Motors & Generators Presentation

7/26/2019 Motors & Generators Presentation

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Girraween High School


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1. Motors use the effect of forces

on current-carrying conductorsin magnetic fields


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The force on a charged particle in a magneticfield is given by:

F is force,B is magnetic field strength,q is the charge,v is particle velocity,q being the angle between the direction of v

and the direction of the magnetic field

qsinqvBF


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The force on a current carrying wire in amagnetic field is given by:

F is force,B is magnetic field strength,I is current,l is length of the wire within the field,q being the angle between the wire and the

direction of the magnetic field

qsinBIlF


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But while it is easy to calculate themagnitude of the force on the wire,

determining the direction is not obvious, sowe have an aid in the right hand rule.

qsinBIlF


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You can visit these sites to learn more and practice with the righthand rules:

http://www.phy.syr.edu/courses/video/RightHandRule/index2.html

http://www.phys.ufl.edu/~phy3054/magnet/mforce/rhr/Welcome.html
http://www.phy.syr.edu/courses/video/RightHandRule/index2.htmlhttp://www.phys.ufl.edu/~phy3054/magnet/mforce/rhr/Welcome.htmlhttp://www.phys.ufl.edu/~phy3054/magnet/mforce/rhr/Welcome.htmlhttp://www.phys.ufl.edu/~phy3054/magnet/mforce/rhr/Welcome.htmlhttp://www.phys.ufl.edu/~phy3054/magnet/mforce/rhr/Welcome.htmlhttp://www.phy.syr.edu/courses/video/RightHandRule/index2.html


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Example

A 15 cm length of wire carrying a current of2.5 A lies at an angle of 30 to a magnetic fieldof 0.6 T. Calculate the force on the wire due to

the field.

L= 0.15 mI

= 2.5 AB = 0.6 Tq = 30

pagetheofoutN1125.0

30sin15.05.26.0

sin

qBIlF

BI30

+

-


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A DC power line 250 m long is oriented NESW inthe Earths magnetic field that has a strength of

1.5 x 10-5 T in the northsouth direction (Hint: TheNorth Pole is a south pole and the South Pole isa north pole). If the current carried by the

powerline is 150 A flowing from the NE calculatethe magnitude and direction of the force exertedon the powerline due to the interaction of thecurrent carrying wire and the magnetic field.


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Directed down


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Two parallel current carrying wires exert forces on eachother:

F is force,k is the magnetic constant 2 x 10-7 T m A-1.I1, I2 are the currents in the wires,l is length of the wires overlap,dis the perpendicular distance between the wires

d

IIklF 21


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Example Problem 1

Find the magnitude and direction of the

magnetic force between two long parallelconducting wires of length 2 m that are 0.1m apart if both carry a current of 5 A when:

a) The currents are in the same direction

b) When the currents are in opposite

directions


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repulsive

attractive

attractive


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The direction of the currents dictates the direction of the force


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Torque is defined as the turning moment of aforce:

F is the applied forcedis the perpendicular distance from the pointof application of the force to the axis ofrotation

Fd


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Torque on a coil is defined as :

n is the number of turns in the coilB is the magnetic field strength

Iis the current flowing through the coilA is the area of the loop

qis the angle between the magnetic field and the plane ofthe coil.

q cosnBIA


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q sinFd


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-1.5

-1

-0.5

0

0.5

1

1.5

0 50 100 150 200 250 300 350 400

Arbitary units

Force and Torque on a Coil in a DC motor

force torque


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qsinBIlF

q cosnBIAThese angles are not the same, as

they lie in different planes!

Angle between direction of wire

and direction of B field

Angle between directionof plane of coil and

direction of B field


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You are to obtain an image of a DC Motorand then label the following parts:

Brushes Commutator Coil or rotor loop

Tabulate the parts and their functions.


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2. The relative motion between a

conductor and magnetic field isused to generate an electrical

voltage


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Induction can be defined as a process where

one object with magnetic or electricalproperties can produce the same propertiesin another object without making physicalcontact.


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Year of discovery 1831

Electromagnetic induction is the generationof an emf and electric current through theuse of a magnetic field.


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When the current was flowing through theprimary coil, the galvanometer moved.


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When the current flows through the primarycoil a magnetic field is induced around that

coil. The second coil, which lies within thismagnetic field has electrons free to movewhich feel the force of the motor effect. Theystart to move, creating an electrical current.

When there is current flowing in the secondcoil the galvanometer will move.


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The word flux comes from theLatin wordfluo meaning flow.

Flux is a state of flowing ormovement.

In physics, flux is therate of flow of a fluid, radiation orparticles.


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Magnetic flux, B, is the amount of magneticfield passing through a given area. In the SI

system, B is measured in weber (Wb).


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Symbol: B

The strength of a magnetic field, B, is alsoknown as the magnetic flux density. In the SIsystem, B is measured in tesla (T) or weberper square metre (Wb m2).


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The generated potential difference is afunction of the time rate of change of the

applied magnetic field


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The induced emf in a circuit is equal in magnitudeto the rate at which the magnetic flux through

the circuit is changing with time.

Faradays law can be written in equation formas:


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An induced emfalways gives rise to a

current that creates a magnetic field

that opposes the original change in flux

through the circuit.


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3. Generators are used to

provide large scale powerproduction


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Comparing a motor to a generator


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Comparing a motor to a generator


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The electrical resistance of the wires is a fixedquantity.

Recall that

= 2

So power consumption by the wires is

proportional to 2.


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Re-do the previous calculation with thevoltage in the wire set at 110,000 Volts.


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4. Transformers allow generated

voltage to be either increasedor decreased before it is used


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A transformer is a magnetic circuit with twomulti-turn coils wound onto a common core.


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nn

VV

s

p

s

p


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nn

II

s

p

p

s


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5. Motors are used in industries

and the home usually toconvert electrical energy into

more useful forms of energy


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Motors & Generators Presentation