Friday, July 20, 2012

Ideal Transformer – Fact or Dream?

We all have dreams.  You may dream of the perfect vacation, winning the lottery or just having a wonderful day in the sunshine.  My dream is a lot more farfetched.  I dream of the ideal power transformer.

Like all dreams, my dream is about as likely to occur as finding the “ideal” man or woman.  A lovely concept but never actually experienced by anyone.

The ideal transformer is a physics concept that refers to perfect magnetic coupling where there is 100% efficiency in the transfer of power between the primary and the secondary windings.   The physics equation for an ideal transformer is;

The ratio voltage of the primary (Vp) to the voltage of the secondary (Vs) is the same ratio as the number of turns on the primary (Np) to the number of turns on the secondary (Ns).

A transformer that takes the primary voltage and increases it is called a “step up” transformer because the secondary voltage is higher than the primary because it has more turns on the secondary.  Likewise, a transformer that has fewer turns on the secondary (compared to the primary) is called a “step down” transformer because the voltage is reduced.

So if you want the voltage to step down by exactly 50% then you need 50% of the windings on the secondary compared to that of the primary.  If only it were this simple.  Apparently God has a keen sense of humor and decided that if things were this easy then all of us would pass physics and that would mean that there would be rampant unemployment in the ranks of teachers and teaching assistants!

There are two factors that interrupt the perfection of an ideal transformer; imperfect coupling and core or power losses.  In order to achieve perfect coupling, all of the magnetic flux produced by the primary must be transferred and captured by the transformer core.  One problem that quickly arises is simply a matter of geography.  The shape of the windings and the core would have to be infinitely close in order to have all of the lines of flux completely cut through the core.  While the transformer and core manufacturers struggle mightily to come up with shapes that maximize the coupling, it is impossible to get all of the flux transferred into the core.  Stray flux lines radiate in all directions thus reducing the efficiency of energy transfer from the primary to the core.

Even if we could get perfect flux transfer from the primary to the core, the core has issues itself.  The purpose of the core is for it to be a conduit of flux to the secondary.  The core absorbs flux form the primary and then by producing a magnetic field for the secondary, creates a voltage in the secondary.  Over the years better and better core materials have been developed to reduce power losses but we still are far from perfection.  Even tiny imperfections know as grain boundaries in the crystals that make up the core slow down the flux and resist its movement.  This resistance becomes heat.  If you want a real life experience of this, Just touch any transformer under power and you will see that they range from warm to very hot (depending on the efficiencies).

I will continue to dream of the ideal transformer while the rest of the world dreams of more achievable goals like world peace or the perfect enchilada.

Written by Denny Wist
President of Butler Winding