Main Body

Basic Operations of Single Phase Transformers

A transformer is an AC device used to transfer energy from one circuit to another:

  • without a change of frequency
  • with a change in voltage, if required
  • without an electrical connection (with the exception of auto-transformers)

Mutual Induction

Transformers operate on the principle of mutual induction, which is the process of inducing a voltage in a coil by changing the current in another coil.

We learned previously that changing the current value in a coil causes the flux lines surrounding the coil to change. This change in flux induces a voltage in the coil called counter-electromotive force (CEMF).

If a second coil is placed next to the first coil, the lines of flux induce a voltage in the second coil without there being any electrical connection.

In a DC circuit, how long will we see an induced voltage in the second coil? For 5τ after the switch is opened or closed. Only when the value of the current is changing will the magnetic flux lines be cutting the second coil. 

In an AC circuit, the value of current is constantly changing so the flux lines are constantly cutting through the coil, inducing a voltage.

Other Characteristics

Transformers are designed to either:

  1. Step up voltage and step down current.
  2. Step down voltage and step up current.

Transformers are very efficient, ranging from 96% to 99% efficient. They require very little maintenance, as there are no moving parts.


Transformers are classified by:

  • Cooling method
    • Air or oil
    • Natural convection or forced
Figure 1. Three-phase air-cooled transformer by Craxd1 is used under the creative commons attribution-Share Alike 3.0 Unported license.


Figure 2. Cooling electric fans by byrev used under the creative commons CC-BY license.
  • Application

    • Power supply (over 500 KVA)
    • Distribution (500 KVA and under, pole mounts)
    • Control
    • Instrument (CTs and PTs)
  • Phases
    • Single-phase
    • Polyphase

    Video Alert! (A Blast from the Past)

    This video from the U.S. Department of Defense gives a fantastic description of how transformers work.



    Transformers video by PublicResourceOrg is under a Creative Commons Attribution License.


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