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Magnetism

was discovered by the chinese around 2637 B.C.

Lodestones

Also known as leading stones, were used as magnets in primitive compasses.

Type of magnets

are man-made, or known as artificial magnets.

Permanent Magnets

Hard magnetic materials (cobalt steel). Retain magnetism even when magnetizing field is removed.

Temporary Magnets

Cannot retain magnetism after magnetizing field is removed

CLASSIFICATION OF MAGNETIC MATERIALS

1. Ferromagnetic Materials

2. Paramagnetic Materials

3. Diamagnetic Materials

Ferromagnetic Materials

e.g Iron, Steel, Nickel, Cobalt, Permalloy. Ferrites are nonmagnetic materials that have ferromagnetic properties of iron. Typically used in RF Transformers. Permeability is in the range of 50 to 3000.

Paramagnetic Materials

e.g Aluminum, Platinum, Manganese, Chromium. Permeability is less than 1.

Diamagnetic Materials

e.g Bismuth, Antimony, Copper, Zinc, Mercury, Gold, Silver. Permeability is less than 1.

Magnetic Fields

Magnets Have two points opposite each other that attracts other magnets. These points are the north and south pole, and similarly with electric charges, like poles repel each other while unlike poles attract each other.

MAGNETIC FLUX

Magnetic field that flows outward from the pole. Denoted by the symbol ϕ (phi). Unit is weber (Wb). 1 weber is equal to 1 x 10⁸ magnetic field lines. Typical usafe of weber is in the form of microweber (μWb).

MAGNETIC FLUX DENSITY

Magnetic flux per unit area of a section perpendicular to the direction of the flux. Denoted by the symbol B. Unit is Tesla (T).

ELECTROMAGNETISM

Electrict current flowing through a conductor produces a magnetic field around the conductor

Hans Christian Ørsted (1819)

a Danish scientist discovered the relationship between magnetism and electric current.

Every section of the wire has a field of force perpendicular to the wire itself

Strength of the magnetic field around a conductor depends on the current, the larger the current, the larger the magnetic field

Polarity of a Single Conductor

Use right hand rule to determine the direction of the lines of force around the conductor.

MAGNETIC FIELD AND POLARITY OF A COIL

If a straight conductor is bent into a loop, the magnetic field lines will be denser inside the loop, while the number of lines are still the same as a straight conductor.

Coil

formed when there is more than one loop or turn.

Thumb points to the north pole of the coil as well as an electromagnet

Polarity depends on the direction of the current flow and direction of winding.

MAGNETOMOTIVE FORCE

The more turns, the stronger the magnetic field. The more turns, the more concentrated are the lines of force.

Product of current times the number of turns of the coil, expressed in Ampere-turns (At)

MAGNETIC FIELD INTENSITY

Depends on the length of the coil. If a coil is stretched out to twice its original length, the intensity will be half as great.

Magnetization Curve

Flux-field curve, known as the BH curve, is used to show how much flux density results as the amount of field intensity increases.

Permeability

Ratio of flux density to field intensity

Denoted by the symbol μ. Unit is Tm/At

Magnetic Circuit

Ohm's Law for Magnetic Circuits

Reluctance

Inversely proportional to permeability. Different forms of electromagnets have different values of reluctance.

Air Gap

Air space between poles of a magnet. Air has high reluctance, which means the size of air gap affects the value of reluctance. The shorter the air gap, the stronger the field intensity. Magnetic Circuit

Electromagnetic Induction

Discovered by Michael Faraday in 1831. States that "If a conductor 'cuts across' lines of magnetic force, or if lines of force cut across a conductor, an emf or voltage, is induced across the ends of the conductor.

Faraday's Law

states that the value of the induced voltage depends upon the number of turns of a coil and how fast the conductor cuts across the lines of force or flux.

Lenz's Law

states that an induced voltage always has such a direction as to oppose the change in magnetic flux that produced it.

