1. Generator and Transformer

Learning objectives

• describe a simple form of a.c. generator (rotating coil or rotating magnet) and the use of slip rings (where needed)

• sketch a graph of voltage output against time for a simple a.c. generator

• appreciate similarities and differences between D.C generator and A.C. generator

• describe the structure and principle of operation of a basic iron-cored transformer as used for voltage transformations

• recall and apply the equation (Vs/Vp ) = (Ns/Np ) and Vs Is = Vp Ip (for ideal transformer) to new situations or to solve related problems

Electromagnetic induction is the basic principle for the working of generators.

How does one of this generator works?

The generator is a device in which a coil is rotated in a magnetic field to produce electricity.

The mechanical energy of a rotating coil in a running generator is converted into electrical energy.

Structure and working of an a.c. generator

As the coil is rotated, one side of the coil moves up while the other side moves down in the magnetic field between the permanent magnets.The magnetic field through the coil changes and therefore induces an e.m.f. between the ends of the coil. The induced current does not flow unless the ends of the coil are connected to an external circuit with an electrical load.

The slip rings ensure that the direction of the induced current flowing in the external circuit changes direction every time the coil passes through the vertical position. Hence the output voltage or current is alternating. The carbon brushes provide sliding contact witht the rotating slip rings.

Graph of a.c. output against time

The induced e.m.f or current of an a.c. generator is increased by increasing the rate of change of magnetic flux linking the coil. This can be done by:-

The following websites provide simulations of generator. You may want to explore theses sites to figure out how a generator works.

http://micro.magnet.fsu.edu/electromag/java/generator/ac.html

https://www.walter-fendt.de/html5/phen/generator_en.htm 

The following websites provide explaination of how an AC generator works.

http://www.hk-phy.org/energy/power/elect_phy/flash/ac_generator_e.html

http://www.gcsescience.com/pme17.htm

Compare and contrast AC and DC generators

http://www.tutorvista.com/content/physics/physics-ii/electricity/electric-generator.php

The transformer is a device for changing high a.c. voltage to a low a.c voltage or vice versa by using the process of mutual induction.

Working of a Transformer

The a.c. flowing in the primary coil changes in terms of magnitude or direction of flow or both. The magnetic field around it changes as well. This causes the secondary coil to experience a changing magnetic field around it, this induces an e.m.f in the secondary coil and hence an induced current if the secondary coil is connected to an external circuit. The direction of induced e.m.f and current depends on the direction of flow of current in the primary coil. 

The voltage of the output current follows the principle of transformers which states that :-

Vs/Vp = Ns/Np

The ratio Ns/Np is called the turns ratio.

The efficiency of a transformer is defined as follows:-

Efficiency = (output power /input power) x 100%

If the efficiency is 100%, then input power = output power

VpIp = VsIs

Most practical transformers have efficiency less than 100%. This is because :-

* some energy is lost in the form of thermal energy produced by current in the coils which have resistance

* some thermal energy is produced in the core due to the flow of eddy currents induced in the core

( The changing magnetic field not only induces currents in the secondary coil, but also currents in the iron core itself. These currents flow in little circles in the iron core and are called eddy currents)

* not all the magnetic lines of forces are linked to the secondary coil

* Hysteresis loss caused by the flipping of magnetic poles in iron core due to the a.c. in primary coil

To improve the efficiency of a transformer, the following featuresshould be taken into consideration:-

• low resistance copper wires should be used for the coil

• A laminated core should be used to reduce the flow of eddy currents

• Effective soft magnetic material should be used to make the core

• A closed core design should be used. It should ensure that the magnetic field produced by the primary coil is linked completely to the secondary coil

The following websites provide more information about the working of transformer.

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/transf.html

http://www.gcsescience.com/pme23.htm

https://interactives.ck12.org/simulations/physics/ac-transformer/app/index.html