AEF Topic 4 -- Magnetism and Inductors

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Types of cores

- Air cores - Ferromagnetic Core

What is the right-hand grip rule?

- Conventional current is flowing in the pointing direction of the right thumb Magnetic field produced due to the current flow is in the direction of the right-hand's gripped finger

How to make a simple electromagnet?

- Use a piece of ferromagnetic material with turns of wire around it - When current flows through the wire, the ferromagnetic materials magnetized - When current flow stops, electromagnet loses its magnetism

What is Eddy Current loss?

- Varying flux in the core of the magnetic circuit induces EMF in the core material and hence causing current to flow - The resultant resistive energy loss is Eddy Current loss

Electric Circuit, Current

Ampere (A)

Magnetic Circuit, MMF

Ampere (A)

Magnetic Circuit, Reluctance, S

Ampere/Weber (A/Wb)

Magnetic Circuit, Magnetic Field Strength, H

Ampere/metre (A/m)

Electric Circuit, Current Density

Ampere/metre2

Unlike poles

Attract

Magnetic shield provides low reluctance path for the interference field by?

Attracting flux lines to it so as to divert the magnetic field away from the sensitive components or any other components

Most common use of electromagnets inductance

Generators

H = MMF/I = IN/I

I is the length of the coil

What is Lenz's law?

Induced EMF generates a current that induces a counter magnetic field that opposes the magnetic field generating the current

What happens if two inductors are positioned so that all the flux of one inductor cuts all the turns of the second?

Inductors have a unity coefficient of coupling

Coefficient of couple SI symbol

K

Magnetic force is strongest _____

Near the poles and visually the magnetic flux are converging at the poles. There is a "neutral zone" at the centre of the magnet.

Properties of magnet

North pole end point to the Earth's magnetic pole and its south pole end to the Earth's south magnetic pole

Electric Circuit, Resistance

Ohm ()

What are unintentional magnetism?

Older versions of Cockpit Voice Recorder (CVR) or Flight Data recorder (FDR) used permanent magnets in their recording heads. thus may introduce unintentional magnetism to the tools that worked on the equipment

What is Magnetic reluctance

Opposition to the creation of magnetic lines of flux in a magnetic circuit.

Electron revolving around its nucleus produces an ________

Orbital magnetic dipole moment

Ferromagnetism

Phenomenon where certain material forms permanent magnet and/or exhibits strong interactions with magnets

Saturation

Phenomenon whereby the material

Inductors are used in ___

Radio receivers where it couples with capacitors to create resonance circuit

What is relative permeability

Ratio of the absolute permeability of a specific medium to the permeability of free space.

The use of soft iron, silicon, steel, nickel-iron alloys and soft ceramic ferrite compounds _____

Reduces Hysteresis loss

Like poles

Repel

How to find the direction of the magnetic field?

Right-hand grip rule

Magnetic reluctance

SI unit of measurement = Ampere per Weber (A/Wb) SI symbol = 'S'

Magnetic field strength

SI unit of measurement = Ampere per metre (A/m) SI symbol = 'H'

Inductance

SI unit of measurement = Henry (H) SI symbol = 'L'

Mutual Inductance SI

SI unit of measurement = Henry (H) SI symbol = 'M'

Comparison between magnetic materials (permeability)

Soft magnetic materials --> High permeability Hard permeability --> Low permeability

Comparison between magnetic materials ( hysteresis loss)

Soft magnetic materials --> Low hysteresis loss Hard magnetic materials --> High hysteresis loss

Comparison between magnetic materials ( value of remnant flux)

Soft magnetic materials --> Low value of remnant flux High magnetic materials --> High value of remnant flux

Comparison between magnetic materials (Saturate value)

Soft magnetic materials --> Saturate at low H value Hard Magnetic Materials --> Saturate at high H value

Comparison between magnetic materials (Coercive force)

Soft magnetic materials --> low coercive force hard magnetic materials --> High coercive force

Magnetic Circuit, Magnetic Flux Density, B

Tesla (T)

What happens when two inductors of mutual inductance M in separate circuits, change in current I in the first inductor in t seconds causes an EMF to be induced in the second inductor?

The induced EMF tends to circulate a current in the second circuit that opposes the increase in flux due to the increase in current in the first circuit. Therefore, it can also be viewed as a rate of change of flux is created by the second circuit.

What happens when the H field is zero?

The residual magnetism retained is the B/H characteristics

What happens when two parallel current flow in the same direction?

They attract each other

What happens when two parallel currents flow in opposite directions?

They repel each other

What is electromagnets?

Type of magnet in which the magnetic field is produced by the flow of an electric current. Magnetic field disappears when the current ceases

Magnetomotive force (MMF)

Unit of measurement = Ampere-turns Symbol = 'F'

Moving coils in speakers

Uses the motor principle, worked pretty much the same as the moving coils in meters

How do you remove the effects of unintentional magnetism?

Using a degausser, a modern equipment in the aircraft has the function built-in

Electric Circuit, EMF

Volt (V)

Magnetic Circuit, Magnetic Flux,

Weber (Wb)

Mutual Inductance

When two coils comes close to each other, the magnetic field of one coil will affect the other. As a current changes in one coil is accompanied by a change of flux, an EMF is induced on the other coil.

Hard magnetic materials have high hysteresis loss they are ____

well-suited to be used as permanent magnets and are commonly found in motors, generators, moving coil microphones/meters. Some examples include carbon steel, tungsten steel, cobalt steel and hard ferrites.

How is Eddy Current loss reduced?

