Friday, March 20, 2009

Electromagnetic Induction

Electromagnetic Induction

In 1820 Oersted discovered the magnetic effect of an electric
current, and the first primitive electric motor was built in the
following year. Faraday's discovery of electromagnetic induction in
1831 completed the foundations of electromagnetism, and the
principles were vigorously exploited in the rapidly growing field of
electrical engineering. By 1890 the main types of rotating electrical
machine had been invented, and the next forty years saw the
development of many ingenious variations, along with refinement
of the basic types. This was the golden age of machine
development. Many machines are now obsolete which were once
made in large numbers. Thus the cross-field DC machines, or rotary
amplifiers, have been replaced by solid-state power amplifiers;
while the Schrage motor and other ingenious variable-speed AC
machines have given way to the thyristorcontrolled DC motor and
the inverter-fed induction motor.
When a conductor moves in a magnetic field, an EMF is
generated; when it caries current in a magnetic field, a force is
produced. Both of these effects may be deduced from one of the
most fundamental principles of electromagnetism, and they provide
the basis for a number of devices in which conductors move freely
in a magnetic field. It has already been mentioned that most
electrical machines employ a different form of construction.

Source ( pdf )

Induced Voltage

An expression can be derived for the voltage induced in a
conductor moving in a magnetic field. As shown in Fig.3.2a, if a
conductor of length l moves at a linear speed v in a magnetic field
B, the induced voltage in the conductor can be obtained with the
help of fraday’s law as shown in the following equation:

e = Blv (3.1)

where B, l, and v are mutually perpendicular. The polarity
(Direction) of the induced voltage can be determined from the
so-called Fleming's Right-Hand Rule as explained in the previous
chapter. The direction of this force is shown in Fig.3.2(b).

Fleming's Right-Hand Rule
“Hold out your right hand with forefinger, second finger, and
thumb at right angles to one another. If the forefinger represents
the direction of the field, and the thumb represents the direction of
the motion then, the second finger represents the direction of the
induced emf in the coil”.

Source ( pdf )

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