomplete
SECTION: Maintenance Basics
UNIT(S): Alternating Current Fundamentals
VG = Very GoodACC = AcceptableNI = Needs ImprovementNA = Not Able to Complete
on this Crew
REQUIREMENTS SUPERVISOR SIGN-OFF
VG ACC NI NA
SEGMENT 1 ALTERNATING CURRENT
1.1 Can explain the differences between direct current and alternatingcurrent ..
SEGMENT 2 INDUCTANCE
2.1 Can define inductance and inductive reactance
2.2 Can differentiate between in-phase and out-of-phase current flow
SEGMENT 3 CAPACITANCE
3.1 Can describe the effects of capacitance on current and voltage ..
SEGMENT 4 AC POWER
4.1 Can differentiate among true power, reactive power and apparentpower ..
SEGMENT 5 SINGLE PHASE AND THREE-PHASE SYSTEMS5.1 Can describe the difference between single-phase and three-phase
AC systems ..
5.2 Can differentiate between delta-connected and wye-connectedthree-phase AC systems ..
______________________________ ______________________________ _______________Employees Signature Supervisors Signature Date
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Performance Notes:
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_____________________________________________________________________________
1910.269(a)(2)(vii) as of July, 2006:
The employer shall certify that each employee has received the training required by paragraph (a)(2) of
this section. This certification shall be made when the employee demonstrates proficiency in the work
practices involved and shall be maintained for the duration of the employees employment.
Note: Employment records that indicate that an employee has received the required training are an
acceptable means of meeting this requirement.
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TABLE OF CONTENTS
Section Title Page
1. Alternating Current 7
1.1 Current Flow and Polarity 7
1.2 Sine Waves 11
1.3 Peak Values, Peak-to-Peak Values, and Effective Values 14
2. Inductance 16
2.1 Inductance and Inductive Reactance 16
2.2 Factors That Affect Inductive Reactance 18
2.3 Effects of Inductance on Current and Voltage 20
3. Capacitance 24
3.1 Capacitors 25
3.2 Effects of Capacitance on Current and Voltage 27
4. AC Power 31
4.1 True Power 31
4.2 Reactive Power 324.3 Apparent Power 35
4.4 Power Factor 36
5. Single-Phase and Three-Phase Systems 38
5.1 Single-Phase Systems 39
5.2 Three-Phase Systems 40
5.2.1 Delta Connections 41
5.2.2 Wye Connections 42
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LIST OF ILLUSTRATIONS
Figure Title Page
1-1 Simple DC Current 8
1-2 Simplified AC Generator 9
1-3 Rotation of a Conductor 10 & 11
1-4 Induced Voltage Graph 12
1-5 Voltage and Current Sine Waves 13
2-1 Magnetic Field Around a Conductor 17
2-2 Magnetic Fields Around a Coiled Conductor 19
2-3 Metal Core Placed Inside Coil 20
2-4 Voltage and Current Sine Waves for a Purely Resistive Circuit 21
2-5 Voltage and Current Sine Waves for a Purely Inductive Circuit 22
3-1 Simplified Capacitor 25
3-2 Charging a Capacitor 26
3-3 Voltage and Current Sine Waves for a Purely Capacitive Circuit 29
4-1 Sine Waves for Voltage, Current, and True Power in a PurelyResistive Circuit 32
4-2 Sine Waves for Voltage, Current, and reactive Power in
a Purely Inductive Circuit 33
4-3 Sine Waves for Voltage, Current, and Reactive Power in
a Purely Capacitive Circuit 34
4-4 Circuit for Apparent Power 36
5-1 Simplified Single-Phase System 38
5-2 Simplified Three-Phase System 39
5-3 Simplified Three-Wire System 40
5-4 Delta-Connected Three-Phase System 41
5-5 Wye-Connected Three-Phase System 42
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ALTERNATING CURRENT FUNDAMENTALS
1. Alternating Current
Most of the electrical equipment used today operates on alternating current (AC). The purposeof this training unit is to review significant terms, concepts, and principles associated withalternating current. Emphasis is placed on what alternating current is, how it works, and what
factors affect the operation and maintenance of AC equipment such as motors, lights, andcommunications equipment.
1. Alternating Current
OBJECTIVES:
Explain the differences between direct current and alternating current.
Explain how current flow and polarity change in AC circuits.
Explain what frequency is and how it is measured.
Define peak value, peak-to-peak value, and effective value with respectto AC voltage and current.
1.1 Current Flow and Polarity
There are two types of current: direct current (DC) and alternating current (AC). In a
DC circuit, current flow is always in one direction. In an AC circuit, current flows first in
one direction, stops, then flows in the opposite direction.
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AC FUNDAMENTALS
1. Alternating Current (continued)
Figure 1-1 shows a simple DC circuit. It has two important parts a power source and a
load. The power source is a battery and the load is a resistor. The battery has twoterminals, one negative and one positive. Current flows from the negative terminal,through the circuit, to the positive terminal. The current flow is always in this direction.
The negative terminal in this, and all other DC power sources, is always the same
terminal and the positive terminal is always the opposite terminal. Their positions do notchange. One way of referring to this is to say that a DC power source has fixed polarity.
When a power source has fixed polarity, the current it produces always flows in the same
direction.
Figure 1-1. Simple DC Circuit
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Sim le DC Circuit
AC power sources, on the other hand, do not have fixed polarity. Their polarity changesperiodically. As the polarity of the power source changes, the direction of the current itproduces also changes.
