**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amplifiers, have been replaced by solid-state power amplifiers;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 )**

http://faculty.ksu.edu.sa/eltamaly/Documents/Courses/EE%20339/

MAGNETIC%20CIRCUITS.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”.

“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 )**

http://faculty.ksu.edu.sa/eltamaly/Documents/Courses/EE%20339/

DC%20Machines2.pdf