Man's physical universe

xanabras

516 MAGNETISM AND ELECTRICITY

headlights. Such automobiles had to be started by cranking, for there

was no spark until the magneto was turned as the automobile was

cranked. The introduction of the storage battery and the induction

coil permitted the use of the self-starter and electric lights.

Alternating-current

Large Installations Today. ^

Generators Are the Type Most Widely Used for

If powerful electromagnets are used to replace the permanent magnet

of a magneto, a greater electromotive force is generated, thus

producing a larger current.

The strength of the current

may be further increased

by greatly increasing the

number of magnetic lines

Fig. 252. Completing the stator for one of

the generators at the Grand Coulee Dam.

(Courtesy of the Westinghouse Electric and

Manufacturing Company.)

of force (by use of four,

six,

or even more electromagnets),

by increasing

the number of turns in the

loop of wire, or by increasing

the strength of the

electromagnetic field.

Inasmuch as the directcurrent

generator produces

direct current, a portion or

all of this current, depending

on the type of generator, may be used to excite the electromagnets.

Direct-current generators differ from alternating-current generators in

that they have commutators and

do not need auxiliary exciting

motors. Alternating-current generators

require auxiliary direct-current

motors to maintain a constant

electromagnetic field. The rotor

and stator of an alternating-current

generator correspond to the armature

and field of the direct-current

generator.

If direct current is desired from

^^^-^^l.e 527-ton rotor for

a generator, the collars or slip rings /'^one

*^

,. .

^

r of the 108,000-kw. generators at the

are split, formmg a commutator,

^^^^^ Coulee Dam. (Courtesy of

» Automobile generators are the direct- the Westinghouse Electric and Manucurrent

type.

facturing Company.)

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