01-07-2014, 09:58 AM
D.C Machines
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Introduction
The steam age signalled the beginning of an industrial revolution. The advantages
of machines and gadgets in helping mass production and in improving the services spurred
the industrial research. Thus a search for new sources of energy and novel gadgets received
great attention. By the end of the 18th century the research on electric charges received
a great boost with the invention of storage batteries. This enabled the research work on
moving charges or currents. It was soon discovered ( in 1820 ) that, these electric currents
are also associated with magnetic field like a load stone. This led to the invention of an
electromagnet. Hardly a year later the force exerted on a current carrying conductor placed
in the magnetic field was invented. This can be termed as the birth of a motor. A better
understanding of the inter relationship between electric and magnetic circuits was obtained
with the enumeration of laws of induction by Faraday in 1831. Parallel research was contem-
porarily being done to invent a source of energy to recharge the batteries in the form of a
d.c. source of constant amplitude (or d.c. generator). For about three decades the research
on d.c. motors and d.c. generators proceeded on independent paths. During the second half
of the 19th century these two paths merged. The invention of a commutator paved the way
for the birth of d.c. generators and motors. These inventions generated great interest in the
generation and use of electrical energy. Other useful machines like alternators, transformers
and induction motors came into existence almost contemporarily. The evolution of these
machines was very quick. They rapidly attained the physical configurations that are being
used even today. The d.c. power system was poised for a predominant place as a preferred
Basic principles
Electric machines can be broadly classified into electrostatic machines and electro-
magnetic machines. The electrostatic principles do not yield practical machines for commer-
cial electric power generation. The present day machines are based on the electro-magnetic
principles. Though one sees a variety of electrical machines in the market, the basic under-
lying principles of all these are the same. To understand, design and use these machines the
following laws must be studied.
1. Electric circuit laws - Kirchoff′s Laws
2. Magnetic circuit law - Ampere′s Law
3. Law of electromagnetic induction - Faraday′s Law
4. Law of electromagnetic interaction -BiotSavart′s Law
Most of the present day machines have one or two electric circuits linking a common
magnetic circuit. In subsequent discussions the knowledge of electric and magnetic circuit
laws is assumed. The attention is focused on the Faraday’s law and Biot Savart’s law in the
present study of the electrical machines.
Law of electro magnetic induction
Faraday proposed this law of Induction in 1831. It states that if the magnetic
flux lines linking a closed electric coil changes, then an emf is induced in the coil. This
emf is proportional to the rate of change of these flux linkages. This can be expressed
mathematically,