Transcript Electromagnetic Principles - hvacr

Electromagnetic
Principles
Definition
Electromagnetic is made up of two words.
1. Electro, which has something to do with
electricity and
2. magnetic, which has something do with
magnetism.
Definition
So, electromagnetic principles deal
with
the
relationship
between
electricity and magnetism, as well as
the relationship between magnetism
and electricity.
There are two electromagnetic
principles that we are going to
review.
Principle # 1
The first principle that we want to review
states that electrons will flow in a
conductor when the conductor MOVES
or PASSES through a magnetic field.
If it were not for this principle, we would
not have ‘electricity’ today. Let’s take a
closer look at this principle.
Generating Current
Magnet
NORTH
Passing a conductor between two magnets and
the magnetic field causes electrons to flow in
the wire.
This produces electrical current in the wire.
SOUTH
Magnet
Conductor
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Alternating Current
Magnet
0º
90º
180º
270º
NORTH
Positive
Negative
SOUTH
Magnet
One cycle
Conductor
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360º
Effective voltage
• Alternating current starts at 0, reaches a peak,
then returns to 0
• Peak voltage at 90° (electrical degrees)
• Effective voltage is .707 times peak voltage
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Effective Voltage
0º
90º
180º
Peak Voltage
170 v
Effective Voltage
120 v
.707 x 170 = 120 v
0v
Effective voltage = .707 x Peak voltage
Note: Meters measure effective voltage
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Effective Voltage
0º
90º
180º
Peak Voltage
340 v
Effective Voltage
240 v
.707 x 340 = 240 v
0v
Effective voltage = .707 x Peak voltage
Note: Meters measure effective voltage
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The ‘effective voltage’ is also
known as the RMS (Root Mean
Square) value or the DC (Direct
Current) equivalent.
Many years ago, there was only
Direct Current.
invented yet.
AC was not
But when it was,
what do you think the reference
current would be?
If you said DC, you would be
correct. So, 120v AC will do the
same ‘work’ that 120 DC will do.
In other words, if you plug a 100w
incandescent light bulb into 120v
AC, it will glow just as bright as if
you plugged it into 120v DC, thus
the DC equivalent.
For another example, if you wired
a 240v AC electric hot water tank
to 240v DC, it will heat the water
just as quickly.
equivalent.
Thus, the DC
Generating Alternating Current (AC)
• Passing a conductor through a magnetic field
• A generator uses many conductors and a large
magnetic field to produce electrical current
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Expressing AC with a Sine Wave
• A sine wave shows how alternating current
flows in one direction, then reverses to flow in
the opposite direction.
• A sine wave is a mathematical derivation of
the trigonometric function – sine. You do not
need to know trig. in order to understand AC
though.
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Sine Wave of Alternating Current
Magnet
0º
90º
180º
270º
NORTH
Positive
Negative
SOUTH
Magnet
One cycle
Conductor
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360º
Cycles and Frequency
• Cycle:
– One complete electrical alternation
• Frequency
– Number of cycles in a second
• Measurement of frequency:
– Hertz (Hz)
– Cycles
• U.S. frequency is 60 hertz, or 60 cycles
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Principle # 2
The second principle that we want to
review states that a magnetic field will be
produced when electrons flow in a
conductor.
If it were not for this principle, we would
not have motors, relays, contactors,
solenoids, timers, etc. Let’s take a closer
look at this principle.
For the first example, let’s take a look at
how electricity produces a magnetic field
and is used on a device known as a
‘solenoid valve.’
Solenoid Valve
Magnetic coil energized
Plunger pulled
up
Power off
Plunger drops
Fluid flows
Plunger
Fluid stops
Seat
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For the second example, let’s take a look
at how electricity produces a magnetic
field and is used on a device known as a
‘contactor.’
Side View
LINE
L1
L2
T1
T2
CONTROL
CIRCUIT
LOAD
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Side View
LINE
Power to
L2 Contactor
L1
CONTROL
CIRCUIT
Magnetic Coil
energized
Contacts
pulled in
Power to
T2 Load
T1
LOAD
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For the third example, let’s take a look at
how electricity produces a magnetic field
and is used on a device known as a
‘motor.’
Two-pole stator motor
Stator
Rotor
(Stationary)
(Rotates)
Polarity reverses (N
to S) on stator
SN
N
S
Stator repels &
attracts rotor into
motion
When stator is energized the
rotor will make a half turn
with each half of current cycle
+ _
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N
S
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Second half of cycle
N
S
Stator continues
motion by repelling
and attracting
SN
Polarity reverses (N
to S) on stator
+ _
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S
N
The alternating current now
changes direction
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In
conclusion,
if
it
were
not
for
electromagnetic principles, we would be
living in the dark ages.