29-05-2012, 03:20 PM
Direct Current Generators
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INTRODUCTION
The linear machine was introduced because of its simple construction and the fact that it served to
demonstrate clearly the principles of electromechanics. It also allowed us to establish a model to show
symbolically the relationships which exist with devices of this kind. At this point we must take a quantum
jump to see how these principles have been implemented in rotating machines, specifically generators and
motors. Historically, d-c machines came into being before a-c machines because the scientists of that time
(about the middle of the 19th century) were only familiar with battery sources and consequently strived to make
motors which operated from batteries, as well as generators to charge the batteries and operate arc lamps.
Although superior in many ways, a-c machines have not completely replaced d-c machines, and will not in the
foreseeable future, since the d-c motor offers a controllability not yet approached by a-c motors. The d-c
generator, on the other hand, is declining rapidly in use since its functions have been largely taken over by solid
state rectification of alternator outputs (in automobiles for example). Nevertheless a thorough study of d-c
generators is worthwhile because the construction of motors and generators is the same, and the bilateral nature
of the energy conversion process means their “inner workings” share much in common.
CONSTRUCTION AND TERMINOLOGY
No mention was made of the origin of the magnetic field in the linear machine; in the loudspeaker it
was furnished by a permanent magnet. In d-c machines the magnetic field is supplied by field coils” of wire
wound around “pole cores” which are part of the magnetic circuit. These terms and others relating to the
construction of a typical d-c generator or motor are explained in the following table.
THE ELEMENTARY GENERATOR AND MECHANICAL COMMUTATION:
A very simple elementary AC generator is discussed first because of the similarity in concepts and
construction between AC and DC generators. Then the process of mechanical commutation is introduced.
Commutation is used to change an AC into a DC machine be it a generator or a motor.
Elementary Generator Construction
The elementary generator of Figure 5.3 consists of a loop of wire free to rotate in a stationary
magnetic field. Relative motion between the wire and the magnetic field will induce a potential difference
between the ends of the loop. Sliding contacts are used to connect the rotating loop to an external circuit in
order to use the induced voltage.
ELEMENTARY GENERATOR OPERATION
Assume that the armature loop is rotating in a clockwise direction and that its initial position is at A
(zero degrees) of Figure 5.4. In position A, the loop is perpendicular to the magnetic field and the black and
white conductors of the loop are moving parallel to the magnetic field. If a conductor is moving parallel to a
magnetic field, it does not cut through any lines of magnetic flux and no voltage is generated in the conductor.
This applies to the conductors of the loop at the instant they go through position A. No voltage is induced in
them and therefore no current flows through the circuit. The current meter registers zero.