Download AAAAMotors and Magnets

Motors and Magnets

Electricity and Magnetism
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When current passes through a piece of wire, it generates a
xxxsmallxxx
magnetic field. The greater the current flowing through the wire, the greater the
field
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The field gets stronger closer to the wire, and decreases further from the wire
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xxxIn the majority of materialsxxx, the magnetic field will
exist only when current is flowing.
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The
field is represented as “lines of force”
around the wire
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The direction of the field will depend on the direction of the current flow through
the wire
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LEFT HAND RULE FOR MOTORS AND RIGHT
HAND RULE FOR GENERATORS.
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Couch this in terms of a motor:
The direction of the field
can be determined by the ‘right hand rule’. This rule shows that if you were to hold
the wire with your four fingers, and use your thumb to point in the direction of the
conventional current flow, your four fingers will show the direction of the magnetic
field
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The ‘left hand rule’ is when you use your thumb to point in the direction
XXXXof the electron flowXXX The rules use
conventional current flow.
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Figure 1a shows the magnetic field around a wire as current is passing through
called the right hand grip rule
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On its own, the magnetic field produced by one wire is usually not much
it
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If two or more wires producing magnetic fields that are travelling in the same
direction are placed close enough to each other, the magnetic fields are added
together
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Figure 1b shows two wires close together. The current is flowing in the same
direction through both wires, so the magnetic fields travel in the same direction.
Both the fields join together to make one field
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Rather than produce multiple strands of wire to create magnetic fields, one long
strand of wire could be wrapped around in a spiral formation
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Because each loop of the spiral passes next to the previous loop, the magnetic field
is greatly increased. This is called a coil
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Figure 2 shows a coil with the magnetic field around it
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The magnetic field will travel down through the centre of the coil, then loop back
outside the coil, then back through, etc
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The magnetic field is not created instantly as soon as current begins flowing through
it. It takes a few moments for the field to build
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When the current stops
flowing, the magnetic field
collapses and produces a current in the coil
(wire) that is in the opposite direction to the
original current. This is the most important
thing about inductors.
XXXis not collapsed instantly either. It will
continue the flow of the current until the field has collapsedXXXX
o XXXXXXThe unit of measurement for magnetic
field strength, it’s inductance,XXXXX NO.
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is called the ‘Henry’, which is represented by the symbol ‘H’
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Producing a magnetic field by means of electricity is an electromagnet
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If the coil is wound around an iron core, the iron core will behave like a bar magnet,
with a North and South end
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Just as electricity can create magnetic fields,
moving
(increasing and/or decreasing)
electricity
magnetic
fields
can be used to create
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If a magnetic field that is encompassing a wire changes
(rapidly),
it will
generate electricity in that wire
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This is the general concept
of how generators work. They move or spin a
magnet which in turn generates electricity

Inductors
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An inductor is a basic
(electronic) component that is simply a coil wrapped
around a core
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Another name for an inductor is a ‘choke’
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The material that makes up the core will vary for different types of inductors. It can
even be air
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Figure 3 shows the schematic symbol for an inductor
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When current first starts flowing through an inductor, the inductor starts to build a
magnetic field
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During the time it takes for the field to build, the inductor will (inhibit)
restrict
or limit the flow of current
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Once the field has been built, current will flow normally through the wire
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When the current stops flowing, the magnetic field will begin to collapse, and it will
xxxsustain the flow of currentxxx OPPOSITE
DIRECTION until the field is gone
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Due to the behaviour of the inductor with building and collapsing fields, it has the
ability to resist any instant change in current
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Due to its ability to resist change in current, an inductor can be effective to filter AC,
but still allow DC to flow
o Inductors have a value called ‘Q’ which is their quality factor. XXXXThe
higher resistance of the coil, the lower the
QXXXXXXX

