Alternating Currents

Abstract

Virtually all the electric power produced in North America is obtained from a-c generators. The simplest of these was described in section 37.03. We saw in that section that a coil rotated in a uniform magnetic field generates an emf of form

$$e = {E_M}\cos wt$$

where ω is the angular velocity of the coil. Practical generators do not have uniform fields. Instead, the rotating coil, or rotor, turns in the field of an electromagnet having several pairs of poles. The result is an emf of the form of (39.01), but ω is now equal to the angular velocity of the rotor times the number of pairs of poles. For this reason ω is renamed and called the angular frequency or the radian frequency. The renaming is also appropriate for emf’s with higher values of ω. These are produced by electronic oscillators which contain no (visibly) moving parts at all.