6CHAPTER 1. BASIC AC THEORYAs useful as transformers are, they only work with AC, not DC. Because the phenomenon ofmutual inductance relies on changing magnetic ﬁelds, and direct current (DC) can only producesteady magnetic ﬁelds, transformers simply will not work with direct current. Of course, directcurrent may be interrupted (pulsed) through the primary winding of a transformer to create achanging magnetic ﬁeld (as is done in automotive ignition systems to produce high-voltage sparkplug power from a low-voltage DC battery), but pulsed DC is not that diﬀerent from AC. Perhapsmore than any other reason, this is why AC ﬁnds such widespread application in power systems.• REVIEW:• DC stands for “Direct Current,” meaning voltage or current that maintains constant polarityor direction, respectively, over time.• AC stands for “Alternating Current,” meaning voltage or current that changes polarity ordirection, respectively, over time.• AC electromechanical generators, known as alternators, are of simpler construction than DCelectromechanical generators.• AC and DC motor design follows respective generator design principles very closely.• A transformer is a pair of mutually-inductive coils used to convey AC power from one coil tothe other. Often, the number of turns in each coil is set to create a voltage increase or decreasefrom the powered (primary) coil to the unpowered (secondary) coil.• Secondary voltage = Primary voltage (secondary turns / primary turns)• Secondary current = Primary current (primary turns / secondary turns)1.2AC waveformsWhen an alternator produces AC voltage, the voltage switches polarity over time, but does so in avery particular manner. When graphed over time, the “wave” traced by this voltage of alternatingpolarity from an alternator takes on a distinct shape, known as a sine wave: Figure 15,1.8+-Time (the sine wave)Figure 1.8: Graph of AC voltage over time (the sine wave).