TABLE OF CONTENTS
CHAPTER PAGE
1. Concepts of Alternating Current
2. Inductance
3. Capacitance
4. Inductive and Capacitive Reactance
5. Transformers .
APPENDIX
Glossary. Alphabet.
III. Square and Square Roots.
IV. Useful AC Formulas
V. Trigonometric Functions
VI. Trigonometric Tables
AIR-CORE TRANSFORMER—A transformer composed of two or more coils, which are wound around a non-metallic core.
ALTERNATING CURRENT—An electrical current which constantly changes amplitude and changes polarity at regular intervals.
APPARENT POWER—That power apparently available for use in an ac circuit containing a reactive element. It is the product of effective voltage times effective current expressed in voltamperes. It must be multiplied by the power factor to obtain true power available.
AVERAGE VALUE OF AC—The average of all the instantaneous values of one-half cycle of alternating current.
CAPACITANCE—The property of an electrical circuit that opposes changes in voltage.
CAPACITOR—An electrical device capable of storing electrical energy in an electrostatic field.
CAPACITIVE REACTANCE—The opposition offered to the flow of an alternating current by capacitance, expressed in ohms. The symbol for capacitive reactance is Xc.
CHARGE CYCLE—The period of time that a capacitor in an electrical circuit is storing a charge.
COIL—An inductive device created by looping turns of wire around a core.
COPPER LOSS (I2
R LOSS)—
The power lost due to the resistance of the conductors. In transformers the power lost due to heating because of current flow (I) through the resistance (R) of the windings.
CORE—Any material that affords a path for magnetic flux lines in a coil.
COUNTER EMF—Counter electromotive force; an electromotive force (voltage) induced in a coil that opposes the applied voltage.
COUPLING, COEFFICIENT OF—An expression of the extent to which two inductors are coupled by magnetic lines of force. This is expressed as a decimal or percentage of maximum possible coupling and represented by the letter K.
CYCLE—One complete positive and one complete negative alternation of a current or voltage.
DIELECTRIC—An insulator; a term applied to the insulating material between the plates of a capacitor.
DIELECTRIC CONSTANT—The ratio of the capacitance of a capacitor with a dielectric between the electrodes to the capacitance of a capacitor with air between the electrodes.
DIELECTRIC HYSTERESIS LOSS—Power loss of a capacitor due to the changes in orientation of electron orbits in the dielectric caused by rapid reversal in polarity of line voltage. The higher the frequency, the greater the loss.
DIELECTRIC LEAKAGE—Power loss of a capacitor due to the leakage of current through the dielectric. Also relates to leakage resistance, the higher the leakage resistance, the lower the dielectric leakage.
DISPLACEMENT CURRENT—The current which appears to flow through a capacitor.
EDDY CURRENT—Induced circulating currents in a conducting material that are caused by a varying magnetic field.
EDDY CURRENT LOSS—Losses caused by random current flowing in the core of a transformer.Power is lost in the form of heat.
EFFECTIVE VALUE—Same as root-mean-square.
ELECTROMAGNETIC INDUCTION—The production of a voltage in a coil due to the change in the number of magnetic lines of force (flux linkage) passing through the coil.
ELECTROMAGNETISM—The generation of a magnetic field around a current carrying conductor.
ELECTROMOTIVE FORCE (emf)—The force (voltage) that produces an electric current in a circuit
ELECTROSTATIC FIELD—The field of influence between two charged bodies.
EXCITING CURRENT—The current that flows in the primary winding of a transformer, which produces a magnetic flux field. Also called magnetizing current.
FARAD—The basic unit of capacitance. A capacitor has a capacitance of 1 farad when a voltage change of 1 volt per second across it produces a current of 1 ampere.
FLUX—Electrostatic or magnetic lines of force.
FREQUENCY (f)—The number of complete cycles per second existing in any form of wave motion;such as the number of cycles per second of an alternating current.
HENRY (H)—The electromagnetic unit of inductance or mutual inductance. The inductance of a circuitis 1 henry when a current variation of 1 ampere per second induces 1 volt.
It is the PEAK VALUE—The maximum instantaneous value of a varying current, voltage, or power. It is equal to 1.414 times the effective value of a sine wave. to the flow of an alternating current by the inductance,capacitance, or both, in any circuit.
Introduction to Alternating Current (AC) and Transformers
Alternating current (AC) and transformers are essential components in electrical power systems, forming the backbone of electricity generation, transmission, and distribution. Understanding these concepts is crucial for anyone studying electrical engineering or working with electrical systems.
