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Middle & High School - Reading Comprehension
3.15 Transformers

Directions: Use the SQRW (Survey-Question-Read-Write) Strategy to read the following informational article. Answer the questions and have the notes you have taken while reading this article reviewed by your parent or teacher.


A transformer is an electrical device that transfers energy from one circuit to another by magnetic coupling with no moving parts. A transformer comprises two or more coupled windings, or a single tapped winding and, in most cases, a magnetic core to concentrate magnetic flux. An alternating current in one winding creates a time-varying magnetic flux in the core, which induces a voltage in the other windings. Transformers are used to convert between high and low voltages, to change impedance, and to provide electrical isolation between circuits.

The transformer is one of the simplest of electrical devices. Its basic design, materials, and principles have changed little over the last one hundred years, yet transformer designs and materials continue to be improved. Transformers are essential for high voltage power transmission, providing an economical means of transmitting power over large distances. The simplicity, reliability, and economy of conversion of voltages by transformers was the principal factor in the selection of alternating current power transmission in the "War of Currents" in the late 1880's. Audio-frequency transformers, then referred to as repeating coils, were used by the earliest experimenters in the development of the telephone. While some early electronics applications of the transformer have been replaced by alternative techniques, transformers are still found in many electronic devices.

Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge gigawatt units used to interconnect large portions of national power grids. All operate with the same basic principles and with many similarities in their parts.

Transformers alone cannot do the following:

  • Convert DC to AC or vice versa
  • Change the voltage or current of DC
  • Change the AC supply frequency.
However, transformers are components of the systems that perform all these functions.

An analogy

The transformer may be considered as a simple two-wheel 'gearbox' for electrical voltage and current. The primary winding is analogous to the input shaft and the secondary winding to the output shaft. In this comparison, current is equivalent to shaft speed, voltage to shaft torque. In a gearbox, mechanical power (speed multiplied by torque) is constant (neglecting losses) and is equivalent to electrical power (voltage multiplied by current) which is also constant.

The gear ratio is equivalent to the transformer step-up or step-down ratio. A step-up transformer acts analogously to a reduction gear (in which mechanical power is transferred from a small, rapidly rotating gear to a large, slowly rotating gear): it trades current (speed) for voltage (torque), by transferring power from a primary coil to a secondary coil having more turns. A step-down transformer acts analogously to a multiplier gear (in which mechanical power is transferred from a large gear to a small gear): it trades voltage (torque) for current (speed), by transferring power from a primary coil to a secondary coil having fewer turns.


Q 1: While the design and materials used to create transformers have improved, the basic design has not changed much over 100 years. Why is that?
There are few electricians who work with transformers.
There is no need for transformers in this day and age.
It is a simple, basic device.
Materials are too expensive.

Q 2: What does "time-varying magnetic flux'"mean?
A magnetic glue that is on a timer.
A magnetic flow that varies in the length of time.
A variety of currents that changes over time.
A timed series of magnetic conduction.

Q 3: Transformers can be used for huge projects, like connecting power grids, to small projects, like the transformer found in a microphone. How is this technology applicable to such diverse projects?
All operate with the same basic principles and with many similarities in their parts.
The larger systems are build on complicated extensions of the basic technology.
It is not applicable.
The parts used for each project are very different.

Q 4: Transformers can be used for all of the following except for ___________.
change the AC supply frequency
converting DC to AC or vice versa
converting heat into electricity
change the voltage or current of DC

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