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