Chapter 17 Powerpoint

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16 Νοε 2013 (πριν από 3 χρόνια και 11 μήνες)

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Chapter 17

Electromagnetic Induction

The prime link between
electricity and magnetism is…

MOTION

Magnetism Creates Electricity Through Motion

Electricity Creates Magnetism Through Motion


Anytime a
charged
particle moves, a
magnetic field is generated


Magnetism Creates Electricity Through Motion

Electricity Creates Magnetism Through Motion


Anytime a
charged
particle moves, a
magnetic field is generated


If the direction of the current changes, the
resulting magnetic field reverses as well

If a conductor (wire) is passed through
a magnetic field a voltage is induced
across the conductor. (Current flows
through the wire)


N

S

This phenomenon is called
electromagnetic

induction

Left Hand Rule

The magnetic field with circular lines of forces is
in a plane perpendicular to the current in the
wire.

Determining Magnetic Polarity
(N
vs

S)


You can also determine magnetic
polarity using the Left Hand Rule.








Using your left hand, curl your fingers
in the direction that current is flowing
through a coil of wire.

Explaining
Left hand rule
vs

Right hand
rule


In our class we use the left hand rule
because we use the electron flow
model.



If we were using the traditional
current flow model, the right hand
rule would be used


It is useful for illustration purposes to denote
when current is going toward you or away
from you.


X means current is going into the board. A dot
means is coming out.






Notice the direction of magnetic field follow
the left hand rule.


Notice current going in the same direction
aids each other’s magnetic field.


This is why current going through a coil
makes a large electromagnet! (See next slide)

Remember! You can go from electricity to
magnetism, or magnetism to electricity.


You can either move a magnet
through a coil of wire inducing
current, or…




You can move current through a coil
of wire inducing a magnetic field.


If the magnet inside the coil of wire
below was stationary, would an
electromagnetic field be induced?

Faraday’s Law

“A
voltage is induced by magnetic flux
cutting the turns of a coil
.”



When current is produced an
induced voltage is also produced.
This occurs when there is relative
motion between a magnetic field
and/or a current carrying conductor


3 factors effect the size of this
induced voltage

1
. The number of turns in the coil

2
. The strength of the magnetic field
(Number of flux lines)

3
. The relative speed between the coil
and the magnetic field.


Example


A magnetic flux of 1000
uWb

cuts
across a coil of 2000 turns in 0.1 s.
Calculate the induced voltage.

Lenz’s Law


“The
induced voltage in a circuit causes
a current to flow. This current’s magnetic
field is in the opposite direction of
motion of the original magnetic field,
thus opposing it
.”



If this law weren’t true, what would
happen?

AC Generators


Converts mechanical energy into
electrical energy (Faraday’s Law)


What does the opposite? (Converts
electrical energy into mechanical)


A Motor

How much voltage can a
generator make?


The
a
mount
of voltage depends
on 4
things:

1.
The
strength of the magnetic field

2.
The
angle at which the conductor
cuts the magnetic field

3.
The
relative change in speed
between conductor and magnetic
field

4.
The
length of the conductor within
the magnetic field.


How an AC generator works…


http://
www.youtube.com/watch?v=u
YfTzCa71SE


http://
www.youtube.com/watch?v=g
qA3WoOunEA&feature=related

See magnetic field handout

N

S

How do
DC

Generators work?

DC generators produce DC voltage. However
this voltage is not a nice flat voltage, it is
pulsating. See video on next slide.





Same as AC generator except slip rings are
replaced with two piece
commutator
.

How do DC generators work?


http://www.youtube.com/watch?v=X
i7o8cMPI0E&feature=related


Generators are not perfectly efficient


Not all 100% of the mechanical energy is converted into
electrical energy.


The efficiency depends on 3 things.

1. I^2*R
or copper losses in the winding


(Energy is lost as heat, heat is reduced by using Small
Awg

wire)

2. Eddy
current loss in the core material


Currents
that go in the opposite direction as the current being
made. This
can cause a circulating flow of
electrons
, or a
current
,
within the body of the conductor. These circulating
eddies

of
current create induced magnetic fields that oppose the change of
the original magnetic field due to
Lenz's
law
.

3. Hysteresis
loss (magnetic friction)


(Magnetic dipoles constantly change directions, this molecular
friction produces heat. Heat goes into armature causing its
resistance to increase.)


What about motors?


Motors are opposite of generators.


Motors convert electrical energy into
mechanical energy.



http://www.youtube.com/watch?v=it
_Z7NdKgmY&feature=related


Types of DC motors

Series Motors:

Windings are connected in series with Armature



Starting torque is greater than other DC motors



Speed varies widely with
load.

Types of DC motors

Shunt Motors:

Windings are connected in parallel
with



Armature. Starting
torque is smaller than



other
DC
motors. Speed
does not vary widely


with load.

Types of DC motors

Compound Motors:

A mixture of series and
shunt.

Stepper Motors:

Motors that turn a specific amount of



time. Used
in printers, scanner, disk



drives… (
Micro
-
controllers drive




stepper
motors
)

Brushless DC Motors

In a BLDC motor, the electromagnets do not
move; instead, the permanent magnets rotate
and the armature remains static.