Understanding AC Circuits (eBook)

Dale Patrick, Stephen Fardo (Autoren)

eBook Download: PDF | EPUB

1999 | 1. Auflage
172 Seiten
Elsevier Reference Monographs (Verlag)
978-0-08-051992-0 (ISBN)

Infos

Understanding AC Circuits covers the second half of a basic electronic circuits theory course, integrating theory and laboratory practice into a single text. Several key features in each unit make this an excellent teaching tool: objectives, key terms, self-tests, lab experiments, and a unit exam. This new edition has been thoroughly revised and updated by the authors to reflect the latest information on electronics. Understanding AC Circuits is designed with the electronics beginner and student in mind. The authors use a practical approach exposing the reader to the systems that are built with AC circuits making it easy for beginners to master even complex concepts in electronics while gradually building their knowledge base of both theory and applications. Each chapter includes easy-to-read text accompanied by clear and concise graphics fully explaining each concept before moving onto the next. The authors have provided section quizzes and chapter tests so the readers can monitor their progress and review any sections before moving onto the next chapter. Each chapter also includes several electronics experiments, allowing the reader to build small circuits and low-cost projects for the added bonus of hands-on experience in AC electronics. Understanding AC Circuits fully covers dozens of topics including single-phase and three-phase AC electronics; electrical generator basics; how to use a multimeter and oscilloscope in AC electronics; troubleshooting and testing circuits; tools and equipment; resistive circuits; inductive circuits; capacitive circuits; vector diagrams; series circuits; transformers; filter circuits; resonant circuits; decibels; waveshaping control; electronic symbols; soldering techniques; plus much more. - Integrates theory and lab experiments - Contains course and learning objectives and self-quizzes - Heavily illustrated

Textauszug

Unit 1

Basics of Alternating Current

Alternating current (ac) electronics is somewhat more complex than direct current (dc) electronics. AC circuits, like dc circuits, have a source of energy and a load in which power conversion takes place. Most of the electric energy produced in the United States is alternating current; therefore, ac systems are used for many applications. In terms of electronic circuits, three important characteristics are present in ac circuits. These characteristics are resistance, inductance, and capacitance. There are two types of ac voltage: single-phase and three-phase. These are discussed in the units that follow.

Unit Objectives

Upon completion of this unit you will be able to do the following:

1. Explain the difference between ac and dc.

2. Define the process of electromagnetic induction.

3. Describe factors that affect induced voltage.

4. Draw a simple ac generator and explain ac voltage generation.

5. Convert peak, peak-peak, average, and RMS-effective values from one to the other.

6. Explain the relation between period and frequency of an ac waveform.

7. Recognize the different types of ac waveforms.

Important Terms

The following terms provide a review of the basics of ac electronics:

Average voltage (Vavg) The value of an ac sine wave voltage; found with the formula Vavg = Vpeak × 0.636.

Cycle A sequence of events that causes one complete sine wave or pattern of alternating current. It begins from a zero reference and goes in a positive direction, back to zero, in a negative direction, and back to zero, a complete 360° sequence.

Delta connection A method of connecting three-phase circuits in which the beginning of one phase is connected to the end of the adjacent phase.

Effective voltage (Veff) A value of an ac sine wave voltage that has the same effect as an equal value of dc voltage; also called RMS (root mean square) voltage; Veff = Vpeak × 0.707.

Frequency (f) The number of ac cycles per second; measured in hertz (Hz).

Hertz The international unit of measurement of frequency; equal to one cycle per second (cps).

In-phase The condition in which two ac waveforms of the same frequency pass through their minimum values at the same time and same polarity.

Instantaneous voltage (Vi) A value of ac voltage at a given instant of time along a waveform.

Peak voltage (Vpeak) The maximum positive or negative value of an ac sine wave voltage; Vpeak = Veff × 1.41.

Peak-to-peak voltage (Vp-p) The value of an ac sine wave voltage from its positive peak to its negative peak.

Period (time) The time required to complete one ac cycle; time = 1/frequency.

Phase angle (θ) The angular displacement (in degrees) between applied voltage and total current flow in an ac circuit.

Root mean square (RMS) voltage Same as effective voltage.

Sine wave A waveform that represents one cycle of ac voltage; see cycle.

Single-phase ac The voltage output produced by a single-phase generator in the form of a series of sine waves.

