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Upon completion of this chapter you will be able to:

  • State the meaning of "transformer action."
  • State the physical characteristics of a transformer, including the basic parts, common core materials, and main core types.
  • State the names given to the source and load windings of a transformer.
  • State the difference in construction between a high- and a low-voltage transformer.
  • Identify transformer symbols as to the type of transformer each symbol represents and the method used to denote transformer phasing.
  • State the meaning of a "no-load condition" and "exciting current" relative to a transformer.
  • State what causes voltage to be developed across the secondary of a transformer and the effect of cemf in a transformer.
  • State the meaning of leakage flux and its effect on the coefficient of coupling.
  • Identify a transformer as step up or step down and state the current ratio of a transformer when given the turns ratio.
  • Solve for primary voltage, secondary voltage, primary current and number of turns in the secondary given various transformer values.
  • State the mathematical relationship between the power in the primary and the power in the secondary of a transformer and compute efficiency of a transformer.
  • State the three power losses in a transformer.
  • State the reason a transformer should not be operated at a lower frequency than that specified for the transformer.
  • List five different types of transformers according to their applications.
  • State the standard color coding for a power transformer.
  • State the general safety precautions you should observe when working with transformers and other electrical components.


The information in this chapter is on the construction, theory, operation, and the various uses of transformers. Safety precautions to be observed by a person working with transformers are also discussed.

A TRANSFORMER is a device that transfers electrical energy from one circuit to another by electromagnetic induction (transformer action). The electrical energy is always transferred without a change in frequency, but may involve changes in magnitudes of voltage and current. Because a transformer works on the principle of electromagnetic induction, it must be used with an input source voltage that varies in amplitude. There are many types of power that fit this description; for ease of explanation and understanding, transformer action will be explained using an ac voltage as the input source.

In a preceding chapter you learned that alternating current has certain advantages over direct current. One important advantage is that when ac is used, the voltage and current levels can be increased or decreased by means of a transformer.

As you know, the amount of power used by the load of an electrical circuit is equal to the current in the load times the voltage across the load, or P = EI. If, for example, the load in an electrical circuit requires an input of 2 amperes at 10 volts (20 watts) and the source is capable of delivering only 1 ampere at 20 volts, the circuit could not normally be used with this particular source. However, if a transformer is connected between the source and the load, the voltage can be decreased (stepped down) to 10 volts and the current increased (stepped up) to 2 amperes. Notice in the above case that the power remains the same. That is, 20 volts times 1 ampere equals the same power as 10 volts times 2 amperes.

Q.1 What is meant by "transformer action?"

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