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AC FUNDAMENTALS
1. Alternating Current (continued)
Figure 2-1 shows a simplified AC generator. This generator produces voltage by means
of induction. The three requirements for inducing voltage are: a conductor, a magneticfield, and relative motion. In this generator, a loop of wire is the conductor, and themagnetic field is provided by a permanent magnet. (The north and south poles of the
magnet are visible in the figure.) The relative motion occurs when the conductor is
rotated through the magnetic field. This simplified generator has two more components:slip rings and brushes. the slip rings are attached to the ends of the conductor; they slide
against the brushes as the conductor rotates. Current produced by the generated voltage
could flow through the brushes and through a circuit connected to the generator.
Figure 1-2. Simplified AC Generator
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The magnetic field, indicated by the blue lines in Figure 1-2, is made up of a number of
lines of flux. (For simplicity, the lines are shown as straight lines. Actually, they are
curved.) When the conductor turns, each half of the loop cuts through the magnetic linesof flux, first in one direction and then in the opposite direction. Because the conductor
moves in a circular pattern, its rotation can be demonstrated by using the 360 degrees that
make up a circle. This movement is illustrated in Figure 1-3, which shows an end view ofthe conductor, without the slip rings or brushes. For ease of explanation, the two ends of
the conductor have been labeled X and Y.
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AC FUNDAMENTALS
1. Alternating Current (continued) Figure 1-3 Rotation of Conductor
At zero degrees, the ends of the conductor
are not cutting across any of the lines offlux. Since there is no relative motion
between the conductor and the magnetic
field, no voltage is induced.
As the conductor starts to rotate, the X end
of the conductor begins to cut the magneticfield in a downward direction, while the Y
end of the conductor cuts the magneticfield in an upward direction. Now, voltage
is being induced. As the conductor moves
toward 90 degrees, more and more fluxlines are cut, so the induced voltage
increases. Maximum voltage is induced at
the instant that the conductor reaches the90-degree point.
As rotation continues towards 180 degrees,
the conductor cuts fewer and fewer flux
lines, so the induced voltage decreases.When the conductor reaches 180 degrees,
the conductor is cutting through no lines of
flux, so no voltage is induced.
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AC FUNDAMENTALS
1. Alternating Current (continued)
At 180 degrees, the polarity changes.The X end of the conductor starts cutting
the magnetic field in an upward direction,
while the Y end cuts the field in adownward direction. From 180 to 270
degrees, the conductor once again cuts
through more and more lines of flux. At
the instant that the conductor reaches 270degrees, it is again cutting the maximum
number of flux lines, so maximum
voltage is induced.
As rotation continues from 270 degreesto 360 degrees, voltage begins
decreasing, because the conductor is
cutting through few and fewer lines offlux. When the conductor completes its
rotation, at 360 degrees, no voltage is
induced, because no flux lines are being
cut.
1.2 Sine Waves
The direction in which a conductor cuts a magnetic field, or the direction in which a
magnetic field cuts a conductor, determines the polarity of the voltage that is induced. as
easy method of showing how the polarity changes is to use a graph.
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AC FUNDAMENTALS
1. Alternating Current (continued)
Figure 1-4 is a graph that represents the voltage induced as the conductor in Figure 1-3
makes a complete rotation through the magnetic field. the vertical line of the graphrepresents the magnitude of the induced voltage. Voltage that is above the horizontal lineis positive, and voltage that is below the horizontal line is negative. Voltage that is on the
horizontal line is neither positive nor negative it is zero. (On this graph, the horizontal
line also represents the time that elapses as the voltage changes.)
Figure 1-4. Induced Voltage Graph
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At zero degrees, the conductor is not cutting through the magnetic fields, so no voltage isinduced. As the conductor rotates toward 90 degrees, more and more lines of flux are
cut, so voltage increases. At 90 degrees, the induced voltage reaches its maximum
positive value. From 90 degrees to 180 degrees, voltage decreases, because theconductor cuts across fewer and fewer lines of flux. At 180 degrees, no flux lines are
being cut, so no voltage is induced. At this point, the conductor starts to cut across the
flux lines in the opposite direction. From 180 degrees to 270 degrees, voltage increasesin the negative direction. At 270 degrees, it reaches its maximum negative direction. At
270 degrees, it reaches its maximum negative value. From 270 degrees to 360 degrees,
voltage decreases again. At 360 degrees, voltage is again zero, because the conductor is
not cutting across any flux lines.
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AC FUNDAMENTALS
1. Alternating Current (continued)
The type of graph shown n Figure 1-4 is called a sinusoidal curve, a sine curve, or a sine
wave. A sine wave has no sharp bens or straight portions; it is just a smooth rise and fall.sine waves are often used to plot electrical quantities.
If the simplified generator shown in Figure 1-2 were part of a complete circuit, the
induced voltage would cause current to flow. In Figure 1-5, a current sine wave has beenadded to the voltage sine wave shown in Figure 1-4. The current sine wave represents
current flow through the complete circuit.
Figure 1-5. Voltage and Current Sine Waves
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When the conductor is at zero degrees, no voltage is induced, so no current can flow. as
the conductor begins to rotate, voltage and current increase. Both voltage and current
reach their maximum values at 90 degrees. As rotation continues, both voltage andcurrent decrease. At 180 degrees, no voltage is induced, so no current flows. From 180
degrees to 270 degrees, voltage and current increase in the negative direction, reachingtheir maximum negative values at 270 degrees. Finally, as the conductor moves from270 to 360 degrees, voltage and current decrease. At 360 degrees, both voltage and
current are again zero.
