1. E1 – Electrical Fundamentals
# 2 – AC and DC Current

2. © 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1
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
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1

3. © 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1
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
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1

4. Alternating Current (AC)
Moves in one direction, then the other
Produced by a generator
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1

5. Alternating Current (AC)
Generator
Generators produce Alternating Current
First, the current goes in one direction
Then it reverses, or alternates, in the opposite direction
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1

6. Alternating Current (AC)
Generator
In the U.S. there are 60 complete cycles per second
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1

7. © 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1
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
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1

8. 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 v1.1

9. 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
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1

10. 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
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1

11. Sine Wave of Alternating Current
Magnet 0º
90º
180º
270º
360º
NORTH
Positive
Negative
SOUTH
Magnet
One cycle
Conductor
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1

12. © 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1
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 v1.1

13. © 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1
Effective Voltage 0º
90º
180º
Peak Voltage
170 v
Effective Voltage
120 v
.707 x 170 = 120 v
0 v
Effective voltage = .707 x Peak voltage
Note: Meters measure effective voltage
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1

14. Generating 3 Phase Alternating Current
3Ø current is generated by passing three conductors through a magnetic field
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1

15. © 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1
Generating 3Ø Current
Magnet
NORTH
Current is generated in each conductor as it passes through the magnetic field.
SOUTH
Magnet
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1

16. Sine Wave of 3 Phase Current
Magnet 0º
120º
240º
360º
NORTH
90º
180º
270º
SOUTH
Magnet
Each sine wave starts 120° after the other.
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1

17. Sine Wave of 3 Phase Current
Magnet 0º
120º
240º
360º
NORTH
90º
180º
270º
SOUTH
Magnet
Windings 120° out of phase give 3Ø motors their high starting torque.
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1

18. Inductance & Reactance
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
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1

19. Inductive Reactance Induction: When voltage leads current Reactance:0º
90º
180º
270º
360º
VOLTAGE
CURRENT
Reactance:
The resistance encountered during the change of flow
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1

20. © 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1
Capacitive Reactance
Capacitors can create an out-of-phase condition between voltage and amperage
The voltage lags behind the amperage
Capacitors increase motor starting torque
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1

21. Capacitive Reactance Capacitors cause voltage to lag behind current 0º
90º
180º
270º
360º
CURRENT
VOLTAGE
Capacitors cause voltage to lag behind current
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1

22. © 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1
END OF
A/C and D/C
Current
© 2005 Refrigeration Training Services - E1#2 AC and DC Current v1.1