Generator and Transformer Models

In a generator, motion of a conductor in a magnetic field induces an EMF. In a transformer, it is the changing field that induces an EMF in a fixed conductor.

Schematic generator

Discussion: Generators

The structure of a simple generator is essentially the same as a motor. The difference is that now mechanical energy is converted into electrical energy. The electrical current to a load is via a commutator for an ac generator or slip rings if ac is required.

Basic ideas can be understood by thinking about a coil rotating in a uniform magnetic field.

Plane of coil at an angle to the magnetic field
Consider a coil of area A with N turns of wire rotating at a constant angular velocity w in a uniform magnetic flux density B. As the coil rotates, it cuts through the lines of flux. Another way to express this is to say that the flux linking the coil is changing.

At what point is the rate of flux-cutting greatest? (When it is horizontal in the diagram above; when it is vertical, the rate of flux cutting is instantaneously zero.)

Rate of flux cutting = induced EMF = BANwcos wt

with a maximum value, Eo = BANw when the coil is parallel to the field.

Demonstrations: A motor in reverse

Show that a motor can operate in reverse, as a generator. One starting point is simply to attach a weight to a small motor and to drop the weight. The motor works in reverse as a generator; the induced EMF can be monitored with a meter

 Practical transformers

Discuss reasons for energy losses in real transformers. These are readily identified as:

ohmic heating of the coils
eddy current heating of the core
hysteresis effects which heat the core
magnetic flux escaping
But even with these it is not unusual to find efficiencies of 95% and higher. Large transformers used in power transmission may be as much as 99.5% efficient.

Where electronics are being used, low voltage ac supplies are usually required so step-down transformers will be an essential part of the power supply. The output from a transformer is ac, so there will have to be some form of rectification (with diodes) and smoothing (with capacitors).

Transformer on the grid

A second widespread use is within the 'Grid' that supplies electricity to the consumer. The connection from a power station to the consumer involves a long length of wire and often, high currents. For a given section of the grid, the resistance, R is fixed and the rate of heating generated in the wire will be I2 R; this energy is wasted. To minimise this energy loss, the current should be as small as possible. To deliver a particular power (VI), a smaller current can be achieved by using as high a voltage as possible. The grid is designed so that transformers are used to step up the voltage at the power station before transmission. Step down transformers reduce the voltage in stages to the level required by industrial and domestic consumers.

Book

• Power System Analysis by Grainger and W.D. Stevenson

• Power System Analysis and Design by J. Duncan Glover,Mulukutla S. Sarma and Thomas J. Overbye.

• Power System Analysis by Hadi Sadat.

• Principles of Power Systems by Mehta.

     
Author        Bilal masood
lecture       04
Type         ppt
Availability Available online
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