Faraday's Law and Lenz's Law

Induced voltage is determined by three factors:

1. Amount of Flux

2. Number of Turns

3. Time Rate of Cutting

Amount of Flux

The more lines of force that cut across the conductor, the higher the value of induced voltage.

Number of Turns

The more turns in a coil, the higher the induced voltage.

Time Rate of Cutting

The faster the flux cuts a conductor or the conductor cuts the flux, the higher the induced voltage because more lines of force cut the conductor within a given period of time

AC Generators

Also called alternators Almost all electric power used in homes and industries are supplied by alternators

Simple alternator consists of:

Strong, constant magnetic field. Conductors that rotate across the magnetic field. Continuous connection to the conductors as they rotate

The amount of generated voltage depends on the field strength and speed of the motor

Types of Alternators

DC Alternators and AC Alternators

DC Alternators

same functions as DC generators, the main difference is the magnetic poles rotate and induce voltage into a fixed or stationary winding. Usually used on light aircraft

AC Alternators

Produces great amounts of power, usually used on larger and military aircraft

Classification of Alternators in terms of Stator and Rotor

Revolving-Armature Type and Revolving-Field Type

Revolving-Armature Type

similar to a DC generator, the stator is the magnetic field, while the rotor is the armature

Revolving-Field Type

the stator in this type is the armature winding, while the rotor is the field winding or the magnet itself.

Construction of AC Generators

The stator is the unit that surrounds the rotor, made up of wire coils around an iron housing

Alternator Stator

Single-Phase Alternator, Two-Phase Alternator, and Three-Phase Alternator

Single-Phase Alternator

stator is made up of several windings connected in series to form a single circuit.

Two-Phase Alternator

have two or more single-phase windings spaced symmetrically around the stator. The AC voltage induced is 90° apart from one another.

Three-Phase Alternator

also known as a polyphase alternator, it has three single-phase windings spaced 120° apart from one another.

Star-Connected

One lead from each winding is connected to form a junction, which is known as star-connected.

Delta Connection

If the stator is connected in a way that the phases are end-to-end, it is known as delta connection

Diode Rectifier Bridge

responsible for the conversion or rectification of ac voltage to dc voltage

Automatic Voltage Regulator

controls the output of the alternator

Brushes

supplies current to the field coils.

Frequency of AC Generators

The frequency of the generated emf depends on the number of field poles and on the speed at which the generator is operated

Voltage Regulation

Regulation of an ac generator is the percentage rise in terminal voltage as load is reduced from the rated full-load current to zero, with the speed and excitation being constant

Losses and Efficiency

Losses of an ac generator are similar to those of a dc generator and include armature copper loss, field-excitation copper loss, and mechanical losses.

Paralleling Generators

Most power plants have several ac generators operating in parallel in order to increase the power available. Before two generators may be paralleled, their terminal voltages must be equal, their voltages must be in phase, and their frequencies must be equal.

Synchronizing

When these conditions are met, the two generators are operating in synchronism. The operation of getting the generators into synchronism is called synchronizing.

Construction: Basic Parts of AC Motors

Enclosure, Stator, Rotor, Bearings, Conduit Box,

Enclosure

consists of a frame/yoke and two end brackets. Holds and protects the internal components of the motor.

Open Closure

permit cooling air to flow through the motor. One type of open closure is called the open drip proof (ODP) enclosure. This type has vents that allow the flow of air, and is positioned at angles up to 15° to prevent solids and liquids falling from above.

Totally Enclosed Enclosure

Non-Ventilated

limits the flow of air, but is not airtight.

Fan-Cooled

similar to non-ventilated, but has an external fan mounted opposite the drive end of the motor.

Explosion-Proof

commonly found in hazardous applications, and must comply with the strictest safety standards for protection.

Stator

has two main parts, the stator core and stator windings. Stator core is made up of thin metal sheets called laminations, used to reduce energy losses. Stator windings are directly connected to the power source during operation.

Rotor

rotating part of the motor. Magnetic field from the stator induces an opposing magnetic field which causes the rotor to "push" away from the stator field.

