AC Power Sources 281

AC

Alternating current. Current that reverses direction at regularly recurring intervals of time. In the U.S., AC alternates 60 times per second, or 60 hertz.

alternating current

AC. Current that reverses direction at regularly recurring intervals of time. In the U.S., AC alternates 60 times per second, or 60 hertz.

armature

A device found in generators that has many separate coils wound around it. When the armature rotates in the generator, it creates a magnetic field and produces current.

bridge rectifier

A type of full-wave rectifier with four diodes. A bridge rectifier converts both the positive and negative cycles of AC voltage to DC.

brushes

Devices that conduct electricity between non-moving wires and moving parts that are moving. Brushes are used in generators to pass current from wires to rotors.

brushless exciter

An AC generator design that mounts a small DC generator on its shaft. When rotated, brushless exciters provide the direct current to excite the magnetic field.

center tap

The point where all three windings in a wye connection connect to one another. Center taps divide secondary windings and alter a transformer's turn ratio.

circuit breakers

A safety device that detects excess current in a circuit to prevent overheating. Circuit breakers often contain a bimetallic strip that bends and trips a switch that opens a circuit.

closed loop

An electrical cycle in which the amount of output is directly proportional to the amount of input. A closed loop is formed in delta connections by connecting the starting coil of one phase to the end of the next.

coil

Loops of conducting wire around a magnetic core. Coils are used to create magnetic fields when current passes through them.

conductor

A material or element that allows free movement of electrons and, therefore, allows the easy flow of electricity. Most conductors are metals.

constant

A value, number, or variable that does not change. In frequency, the number of poles is a constant.

copper loss

A power loss due to current flowing through wire. Copper loss is proportional to the resistance of the wire and the square of the current.

copper loss

A power loss due to current flowing through wire. Excessive copper loss can lead wires to overheat and burn through the insulator.

core

A component of a transformer. A core that is iron or steel provides a controlled path for the magnetic flux generated in the transformer by the current flowing through the windings in the transformer.

current

The flow of electricity. Current is measured in amperes (A).

cycles

The course of travel of a sine wave. During one cycle, a sine wave travels from its peak positive voltage to 180°, then to its peak negative voltage and back to 0.

DC

Direct current. Current that flows in one direction. DC is expensive and inefficient, making AC the better choice for power.

delta

A connection of three components where a triangular series circuit is formed. Delta connections are used in three-phase power systems.

delta/delta

A transformer with both the primary and the secondary connected as a delta. The delta/delta is a type of three-phase transformer.

delta/wye

A transformer with the primary connected as a delta and the secondary connected as a wye. The delta/wye is a type of three-phase transformer.

diodes

A two-terminal semiconductor device that allows current to flow through it in only one direction. Diodes have low resistance in one direction and a high resistance in the other direction.

direct current

DC. Current that flows in one direction. DC is expensive and inefficient, making AC the better choice for power.

distribution substation

A bank of step down transformers near the end users. The distribution substation converts voltage closer to the required usable levels.

distribution system

A circuit of users linked to a generating station and substations. Distribution systems are typically arranged as either radial or interconnected systems.

efficient

Having an energy output that is close to the total energy supplied. An efficient system has very little resistance loss.

electric shock

The flow of electricity through the body. Electric shock can be fatal.

electrical energy

A form of power created by the movement of electrons. Mechanical energy is converted into electrical energy by generators.

electromagnets

A magnet that gains an attractive force only when current passes through it. Electromagnets are extremely powerful.

electrons

A negatively charged particle that orbits the nucleus of an atom. Electrons flowing between albums causes electricity.

enclosure

A physical barrier used as protection. An enclosure provides mechanical or electrical protection to components used in a system.

excitation current

The DC required to magnetize a piece of metal. Excitation current creates the magnetic field in a revolving field generator.

exciter

An AC generator that is used with another generator or motor to generate a magnetic field. Exciters provide additional current through electromagnets to create more lines of flux.

filtered

The state caused by a rectifier in which the ripple effect has been removed. Filtered AC is kept as close to its peak value as possible.

filters

A device that diminishes the ripple present in rectified AC. Filters keep rectified AC current (DC) as close to peak value as possible.

flux leakage

Flux lines that are lost to the surrounding air. Flux leakage occurs with flux lines that do not follow the core.

frequency

A measurement of the number of complete AC cycles that occur in one second. Frequency is measured in hertz (Hz).

full capacity

The maximum amount of electricity that can flow without damaging equipment. Generators are run at full capacity.

full-wave rectifier

A rectifier that flattens both positive and negative halves of the incoming AC to pulsating DC. The full-wave rectifier is used more often than other designs because it is more efficient than a half-wave rectifier.

