CHAPTER XII

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.

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_.

=Essential Parts of a Dynamo.=--The dynamo in its simplest form consists of two principal parts:

1. The field magnet; 2. The armature.

=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.