Engineering Expert Witness Blog

     Last time we learned how George Westinghouse’s chief engineer, William Stanley, developed the first practical transformer for electric utility use.  Now let’s see how it works, as illustrated in Figure 1.

Figure 1 – The Basic Electrical Transformer

     What we have here is an alternating current (AC) power source.  And much like an electrical generator in a utility power plant, it is connected via power lines to the primary coil wires of a transformer, such as the one which feeds power to your house.  The voltage applied by the source, that is, the power plant, to the primary coil is known as V_P, and the electrical current flowing through the primary coil is referred to as I_P.

     As we learned last week, the continually varying electrical current flowing through the coil creates lines of magnetic flux, which also continually vary.  In our diagram the lines of flux flow around the core of the transformer.  The magnetic flux present in the core then induces AC voltage, or V_S, and current, or I_S,  to the secondary coil when its wires are connected to something requiring an electrical load to operate.  Some examples would be light bulbs, TV’s, and most appliances found in the average home.

     As was mentioned last week, the number of wire turns, or loops, in the secondary coil as compared to the primary coil determine how the transformer will change the voltage that is applied to it.  An example of this phenomenon can be observed in the power lines supplying electricity to our homes.  Voltage from power plants is too high to be introduced into our homes, so transformers convert it to a lower voltage, one which can be used by the myriad of electrical devices we couldn’t live without.

     To get an idea of how this voltage changing works, let’s consider Figure 2.

Figure 2 – Basic Transformer Example

     We can see that the primary coil has 17 turns of wire and the secondary coil has 8.  For purposes of our example and to keep the numbers workable, let’s arbitrarily say that V_P = 5 volts AC.  By the way, the power initially coming into the transformer feeding your home is typically measured in the thousands of volts. 

     Now it’s time to do some math.  Based on this input voltage and the number of wire turns in each coil, what would the voltage be on the secondary coil? As William Stanley discovered, it’s a matter of ratios and algebra, and it works according to this formula:

N_P ÷ N_S = V_P ÷ V_S

     Here N_P and N_S are the number of turns of wire in the primary and secondary coils, respectively.  So plugging in the numbers we get:

17 turns ÷ 8 turns = 5 volts ÷ V_S

[(8 turns ÷ 17 turns) x 5 volts] = V_S = 2.3 volts

     This tells us that the transformer in our example reduces, or “steps down” 5 volts AC to just under half the voltage.   So the transformer changed a higher voltage to a lower voltage.  By the same token, a large utility transformer can be used to reduce transmission line voltage to one which can be used safely within our homes.

     Like magic, this mechanism also works in reverse.  For example, you can apply 2.3volts AC to the 8 turn coil and you will get 5 volts AC out of the 17 turn coil, resulting in a “stepping up” of voltage.

     But wait a minute.  How can you possibly get more voltage out than you put in?  Next time we’ll find out.

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