fuses

A safety device that detects excess current in a circuit to prevent overheating. Fuses often have a component that melts and opens the circuit.

generating station

A building where a utility company houses large linked AC generators. Generating stations can create large amounts of power for widespread use.

generators

A device that converts mechanical energy into electrical energy. Generators use the process of electromagnetic induction to convert energy.

generators

Devices that convert mechanical energy into electrical energy. Generators that convert energy using magnetic induction.

half-wave rectifier

A rectifier that flattens only the positive or negative half of the incoming AC to pulsating DC. The half-wave rectifier is less efficient and needs more filtering than a full-wave rectifier.

hertz

Hz. A unit of frequency equal to one cycle per second. Hertz are used to express AC cycles.

hollow core

A square-shaped laminated iron transformer core with an open center. In hollow core transformers, the windings surround the sides of the core.

H-type core

A type of transformer core that surrounds the windings on four sides. The construction of the H-type core helps to lessen flux leakage.

in-phase

The condition in which generators rotate so they have corresponding points that cross the same places at the same time. When generators are in-phase, they may be considered synchronized.

insulator

A material that does not readily conduct electricity. Insulators can be constructed from rubber and glass.

interconnected systems

A distribution system with multiple available power sources that can loop throughout the network. In interconnected systems, if one source goes down, a different source can be activated to maintain service.

iron

A silver-white, metallic element that is highly magnetic. Pole pieces are often constructed of iron.

kilovolt-amps

kVA. Units used to rate transformer and generator strength. A kilovolt amp is one thousand volts.

kilovolts

kV. A unit of measurement of potential energy in a circuit at a specific point. A kilovolt is equal to one thousand volts.

kVA

Kilovolt-amps. Units used to rate transformer and generator strength. One kVA is one thousand volts.

laminated

Sealed together in multiple layers of thin sheets to construct the core of a transformer. Laminating helps to reduce resistance losses.

lines of flux

Imaginary lines of force that surround a magnet. Lines of flux form a magnetic field.

load

A component in a circuit that converts electrical energy into light, heat, or mechanical motion. Examples of loads are motors, appliances, and light bulbs.

load

A component in a circuit that converts electricity into light, heat, or mechanical motion. Examples of loads include light bulbs, appliances, or machines.

lockout/tagout

A method of protecting employees from accidental machine startup through proper locking and labeling of machines. Lockout/tagout is an essential practice for safe repair of machines.

magnetic field

The area in and around a magnet in which a magnetic force exists. Magnetic fields exhibit the powers of attraction and repulsion.

magnetic flux

A measure of the strength of the field formed around a magnet. Magnetic flux is expressed in webers (Wb).

magnetic induction

The process in which a material becomes magnetized in a magnetic field. Magnetic induction commonly takes place within metals.

mechanical energy

A combination of kintetic and potential energy resulting from the force of gravity or movement of a machine component. Mechanical energy is created by generators using magnetic flux.

mutual induction

The ability of one coil to induce a voltage into another coil. Mutual induction is used in transformers to step up or step down voltage.

network system

A distribution system where multiple power sources are connected to each user to guarantee uninterrupted power. Network systems are expensive, so they are only used when constant power is vital.

north pole

The negative side of a magnet. The differences between the north pole and south pole of a magnet create lines of flux.

open delta

A transformer connection that does not complete the triangle, using only two of the three sides. An open delta is used when full power is not needed.

output

The generation of a result from a particular process. Electrical output is the voltage or current produced.

output voltage

The amount of electrical pressure or potential that is generated from a process or machine. In a generator, output voltage is generated in proportion to the turns that exist in the armature conducting wire.

parallel circuits

An electrical system that has multiple paths for the flow of electricity. Parallel circuits can be formed by linking generators together.

peak negative voltage

The lowest negative point on a wave. The peak negative voltage indicates the maximum negative voltage of the wave.

peak positive voltage

The highest positive point on a wave. Peak positive voltage indicates the maximum positive voltage of the wave.

personal protective equipment

PPE. Any of the various articles of clothing or safeguarding devices that workers wear to ensure their safety. Common personal protective equipment worn during electrical work includes safety glasses, rubber-soled footwear, and gloves.

phase

A measurement of the number of voltages in one cycle. Most AC generators are three-phase.

polarity

Having a positive or negative charge. Polarity determines the direction in which current flows.

pole pieces

Magnetic devices mounted on the inside of a generator armature. Pole pieces form electromagnets that create lines of flux when connected to windings.

poles

Opposite ends of an axis. Poles also refer to the opposite ends of a magnet.

poles

Two oppositely charged ends of a magnet. The relationship between the poles causes magnetic flux.

power distribution

The circuit that travels from generating stations to end users to provide electrical power. Power distribution is expressed in hertz.

