Definitions Values of AC Alternating Current (AC) Introduction  Instantaneous  Peak  Effective  9-9 Figure 9-12.  DC and AC voltage curves. AC is constantly changing in value and polarity, or as the  name implies, alternating.  Figure 9-12  shows a graphic  comparison of DC and AC. The polarity of DC never  changes, and the polarity and voltage constantly change in  AC. It should also be noted that the AC cycle repeats at given  intervals. With AC, both voltage and current start at zero,  increase, reach a peak, then decrease and reverse polarity.  If one is to graph this concept, it becomes easy to see the  alternating wave form. This wave form is typically referred  to as a sine wave.  The effective values for voltage are always less than the  peak (maximum) values of the sine wave and approximate  DC voltage of the same value. For example, an AC circuit of  24 volts and 2 amps should produce the same heat through a  resistor as a DC circuit of 24 volts and 2 amps. The effective  value is also known as the root mean square, or RMS value,  which refers to the mathematical process by which the value  is derived.  The peak value is the largest instantaneous value, often  referred to as the maximum value. The largest single positive  value occurs after a certain period of time when the sine wave  reaches 90°, and the largest single negative value occurs  when the wave reaches 270°. Although important in the  understanding of the AC sine wave, peak values are seldom  used by aircraft technicians.  AC can be transmitted over long distances more readily  and more economically than DC, since AC voltages can be  increased or decreased by means of transformers. Because  more and more units are being operated electrically in  airplanes, the power requirements are such that a number of  advantages can be realized by using AC (especially with large  transport category aircraft). Space and weight can be saved  since AC devices, especially motors, are smaller and simpler  than DC devices. In most AC motors, no brushes are required,  and they require less maintenance than DC motors. Circuit  breakers operate satisfactorily under loads at high altitudes in  an AC system, whereas arcing is so excessive on DC systems  that circuit breakers must be replaced frequently. Finally,  most airplanes using a 24-volt DC system have special  equipment that requires a certain amount of 400 cycle AC  current. For these aircraft, a unit called an inverter is used to  change DC to AC. Inverters are discussed later in this book.  An instantaneous voltage is the value at any instant in time  along the AC wave. The sine wave represents a series of  these values. The instantaneous value of the voltage varies  from zero at 0° to maximum at 90°, back to zero at 180°,  to maximum in the opposite direction at 270°, and to zero  again at 360°. Any point on the sine wave is considered the  instantaneous value of voltage.  Alternating current (AC) electrical systems are found on most  multi-engine, high performance turbine powered aircraft and  transport category aircraft. AC is the same type of electricity  used in industry and to power our homes. Direct current (DC)  is used on systems that must be compatible with battery  power, such as on light aircraft and automobiles. There are  many benefits of AC power when selected over DC power  for aircraft electrical systems. There are three values of AC that apply to both voltage and  current. These values help to define the sine wave and are  called instantaneous, peak, and effective. It should be noted  that during the discussion of these terms, the text refers to  voltage. But remember, the values apply to voltage and  current in all AC circuits.  Closed switch Open switch Operation of circuit Wave form for DC Volts Time Wave form for AC Volts + Time − 0° 90° 180° 270° 360°