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Direct-Current Machinery
The Direct-Current Dynamo
A generator of electric current can be
defined as a machine which converts
mechanical energy into electrical energy,
the direct-current generator, or dynamo,
being
one
which
supplies
a
unidirectional, as distinct from an
alternating current. It is, therefore,
necessary that the dynamo shall be
driven by some kind of prime mover
which has to provide an amount of energy equal to that supplied by
the dynamo, plus the various losses that occur in the dynamo.
When the dynamo is on no-load, that is when it is not supplying
any energy to the circuit to which it is connected, the prime mover
has to supply energy sufficient for the losses that take place in the
dynamo at no-load.
Essentials for Production of a Current. So that an electric
current may flow through a circuit it is necessary that there shall be an
electro-motive force (e. m. f.) acting in the circuit, and in the
dynamo
this e. m. f. is produced by electromagnetic induction. For an e. m. f.
to be produced in this manner it is necessary that conductors of
electricity shall be moved in such a way a s t o c u t t h e l i n e s o f
f o r c e o f a magnetic field; in other words, there must be three
things, namely, a magnetic field, a system of conductors, and motion
of those conductors in the magnetic field, the motion being such that
the conductors move across, and not along, the magnetic lines of
force. That part of the dynamo which produces the magnetic field is
called the field-magnet, while the part which carries the system of
moving conductors is called the armature. We shall see that the
dynamo gives an unidirectional, and not an alternating, voltage at
its terminals, but to transfer this voltage to fixed terminals a third
part called the commutator is necessary.
To understand the modern dynamo it is advisable to consider the
simplest possible form, since this is the easiest way of obtaining a
grasp of the principles which are the
same for the more complicated types
necessitated by commercial requirements.
We will therefore consider, first of all, a
single-turn rectangular coil rotating in a
bi-polar field as shown in Fig. 16. The
diagram shows the coil ABCD rotating
between the poles N. and S. in a
clockwise direction when looked at
from the end AC. In the position shown,
the coil side AB is moving under the N.
pole in a direction from left to right (in the end view) so that it is
cutting the magnetic lines of force, which are represented by the
dotted lines.
Fleming's Right-Hand Rule. — The direction of the e. m. f.
induced in AB is given by Fleming's Right-Hand Rule, which is
applied as follows: Arrange the thumb and the first and second
fingers of the right hand mutually at right angles. Point the first
finger in the direction of the magnetic lines of force, and the thumb in
the direction of motion of the conductor: then the direction in which
the second finger is pointing gives the direction of the e. m. f. induced
in the conductor (Fig. 17). Applying this rule to the coil side AB in
Fig. 16, and making use of the end view, we point the first finger
downwards, i.e. from TV". to S., the thumb to the right, and the
second finger then points into the paper. The e. rn. f. induced in AB
is, therefore, into the paper in the end view, or from A to B in the
side view, as indicated by the arrow head. Similarly, we find that the e.
m. f. induced in the coil side CD is from D to C. Following these
directions round the coil we see that the e. m. f. s induced in the two
coil sides act in the same direction round the coil, a condition
which, as we shall see, applies to the coils in commercial armature
windings. Also, with the coil in the position shown in the figure, if the
two ends E and F are connected to an external circuit, the
current set up by the induced e. m. f. will leave the coil at E and reenter the coil at F. In other words, E will be the positive and F the
negative terminal of the coil.
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