. Farm machinery and farm motors. nal circuit is thrown around these machines are designed especially for conditions inwhich the load is very variable, as street car work, in-candescent lighting and for commercial power purposes. 648. Classification of dynamos. — Dynamos may beclassified according to their mechanical arrangement asfollows: 1. Stationary field magnet with revolving armature. 2. Stationary armature with revolving field magnet. 3. Stationary armature and stationary field magnet with re-volving core. They may also be classified by mechanical designs asfollows: 1. Direc
. Farm machinery and farm motors. nal circuit is thrown around these machines are designed especially for conditions inwhich the load is very variable, as street car work, in-candescent lighting and for commercial power purposes. 648. Classification of dynamos. — Dynamos may beclassified according to their mechanical arrangement asfollows: 1. Stationary field magnet with revolving armature. 2. Stationary armature with revolving field magnet. 3. Stationary armature and stationary field magnet with re-volving core. They may also be classified by mechanical designs asfollows: 1. Direct-current machines. 2. Alternating-current machines. And by electrical arrangement as 3. Shunt-wound. 4. Series-wound. 5. Compound-wound. 649. Armatures.—The armature core introduced intothe magnetic circuit to lielp lower the reluctance is alsoan electrical conductor, and when rotated in a magneticfield will have currents set up within itself. These cur-rents are independent of the external circuit, hence are 484 FARM MOTORS. i FIG. 365—BIPOLAR DIRECT-CURRENT DYNAMO a loss. They are known as eddy currents and the lossis termed eddy current loss. Fig. 366 shows a sectionof a solid armature and the direction of these only do these currents create a loss themselves butthey heat the armature windings and thus increase thearmature resistance. If these large eddy currents can bebroken up into smaller ones the loss will not be so break up these eddies armatures are now generallybuilt up of a large number of sheets of iron with insula-tion between the sheets. The insulation used for this ELECTRICAL MACHINERY 485 purpose is generally a coat of rust or a sheet of tissuepaper. 650. Hysteresis.—Another source of loss in an arma-ture is due to the fact that every time the current alter-nates the polarity of the magnetism is reversed. If thearmature is making 2,000 revolutions a minute and thereare two alterations in each revolution there would be4,000 alterati
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