EEE/ETI 1202
CIRCUITS & NETWORK THEORY I
LECTURE 7
, A.C. Fundamentals
• The use of direct currents is limited to a few applications e.g. charging of batterie
electroplating, electric traction, dc machines operation etc.
• For large scale power distribution there are, however, many advantages in usin
alternating current (a.c.).
• In an a.c. system, the voltage acting in the circuit changes polarity at regul
intervals of time and the resulting current (called alternating current) change
direction accordingly.
• Three principal advantages are claimed for a.c. system over the d.c. system.
1. First, alternating voltages can be stepped up or stepped down efficiently by mean
of a transformer. This permits the transmission of electric power at high voltage
to achieve economy and distribute the power at utilisation voltages.
2. Secondly, a.c. motors (induction motors) are cheaper and simpler in constructio
than d.c. motors.
3. Thirdly, the switchgear (e.g. switches, circuit breakers etc.) for a.c. system
simpler than the d.c. system.
, AC Fundamentals
➢ Previously you learned that DC sources have fixed polarities and constant
magnitudes and thus produce currents with constant value and unchanging
direction
➢ In contrast, the voltages of ac sources alternate in polarity and vary in
magnitude and thus produce currents that vary in magnitude and alternate in
direction.
3
CIRCUITS & NETWORK THEORY I
LECTURE 7
, A.C. Fundamentals
• The use of direct currents is limited to a few applications e.g. charging of batterie
electroplating, electric traction, dc machines operation etc.
• For large scale power distribution there are, however, many advantages in usin
alternating current (a.c.).
• In an a.c. system, the voltage acting in the circuit changes polarity at regul
intervals of time and the resulting current (called alternating current) change
direction accordingly.
• Three principal advantages are claimed for a.c. system over the d.c. system.
1. First, alternating voltages can be stepped up or stepped down efficiently by mean
of a transformer. This permits the transmission of electric power at high voltage
to achieve economy and distribute the power at utilisation voltages.
2. Secondly, a.c. motors (induction motors) are cheaper and simpler in constructio
than d.c. motors.
3. Thirdly, the switchgear (e.g. switches, circuit breakers etc.) for a.c. system
simpler than the d.c. system.
, AC Fundamentals
➢ Previously you learned that DC sources have fixed polarities and constant
magnitudes and thus produce currents with constant value and unchanging
direction
➢ In contrast, the voltages of ac sources alternate in polarity and vary in
magnitude and thus produce currents that vary in magnitude and alternate in
direction.
3
