Transcript E1 Subject 2

E1 – Electrical Fundamentals
# 2 – AC and DC Current
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Direct Current
• Moves in one direction
– Negative to positive
• Can be produced through chemical action
– Chemical electrolyte produces electrons
– Negative cathode gives away electrons
– Positive cathode collects electrons
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Direct Current (DC)
Electrolyte
Zinc Case
(Negative Electrode)
Carbon Rod
(Positive Electrode)
Dry Cell Battery
When load is attached,
the chemical reaction in
the electrolyte causes
current flow
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Alternating Current (AC)
• Moves in one direction, then the other
• Produced by a generator
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Alternating Current (AC)
First, the current goes
in one direction
Generators
produce
Generator
Alternating
Current
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Then it reverses, or
alternates, in the
opposite direction
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Alternating Current (AC)
Generator
In the U.S.
there are 60
complete
cycles per
second
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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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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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Generating Alternating Current
Magnet
NORTH
Passing a conductor between two magnets
causes electrons to flow in the wire.
This produces electrical current in the wire.
SOUTH
Magnet
Conductor
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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
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Sine Wave of Alternating Current
Magnet
0º
90º
180º
270º
360º
NORTH
Positive
Negative
SOUTH
Magnet
One cycle
Conductor
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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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Generating 3 Phase Alternating Current
• 3Ø current is generated by passing three
conductors through a magnetic field
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Generating 3Ø Current
Magnet
NORTH
Current is generated in each conductor as it
passes through the magnetic field.
SOUTH
Magnet
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Sine Wave of 3 Phase Current
Magnet
0º
NORTH
90º 120º
180º
240º270º
360º
SOUTH
Magnet
Each sine wave starts 120° after the other.
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Sine Wave of 3 Phase Current
Magnet
0º
NORTH
90º 120º
180º
240º270º
360º
SOUTH
Magnet
Windings 120° out of phase give 3Ø motors
their high starting torque.
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Inductance & Reactance
• Inductance:
– The out-of-phase condition between voltage and
amperage
• Reactance:
– The resistance voltage encounters when it
changes flow
• Inductive reactance:
– A combination of inductance and reactance
– Necessary for the starting and running of an
induction motor
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© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.2
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