Transformers

The English Scientist Michael Faraday discovered the principles of induction and published his findings in 1831.

The American Scientist Joseph Henry independently devised his first transformer in 1832.

Gaulard and Gibbs patented a practical transformer in England in 1882 and George Westinghouse purchased the American rights to it in 1886. Westinghouse needed the transformer for use in the development of the AC power system that Tesla had proposed.

  In today’s electronic world, small transformers are used to charge the batteries of many devices; cell phones, media players, and toys.  Transformers are used in communications devices of all types and large power transformers are required for the transmission of AC power.

Transformers use the properties of induction to either step up or step down AC voltage.  The wire used in power transmission lines has resistance that increases with the length of the line, and over long distances, the resistance in the wire accounts for a loss of some of the transmitted power. 

Edison’s DC power transmission scheme was not effective because of the large amount of copper and large size of the wire required to transmit power.  20 miles was his limit.  In the transmission of AC power, using a transformer to step up the voltage produced by the alternator reduced the losses caused by the resistance of the transmission lines.  Then stepping the voltage down at the destination provided the desired power for the end user.

A simple transformer consists of two coils separately wound around a common core made of iron.  The coils are linked by magnetic lines of force.  The AC power that is applied to the primary winding is transferred to the secondary winding which is connected to the electrical load. 

The turns ratio between the primary winding and the secondary winding determines the AC voltage output.  In the case of a cell phone charger, the turns ratio would be 40:1 for an input of 120 volts and an output of  3 volts.  Inside the cell phone charger are two small diode rectifiers that change the 3 volts AC to a pulsating DC used to recharge the cell phone battery.

A transformer that reduces the output voltage is called a step down transformer.  If you feel the case of a cell phone charger that has been charging a battery, you probably noticed that it was warm to the touch; that is because of the power conversion losses in these components.

The photo at left is a cell phone charger.                                   The case has been cut away to show the Transformer that converts household Electricity to 3.3 volts. The blue capacitor And small circuit board containing solid State diode rectifiers convert the AC to DC to charge the cell phone battery.

In the electrical utility industry, transformers are large.  They vary in size according to the amount of power they must convert.  At the power house the alternator may generate an AC voltage of 6000 volts  This voltage can be stepped up to 60,000 with a  1:10 step up transformer that is connected to the distribution system for transmission over long distances. 

The power company determines the routes of transmission and the high voltages required for efficient transfer of power.  There is a transformer in your neighborhood that steps down the high voltage.  This transformer conditions the voltage that supplies electrical power to your home.
 
In a transformer, the voltage induced into the secondary winding by a change of current in the primary is said to be induced by “mutual induction”.  The turns ratio is the number of turns in the primary winding compared to the number of turns in the secondary winding.  The turns ratio will indicate whether the transformer is a step up or step down type.  If the primary winding has twice as many turns as the secondary winding, then the turns ratio would be 2:1 and the transformer would be a step down  type.

If the turns ratio is 1:2, then the transformer would double the input voltage at the output leads.  The power consumed by the load in the circuit is transferred from the primary winding to the secondary winding by means of the magnetic flux. While voltage is proportional to the turns ratio, current varies inversely with the voltage.  A transformer is classified as step up or step down only in relation to voltage. 

Transformers do not generate electrical power; they only transfer electric power from one coil to the other by mutual induction.  The input power equals voltage times current  (P=E X I) of the primary coil.  Output power equals voltage times current of the secondary coil. 

Transformer efficiency is high, possibly 98%, the losses in a transformer are due to two factors, copper loss and core loss.  Copper loss is due to the resistance of the wire in the windings of the primary and secondary coils.  Core loss involves the iron that the windings are wound around.  Core losses are due to eddy current loss and hysteresis  loss.

Eddy currents are currents that are induced into the iron core; this flow of current is dissipated as heat.  Cores are made of insulated laminated steel  sections to reduce the eddy currents. 

Hysteresis loss is energy expended by the rapidly changing realignment of the molecules that comprise the core material as they attempt to change polarity with each cycle of the applied AC. 
   
When the primary and secondary coils are wound in the same direction on the iron core, the polarity of the secondary coil output will have a phase reversal of 180 degrees from the primary coils input voltage.

When the primary and secondary coils are wound in the opposite direction, the polarity of the input and output voltage will be the same.