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AC FUNDAMENTALS
1. Alternating Current (continued)
In this example, each time the conductor rotates a full 360 degrees, it completes a cycle.
In a typical AC power system, 60 cycles are completed every second. The number ofcycles completed each second by a given AC voltage is called frequency. Frequency ismeasured in units called hertz. Sixty cycles per second can, therefore, be referred to as
60 hertz, or 60 Hz.
1.3 Peak Values, Peak-to-Peak Values, and Effective Values
The amount of voltage or current at the maximum positive or negative point on a sine
wave is called the peak value of the voltage or current. Peak values occur twice in eachcycle: once positive and once negative. The amount of voltage or current represented by
the distance between the positive peak and the negative peak is called the peak-to-peak
value.
Peak values and peak-to-peak values are not commonly used for AC current or voltage
except when designing AC equipment (for example, the insulating rating on equipment isbased on peak voltage). Most often, effective values are use. The reason for using
effective values instead of peak values is that alternating current does not maintain a
constant value. It does not build up to a peak and then stay there, like direct current does.
A peak AC value is not equivalent to a DC value with the same numbers: 120 volts peakAC voltage is not the same as 120 volts DC. When scientists conducted tests to find the
exact relationship between peak AC values and DC values, they discovered that one
ampere, peak value, of alternating current produces the same heating effect as .707amperes of direct current. This relationship, which applies to both current and voltage(because voltage produces current) is the basis for effective values. Effective AC values
are equal to peak AC values multiplied by .707.
Effective values for AC are often called RMS values. RMS stands for root-mean-square,
which refers to the mathematical formula used to determine effective values. The
formula itself is not important here. What is important is to understand that RMS valuesare used to rate operating voltages on almost all AC equipment. Most meters read RMS
values, too. Unless the data plate on a meter or piece of equipment indicates otherwise,
all AC values are RMS (effective) values.
In summary, Peak AC values and peak-to-peak AC values are related as follows:
Peak-to peak = 2 x peak
And RMS values are related to peak values like this:
RMS = Peak x .707
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AC FUNDAMENTALS
1. Alternating Current (continued)
Questions
1-1. The ____________ of an AC power source changes periodically.
1-2. List the three requirements for inducing a voltage.
a. ___________________________________
b. ___________________________________
c. ___________________________________
1-3. Circle the correct answer.
When a conductor rotating in a magnetic field is cutting through the maximumnumber of flux lines,
a. The conductor stops moving
b. No voltage is induced
c. Maximum voltage is inducesd. None of the above
1-4. True or False. When a sine wave is used to represent voltage, voltage below the
horizontal line is negative
1-5. The number of cycles completed each second by a given AC voltage is called
(a) _____________ and is measured in (b) ______________.
1-6. The letters used to express effective AC values are __________________.
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AC FUNDAMENTALS
2. Inductance
OBJECTIVES:
Define inductance and inductive reactance.
Explain how inductive reactance limits current flow.
Differentiate between in-phase and our-of-phase currents and voltages.
Ohms Law states that current is equal to voltage divided by resistance. In DC circuits,
Ohms Law holds true for all applications, because the only two factors that affect DC
current are resistance and voltage. AC current, however, is affected by additional factors,which must be taken into account. Like DC current, AC current is affected by voltage
and resistance, but AC current is also affected by inductance and capacitance. Inductance
is covered i this section; capacitance is covered in Section 3.
2.1 Inductance and Inductive Reactance
Inductance is a physical property of all AC circuits that opposes any change in current
flow. It is measured in units called henrys. The symbol for inductance is a capital L.
Inductive reactance is the measure of the opposition to current flow that is created byinductance. Since inductive reactance, like resistance, limits current flow, it is measuredin ohms.
The common symbol for inductive reactance is XL. The value of inductive reactance, inohms, can be calculated by using the following formula:
XL = 2 f L
where: is the constant 3.14f is the frequency, in hertz
L is the inductance, in henrys
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AC FUNDAMENTALS (continued)
2. Inductance
To understand how inductive reactance limits current flow, it is first necessary tunderstand a process call self-induction. Current flow is actually limited by an induced
voltage that opposes the applied voltage. This induced voltage is called counter voltageor, more commonly, counter electromotive force (CEMF). Counter electromotive forceis caused by self-induction, which is the induction of voltage i a conductor by AC current
following through that same conductor.
When voltage is applied to a conductor, current starts to flow through the conductor. The
current flow causes a magnetic field to build up around the conductor, as shown in Figure
2-1. The magnetic field continues to expand outward from the center of the conductor
until the current that is producing it reaches its peak value.
Figure 2-1 Magnetic Field Around a Conductor
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Conductor
While the magnetic field is building up, there is relative motion between it and the
conductor, because the field itself is moving. And, whenever there is a conductor, amagnetic field, and relative motion, voltage is induced. In this case, as the magnetic field
builds up, its motion induces a voltage in the conductor. Since the current-carrying
conductor is inducing a voltage in itself, the process is called self-induction. the inducedvoltage, which is opposite in polarity to the applied voltage, is counter electromotive
force. Because the CEMF opposes the applied voltage, it limits current.
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AC FUNDAMENTALS (continued)
2. Inductance
After the current flowing through the conductor reaches its peak value, it decreases untilit reaches zero. The decreasing current causes the magnetic field to collapse. The motion
of the magnetic field collapsing is opposite to the motion of the field building up. Sincethe motion is opposite, the self-induction is also opposite. The voltage that is induced inthe conductor now has the same polarity as the applied voltage.