1. What is Alternating Current (AC)?
Alternating current (AC) is a type of electrical current where the flow of electric charge periodically changes direction. Unlike direct current (DC), where electrons flow in a single, consistent direction, AC alternates back and forth, typically in the form of a sine wave.
Key Characteristics of AC:
- Frequency: The number of cycles per second is called the frequency, measured in Hertz (Hz). For example, in most countries, the standard frequency of AC power is 50 Hz or 60 Hz.
- Amplitude: The maximum voltage reached during one cycle of the AC waveform.
- Waveform: The most common waveform for AC is a sine wave, but it can also be triangular or square, depending on the application.
Advantages of AC Over DC:
- Efficient Transmission: AC can be easily transformed to higher or lower voltages using transformers, making it more efficient for long-distance power transmission.
- Lower Power Losses: High-voltage AC transmission reduces current, which minimizes power losses in the form of heat.
- Ease of Generation: AC is simpler to generate using alternators, especially for large-scale power generation.
2. Generation of Alternating Current
AC is typically generated using an alternator or an AC generator, which operates on the principle of electromagnetic induction. In an alternator:
- A rotor (rotating part) spins inside a stator (stationary part), creating a changing magnetic field.
- The changing magnetic field induces an alternating voltage in the stator's windings, producing an AC output.
The mechanical energy required to rotate the alternator can come from various sources, including steam turbines, water turbines (in hydroelectric power plants), wind turbines, or internal combustion engines.
3. Applications of Alternating Current
AC is the primary form of electrical power used for:
- Household Appliances: From lighting and heating to electronic devices like televisions and refrigerators.
- Industrial Machinery: Motors, pumps, and conveyor systems in factories.
- Power Distribution: The entire electrical grid relies on AC for efficient power transmission and distribution.
- Electric Motors: Many types of AC motors (e.g., induction motors, synchronous motors) are widely used in both domestic and industrial applications.
4. What is a Transformer?
A transformer is an electrical device used to change the voltage level of alternating current without altering its frequency. It works on the principle of mutual induction, where a change in current in one coil induces a voltage in another coil.
Basic Components of a Transformer:
- Primary Coil: The coil connected to the input voltage source.
- Secondary Coil: The coil connected to the output load.
- Core: Made of iron or another magnetic material, the core enhances the magnetic coupling between the primary and secondary coils.
Types of Transformers:
- Step-Up Transformer: Increases the voltage from the primary coil to the secondary coil. Used in power transmission to reduce current and minimize power losses over long distances.
- Step-Down Transformer: Decreases the voltage from the primary coil to the secondary coil. Used in power distribution to provide a safe voltage level for homes and businesses.
- Isolation Transformer: Provides electrical isolation between the primary and secondary coils without changing the voltage level, enhancing safety in sensitive electronic equipment.
5. Working Principle of a Transformer
The transformer operates based on Faraday's Law of Electromagnetic Induction:
- When an alternating current passes through the primary coil, it creates a changing magnetic field in the core.
- This changing magnetic field induces a voltage in the secondary coil, producing an AC output.
- The voltage change is determined by the turns ratio of the transformer:
Where:
- = Voltage in the secondary coil
- = Voltage in the primary coil
- = Number of turns in the secondary coil
- = Number of turns in the primary coil
6. Importance of Transformers in Power Systems
Transformers play a vital role in power systems by enabling efficient transmission and distribution of electricity:
- Voltage Step-Up for Transmission: At power generation stations, step-up transformers increase the voltage to several hundred kilovolts (kV) for long-distance transmission. Higher voltage reduces the current and minimizes energy losses due to the resistance of transmission lines.
- Voltage Step-Down for Distribution: Near end users, step-down transformers decrease the voltage to safer levels (e.g., 240V or 120V) for use in homes and businesses.
- Adaptation to Load Requirements: Transformers can adjust voltage levels according to the specific needs of industrial equipment or sensitive electronic devices.
7. Applications of Transformers
Transformers are used in various applications across different sectors:
- Power Grid: For stepping up and stepping down voltage levels in the electrical transmission and distribution system.
- Electronic Devices: In power adapters and chargers to convert AC mains power to lower voltages suitable for electronic circuits.
- Isolation and Safety: In medical equipment and sensitive electronics to provide electrical isolation and protect against electrical shocks.
Conclusion
Alternating current (AC) and transformers are fundamental elements of modern electrical systems. AC provides an efficient way to transmit electrical power over long distances, while transformers enable the voltage to be adjusted as needed for safe and efficient distribution. Together, they form the backbone of the electrical infrastructure that powers homes, industries, and cities worldwide. Understanding their principles, operation, and applications is essential for designing, operating, and maintaining reliable electrical systems.
