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AC Fundamentals Solved Problems

AC Fundamentals Questions

  1. What is the definition of a generator?
    A generator is a machine which converts mechanical energy to electrical energy.
  2. What is the definition of an alternator?
    An AC generator is also called an alternator.
  3. With a six pole alternator spinning at 1200 rpm, what would the frequency of the voltage be?

         \[f=\frac{n*p}{120}=\frac{1200*6}{120}=60Hz\]

  1. To generate an output frequency of 6oHz, what would be the rpm speed need to be with an eight pole alternator?

       \[n=\frac{120*f}{p}=\frac{120*60}{8}=900RPM\]

  1. In North America what is the standard frequency of AC voltage generated?
    The standard frequency in North America is 60 Hz
  2. With a voltage of 240-V RMS, what would be the peak voltage?

${{V}_{p}}=\sqrt{2}{{V}_{rms}}=1.414*240=339V$

  1. Using an AC volt meter we found the AC sine wave voltage to be 10-Volts.
    1. Determine the peak value of this voltage.
      ${{V}_{p}}=\sqrt{2}{{V}_{rms}}=1.414*10=14.14V$
    2. Determine the RMS value of this voltage.
      Measured voltage is RMS voltage; 10-V rms.
    3. Determine the peak to peak value of this voltage.
      Peak x 2 = peak to peak; 14.14 x 2 = 28.28 volts peak to peak.
  2. List the two single phase voltages in a residential system.
    120-V and 240-V AC
  3. The direction of rotation of an induction motor is affected in what way by interchanging the supply phases?
    By interchanging the supply phases the motor will run in reverse.
  4. When connecting a three phase alternator to the power system, what conditions must be met?
        1.The phase sequence or rotation of the machine must be the same as that of the system.

              2.The alternator voltage must be in phase with the grid system.

             3.The alternator frequency must be the same as the grid system frequency.

  1. The three stator coils of a three phase alternator are positioned apart by how many electrical degrees?
    The stator coils must be set to 120 degrees apart.
  2. There are two basic types of three phase alternator coil connections, what are they?
    The Wye and Delta connections.
  3. In an AC resistive circuit, what is the phase relationship between the voltage and current?
    The current and voltage are in phase.
  4. A 10 ohm heater is connected to an AC 340-V peak-to-peak voltage determine:
    1. The peak value of the voltage.
      340-V/2 = 170-V peak
    2. The effective value of the voltage.
      $\frac{170}{\sqrt{2}}=120V$
    3. The wattage.
         120V/10Ω = 12Amps;

                  12A x 120V = 1440 Watts