Relays
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A relay is a switch that is operated electrically
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Figure 4 shows the schematic symbol for a relay
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When current passes through a coil within the relay, the coil creates a magnetic field
which causes a lever (the switch) to move and switch contacts
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Relays can have multiple switch poles
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The relay will typically draw a relatively large current of about 30mA for a 12v relay.
They can go higher though
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If an IC can’t supply this much current, a transistor can be used to amplify the
current
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The terminals on a relay are marked COM, NC and NO
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COM is the common terminal. NC means normally Closed, and NO means normally
open
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NO is very common, and means that the switch is normally in an open state, and the
relay can switch it to closed, to allow current to flow through it. COM will connect to
this when the relay coil is on
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NC means that it is normally a closed circuit, and the relay can open it to stop the
current flow. COM will connect to this when the relay coil is off
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When relays are switched off, they can supply high voltage spikes which can destroy
other components
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To protect the other components, a protection diode is used
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Figure 5 shows a relay with a protection diode
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When the relay is switched off, its magnetic field has to collapse. However, it
collapses too quickly which causes a high voltage spike
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Putting the diode in parallel allows the magnetic field to collapse more slowly, which
avoids the voltage spike

Transformers
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A transformer is used to convert an AC voltage to a different voltage level
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The AC frequency stays the same throughout the conversion
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A transformer is made up of two or more coils (inductors) that use a single core
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Figure 6 shows a schematic symbol for a transformer
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One coil is the primary coil, and the other coils are secondary coils
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The input is the primary coil, and the secondary coil is for the output
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Figure 7 shows the primary and secondary coil around a common core
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The alternating current in the primary coil creates a magnetic field that changes
direction with each cycle
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The alternating magnetic field creates an alternating
VOLTAGE
in the
secondary coil
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The voltage at the secondary coil is determined by the ratio of the number of turns
on the primary coil, to the number of turns on the secondary coil
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Eg, if there is 240v across the primary coil, which consists of 100 turns, and the
secondary coil has 20 turns, the secondary will have 48v across it

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
240 x 20 / 100 = 48v
The term for reducing the voltage is called ‘step down’
Speakers
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Figure 8 shows a schematic symbol for a speaker
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Speaker Structure:

Basic speakers are made up of a cone, a coil (called a voice coil) and a
magnet

The speaker makes sound by rapidly vibrating the cone

The cone produces sound by vibrating the air around it. When your ear
detects these vibrations, your brain interprets it as sound

The cone is attached to the voice coil, and the voice coil is placed in the
magnetic field, created by the magnet

When current flows through the coil it produces its own magnetic field. The
magnetic field from the coil is affected by the magnetic field from the
magnet. The two fields will attract or repel each other

This interaction causes the coil and the cone to move

The current going into the coil is AC, which means it is rapidly changing
direction. The magnetic field created by the coil will be rapidly changing too

The two fields will repel each other, then attract, then repel and so on very
rapidly

This means that the coil will be rapidly moving back and forth. This is what
causes the cone to vibrate

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Speakers will have red and black terminals on them
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An AC signal is fed into the speaker, which is converted to sound
Motors
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An electric motor uses electricity and magnetic fields to create movement
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The motor has a coil with a core. The core is called the armature
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The armature is held to an axle that it spins on, and is suspended in a magnetic field
that is created by a permanent magnet
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As current flows through the coil it creates another magnetic field in the armature
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This magnetic field is attracted to the magnetic field surrounding it (south pulling
north), which pulls the ends of the armature towards it
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As the armature moves around, the fields are opposed (North to North), so the fields
push each other away, continuing the motion
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As the armature continues around, it continues the cycle of being pushed and
pulled, and the armature continues to spin
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Figure 9 shows a basic look at how motion is produced
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There is a shaft attached to the armature which turns. This is the part that protrudes
from the motor casing that you would attach something to (eg a wheel)
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The motor has a positive and negative terminal which the power supply connects to
Fig. 1a – Magnetic Field Around One Wire
Fig. 1b – Magnetic Field Around Two Wires
Fig. 2 – Coil With Magnetic Field
Fig. 3 – Inductor
Fig. 4 – Relay
Fig. 5 – Relay with a Protection Diode
Fig. 6 – Transformer Symbol
Fig. 7 – Transformer Coils
Fig. 8 – Speaker
Fig. 9 – Motor
Fig. 10
– XXXSawtoothXXX / Square Wave