power supply

A device that converts AC to DC. A power supply contains a rectifier to convert current.

primary winding

The first winding that current passes through in a transformer. The primary winding contains multiple thin wires that are wrapped into a coil.

prime mover

A device that supplies the turning force necessary to turn the shaft of a generator or alternator. Examples of prime movers include steam turbines and diesel engines for AC generators.

radial system

A distribution system with only one power source. In a radial system, if the power source goes out, power is lost to the entire circuit.

rectifiers

A device used in an electrical circuit to convert AC to DC. Rectifiers use diodes to prevent current from flowing in more than one direction.

rectify

When a diode changes AC. When AC is rectified, it is converted to DC.

regulation

A system of control. Voltage requires regulation to stay at a constant level.

reserve generator

A generator that is used when the primary generator or generators cannot generate enough voltage to meet demand. Reserve generators are activated when more power is needed.

ripple

Current that is not stable and characterized by a varying value. AC output constantly ripples, unlike the constant values of DC.

rotating armature generators

A device that generates alternating current by rotating an armature through a stationary magnetic field. Rotating armature generators use slip rings and brushes to conduct current from the armature to the load being powered.

rotating field coil

An electromagnet that rotates to produce a magnetic field. In a rotating field generator, rotating field coils turn instead of the armature.

rotating field generators

A device that generates alternating current through a rotating magnetic field and a stationary armature. Rotating field generators are the most commonly used type of AC generator.

rotor

A device found in alternators that contains many windings. When a rotor rotates inside an alternator, it creates a magnetic field, which produces alternating current.

rotor

A device found in generators that rotates and creates a magnetic field that produces AC. Rotors contain wire windings.

secondary winding

The second winding that current passes through in a transformer. The secondary winding contains fewer, but thicker wires that are wrapped into a coil.

shell core

A type of core that surrounds the windings to create a strong magnetic coupling. Shell cores are shaped like the letter E.

short circuit

A circuit where current takes a shorter, unintended path between two conductors. Short circuits can cause fires, but fuses and circuit breakers prevent them.

sine wave

The most common type of AC waveform. A sine wave consists of 360 electrical degrees and is produced by rotating machines.

single-phase

AC voltage that has only one sine wave. Single-phase is a continuous, single, alternating current cycle.

slip rings

A conductive device attached to the end of a generator rotor. Slip rings conduct current to brushes in generators.

south pole

The positive side of a magnet. The differences between the south pole and north pole of a magnet create lines of flux.

spike

A sharp, sudden increase in voltage in a circuit that can damage equipment. Spikes are caused by malfunctions or changing load conditions.

stator

The stationary windings of a motor. Stators are usually found inside of an AC motor.

step down

In electricity, a phrase used to describe voltage adjustment. To step down voltage means to decrease voltage.

step up

In electricity, a phrase used to describe voltage adjustment. To step up voltage means to increase voltage.

synchronized

Meeting the necessary conditions to operate in exact unison. Generators are considered synchronized if they have identical voltage, frequency, and phase rotation.

terminals

A connecting point in a circuit to which a wire can be attached. Terminals help to connect electrical components to one another.

three-phase

A continuous series of three overlapping AC cycles offset by 120 degrees. Three-phase power is used for all large scale distribution systems.

toroid core

A ring-shaped electromagnetic transformer core. The toroid core is very efficient because it minimizes flux leakage.

transformer

A device that uses electromagnetic induction to transfer electrical energy from one circuit to another without changing its frequency. A transformer is most often used to change line voltage.

transformer substation

A bank of step up transformers near the generating station. Transformer substations increase voltage as high as 800kV for widespread distribution.

transformers

A device that transfers electrical energy from one circuit to another, without changing the frequency, using electromagnetic induction. A transformer is most often used to change the line voltage.

turbine

A machine that uses the motion or energy of moving water or wind to produce movement in paddles, buckets, or other devices arranged in a circle. Turbines convert this mechanical motion to electricity.

turns ratio

A comparison of the number of turns in the primary versus the number of turns in the secondary windings. Based on the turns ratio, transformers step voltage up or down.

voltage

A measure of electrical pressure or potential. Voltage is measured in volts (V).

windings

The conducting coils that are wound around the armature in which voltage is induced if moved within a magnetic field. Windings are also used to transfer voltage in transformers.

wire resistance

The opposition to current flow in a conductor. Constant wire resistance is a benefit of the usage of alternating current.

wye

A connection of three components where one end of each component is connected at a common point. Wye connections are used in three-phase power systems.

wye/delta

A transformer with the primary connected as a wye and the secondary connected as a delta. The wye/delta is a type of three-phase transformer.

wye/wye

A transformer with both the primary and the secondary connected as a wye. The wye/wye is a type of three-phase transformer.