THE DYNAMO
The dynamo is a machine which converts mechanical energy into electrical
energy by electromagnetic induction.
The word dynamo is used to designate a machine which produces _direct
current_ as distinguished from the _alternator_ or machine generating an
_alternating current_. In a broader sense, the word _generator_ is used to
denote any machine generating electric current by electromagnetic
induction; the term therefore includes both dynamos and alternators.
=Operation of a Dynamo.=--A dynamo does not create electricity, but
generates or produces an _induced electromotive force_ which causes a
current of electricity to flow through a circuit of conductors in much the
same way as a force pump causes a current of water to flow in pipes. The
electromotive force generated in the dynamo causes the current of
electricity to pass from a lower to a higher potential in the machine, and
from the higher back to the lower potential in the external circuit; that
is, the dynamo generates electrical pressure which overcomes the
_resistance_ or opposition to the current flow in the circuit. The pump
produces a mechanical pressure which, for instance, may be used to force
water into an elevated reservoir against the back pressure due to its
weight.
[Illustration: FIG. 162.--Holzer Cabot type “M” dynamo. The design of the
base is such that it allows the field ring or frame to drop down, lowering
the center of gravity, which gives increased stability. The pedestals are
bolted directly to the base. Both front and rear pedestals are removable,
so that the armature may be taken out from either end without disturbing
the brushes or connections. The journals are provided with oil rings which
keep the oil in continual circulation around the shaft by means of oil
grooves in the journal. The pole pieces are cylindrical in shape and are
fitted with shoes which retain the field coils in place and assist
commutation. The field coils are former wound, the insulation being
reinforced with mica. They are soaked in varnish and baked for 24 hours at
225° Fahr. The armature is wound as desired, series, shunt or compound.
The armature core is of the drum type and is laminated, the discs being
held by end plates locked without through bolts. The armature coils are
formed of round, ribbon or bar copper, and are without joint except at the
commutator; they lie in troughs of insulating material, the upper layers
being insulated from the lower layers; they are retained in place by maple
wedges secured by binding wires, soldered throughout their length. The
commutator segments are drop forged in the smaller, and hard drawn in the
larger sizes. Radial brushes are used. The efficiency of this type machine
is stated by the maker at from 80% to 90%, according to size.]
The point to be emphasized is that _the dynamo does not create
electricity_ (nor the pump water) _but sets into motion something already
existing by generating sufficient pressure to overcome the opposition to
its movement_.
[Illustration: FIG. 163.--General Electric 16 KW multi-polar dynamo
designed to operate at moderate and slow speeds. The outer structure of
the machine consists of a magnet frame having feet in one casting.
Adjustment is provided for moving the machine on its bed plate to tighten
the belt. The field coils are former wound and the series windings permit
of any degree of compounding up to 10% by the use of suitable German
silver shunts connected across the series field.]
=Essential Parts of a Dynamo.=--The dynamo in its simplest form consists
of two principal parts:
1. The field magnet;
2. The armature.
[Illustration: FIG. 164.--General Electric dynamo with end shield and
armature removed showing construction. The core of the armature consists
of laminations keyed to spider with space blocks inserted at intervals to
provide ventilating ducts for cooling the core and windings. The armature
is _former wound_--that is, the inductors are bent to the proper shape on
a form; they are, therefore, interchangeable.]
=Ques. What is the object of the field magnet?=
Ans. To provide a field of magnetic lines or lines of force to be _cut_ by
the armature inductors as they revolve in the field.
=Ques. What is an armature?=
Ans. A collection of _inductors_ mounted on a shaft and arranged to rotate
in a magnetic field with provision for collecting the currents induced in
the inductors.
A simple loop or turn or wire may be considered as the simplest
form of armature.
=Ques. How do armatures and field magnets differ in dynamos and
alternators?=
Ans. A characteristic feature is that in the dynamo the field magnet is
the stationary part and the armature the rotating part, while in the
alternator the reverse conditions usually obtain.
=Ques. With respect to this feature, what names are sometimes given to the
armature and field magnet?=
Ans. The _stator_ and the _rotor_ depending on which moves.
=Ques. What is the real distinction between an armature and a field
magnet?=
Ans. The name field magnet is properly given to that part which, whether
stationary or revolving, _maintains its magnetism steady during
operation_; the name armature is properly given to that part which,
whether revolving or fixed, _has its magnetism changed in a regularly
repeated fashion when the machine is in motion_.
=Construction of Dynamos.=--In the make up of a dynamo, as actually
constructed, there are five principal parts, as follows:
1. Bed plate;
2. Field magnets;
3. Armature;
4. Commutator;
5. Brushes.