=Coil Retaining Devices.=--In the operation of a dynamo there are two forces which tend to throw the inductors out of position:
Both of these forces are present with smooth core armatures, but only centrifugal force with slotted armatures. The devices used to hold the inductors in position against these forces are:
1. Driving horns; 2. Binding ribbons; 3. Retaining wedges.
Ans. They are simply pins or strips projecting from the surface of a smooth core as shown in fig. 251.
[Illustration: FIG. 387.--Armature coil taping machine. Numerous machines have been invented for taping armature coils. They consist essentially of a device which revolves a roll of tape around the coil, in such a direction that the tape is unwound from the roll and rewound on the coil. The speed at which the coil is fed through the machine will determine the overlapping of the tape.]
=Ques. What other kinds of retainer are used on smooth core armatures?=
Ans. They require several binding ribbons or brass bands placed around the winding to prevent the inductors being thrown off the core by centrifugal force.
=Ques. With slotted armatures what provision must be made for retaining the inductors in position?=
Ans. Retaining wedges must be inserted into the notches or between the projecting tops of the teeth.
[Illustration: FIG. 388.--Front view of large armature for direct connected dynamo, built by the General Electric Co.]
Ans. They are usually made of well baked hard wood, such as hornbeam, or hard white vulcanized fibre. Sometimes a springy strip of German silver is used.
An electric motor is just the reverse of a dynamo; it is _a machine for converting electrical energy into mechanical energy_.
The electrical energy delivered by the dynamo must be obtained from a steam engine, gas engine, or other power; the mechanical energy obtained from the motor comes from the energy of the current flowing through its armature.
=Ques. What is the construction of a motor?=
Ans. It is constructed in the same manner as a dynamo.
Any machine that can be used as a dynamo will, when supplied with electrical power, run as a motor, and conversely, a motor when driven by mechanical power, will supply electrical energy to the circuit connected to it. Dynamos and motors, therefore, are convertible machines, and the differences that are found in practice are largely mechanical; they arise chiefly from the conditions under which the motor must work. Hence, the study of the motor begins with a knowledge of the dynamo, and accordingly the student should understand thoroughly all the fundamental principles of the dynamo, as already given, before proceeding further with the study of the motor.