AC Electrics -Introduction to AC — Page 168, Lesson 166

I want to walk you through the introduction to AC Electrics. We're starting from the very beginning, so let's build this up step by step. First, the magnitude of the voltage that gets induced in this system depends on two things: the speed of rotation and the field strength. In other words, it depends on the rate of change of flux. Faster rotation or a stronger magnetic field means a higher induced voltage. Now, imagine we take that loop of wire — which we call the armature — and connect it to a load, like a resistor, forming a closed circuit. We make that connection through slip rings and carbon brushes. When the armature rotates, a current will flow around that circuit, and that current is proportional to the induced voltage. Let's look at what happens as the armature rotates. If you refer to Figure 11.2, you'll see the flux is constantly changing. In positions 1, 3, and 5, the two sides of the loop are moving parallel to the magnetic field. When they move parallel, there is no rate of change of flux, so no voltage is induced. In positions 2 and 4, the two sides of the armature are moving at right angles to the field. That gives the maximum rate of change of flux, so the maximum voltage is induced. In between those positions, the induced voltage is somewhere between maximum and zero. The polarity of the induced voltage changes as it passes through zero. Why? Because the direction that each side of the armature moves through the field reverses. If the polarity reverses, then the current through the external circuit must also reverse. That current flowing backwards and forwards about a mean position is what we call alternating current. To determine the direction of current flow through each side of the armature at any point, you use Fleming's Right Hand Rule for generators. Figure 11.2 shows one complete revolution of the generator armature and the associated rise and fall of induced voltage. Now, let's move to Figure 11.3, which illustrates the production of AC in a different way. There's a blue vector arrow labelled OP. That arrow represents one half of the coil of the generator, pivoted at O and rotating in an anti-clockwise direction. The EMF — that's electromotive force, or voltage — induced in the coil is proportional to the ordinate ON. Alternatively, you can calculate it by multiplying the maximum value by the sine of the phase angle at that point. If you take successive ordinates and plot them to a time scale that corresponds to the rate of rotation of OP, you produce a sine wave. That sine wave represents an alternating current or voltage. Now, let's define some key terms used to describe alternating current, all illustrated in Figure 11.3. First, a cycle. A cycle is one complete series of values. For example, the entire graph in Figure 11.3 from start to finish represents one cycle. Second, phase. A sine wave can be given an angular notation called phase. One complete cycle represents from 0° to 360° of phase. So to summarise: we generate AC by rotating a loop of wire in a magnetic field. The voltage rises and falls in a sine wave pattern, and the current reverses direction each half-cycle. The key terms you need to know are cycle and phase, and the key tool for determining current direction is Fleming's Right Hand Rule for generators.