Chapter XIII: The Dynamo: Basic Principles

A dynamo is a machine for converting mechanical energy into electrical energy, by means of electromagnetic induction, the amount of electric energy thus obtained depending upon the mechanical energy originally supplied.

The word dynamo is properly applied to a machine which generates[14] direct current, as distinguished from the alternator, which generates alternating current.

=Ques. Define a dynamo with respect to its principle of operation.=

Ans. A dynamo is _a machine for filling and emptying conducting loops with magnetic flux, and utilizing the electromotive force thus induced in them for the production of current in the external circuit_.

The fitness of this definition is apparent, having in mind the
principles of electromagnetic induction.

=Ques. What are the three essential parts of a dynamo?=

Ans. The field magnet, armature, and commutator.

=Ques. What is the object of the field magnet?=

Ans. To provide a magnetic field, through which the conducting loops arranged on a central hub and forming the _armature_ are carried, or the flux carried through them, so that they are successively filled and emptied of magnetic lines.

=Ques. What is a commutator?=

Ans. A device for causing the alternating currents generated in the armature to flow in the same direction in the external circuit.

=Ques. Upon what does the voltage depend?=

Ans. Upon the _rate_ at which each conducting loop is filled and emptied of lines of force and the number of such loops with their grouping or connection.

=Ques. How is the operation of a dynamo best explained?=

Ans. By considering first the action of the simplest form of current generator, or elementary alternator.

=Ques. Describe an elementary alternator.=

Ans. It consists, as shown in fig. 165, of a single rectangular loop of wire A B C D, one end being attached to a ring F and the other to the shaft G, and arranged so as to revolve around the axis X X′, which is located midway between the two poles of the magnet. Two metallic strips or _brushes_ M and S connected with the external circuit, bear on the ring F and shaft G, respectively, in order to “collect” the current generated in the armature when the machine is in operation. The long, straight, horizontal arrows joining the two poles of the magnet, represent the _lines of force_ which make up the magnetic field between the poles. The field is here assumed to be uniform, as indicated by the equal spacing of the arrows.

=Ques. What happens when the loop is rotated?=

Ans. According to the law of electromagnetic induction, when the loop is rotated around its horizontal axis in the direction indicated by the curved arrow, an electromotive force will be induced in the loop, the magnitude of which depends on the _rate_ of change of the number of lines of force threading through, or embraced by the loop.

That is, if the number of lines embraced by the loop be
increased from, say, 0 to 1000, or decreased from 1000 to 0, in
one second, the electromotive force generated will be two times
as great as if the increase or decrease were only 500 lines per
second.

=Ques. Upon what does the direction of the induced current depend?=

Ans. Upon the direction of the lines of force and direction of rotation of the loop.

=Ques. How is Fleming’s rule applied to determine the direction of current?=

Ans. In applying this rule, the horizontal portion of the loop, such as A B or C D (fig. 165), is to be considered as moving up or down; that is, the component of its motion at right angles to the lines of force is taken as the direction of motion. When the loop is in the position A B C D, such that its plane is vertical or perpendicular to the lines of force, the maximum number of magnetic lines thread through it, but when it is in a horizontal position, A′ B′ C′ D′, so that its plane is parallel to the lines of force, no lines pass through the loop. During the rotation from position A B C D to A′ B′ C′ D′, the number of lines passing through the loop is _reduced_ from the maximum to zero, the reduction taking place with _increasing rapidity_ as the loop approaches the horizontal position, the electromotive force thus induced _increasing in like proportion_. Continuing the rotation from the horizontal position A′ B′ C′ D′ to the inverted vertical position A B C D (fig. 166), the number of lines passing through the loop is increased from zero to the maximum, the increase taking place _with decreasing rapidity_ as the loop approaches the inverted vertical position, the electromotive force thus induced _decreasing in like proportion_.

=Ques. How does the current flow during the first half of the revolution of the loop?=

Ans. It flows in the direction A B C D (fig. 165), as is easily ascertained by aid of Fleming’s rule.

=Ques. What is the path of the current to the external circuit?=

Ans. It flows out through brush M (fig. 165) and returns through brush S, thus making M positive and S negative.

=Ques. What occurs during the second half of the revolution?=

Ans. The wire A B (fig. 166), which before was moving in a downward direction, moves in an upward direction; hence, the current is reversed and flows around the loop in the direction A D C B (fig. 166), going out through brush S and returning through brush M. This makes M negative and S positive.

=Ques. What may be said of the electromotive force during the second half of the revolution?=

Ans. It varies in a similar manner as in the first half of the revolution; that is, the magnetic lines are cut _with increasing rapidity_ during the third quarter, _and with decreasing rapidity_ during the fourth quarter of the revolution, which causes the electromotive force to increase and decrease during these intervals.

The cycle of events just described may be summed up as follows: During the revolution of the loop:

1. From 0° to 90°, the electromotive force increases from 0 to maximum;
2. From 90° to 180°, the electromotive force decreases from maximum to
zero;
3. From 180° to 270°, current reverses and the electromotive force
increases from zero to maximum;
4. From 270° to 360°, the electromotive force decreases from maximum to
zero.

