AC Electrics -Introduction to AC — Page 174, Lesson 175

Let’s pick up where we left off. I’m going to walk you through what happens when we remove the secondary circuit from a transformer but keep the AC supply connected to the primary coil. Looking at Figure 11.8, the secondary circuit has been removed, so there’s no load drawing power from the secondary side. But the AC supply is still connected to the primary coil, and because it’s alternating current, it generates an ever-changing magnetic field. That changing magnetic field doesn’t just affect a secondary coil — it also affects the coil that created it. This effect is called self-induction. Self-induction means that the changing magnetic field induces a voltage in the same coil that produced the field. And according to Lenz’s law, the voltage induced will always oppose any change of current in the circuit. So if the current is trying to increase, the induced voltage pushes back against that increase. If the current is trying to decrease, the induced voltage tries to keep it flowing. This self-induced voltage is often referred to as the Back EMF — the electromotive force that opposes the change. Now, the amount of inductance in any circuit can be measured by the size of the induced voltage. Several factors affect how much voltage is induced: - The number of turns in the coil — more turns create a stronger magnetic field. - The addition of a soft iron core inside the coil — that also strengthens the magnetic field. - An increase in the rate of change of current — which means an increase in frequency. The first two factors — number of turns and the presence of a soft iron core — relate to the construction of the coil itself. Together, for a given frequency, they determine the value of self-inductance. This is simply called the Inductance of the coil, and it’s a measure of the coil’s ability to produce a Back EMF. A coil with a high value of inductance will produce a greater Back EMF than one with a small value, assuming the same supply frequency. Any device that has inductance can be referred to as an inductor. The unit of inductance is the henry, abbreviated with the symbol H. In practice, the henry is too large a unit for most circuits, so inductance is usually expressed in millihenries — thousandths of a henry — or microhenries — millionths of a henry. Here’s the formal definition: a circuit has an inductance of one henry if a current change of one ampere per second induces a back EMF of one volt. Now, what does inductance actually do in an AC circuit? The key effect is that it causes the voltage and current to be out of phase. Because the inductance opposes changes in current flow, the rise in current is held back behind the rise in voltage. In other words, current lags voltage. In a circuit that has only inductance — a purely inductive circuit — the current lags the voltage by exactly 90 degrees. That’s a quarter of a cycle. This phase relationship is illustrated in Figure 11.9, which shows the voltage waveform reaching its peak a full 90 degrees before the current waveform does. So to summarise: self-induction in an AC circuit produces a Back EMF that opposes current changes. The amount of inductance depends on the coil’s construction and the frequency. The unit is the henry, and the practical result is that current lags voltage — by 90 degrees in a purely inductive circuit.