Bearings

mounted on the shaft and supports the rotor to turn. Must have certain qualities such as load carrying capacity in the axial and radial direction, over speed and duration, rotating speed, and bearing life.

Conduit Box

metal box which protects on or more terminals or terminal boards

Single-Phase Motors

Field windings are directly connected to a single-phase source.

Commutator Motor

AC Series Motor and Repulsion Motor

AC Series Motor

Also known as universal motor. It is when an ordinary dc series motor is connected to an ac supply. It has low running torque due to low current. Its operating characteristics are similar to that of a dc series motor, meaning the speed increases to a high value as the load decreases.

Repulsion Motor

It has an armature and commutator, similar to a dc motor, however the brushes are short-circuited, not connected to the supply. The stator windings produce a current in the rotor windings by induction.

Single-Phase Motors

Field windings are directly connected to a single-phase source.

Induction Motor

The magnetic field set up in the stator by the ac power supply stays lined up in one direction. Even if it is stationary, it pulsates like a voltage sine wave. The pulsation induces voltage in the rotor windings. It is not self-starting, which is why it is necessary to use an auxiliary device to turn the rotor.

Split-Phase

Two stator windings (main and starting winding) of unequal impedance are spaced 90 degrees apart but connected in parallel to a single-phase source, the field produced will appear to rotate.

Capacitor Start Motor

a capacitor is placed in series with the starting winding, which improves starting characteristics

Permanent Capacitor Motor

permanent capacitor operates with an auxiliary winding and series capacitor

Repulsion Start Induction Motor

operates similarly to a dc motor, wherein the rotor of the repulsion-start induction motor has windings connected to a commutator. As the starting brushes make contact with the commutator, the motor starts as a repulsion motor.

Shaded-Pole Motor

produced by a short-circuited coil wound around a part of each pole of a motor. The coil produces a small sweeping motion as the field pulsates. As the field in the pole piece increases, a current is induced in the shading coil, which creates an opposing magnetic field.

Synchronous Motor

these are used to drive electric clocks, phonograph turntables, and other devices that requires precise rotation.

Warren synchronous motor

which starts by using shading coils in the pole piece. Its principal usage is in clocks and other timing devices

Polyphase Motors

Induction Motors

most commonly used type of ac motor due to its simple, rugged construction, and good operating characteristics. Most important characteristic is it is a three-phase motor.

Three-phase motor

machines have three windings and deliver an output between several pairs of wires.

Squirrel-Cage Motor

the rotor has a laminated core, with the conductors placed parallel to the shaft and embedded in slots around the perimeter of the core. Rotor conductors are not insulated from the core. If laminations were not present, the rotor conductors and end rings would resemble a squirrel cage.

Wound Rotor Motor

the rotor is wound with an insulated winding similar to a stator winding. The rotor winding is not connected to the supply, as it uses slip rings and brushes to provide a connection

Synchronous Motors

operates similarly to an induction motor, however the main difference is the rotor circuit of a synchronous motor is excited by a dc source.

Speed and Slip

The speed of the rotating magnetic field is called the synchronous speed of the motor.

Rotor Frequency

For any value of slip, the rotor frequency is equal to the stator frequency times the percent slip

Torque

Depends on the strength of the interacting rotor and stator fields, and the phase relations between them.

DC MACHINES

DC Generators

Electrical components that convert mechanical energy into electrical energy by electromagnetic induction. Supplies the needed electrical energy on electrical components for aircraft operation.

Types of Generators

AC DC Generator

DC Generator

also known as Dynamo, converts alternating current to direct current using a commutator. Flows in one direction

AC Generator

also known as Alternators, converts mechanical energy into electrical energy in the form of alternating current. Current periodically reverses direction.

Construction/Components

Yoke/Field Frame, Electromagnet, Armature Assembly: Armature Core and Armature Coil/Armature Winding, Lap Winding, Wave Winding, Armature Assembly: Commutator and Brushes

Yoke/Field Frame

outer frame of a dc generator. Provides mechanical strength to whole assembly. Carries the magnetic flux produced by the field winding.