The transformer is essentially just two (or more) inductors, sharing a common magnetic path. Any two inductors placed reasonably close to each other will work as a transformer, and the more closely they are coupled magnetically, the more efficient they become.
When a changing magnetic field is in the vicinity of a coil of wire (an inductor), a voltage is induced into the coil which is in sympathy with the applied magnetic field. A static magnetic field has no effect, and generates no output. Many of the same principles apply to generators, alternators, electric motors and loudspeakers, although this would be a very long article indeed if I were to cover all the magnetic field devices that exist.
When an electric current is passed through a coil of wire, a magnetic field is created - this works with AC or DC, but with DC, the magnetic field is obviously static. For this reason, transformers cannot be used directly with DC, for although a magnetic field exists, it must be changing to induce a voltage into the other coil.
Try this experiment. Take a coil of wire (a loudspeaker crossover coil will do nicely for this), and a magnet. Connect a multimeter - preferably analogue) to the coil, and set the range to the most sensitive current range on the meter. As you move the magnet towards or away from the coil, you will see a current, shown by the deflection of the meter pointer. As the magnet is swung one way, the current will be positive, the other way - negative. The higher the coil's inductance and the stronger the magnet (and/ or the closer it is to the coil), the greater will be the induced current.
Move the magnet slowly, and the current will be less than if it is moved quickly. Leave it still, and there is no current at all, regardless of how close the magnet may be. This is the principle of magnetic induction, and it applies to all coils (indeed to all pieces of wire, although the coil makes the effect much greater).
If you now take a handful of nails and place them through the centre of the coil, you will see that the current is increased many times - the magnetic field is now more concentrated because the steel nails make a better magnetic path than air.
The ability of a substance to carry a magnetic field is called permeability, and different materials have differing permeabilities. Some are optimised in specific ways for a particular requirement - for example the cores used for a switching transformer are very different from those used for normal 50/60Hz mains transformers.
The permeability of transformer cores varies widely, depending on the material and any treatment that may be used. The permeability of air is 1, and most traditional cores have a much higher (i.e. > 1) permeability. A couple of notable exceptions are aluminium and brass, which are sometimes used to reduce the inductance of air cored coils in radio frequency (RF) work. This is much less common than a ferrite "slug" core, which increases the inductance and is used to tune some RF transformers.
As well as permeability, magnetic cores (with the exception of air) have a maximum magnetic flux they can handle without saturation. In this context, saturation means the same as in most others - when a towel is saturated, it can hold no more water, and when a magnetic core is saturated, it can carry no more magnetic flux. At this point, the magnetic field is no longer changing, so current is not induced into the winding.
You will be unable to saturate your nails with the magnet, as there is a very large air gap between the two pole pieces. This means that the core will always be able to support the magnetic flux, but the efficiency is also very much lower because the magnetic circuit is open. Nearly all the transformers you will see have a completely closed magnetic circuit, to ensure that as much of the magnetism induced into the core as possible will pass through the winding(s).
There are some cases where a tiny air gap will be left deliberately, and this is done routinely when a transformer or coil must sustain a significant DC component as well as the AC. This is covered briefly below, but there is more on this subject in the second section of the article.
Figure 1.1 - Essential Workings of a Transformer
Figure 1.1 shows the basics of all transformers. A coil (the primary) is connected to an AC voltage source - typically the mains for power transformers. The flux induced into the core is coupled through to the secondary, a voltage is induced into the winding, and a current is produced through the load.
The diagram also shows the various parts of a transformer. This is a simple transformer, with two windings. The primary (denoted as such during the design) will induce a magnetic field into the core in sympathy with the current produced by the applied AC voltage. The magnetic field is concentrated by the core, and nearly all of it will pass through the windings of the secondary as well, where a voltage is induced. The core in this case is typical of the construction of a "C-Core" transformer, where the primary and secondary are separated. More common is the "traditional" EI (ee-eye) type, which although somewhat out of favour these days is still used extensively. This is shown below.
The magnitude of the voltage in the secondary is determined by a very simple formula, which determines the "turns ratio" (N) of the component - this is traditionally calculated by dividing the secondary turns by the primary turns ...
- 1.1.1
N = Ts / Tp- 1.1.2
Vs = Vp * N- 1.1.3
N = Vs / Vp- 1.1.4
Is = Ip / NSmaller transformers will always have a lower efficiency, but the units commonly used in power amplifiers can have efficiencies of up to 90% for larger sizes. The reasons for the lost power will become clear (I hope) as we progress. For the time being, we shall consider the transformer to be "ideal" (i.e. having no losses) for simplicity.
Figure 1.2 - E-I Laminations
The conventional E-I lamination set is still extensively used, and a few pertinent points are worth mentioning. The centre leg is always double the width of the outer legs to maintain the cross-sectional area. Likewise, the "I" lamination and the "back" of the E are the same width as (or sometimes slightly larger than) the outer legs. The winding window is where the copper windings live, and in a well designed transformer will be almost completely full. This maximises the amount of copper and reduces resistive losses because the windings are as thick as they possibly