When the magnetic field has collapsed completely, there is no motion and therefore, noself-induction. Then, as the current again increases towards its peak value, this time with
the opposite polarity, the magnetic field again builds up. Whenever there is a change in
the current, the magnetic field also changes. The voltage that is induced in the conductor
by the changing magnetic field is always in such a direction as to oppose the currentchange. So, if current is trying to increase, the induced voltage opposes the applied
voltage; if current is trying to decrease, the induced voltage aid the applied voltage.
2.2 Factors that affect Inductive Reactance
There are several factors that affect the amount of inductive reactance the amount that
current flow is limited by inductance in a circuit. For example, inductive reactance canbe increased by coiling a conductor. It can be increased even more by placing a metal
core inside the coil. The basis behind both of these factors is that the number of lines offlux that cut a conductor affects the amount of inductive reactance. Anything that
increases the number of magnetic lines of flux cutting a conductor increases the inductive
reactance. Likewise, anything that decreases the number of magnetic lines of flux cuttinga conductor decreases the inductive reactance.
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AC FUNDAMENTALS (continued)
2. Inductance
A conductor that is wound into a coil provides more inductive reactance than a straight
conductor. A straight conductor is cut only once by its magnetic field when the magneticfield changes. In a coiled conductor, as shown in Figure 2-2, the magnetic field from
each turn cuts across the other turns. The more turns there are in the coil, the higher theinductive reactance will be.
Figure 2-2. Magnetic Fields Around a Coiled Conductor
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AC FUNDAMENTALS (continued)
2. Inductance
The inductive reactance that a coil provides can be further increased by placing a metal
core inside the coil, as shown in Figure 2-3. The magnetic field produced by the coil isconcentrated and directed by the metal core, also more line of flux cut across the
conductor as current changes.
Figure 2-3. Metal Core Placed Inside Coil
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Since inductive reactance limits current, a change in inductive reactance also means achange i current. Increasing the inductive reactance decreases the current, and decreasing
the inductive reactance increases the current.
2.3 Effects of Inductance on Current and Voltage
The current and voltage sine waves shown in Section 1 were associated with a circuit thathad only resistance as a current-limiting factor. Any inductance in the circuit was
considered to be so small that it was insignificant. Such a circuit is called a purely
resistive circuit. the term purely resistive means that resistance is the only factor thatlimits current flow.
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AC FUNDAMENTALS (continued)
2. Inductance
In purely resistive circuits, when voltage increases, current also increases. Since voltageand current stay together, always increasing or decreasing in the same direction at the
same time, they are said to be in phase. Figure 2-4 shows in-phase voltage and currentsine waves for a purely resistive circuit.
Figure 2-4. Voltage and Current Sine Waves for a Purely Resistive Circuit
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AC FUNDAMENTALS (continued)
2. Inductance
Figure 2-5 shows sine waves for voltage and current in a purely inductive circuit. Apurely inductive circuit is a circuit that has only inductive reactance as a current-limiting
factor. Actually, there is no circuit that is purely inductive, because there is always someresistance in a circuit. However, the idea of a purely inductive circuit is helpful inunderstanding the effects of inductance on the relationship between voltage and current.
Figure 2-5. Voltage and Current Sine Waves for a Purely Inductive Circuit
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In a purely inductive circuit, when voltage starts to increase, current does not changeright away. The counter EMF keeps current from increasing immediately. Therefore, the
increase in current takes place later than the increase in voltage.
When voltage starts to decrease the induced voltage opposes a decrease in current.
Therefore, the decrease in current takes place later than the decrease in voltage. Because
the changes in current always take place later than the changes in voltage, it can be said
that current lags behind voltage by 90 degrees during the entire cycle.
Whenever voltage and current increase and decrease at different time, they are said to be
out of phase. In a purely inductive circuit, inductance causes voltage and current to be
out of phase.
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AC FUNDAMENTALS (continued)
2. Inductance
Questions
2-1 The physical property of all AC circuits that opposes any change in ___________
is called inductance.
2-2. _________________________ is the measure of the opposition to current flow
that is created by inductance.
2-3 Circle the correct answer.When current flowing through a conductor is increasing,
a. The magnetic field around the conductor is building up
b. The magnetic field around the conductor is collapsingc. No voltage is induced
d. There is no magnetic field around the conductor
2-4 True or False. When CEMF opposes the applied voltage in an AC circuit, it hasthe effect of increasing current.
2-5 List two ways to increase inductive reactance in an AC circuit.
a. ____________________________________________________
b. ____________________________________________________
2-6 In a purely inductive circuit, ______________________ is the only current-
limiting factor.
2-7 Whenever voltage and current increase or decrease at different times, they are
said to be _______________.
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AC FUNDAMENTALS (continued)
3. Capacitance
OBJECTIVES:
Define capacitance and capacitive reactance.
Name the basic components of a capacitor.
Explain the effects of capacitance o current and voltage.
Capacitance is a physical property of all AC circuits that opposes a change in voltage.Capacitance is measured in units called farads. The symbol for capacitance is a capital C.
Capacitive reactance is the measure of the opposition to current flow that is created by
capacitance. Capacitance and capacitive reactance are related in the same way thatinductance and inductive reactance are related. Capacitive reactance, like inductive
reactance, is measured in ohms.