It was stated that, during the revolution of the loop, the
magnetic lines were cut “with increasing or decreasing
rapidity,” causing the electromotive force to rise or fall. The
reason for this is illustrated in fig. 167. The loop is here
shown in a horizontal position at right angles to the direction
of the magnetic field; the latter, as indicated by the even
spacing of the vertical arrows representing the magnetic lines,
is assumed to be uniform.

The wire C D of the loop, as it rotates at _constant speed_,
cuts the magnetic lines at the points 0, 1, 2, 3, etc., but the
distances 0-1, 1-2, 2-3, etc., between these points, are
unequal; that is, the wire C D travels farther in cutting the
lines 0 and 1, than it does in cutting 1 and 2, and still less
in cutting the lines 2 and 3. After cutting the line 4, which
passes through the axis of revolution, the opposite conditions
obtain.

If the arcs 0-1, 1-2, etc., of the dotted circle, which are
intercepted by the magnetic lines and passed through by the
wire, be rectified and laid down under each other, as lines 0-1,
1-2, etc., the time of passage of the wire between successive
magnetic lines will vary as the length, since the speed is
uniform. Thus the wire in passing from line 0 to line 1, takes
much more time than in passing from 1 to 2, as indicated at the
left of the figure by 0-1 and 1-2, and still less in passing
from 2 to 3; that is, the rate of cutting the lines increases as
C D rotates from 0 to 4 and decreases from 4 to 8.

Since similar conditions prevail with respect to A B, for its
corresponding movement, it is evident that the number of lines
which thread through the loop are _decreased with increasing
rapidity_ as the loop rotates through the first quarter of a
revolution, and _increased with decreasing rapidity_ during the
second quarter of the revolution. Moreover, it must be evident
that the reverse conditions obtain for the third and fourth
quarters of the revolution.

=The Sine Curve.=--In the preceding paragraph it was shown that an alternating current is induced in the armature of either an alternator or dynamo; that is, the current: 1, begins with zero electromotive force, 2, rises to a maximum, 3, decreases again to zero, 4, increases to a maximum in the opposite direction, and 5, decreases to zero.

A wave-like curve, as shown in fig. 168, is used to represent these several changes, in which the horizontal distances represent time, and the vertical distances, the varying values of the electromotive force. It is called the sine curve because a perpendicular at any point to its axis is proportional to the sine of the angle corresponding to that point.

=Ques. Describe the construction and application of the sine curve.=

Ans. In fig. 168, at the left, is shown an elementary armature in the horizontal position, but at right angles to the magnetic field. The dotted circle indicates the circular path described by A B or C D during the revolution of the loop. Now, as the loop rotates, the induced electromotive force will vary in such a manner that _its intensity at any point of the rotation is proportional to the sine of the angle corresponding to that point_. Hence, on the horizontal line which passes through the center of the dotted circle, take any length, as 08, and divide it into any number of parts representing fractions of a revolution, as 0°, 90°, 180°, etc. Erect perpendiculars at these points, and from the corresponding points on the dotted circle project lines parallel to 08; the intersections with the perpendiculars give points on the sine curve. Thus the loop passes through 2 at the 90° point of its revolution, hence, projecting over to the corresponding perpendicular gives 2 2′, a point whose elevation from the axis is proportional to the electromotive force at that point. In like manner other points are obtained, and the curved line through them will represent the variation in the electromotive force for all points of the revolution.

At 90°, the electromotive force is at a maximum; hence, by using
a pressure scale such that the length of the perpendicular 2 2′
for 90° will measure the maximum voltage the length of the
perpendicular at any other point will represent the actual
pressure at that point.

The curve lies above the horizontal axis during the first half
of the revolution, and below it during the second half, which
indicates that the current flows in one direction for a half
revolution and in the opposite direction during the remainder of
the revolution.

The application of the sine curve to represent the alternating cycle, is further illustrated in figs. 169 to 173, which show the position of the armature at each quarter of the revolution.

In fig. 179, the loop A B C D is in the vertical position at the
beginning of the revolution. At this instant the electromotive
force is zero, hence the sine curve as shown begins at E, the
zero point--that is, on the axis or line of no pressure.

As soon as the loop rotates out of the vertical plane, the
electromotive force rises and the current begins to flow in the
direction indicated by the arrows, going out to the external
circuit through brush M, and returning through brush S.

Continuing the rotation, the electromotive force increases in
proportion to the sine of the angle made by the plane of the
loop with the horizontal, until the loop comes into the
horizontal position illustrated in fig. 170. This increase is
indicated by the gradual rise of the sine curve from E to F. The
loop has now made one quarter of a revolution and the
electromotive force reached its maximum value.

As the loop rotates past the horizontal position of fig. 170,
the electromotive force gradually decreases in intensity,
reaching the zero point at the end of the second quarter--that
is, when the loop has turned one half revolution. This is
indicated by the gradual fall of the curve from F to G.

When the loop turns out of the vertical position shown in fig.
171 the current reverses, because the movement of A B and C D is
reversed; at this instant the brush M becomes negative, and S
positive. This reversal of current is indicated by the curve
falling _below_ the axis from G to I.

During the second half of the revolution, figs. 171 to 173, the
changes that occur are the same as in the first half, with the
exception that the current is in the reverse direction; these
changes are as shown by the curve from G to I.