Monday, May 30, 2016

2.4 Generating AC Voltage

This is the fourth week of Module 2 of the Navy Basic Electricity and Electronics series. The last few weeks were:

2.1 Electromotive Force
2.2 Magnetism
2.3 Electromagnetic Induction

This section is about how alternating current (such as what comes out of our wall plugs) can be generated.

1. There are two kinds of current: direct current and alternating current. Direct current (DC) is the kind of current you might get from a battery. Its electromotive force (EMF) does not change in polarity but is constant. Its voltage remains at about the same value and moves in the same direction constantly.

AC symbol
By contrast, alternating current changes directions and polarity at regular intervals. It's symbol in a schematic is:

2. If we were to put a loop of wire such as appears in the following diagram (Fig 1) into a magnetic field and were to rotate it, it would generate a current that would alternate in direction and polarity. You can see this using the left hand rule for generators that was introduced in the previous section.

Fig 1
When the wires are parallel to the magnetic field, no current will be generated (0 voltage). When they are moving exactly perpendicular to the magnetic field, a maximum voltage will be generated in one or the other direction.

3. There are two basic parts to a generator (or alternator): the part that moves and the part that doesn't move. The moving part is called the rotor. The part that doesn't move is called the stator.

4. Basically, if we graph the voltage produced, we are going to get a sine wave that alternates from zero to a maximum positive value back to zero to a maximum negative value than back to zero before the cycle repeats itself again (see Fig 2 below).

So here are some relevant concepts:
  • cycle - One cycle of the sine wave is a complete rotation of the loop in Fig. 1
  • frequency - the number of cycles per second
  • Hertz (Hz) - the unit for cycles per second (1 Hz = 1 cycle per second)
  • amplitude - The value of the voltage (or amps) at a given time (how high or low the sine graph is at a given point in time)
  • peak to peak amplitude - the difference between the maximum and minimum values of the amplitude
    Fig 2
  • positive alternation - the positive part of the sine wave
  • negative alternation - the negative part of the sine wave
5. The frequency of the voltage from a generator depends on the speed of rotation of the loop in Fig. 1. If you add additional magnetic poles, that can also increase the frequency (e.g., a "four pole" generator).

When a coil rotates one complete cycle, it is said to have made one revolution. In degrees, this is a 360 degree turn (written 360°).

7. One final concept in this section is "effective AC" (RMS value). [1] Because AC is constantly changing values, the average amount of work done by such current is not the same as its peak value. The effective AC value is the amount of voltage you would get from an equivalent DC source. So if the effective value of an AC current is 10 volts, then you can get the same amount of work from that AC source as you could get from a 10 volt (DC) battery.

[1] RMS stands for "root mean square," 70.7% of the peak value.

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