AEF Topic 4 -- Magnetism and Inductors
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