The common symbol for capacitive reactance is XC . The value of capacitive reactance,
in ohms, can be calculated by using the following formula:
XC = 1 2 f C
where: is the constant 3.14f is the frequency, in hertzC in the capacitance, in farads
The effects of capacitance, like the effects of inductance, cause current and voltage to be
out of phase. However, as will be explained in this section, the effects of capacitance are
not the same as the effects of inductance. In fact, capacitance is often added to ACcircuits to counter the effects of inductance. For example, when the inductance in a
circuit would limit current flow more than a desirable amount, additional capacitance can
be added to that circuit to bring current flow up to the level that is needed. The deviceused to do this is called a capacitor.
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AC FUNDAMENTALS (continued)
3. Capacitance
3.1 Capacitors
Capacitors are devices that store energy. A simplified capacitor is shown in Figure 3-1.It has three main components: two plates and an insulator, which is called a dielectric.
The purpose of the dielectric is to keep electrons from flowing from one plate to the
other. The dielectric can be made of any good insulating material, including air. In fact,air acts as a dielectric whenever two conductors are side-by-side for any significant
distance. the two conductors act like capacitor plates.
Figure 3-1 Simplified Capacitor
Dialectric(insulating material)
OSHA Regulations Snap-Shot
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Conducting
Plates
1910.333 (as of January, 2007)
Selection and use of work practices.
(a) General. Safety-related work practices shall be employed to prevent electric shock or other injuriesresulting from either direct or indirect electrical contacts, when work is performed near or on
equipment or circuits which are or may be energized. The specific safety-related work practices shall
be consistent with the nature and extent of the associated electrical hazards.
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AC FUNDAMENTALS (continued)
3. Capacitance
The flow of electrons from the power source to the negatively charged plate of the
capacitor, and the flow of the capacitor, and the flow of electrons from the positivelycharged plate of the capacitor back to the power source continues until the peak voltage isreached. When the peak voltage is reached, the negative plate has gained a certain
number of electrons, and the positive plate has lost the same number of electrons.
For any given voltage, the specific number of electrons that the negative plate can hold is
called the capacity of the capacitor, or simply, the capacitance. Knowing the capacity of a
capacitor is important. If too much voltage is supplied to a capacitor, two many electrons
will be forced onto the negative plate. As a result, the dielectric could break down. If thedielectric breaks down, current can flow through the capacitor from one plate to the other.
In most cases, current flowing through a capacitor will destroy the capacitor.
3.2 Effects of Capacitance on Current and Voltage
When a capacitor is being charged, a difference in potential develops across it. Each
electron that is added to the negative plate makes that plate more negative, and each
electron that leaves the positive plate makes that plate more positive. Since a differencein potential is voltage, it can be said that voltage builds up across a capacitor as it is
charged. The polarity of this voltage is such that it opposes the source voltage. As thecapacitor continues to be charged, the voltage across the capacitor increases until it is
equal to the source voltage. At this point, the capacitor is fully charged. Since the source
voltage and the voltage across the capacitor are equal, but opposite in polarity, they havethe effect of canceling each other out, and the current stops flowing.
OSHA Regulations Snap-Shot
1910.269 (w) (as of February, 2007)
Special conditions.
(1) Capacitors. The following additional requirements apply to work on
capacitors and on lines connected to capacitors.
(i) Before employees work on capacitors, the capacitors shall be disconnectedfrom energized sources and, after a wait of at least 5 minutes from the time of
disconnection, short-circuited.
(ii) Before the units are handled, each unit in series-parallel capacitor banks shallbe short-circuited between all terminals and the capacitor case or its rack. If the
cases of capacitors are on ungrounded substation racks, the racks shall be bondedto ground.
(iii) Any line to which capacitors are connected shall be short-circuited before it
isconsidered deenergized.
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AC FUNDAMENTALS (continued)
3. Capacitance
When the source voltage passes its peak, the capacitor starts to discharge. Discharging isthe reverse of charging: electrons flow onto the positively charged plate and electrons
leave the negatively charged plate. Current now flows in the opposite direction. When
the source voltage reaches zero, current flow is maximum in the opposite direction.
After the source voltage reaches zero, it again begins to increase toward its peak value,
but with the opposite polarity. The capacitor is again being charged, but in the opposite
direction. When the source voltage reaches its peak value, the opposing voltage is at itspeak value. At this point, the capacitor is completely charged in the opposite direction;
the source voltage and the opposing voltage have the effect of canceling each other out,
and current flow is again zero. After the source voltage passes its peak, the capacitor
again starts to discharge. When the source voltage reaches zero again, current flow ismaximum in the opposite direction.
The sine waves show in Figure 3-3 indicates the relationship between the source voltage
and the current that is produced during a full AC cycle in a purely capacitive circuit. At
the beginning of the cycle, as the source voltage rises from zero, current is at its peak
positive value. By the time the source voltage reaches its peak positive value, theopposing voltage has built up to the same value (but opposite polarity), so current is zero.
During the whole cycle, the changes in current take place ahead of the changes in
voltage. Current and voltage are out of phase, because they do not increase and decreasein the same direction at the same time.
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AC FUNDAMENTALS (continued)
3. Capacitance
Questions
3-1. True or False. Capacitance is a physical property of all AC circuits thatopposes a change in voltage.
3-2. What is capacitive reactance?
3-3. Capacitive reactance is measured in ___________________.
3-4. Circle the correct answer.
A dielectric in a capacitor:
a. Increases the flow of protons to the positive terminalb. Keeps electrons from flowing from one plate to the otherc. Aids electrons in their flow from positive to negatived. Decreases the number of electrons on any plate
3-5. Before a capacitor can store energy, it must first be ______________________.
