AC unit 10

• DC (direct current)


  In a DC circuit, the polarities of the voltages remain the same throughout, and the current directions also remain the same
• The voltmeter shows no voltage is detected in a DC circuit
• AC (alternating current)


  The voltage across each component of a circuit changes (or alternates) its polarity repetitively and/or the current direction changes (or alternates) repetitively
• Examples of some common AC waveforms

  • Sinusoidal
  • Triangular
  • Square
  • Saw-tooth
• Generating AC voltage and current

  1. Electricity comes from the power station

  2. Electricity is generated by changing flux linkage through coils in a power generator

  3. Generating an AC waveform is achieved through the cutting of magnetic flux lines by the rotating coils of wire called the rotor

  4. The direction of the current flow follows Right Hand Rule or "Generator Rule"

  5. The direction reverses as the coil changes position
• Cutting of magnetic flux lines

  • The effective rate of cutting flux lines depends on the instantaneous position of the coil conductor
  • If the coil is positioned horizontally, the conductor is moving vertically to the flux lines and will have the maximum cutting rate
  • If the coil is now positioned vertically, the conductor is moving in parallel to the flux lines and will have zero cutting rate
  • If the coil is positioned at an angle tilted to the flux lines, the rate of cutting is lying between zero and the maximum rate
• Motion that produces the most voltage across the conductor

  • Motion (C)
• Right Hand Rule


  Memorising the directions of the three variables: field, motion and current can be made simple by using the right hand rule<|>If the first finger of the right hand is pointing the direction of the magnetic field, the thumb pointing the direction of the motion, then the second finger is pointing the direction of the induced current
• Frequency (f) and Rotational Speed (n)


  One revolution of conductor through magnetic field: One cycle of induced sinusoidal voltage<|>Conductor's rotational speed determines time to complete of one cycle<|>Frequency of a periodic waveform is the number of cycles per second<|>The unit of frequency is hertz (Hz)<|>Faster conductor's rotational speed, higher frequency of induced voltage<|>Frequency of the AC voltage can also be increased by increasing number of magnetic poles
• The frequency of the electricity supply is usually fixed at 50 Hz or 60 Hz
• Example 2: The conductor loop (rotor) of a simple two-pole generator rotates at a rate of 50 rev/s. What is the frequency of the output voltage?

  • Frequency = 50 rev/s x 1 pole pair = 50 Hz
• Example 3: In order to produce a 400 Hz sinusoidal voltage, at what speed must a four-pole generator be operated?

  • Rotational speed = 400 Hz / (2 pole pairs) = 200 rev/s
• AC voltage source symbol

  Its voltage is in root mean square (rms) value
• Two main types of AC sources


  • AC generator
  • Function generator
• AC generator


  • Supplies sinusoidal voltage and current to power up our lights, home appliances and electronic gadgets
  • Can be generated from power plants, or standby power generators
• Function generator


  • Can generate a few types of periodic signals, such as sinusoidal wave, square wave and triangular wave
  • The output signal is too weak to drive loads that require heavy current
  • Found in laboratories for lab work, repairing and R & D