Electromagnetic Principles. Definition Electromagnetic is made up of two words. 1.Electro, which has...
-
Upload
annis-wheeler -
Category
Documents
-
view
214 -
download
1
Transcript of Electromagnetic Principles. Definition Electromagnetic is made up of two words. 1.Electro, which has...
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.
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.
Principle # 1
If it were not for this principle, we would
not have ‘electricity’ today. Let’s take a
closer look at this principle.
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.2
Magnet
Conductor
SOUTH
NORTH
Magnet
Passing a conductor between two magnets and the magnetic field causes electrons to flow in the wire.
This produces electrical current in the wire.
Generating Current
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.2
8
0º 90º 180º 270º 360º
Positive
Negative
One cycle
Magnet
SOUTH
NORTH
Magnet
Conductor
Alternating Current
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.2
9
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
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.2
10
0º 90º 180º
Effective Voltage
Peak Voltage 170 v
120 v
0 v
Effective voltage = .707 x Peak voltage
.707 x 170 = 120 v
Note: Meters measure effective voltage
Effective Voltage
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.2
11
0º 90º 180º
Effective Voltage
Peak Voltage 340 v
240 v
0 v
Effective voltage = .707 x Peak voltage
.707 x 340 = 240 v
Note: Meters measure effective voltage
Effective Voltage
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. AC was not
invented yet. 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. Thus, the DC
equivalent.
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.2
17
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
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.2
18
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.
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.2
19
0º 90º 180º 270º 360º
Positive
Negative
One cycle
Magnet
SOUTH
NORTH
Magnet
Conductor
Sine Wave of Alternating Current
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.2
20
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
The second principle that we want to
review states that a magnetic field will be
produced when electrons flow in a
conductor.
Principle # 2
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.’
© 2005 Refrigeration Training Services - E1#4 Symbols and Wiring Diagrams v1.2
24
Solenoid Valve
Magnetic coil energized
Plunger pulled up
Fluid flowsFluid stops
Power off
Plunger drops
Plunger
Seat
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.’
© 2005 Refrigeration Training Services - E2#3 Contactors and Motor Starters v1.2
26
Side View
L1 L2
T1 T2
LINE
LOAD
CONTROL CIRCUIT
© 2005 Refrigeration Training Services - E2#3 Contactors and Motor Starters v1.2
27
L1 L2
T1 T2
LINE
LOAD
Power to Contactor
Power to Load
CONTROL CIRCUIT
Contacts pulled in
Magnetic Coil energized
Side View
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.’
© 2005 Refrigeration Training Services - E2#1 Fan Motors v1.2
29
Two-pole stator motor
NN S
_+
SN S
Stator (Stationary)
When stator is energized the rotor will make a half turnwith each half of current cycle
Rotor(Rotates)
Polarity reverses (N to S) on stator
Stator repels & attracts rotor into motion
© 2005 Refrigeration Training Services - E2#1 Fan Motors v1.2
30
Second half of cycle
NS
+ _
N S
The alternating current now
changes direction
NSPolarity reverses (N to S) on stator
Stator continues motion by repelling and attracting
In conclusion, if it were not for
electromagnetic principles, we would be
living in the dark ages.
T h eE n d