3-6. True or False. When a capacitor is fully charged, the source voltage and thevoltage across the capacitor are equal in amount but opposite in polarity.
3-7. Capacitance causes (a) ______________ (b) _______________ to beout of phase.
Notes:
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
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AC FUNDAMENTALS (continued)
4. AC Power
OBJECTIVES:
Differentiate between true power, reactive power, and apparent power.
Explain how power factor is used in calculating true power in AC Circuits.
In DC circuits, power is equal to voltage times current (P=EI). The only factor that limits
current in a DC circuit is resistance, so the only factors that affect DC power are current,
voltage, and resistance. In AC circuits, however, inductance and capacitance must alsobe considered, so AC power calculations can be much more complicated than DC
calculations. Because inductance and capacitance cause AC current and voltage to be out
of phase, there are three different kinds of power in AC circuits: true power, reactivepower, and apparent power.
4.1 True Power
True power in an AC circuit is the power actually used to do work. The power used in a
purely resistive circuit is true power. (A purely resistive AC circuit is one in which
inductance and capacitance are not large enough to be significant.) Figure 4-1 showssimplified voltage, current, and true power waves for a purely resistive circuit.
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AC FUNDAMENTALS (continued)
4. AC Power
Figure 4-1. Sine Waves for Voltage, Current, and True Powerin a Purely Resistive Circuit
Voltage and current sine waves like the ones shown in Figure 4-1 can be used todetermine the true power in the circuit at any instant. This is done by multiplying the
voltage at any instant by current at that same instant. Since voltage and current are in
phase, they are both positive at the same time and negative at the same time. Therefore,their product, true power, will always be positive, since two positive numbers or two
negative numbers multiplied together will always yield a positive result-positive power.
The term positive power is used as a convention. Positive power is power that is going
to a load from a power source. Negative power, the, is power that is returning to a power
source from a load.
4-2. Reactive Power
Reactive power is the type of power that is found in a purely inductive circuit or a purelycapacitive circuit. Unlike true power, reactive power does no useful work.
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AC FUNDAMENTALS (continued)
4. AC Power
Figure 4-2 shows simplified sine waves for voltage, current, and reactive power in apurely inductive circuit. Voltage and current are out of phase, with current laggingbehind voltage.
Figure 4-2. Sine Waves for Voltage, Current, and Reactive Power
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in a Purely Inductive Circuit
In the first quarter of the cycle represented in Figure 4-2, voltage is positive and current isnegative. Multiplying the two together will yield a negative value, because the product of
a positive number and a negative number is always a negative number. Thus, at this
point, voltage times current equals negative power.
During the second quarter of the cycle, voltage is still positive, but current is now also
positive. If voltage and current are multiplied together during this portion of the cycle,the result is positive power.
The same relationships can be seen in the second half of the cycle. When voltage
becomes negative, current is still positive, so the product of voltage and current isnegative power. When voltage and current are both negative, in the final quarter of the
cycle, their product is positive power, because two negative numbers multiplied together
give a positive result.
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AC FUNDAMENTALS (continued)
4. AC Power
As defined earlier, positive power is power that goes from a power source to a load. In a
purely inductive circuit, positive power goes from the power source to the inductance.The inductance absorbs power from the power source as its magnetic field builds up.Negative power, as defined earlier, is power returning to the power source from a load.
In an inductive circuit, the negative power periods are those during which the power
absorbed by the inductance returns to the power source as the magnetic field collapses.
As indicated by the sine waves in Figure 4-2, the amount of power that is returned from
the inductance to the power source is equal to the amount of power that is supplied by the
power source to the inductance. In a purely inductive circuit, then, power just goes backand forth between the power source and the inductance. Since no power is used to do
work, there is no power that can be identified as true power. The power in a purely
inductive circuit is only reactive power.
The power in a purely capacitive circuit is also reactive power. Figure 4-3 shows
voltage, current, and reactive power sine waves for purely capacitive circuit. As wasexplained earlier, in a purely capacitive circuit, current leads voltage.
Figure 4-3. Sine Waves for Voltage, Current, and Reactive Power
in a Purely Capacitive Circuit
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AC FUNDAMENTALS (continued)
4. AC Power
During the first quarter of the cycle represented in Figure 4-3, both voltage and current
are positive, so their product, power, is positive. In the second quarter of the cycle,current is negative and voltage is positive, so power is negative. During the third quarterof the cycle, both current and voltage are negative, which makes power positive again.
Finally, in the last quarter of the cycle, current is positive and voltage is negative, so
power is again negative.
In a purely capacitive circuit, when power is positive, the capacitor is charging, so it is
storing up power. When power is negative, the capacitor is discharging; it is returning
power to the power source. The effect is the same for a purely capacitive circuit as for apurely inductive circuit: the amount of power that is supplied by the power source to the
capacitor is equal to the amount of power that is returned from the capacitor to the power
source. The power in a capacitive circuit does not do any work, so it is reactive powerrather than true power.
4.3. Apparent Power
Apparent power is the power used to do work plus the power stored during part of a cycle
by inductance and capacitance and then returned to the power source. Apparent pow3r isvoltage times current in any circuit. (In a purely resistive circuit, apparent power and true
power are the same.) Figure 4-4 shows a circuit that includes a power source, a resistor,an inductor, and a capacitor. The product of voltage times current in this circuit cannot
be the true power of the circuit, because true power can be calculated this way only for
purely resistive circuits. The product cannot be reactive power, either, because thee is aresistor in the circuit. The product of voltage times current in this circuit is apparentpower.
