Transcript Chapter 17 Powerpoint

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 a changing
or moving 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
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?
Lenz’s law in action

http://www.youtube.com/watch?v=k
eMpUaoA3Tg
Magnetic Field of and
Electromagnet

Depends on 3 things. The 3 C’s
◦ Current
◦ Coils
◦ Core
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 amount 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.listenonrepeat.com/watc
h?v=uYfTzCa71SE
 http://www.listenonrepeat.com/watc
h?v=gqA3WoOunEA&feature=relate
d

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.listenonrepeat.com/watc
h?v=Xi7o8cMPI0E&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. I2*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.
 Note: The DVD movie after lunch will
do a good visual job of explaining
how motors work

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:
Stepper Motors:
A mixture of series and shunt.
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.