Theta (θ) The Greek letter used to represent the phase angle of an ac circuit.

Three-phase ac The voltage produced by a three-phase generator in the form of a series of three sine waves separated in phase by an angle of 120°.

Waveform The pattern of an ac frequency derived by looking at instantaneous voltage values that occur over a period of time. A waveform is plotted on a graph with instantaneous voltages on the vertical axis and time on the horizontal axis.

Wavelength The distance from a point on a waveform to a corresponding point on an adjacent waveform.

Wye connection A method of connecting three-phase circuits in which the beginnings or ends of each phase are connected together to form a common or neutral point.

Alternating Current (AC) Voltage

When an ac source is connected to some type of load, the direction of the current changes several times in a given unit of time. Remember that direct current (dc) flows in one direction only. A diagram of one cycle of ac is compared with a dc waveform in Fig. 1-1. This waveform is called an ac sine wave. When an ac generator shaft rotates one complete revolution, or 360°, one ac sine wave is produced. AC voltage generation is discussed at the end of this unit. The sine wave has a positive peak at 90° and decreases to zero at 180°. It increases to a peak negative voltage at 270° and decreases to zero at 360°. The cycle repeats itself. Current flows in one direction during the positive alternation and in the opposite direction during the negative half-cycle.

Fig. 1-1 Comparison of ac (a) and dc (b) waveforms.

Figure 1-2 shows five cycles of ac. If the time required for an ac generator to produce five cycles were 1 second, the frequency of the ac would be 5 cycles per second (cps). AC generators at power plants in the United States operate at a frequency of 60 cps, or 60 hertz (Hz). The hertz is the international unit for frequency measurement. If 60 ac sine waves are produced every second, a speed of 60 rev/s is needed. This produces a frequency of 60 cps.

Fig. 1-2 Five cycles of alternating current.

AC voltage is measured with a volt-ohm-milliammeter (VOM), also called a multimeter. Polarity of the meter leads is not important. This is because ac changes direction. Polarity is important, however, in measuring dc, because the current flows only in one direction. Some VOMs cannot be used to measure ac current. They have ranges for ac voltage only.

Figure 1-3 shows several voltage values associated with ac. Among these are peak positive, peak negative, and peak-to-peak ac values. Peak positive is the maximum positive voltage reached during a cycle of ac. Peak negative is the maximum negative voltage reached. Peak-to-peak is the voltage value from peak positive to peak negative. These values are important to know for work with radio and television amplifier circuits. For example, the most important ac value is called effective, or measured, value. This value is less than the peak positive value. A common ac voltage is 120 V, which is used in homes. This is an effective value voltage. Its peak value is approximately 1 70 V.

Fig. 1-3 Voltage values of an ac waveform.

The effective value of ac is defined as the ac voltage that will do the same amount of work as a dc voltage of the same value. For example, in the circuit in Fig. 1-4 if the switch is placed in position 2, a 10 V ac effective value is applied to the lamp. The lamp should produce the same amount of brightness with a 10 V ac effective value as with 10 V dc applied. When ac voltage is measured with a meter, the reading is effective value.

Fig. 1-4 Comparison of effective ac voltage and dc voltage.

In some cases it is important to convert one ac value to another. For instance, the voltage rating of electronic devices must be greater than the peak ac voltage applied to them. If 120 V ac is the measured voltage applied to a device, the peak voltage is about 170 V. The device must be rated over 170 V rather than 120 V.

To determine peak ac when the measured or effective value is known, the following formula is used:

=1.41×effectivevalue

When 120 V is multiplied by the 1.41 conversion factor, the peak voltage is found to be about 170 V.

Two other terms that should be mentioned are RMS value and average value. RMS stands for root mean square, and it is equal to 0.707 × peak value. RMS refers to the mathematical method used to determine effective voltage. RMS voltage and effective voltage are the same. Average voltage is the mathematical average of all instantaneous voltages that occur at each period of time throughout an alternation. The average value is equal to 0.636 times the peak value.

Single-Phase and Three-Phase AC

Single-phase ac...

Produkt-Details

• Erscheint lt. Verlag: 20.12.1999
• Sprache: englisch
• Themenwelt: Kunst / Musik / Theater ► Design / Innenarchitektur / Mode
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• ISBN-10: 0-08-051992-X / 008051992X
• ISBN-13: 978-0-08-051992-0 / 9780080519920