AC Electrics - Transformers — Page 231, Lesson 250

I want to walk you through one of the biggest advantages of AC power over DC — the transformer. Let's start with the core idea. A transformer is a device that lets you raise or lower the value of an alternating voltage with extreme efficiency. That's the key benefit: you can change the voltage level with very little energy loss, and you can only do that with AC, not DC. The simplest transformer has two electrically separate coils wound over a stack of iron laminations. Those laminations form a common core, and together they create a completely closed magnetic circuit. One coil is called the primary winding, and it's connected to the AC supply. The other coil is the secondary winding, and that's where you take the output from. Here's how it works. The alternating voltage and current in the primary winding create an alternating magnetic flux — a magnetic field that changes direction and strength with the AC cycle. That flux links across the core to the secondary winding. Because the flux is alternating, it induces an electromotive force — an EMF — in the secondary winding. That's called mutual inductance. That induced EMF becomes the output voltage. One important detail: the output voltage is 180 degrees out of phase with the input voltage. So when the input voltage is at its positive peak, the output voltage is at its negative peak, and vice versa. If you connect a load — any electrical device — across the terminals of the secondary winding, then a current will flow in that secondary circuit. Now, let's talk about the transformation ratio. This is the ratio of the number of turns of wire on the secondary winding to the number of turns on the primary winding. We label the number of turns on the secondary as N2 and the number on the primary as N1. The transformation ratio, often written as 'r', equals N2 divided by N1. And that same ratio also equals the secondary voltage E2 divided by the primary voltage E1. So the formula is: r = N2 / N1 = E2 / E1 If the transformation ratio is greater than one, the transformer is a step-up transformer — it increases the voltage. If the ratio is less than one, it's a step-down transformer — it decreases the voltage. What about power? If we ignore the very small losses that do occur in a real transformer — and they are very small — we can say that the power going into the transformer equals the power coming out. Power in either the primary or the secondary winding is the product of voltage times current in that winding. So if voltage goes up, current must come down to keep the power the same, and vice versa. Now, for three-phase systems. The output of a three-phase alternator can be transformed in one of two ways. You can use three separate single-phase transformers, one for each phase. Or you can use one three-phase transformer. A three-phase transformer has the primary and secondary windings of each phase wound on one of three laminated iron limbs — so all three phases share a single core structure. Finally, let's look at autotransformers. On an aircraft, you often need 26 volts AC for operating instruments. An autotransformer can be used to step down — or sometimes even step up — the source supply to that voltage. An autotransformer is different from a simple transformer. It has just a single winding on a laminated core, forming a closed magnetic circuit. Part of that single winding is common to both the primary and the secondary. That means part of the winding carries both the primary current and the secondary current at the same time. That's the key distinction: in an autotransformer, the primary and secondary are not electrically separate — they share a portion of the same winding.