Archive for December 12th, 2010

Transformers – Alternating Current Does the Trick

Sunday, December 12th, 2010

     If you’ve seen the movie The Prestige, you know just how “tricky” electricity can be, and if you haven’t seen it yet, you’ve yet to see a great movie.  In it, Hugh Jackman uses the magical properties of electricity to pull off a magic trick the likes of which the world has never seen.  But that’s all I’ll say about that… see the movie.

     In 1886, a young American inventor named William Stanley did some serious thinking about Michael Faraday, the British scientist we introduced last week, and his work with electricity and magnetism.  Stanley figured out how to put it all together.  The result was the world’s first electrical transformer.

     What fueled Stanley’s curiosity?  Like most good inventors, he perceived a need and sought to fill it.  At the time George Westinghouse was developing his alternating current (AC) electric utility power system, the same basic system we use today.  As Westinghouse’s chief engineer, Stanley was given the task of figuring out a way to efficiently change voltage levels on an AC power grid.  The industrial revolution was in full swing, and for various industrial purposes factories needed to operate on voltage levels different from those produced by the Westinghouse generators.

     Stanley approached the task before him with the benefit of knowledge supplied by Faraday’s experimentation.  He knew that Faraday was able to cause current to flow through a wire by moving a magnet near it back and forth.  This phenomenon occurred because lines of magnetic flux were varying over time with respect to the wire through the magnet’s movement.  Being aware of the vicissitudes of alternating current, the way it varies in intensity and direction, Stanley was able to conclude that any lines of magnetic flux generated by AC current flowing through a coiled wire would also tend to vary over time.  Armed with this knowledge, Stanley replaced the DC battery used in Faraday’s experiment with an AC generator.  This modified setup is shown in Figure 1. 

Figure 1 – Faraday’s Experiment Modified With An AC Power Source

     In the modified setup the switch is closed, causing the AC power flowing through the first coiled wire to create lines of magnetic flux in the iron rod.  These lines of flux continually vary and thus induce AC flow in the second coil.  The action taking place is duly recorded by a Galvanometer needle, which keeps moving so long as the switch remains closed. 

     Stanley also knew that the voltage created in the second coiled wire was dependent on how many turns, or loops, of wire were present in it compared to the number of turns of wire in the first coil.  He made the observation that if less turns were present in the second coiled wire as compared to the first, less voltage would also be emitted from the second coiled wire.  This demonstrates the phenomenon of changing voltage with respect to supply delivered by the AC generator to the first coil.

     Putting these findings together, Stanley was able to develop the first practical electrical transformer, whose basic design is shown in Figure 2.  Here we see that the iron rod from Faraday’s experiment has been replaced with an iron transformer core resembling a squared off doughnut.  

Figure 2 – A Basic Electrical Transformer

     Next time we’ll get into the math behind this discussion, and we’ll see how Stanley’s transformer worked.