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AC FUNDAMENTALS (continued)
4. AC Power
Figure 4-4. Circuit for Apparent Power
4.4 Power Factor
In a circuit like the one shown in figure 4-4, the effects of resistance, inductance, andcapacitance must all be considered in determining true power. Taken together, the
combined effect of these three factors on current flow and, therefore, on power, is called
impedance. Impedance can be calculated, but this is not done very often by maintenancepersonnel. In most cases, the true power in an AC circuit is the ratio of the true power to
the apparent power in that circuit. Power factor is usually expressed as a decimal value.
Power factor is used as follows: When the apparent power of a circuit and the power
factor for that circuit are known, true power is calculated by multiplying the apparent
power times the power factor. In other words, the true power in an AC circuit is equal tovoltage times current times the power factor. In mathematical terms, this is expressed as
P = E x I x PF.
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AC FUNDAMENTALS (continued)
4. AC Power (continued)
Questions:
4-1. True or False. True power is the amount of power actually used to do work.
4-2. Circle the correct answer.Positive power is power that is
a. Going to a load from a power source.b. Returning to a power source from a loadc. The product of a negative current and a positive voltaged. Equal to voltage times resistance
4-3. Reactive power is power that _____________ do useful work.(does, does not)
4-4. The voltage and current sine waves for reactive power are always__________ phase.
(in, out of)
4-5. True or False. Apparent power is the result of multiplying voltage times currentin any circuit.
4-6. The combined effect of resistance, inductance, and capacitance on current flow inan AC circuit is called _________________.
4-7. In most cased, true power in AC circuits is calculated with the aid ofthe ________________ associated with the specific circuit.
4-8. The power factor for a certain AC circuit is .8. If the voltage is 480 volts and thecurrent is 50 amps, what is the true power used by the circuit?
4-9. Given the following values, calculate true power.
E = 110 volts
I = 10 ampsPF = .5
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AC FUNDAMENTALS (continued)
5. Single-Phase and Three-Phase Systems
OBJECTIVES:
Explain the difference between single-phase and three-phase AC systems
Explain how a three-wire single-phase AC system supplies two differentvoltages.
Differentiate between delta-connected and wye-connected three-phase ACsystems.
There are two common types of AC power systems: single-phase systems and three-
phase systems. The purpose of this segment is to introduce some terms that areassociated with these systems.
A simplified single-phase system is illustrated in Figure 5-1. It consists of two wires,
a voltage source, and a load, which is represented by a resistor.
Figure 5-1. Simplified Single-Phase System
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AC FUNDAMENTALS (continued)
5. Single-Phase and Three-Phase Systems (continued)
A simplified three-phase system is shown in Figure 5-2. This system consists of threewires, a voltage source (indicated by the three coils in the circle), and a load (indicated bythe three resistors).
Figure 5-2. Simplified Three-Phase System
5.1 Single-Phase Systems
Single-phase systems are the most commonly used AC power systems for generalelectrical needs. There are two basic types of single-phase systems: the two-wire
system and the three-wire system. In the two-wire system, the voltage supplied has
only one value. Since it is often desirable to have more than one voltage for home
and office use, the three-wire system was developed. The three-wire system makes itpossible to have two different voltages from one voltage source.
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AC FUNDAMENTALS (continued)
5. Single-Phase and Three-Phase Systems (continued)
The component that changes a two-wire system into a three-wire system is a transformer.As Figure 5-3 illustrates, in a three-wire system, two lines come into the transformer on
the primary side, and three lines come off the transformer on the secondary side. The
middle line on the secondary side is called the neutral line. Voltage between the neutralline and either the top line or the bottom line is 110 volts. Voltage between the top and
bottom lines is 220 volts. Thus, it is possible to get either 110 volts or 220 volts from this
three-wire system. Because the three-wire system provides more than one voltage, it has
many applications for general electrical use.
Figure 5-3. Simplified Three-Wire System
One of the lines in a two-wire system and the neutral line in a three-wire system areusually grounded as a protective measure. The, if an un grounded line accidentally
becomes grounded, a short circuit will occur and the circuits fuses or circuit breakerswill open the circuit.
5.2 Three-Phase Systems
Three-phase systems are most often found in large industrial installations where large
amounts of power are used. The two types of connections commonly used for powersources and for loads in three-phase systems are delta connections and wye connections.
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AC FUNDAMENTALS (continued)
5. Single-Phase and Three-Phase Systems (continued)
5.2.1 Delta Connections
Figure 5-4 shows the wiring for a delta connection. In this example, the three coils
represent a three-phase transformer. The ends of each coil are connected to the ends of
the other two coils. The three wires coming out of the transformer are connected to threeresistors, which are also delta-connected.
Figure 5-4. Delta-Connected Three-Phase System
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The current and voltage in the cols and the resistors of a system like this are not always
the same as the current and voltage in the wires. The current that flows through the coils
or resistors is called phase current. The voltage that is applied across the resistors orinduced in the coils is called phase voltage. The current that flows through the wires is
called line current, and the voltage that is applied to the wires is called line voltage.
