Flemming ' Rule

7/31/2019 Flemming ' Rule

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Three-Phase AC machines

Introduction to Motors and Generators

Resource 1


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Three-Phase AC MachinesResource 1

Aims


To provide an understanding of the motor and generator effect that links electricityto magnetism

To provide an understanding of how to apply Flemings left and right hand rules.


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Objectives At the end of this lesson you should be able to: Describe the effects of placing a current carrying conductor in a magnetic field Perform simple calculations for the force on a conductor in a magnetic field Apply Flemings Left Hand Motor rule Describe the effects of moving a conductor through a magnetic field

Perform simple calculations for the induced EMF across a conductor moving through amagnetic field

Apply Flemings Right Hand Generator Rule Describe the effects of passing a current through a coil of wire to form an

electromagnet

Three-Phase AC MachinesResource 1



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L

F = B I L [Newtons]

B = Density of the magnetic flux in Teslas

I = Induced current in Amps

L = Length of conductor in field in metres

Example 1If a conductor of length 0.4m carrying a currentof 10.6A is placed in a magnetic field with a fluxdensity of 0.03T, determine the forceexperienced by this conductor in newtons.

F = 0.03 x 10.6 x 0.4

= 0.1272 N

B I

F

The Motor Effect

Force

North pole

South pole


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South pole

f irst finger

se cond

finger

thu mb

Each digit of your hand must be at rightangles to both of the other two

currentfield

motion

Flemings Left Hand Rule

If the current is reversed, the direction of motion will change

The Motor Effect

L

B

North pole

Force

I


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Force

IB

F

North pole

South pole


f irst finger

se condfinger

thu mb current

field

motion

If the current is reversed, the direction of motion will change

The Motor Effect



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If the field is reversed, the motion will bein the opposite direction

The Motor Effect


Force

IB

F

North pole

South pole

f irst finger

se condfinger

thu mb current

field

motion


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IB

F first finger

thu m b


field

motion

If the field is reversed, the motion will be inthe opposite direction

se c ond finger current

The Motor Effect


South pole

North pole

Force


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field is clockwise

Current into page

field is anticlockwise

Current out of page

Using the following convention, we can show why Flemings left hand rule works

The Motor Effect


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Field lines in the same direction cause repulsion, field lines in opposite

directions cause attraction

Force

Forceattraction

repulsion

repulsion

attraction

The Motor Effect

South Pole

North Pole

South Pole

North Pole


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The force on a conductor can be increased by forming a single turn coil

Blue spot representsthe central pivotpoint

The Motor Effect

North Pole

South Pole


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Top conductorexperiencesforce to left

The Motor Effect

North Pole

South Pole

Force


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Top conductorexperiences force toleft

Bottom conductorexperiences force toright

The Motor Effect

North Pole

South Pole

Force

Force


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Combined action causes rotation

The Motor Effect

Top conductorexperiences force toleft

Bottom conductorexperiences force toright

North Pole

South Pole

Force

Force


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For a multi-turn coil

n = number of coil turns

Torque producedT = 2 n F r

F = force on single conductor

r = radius of coil

The Motor Effect

T T

North Pole

South Pole


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Example 2A 100 turn coil has a radius of 0.1m and a lengthof 0.15m. It is placed at right angles in a magneticfield of flux density 0.08T and carries 12A,

calculate the force on each conductor and thetotal torque produced by the coil.

Torque producedT = 2 n F r

F = B I L = 0.08 x 12 x 0.15

= 0.144 N

T = 2 n F r = 2 x 100 x 0.144 x 0.1

= 2.88 Nm

The Motor Effect

For a multi-turn coil

T T

North Pole

South Pole


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e = B L v [Volts]

B = Density of the magnetic flux in Teslas

v = velocity in metres per second

L = Length of conductor in field in metres

Example 3Calculate the EMF induced across the ends of awire of length 0.3m when it is moved through amagnetic field of flux density 0.015T at a speedof 50m/s..

e = 0.015 x 0.3 x 50

= 0.225 Volts

I

The Generator Effect

L

e

B

v

+

-

North pole

South pole

Velocity


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If the motion is reversed, the polarity of EMF will change and the current will bereversed

I


f irst finger

se condfinger

thu mb


current

field

motion

Flemings Right Hand Rule

L

e

B

v

+

-

North pole

South pole

Velocity


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f irst finger

se cond finger thu mb


current

field

motion

If the motion is reversed, the polarity of EMF will change and the current will bereversed



L

e

B

v

I

-

+

South pole

North pole

Velocity


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f irst finger

se condfinger

thu mb


current

field motion


If the field is reversed, the polarity of EMFwill change again and the current will bereversed again

L

e

B

I

+

-


Velocity

North pole

South pole


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An EMF can be generated in a rotational motion by forming a coil

EMF generated in both sides of the coil add up


North Pole

South Pole

Motion

Motion


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An EMF can be generated in a rotational motion by forming a coil

Example 4A 200 turn coil has a radius of 0.12m and alength of 0.23m.It is placed in a magnetic field of fluxdensity 0.06T and rotated at 3000rpm.When the coil is in its vertical position atright angles to the field, calculate (a) theEMF on each conductor (b) the total EMFproduced by the coil.

v = 2 r N m/s60

E = 2 n e Volts

e = B L v Volts

v = 2 x 0.12 x 3000

60

= 37.7 m/s

e = 0.06 x 0.23 x 37.7

= 0.52 Volts

E = 2 x 200 x 0.52

= 208.1 Volts


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Electromagnetism

When a coil is formed of many wire turns, the magnetic fields around each wire add up toproduce a strong electromagnet.

One side of this magnet will be a North Pole while the other side will be a South Pole

If the current in the electromagnet is reversed, the magnetic poles will swap sides.


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Electromagnets are used in motors and generators so that the strength of the field can be

varied.

If the coil is wrapped around a soft iron core, the electromagnetic field becomes much stronger.

In a motor, this affects the speed and torque produced. In a generator, it affects the voltagegenerated.

Electromagnetism


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DC motors

AC induction

AC synchronous

Series Field

Shunt Field

Compound Field

Squirrel Cage

Slip ring wound rotor

Salient Pole

Cylindrical

Further Study Types of motor


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Shunt Field Compound Field

Torque

Speed

Series Field

Further Study - DC Motor Performance


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Further Study - AC Motor Performance

Synchronous Wound inductionCage Induction

Speed Speed

Flemming ' Rule

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