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Magnetism lecture
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L11 Machines using magnetism
At the end of this lecture, you should be able to: At the end of this lecture, you should be able to:
1. Give examples of where magnetism is used
2. Describe the relation between electric current and magnetism
3. List the factors to consider when choosing a motor
Examples of magnetism and
its uses
Electric power generation
Motors for heating, air-conditioning,
pumping water / gas / oil, machinery etc.pumping water / gas / oil, machinery etc.
Wire-free charginghttp://www.pcworld.com/article/248198/qualcomm_halo_wireless_inductive_charging_for_electric_vehicles.html
RFID http://www.techweekeurope.co.uk/news/scientists-print-cheap-rfid-tags-on-paper-59911
Electric car http://www.youtube.com/watch?v=G-oK-3SRPic
MRI http://www.youtube.com/watch?v=6_2D3Lh1v74http://www.youtube.com/watch?v=4uzJPpC4Wuk
Magnetism
Current - moving charge - creates a magnetic
field
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfie.html#c1
http://www.youtube.com/watch?v=ys9xL3mw8tI
Why does moving charge create magnetism?http://galileo.phys.virginia.edu/classes/252/rel_el_mag.html
Magnetic flux density B= mag flux per m2
(Webers per m2, or Tesla, T)
Total magnetic flux (Webers, Wb)
Magnetic field strength H (A m-1)like voltage (gradient)
(Webers, Wb)like current
Magnetic flux density B= mag flux per m2
(Webers per m2, or Tesla, T)
Magnetic field strength H (A m-1)like voltage (gradient)
Iron / Steel
B = HMagnetic permeability
= o x r
4x10-7 pi H/m relative permeability
Magnetic circuit
Electric circuit
A piece of wire, length l in a magnetic flux density Bcarrying a current Iexperiences a force F
F = (I l) BF = (I l) B(cross product of current vector and magnetic flux density)
(2) A company advertises a "1 Tesla" magnet for sale.
How much magnetic flux does it produce? We dont know : total flux = flux density (Tesla)area
(3) In an electric motor, a single turn of the rotating
armature coil is 20 cm long, in a magnetic flux density
of 0.3 Tesla, carrying a current of 20 A. How much force
Questions
20 cm long, 10 A
0.3 T
of 0.3 Tesla, carrying a current of 20 A. How much force
is created on it, and in what direction?
F = IlB = 200.20.3 = 1.2 N20 cm long, 10 Acurrent
Magnetic induction
If the magnetic flux through a loop of wire is , then a voltage will be induced in the voltage will be induced in the loop if changes :
V = d/dt(normally written with a sign to show the
voltage opposes the flux change)
Equation not needed for class test or exam
Summary of theory
1. Current moving charge creates a magnetic field
2. Magnetic field strength (H, A/m) is like voltage
3. Magnetic flux (, Webers) is like current4. Magnetic flux density is (flux / area) (B, Tesla)4. Magnetic flux density is (flux / area) (B, Tesla)
5. Time-varying flux induces a voltage in a coil enclosing it
6. Think carefully before designing something that has high
current or high frequency cables next to metal or other
cables
Applications : Relays
Use the magnetic field from a
small current to close a
switch which can carry a large
currentV
S C B
L
Mechanical - can fail
eventually
Slow switching speed
Can handle very large current
/ high voltage
Very low loss
V0
A
L
RL RC
Relays : Domestichttp://www.maplin.co.uk/Module.aspx?ModuleNo=37495
Maximum current 30AMaximum voltage 300Vac, 28VdcMaximum switching power
9000VA resistive840W resistive
Contact resistance 10 million operationsElectrical life >100,000 operations at full loadContact material AgCdO (Silver Cadmium oxide)Coil nominalvoltage
Operaterange(V)
Coilresistance
Current atnominal voltage
12Vdc 9.6-13.2 1205% 100mA
5.20
Relays : High voltage
http://library.abb.com/global/scot/scot245.nsf/veritydisplay/ec0a90a6b482e53d85257554005348c1/$File/121-169PMI_2GNM110055_new.pdf
http://etecindsvc.com/index_002.htm
Transformers and Induction
AC in coil 1 creates an alternating magnetic flux,
shared by both coils
This induces voltages in coils 1 and 2
V1/n1 = V2/n2 I1n1 = I2n2 V1I1 = V2I2
1 2
http://www.youtube.com/watch?v=nWTSzBWEsms
Transformers
Type Secondary Size(WxHxD)*6V 6-0-6V 36 x 31 x 34mm
All types have a 250mA output, centre-tapped. All primaries are 0 to 240V. Available in 6V, 9V, 12V and 15V types
http://www.maplin.co.uk/Module.aspx?ModuleNo=3688
http://www.meppi.com/Products/Transformers/Power/Pages/Core-formTransformers.aspx
3
Inductive charging?
http://en.wikipedia.org/wiki/File:Inductive_charging.svg
Over 95%efficiency is possible http://www.witricity.com/pages/faq.html
DC / AC Motors + Generators motor types overview at
http://zeva.com.au/Tech/
DC motors typically need brushes to
supply current to the armature and a
commutator to reverse it every cycle
Different designs have different Different designs have different
torque / speed characteristics
They can easily run at variable speed
AC motors can work without brushes
They require electronic control to run
at variable speed
http://www.youtube.com/watch?v=Xi7o8cMPI0E
http://web.ncf.ca/ch865/englishdescr/DCElectricMotor.html
AC Motors / Generators
Unfortunately there are many types of motors
Key point : Do they have sliding contacts (brushes), which waste power and
wear out, requiring maintenance?
Yes : Brushed DC motor; slip-ring or universal AC motor*
No : Brushless DC motor; AC induction motor; AC synchronous motor*
Brushed DC motors have simple speed control : more voltage = more speed
Brushless DC motors need an external circuit to work at all.
1919
Brushless DC motors need an external circuit to work at all.
Brushless AC motors run at (almost) fixed speed relative to the frequency of the
supplied AC. They need a variable-frequency AC supply (an inverter) to give
good variable speed control.
Three phase AC motors run more smoothly and give more power per size &
mass than single phase AC.
When choosing, consider the lifetime cost of a motor system performance,
initial cost, energy used (including power factor effects), maintenance.
* dont need to know stuff in grey