In a delta-connected system, the phase voltage equals the line voltage, but the phase
current does not equal the line current. Each coil or resistor shown in Figure 5-4 is
connected across two wires. Therefore, the voltage in each coil or resistor (the phasevoltage) is equal to the voltage in the wires (the line voltage). However, the ends of two
coils, or two resistors, are connected to one wire, so the phase currents add together to
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AC FUNDAMENTALS (continued)
5. Single-Phase and Three-Phase Systems (continued)
form the line current. The line current is actually equal to the square root of three (whichis 1.73) times the phase current. Then, multiplying the phase current time 1.73 equals the
line current.
The relationships between voltage and current in a delta connected system can be
summarized in the following formulas:
EL = EP
IL = IP 3
In these formulas, EL is the line voltage, EP is the phase voltage, IL is the line current, and
IP is the phase current.
5.2.2 Wye Connections
The wiring for a typical wye-connected three-phase system is shown in Figure 5-5. (A
wye connection is also known as a star connection.) In a wye connected system, one endof each coil or resistor is connected to one end of both of the other coils or resistors. The
free ends of the coils or resistors are connected to the three phase lines.
Figure 5-5. Wye Connected Three-Phase System.
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AC FUNDAMENTALS (continuted)
5. Single-Phase and Three-Phase Systems (continued)
Wye connections have different effects on voltage and current than delta connections do.In a wye-connected system, the phase current is equal to the line current, but the phase
voltage is not equal to the line voltage. As shown in Figure 5-4, the current that flows
through each line has to flow through the coil or the resistor in the line. Therefore, thecurrent in a wye-connected system cannot split or add together the way it does in a delta-
connected system. However, the voltage in each coil or resistor (the phase voltage) has
to be added to the voltage in one of the other coils or resistors to form the voltage across
any two wires (the line voltage). In wye-connected systems, the phase voltage times 1.73equals the line voltage.
The relationships between voltage and current in a wye-connected system can besummarized in the following formulas:
EL = EP 3
IL = IP
In these formulas, EL is the line voltage, EP is the phase voltage, IL is the line current, and
IP is the phase current.
Notes:
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
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AC FUNDAMENTALS (continued)
5. Single-Phase and Three-Phase Systems (continued)
Questions:
5-1. The component that changes a two-wire single-phase system into a three-wiresingle phase system is a __________________.
5-2. Circle the correct answer.
The middle line on the secondary side of a three-wire single-phase system is
a. Called the neutral lineb. Connected in a delta patternc. Never usedd. 110 volts.
5-3. In a three-phase system, the current that flows through the coils or resistors iscalled _____________ current.
(phase, line)
5-4. True or False. In a delta-connected system, phase voltage equals line voltage, but
phase current does not equal line current.
5-5. To calculate the line voltage of a wye-connected three-phase system, multiply thephase voltage by __________________.
5-6. In a wye-connected three-phase system, phase current and line current are______________.
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AC FUNDAMENTALS (continued)
Glossary
Alternating current (AC) - Current that flows in one direction, stops, and then
flows in the opposite direction.
Apparent Power - Voltage times current in any circuit. Power used
to do work plus power stored during part of a
cycle by inductance and capacitance and thenreturned to power source.
Capacitance - A physical property of all AC circuits that opposesa change in voltage; measured in farads.
Capacitive reactance - The measure of the opposition to current flow that
is created by capacitance; measured in ohms.
Delta connection - A connection used in three-phase systems in which
three coils (or three resistors) are connected end-toend so that they effectively form a triangle.
Direct current (DC) - Current that always flows in the same direction.
Effective values - AC values for current and voltage based on the
relationship that one ampere, peak value, of AC
current produces the same heating effect as .707amperes of DC current; also called RMS values.
Frequency - The number of cycles completed each second by agiven AC voltage.
Impedance - The combined effect of resistance, inductance, andcapacitance on current flow.
Inductance - A physical property of all AC circuits that opposesa change in current; measured in henrys.
Inductive reactance - The measure of the opposition to current that is
created by inductance; measured in ohms.
Line current - The current that flows through the wires in a three-phase system.
Line voltage - The voltage that is applied to the wires in a threephase system.
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AC FUNDAMENTALS
Glossary (continued)
Negative power - Power that is returning to a power source from aload.
Peak value - The amount of voltage or current at the maximumpositive or negative point on a sine wave.
Peak-to peak value - The amount of voltage of current represented bythe distance between the positive peak and the
negative peak on a sine wave.
Phase current - The current that flows through the coils or resistorsin a three-phase system.
Phase voltage - The voltage that is applied across the resistors orinduced in the coils of a three-phase system.
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AC Fundamentals Review
Workbook Section Quiz Answers
1-1. Polarity
1-2. (These answers may be in any order.)a. Conductorb. Magnetic fieldc. Relative motion
1-3. c
1-4. True
1-5. a. Frequency
b. Hertz
1-6. RMS
2-1. Current flow
2-2. Inductive reactance
2-3. a
2-4. False
2-5. (These answers may be in any order.)a. Coil the conductorb. Add a metal core to a coiled conductor
2-6. Inductive reactance
2-7. Out of phase
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AC Fundamentals Review
Answers (continued)
3-1. True
3-2. Capacitive reactance is the measure of the opposition to current flow that is
created by capacitance.
3-3. Ohms
3-4. b
3-5. Charged
3-6. True
3-7. (These answers may be in either order.)
a. Voltageb. Current
4-1. True
4-2. a
4-3. Does not
4-4. Out of
4-5. True
4-6. Impedance
4-7. Power factor
4-8. 19,200 watts
4-9. 550 watts
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AC Fundamentals Review
Answers (continued)
5-1. Transformer
5-2. a
5-3. Phase
5-4. True
5-5. 1.73
5-6. Equal