Alternating Current Fundamentals

8/7/2019 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

T & D PowerSkills Edition II

Page 2

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:

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

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


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


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


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


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


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


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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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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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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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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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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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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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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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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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)



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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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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.


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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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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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 ___________________.


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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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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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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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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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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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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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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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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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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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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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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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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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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Page 43

AC FUNDAMENTALS (continuted)


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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Page 44



Questions:

5-1. The component that changes a two-wire single-phase system into a three-wiresingle phase system is a __________________.


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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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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Page 46

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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Page 47

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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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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Answers (continued)

5-1. Transformer

5-2. a

5-3. Phase

5-4. True

5-5. 1.73

5-6. Equal

Documents

Alternating Current Fundamentals