- Core of a magnetic circuit is good electrical conductor, large amount of current will flow and this causes excessive heating resulting in significant energy loss. - Core is made of __ that are insulated from each other

What are the factors affecting inductance

- Cross-sectional area of the coil - Number of turns in the coil, N - Length of the coil - Permeability of the core material - Space between the turns of the coils

Factors affecting mutual inductance between two coils

- Cross-sectionals area of the coils - Number of turns in each coil - Length of the coils - Permeability of the core materials - Space between the turns of the coils

Hysteresis loss

- Energy wasted to "reverse" the retention of magnetism in material

Air core

- Have advantage of linear B-H characteristics - Due to low permeability, the inductance is low - Used in high frequency application

F= IN

- I is the current flowing through the coil N is the number of turns in the coil

Magnetic sources

- Intrinsic magnetism of elementary particles - Motion of electric charges

The permeability constant

- Magnetic constant or the permeability of free space, is a measure of the amount of resistance encountered when forming a magnetic field in a classical vacuum. - Magnetic constant has the exact value 0 = 4π×10−7 Hm−1 or NA−2

What happens when the H field is reduced?

- Magnetism is retained by the material

How should magnets be handled or stored?

- Never allow two magnet to come together abruptly - Be careful not to let your fingers get caught in between two magnets - Never allow magnets near computer disks, displays, recording tapes of any kind, or any other device that uses magnetism to record information - Should be stored to avoid high temperature or high humidity as well as avoid storing near magnetically sensitive materials

Metal detectors contain

- One of more inductor coils and when metal passes through the magnetic field generated by the coil or coils, the field induces eddy current in the metal - Current in turn produces a field that generates current in the detector to power a signal indicating the presence of metal

What to do when unwanted magnetism is suspected on an aircraft?

- Position is identified using a magnetometer - Once located, portable degaussing tools may be used to demagnetize the affected components

Ferromagnetic core

- Produce very high inductance - Due to high permeability B-H characteristics are not linear - Inductance varies with current flows - Have pretty good B-H linearity - Limited to low frequency applications

Coefficient of coupling

-Factor that must be taken into account in mutual inductance - Ratio of flux cutting one inductor to the flux originating in the other inductor

Basic properties of the magnetic flux

1. The direction of a line of magnetic flux is "out-flowing" from the north pole of a magnet and "in-flowing" at the south pole of a magnet. 2. Lines of magnetic flux never intersect. 3. Each line of magnetic flux is continuous from the north pole to the south pole or is a closed loop. 4. Lines of magnetic flux that are parallel and in the same direction repel each other (repulsion). 5. Lines of magnetic flux always trying to shorten as far as possible (attraction)

Solenoid

A coil of wire with a length significantly larger than its diameter. When current flows in it, the magnetic fields from each turn add up to give a resultant magnetic field with lines entering one end and leaving at the other. Looking at the left hand side, the current flow is anti-clockwise hence the left hand side is a North pole and vice-versa

Retentivity

Capacity for a material to remain magnetized after the magnetizing field is removed.

What is faraday's law of induction?

Changing magnetic flux passing through a conductor will induce an EMF and the size of the induced EMF is proportional to the rate of change of flux.

What is Eddy Current loss and hysteresis loss known as?

Core losses

Moving cells in microphone

Coupled with a permanent magnet, uses the electromagnetic induction principle where a B field together with the motion caused by vibration from sound energy generates an induced EMF

Moving coils in meters -- Inductor applications

Coupled with permanent magnet, uses the motor principle where a B field together with a current results in a force

Electric Circuit, Current formula

Current = EMF/Resistance

Middle finger electromagnetic induction

Direction of conventional current flow

Index finger electromagnetic induction

Direction of field (from north to south)

Thumb electromagnetic induction

Direction of motion (force)

"Unintentionally magnetized" tools can cause ____

Disorientation in other cockpit equipment when they are used to repair the equipment, such as causing displays to exhibit a "rainbow" effect

Imaginary lines of magnetic flux

Drawn to aid visualizing the force exerted by the magnet

In a magnetic circuit, the magnetic flux is due to _____

Existence of a magnetomotive force (MMF) cause by current flowing through one or more turns.

Magnetic Circuit, Flux formula

Flux = MMF/Reluctance

Reluctance is directly proportional to _____

Length of the magnetic circuit, and inversely proportional to the permeability and cross-sectional area of the material

What does a high retentivity ensure?

Longer life for signals recorded on the media

What happens if the magnetic circuit is uniform, with the same cross-sectional area?

MMF per unit length is known as the magnetic field strength

Hysteresis loop

Magnetic energy is required to overcome this residual magnetism when the H field is applied in the opposite direction and the magnetic field strength is the _____________. This is true when the H field is reversed again and the alternating magnetic characteristics is the hysteresis loop.

What happens when electric current is flowing though a conductor?

Magnetic field is produced around the conductor.

Magnetic field is also known as ____

Magnetic flux density

Metal detectors operations based on

Principle of electromagnetic induction

What is degaussing and demagnetizing?

Process of reducing or removing unwanted magnetism retained in certain materials caused by magnetic hysteresis.

What is self inductance

Property of a circuit that induces an EMF in itself due to changing current and changing flux around the same circuit

What is magnetic shielding required to do?

Protect electronic equipment DC and low frequency AC circuits from interfering magnetic fields

Permeability

SI unit of measurement = Henry per metre (Hm−1) or Newton per ampere squared (NA−2) SI symbol = ' '

Magnetic flux density

SI unit of measurement = Tesla (T) SI symbol = 'B' where 1 T = 1 Wb/m2

Magnetic flux

SI unit of measurement = Weber (Wb) SI symbol = ''

Why is B/H characteristics non-linear when it reach a certain point?

Saturation

Comparison between magnetic materials (difficulty to magnetize/demagnetize)

Soft magnetic materials --> Easy to magnetize/demagnetize Hard magnetic Material --> Difficult to magnetize / demagnetize

What is Permeability

ratio of magnetic flux density to magnetic field strength


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