neutral point and the brushes are in the fringes of flux so that it is difficult to obtain satisfactory
commutation. The most satisfactory solution has been to introduce a separate flux to produce a desired
flux density in the neutral zone. This is done by small poles called interpoles located between the main
poles.
Field armature cores are built up of layers of sheet steel to reduce strap currents called eddy
currents. Since iron is almost as good a conductor of electricity as copper, electrical currents tend to
flow almost as readily in the iron as in the copper. Lacking an outside circuit, however, they circulate
within the cores and generate excessive heat resulting in a loss of power. The heat is much less in a
laminated core than it would be in a solid core because the laminations provide passages through which
air can circulate and carry away heat.
Armature coils are connected in either lap windings or wave windings. Coils in a lap winding
overlap each other and are connected in parallel, while a wave winding is in series around the armature.
Voltage is higher but current is lower in lap-wound coils than in wave-wound coils.
a. Alternating-current generator. In an ac generator, illustrated in figure 1.16, the ends of the
rotating coils are connected to continuous metal rings, called sliprings, mounted on the axis (armature
shaft) and insulated from each other. Metal or carbon blocks, called brushes, ride on the rings to make
contact with the external circuit. The number of complete waves or cycles generated per second is called
the frequency and is determined by the speed and design of the generator.
A generator's emf is affected most by a change in speed of rotation of the armature; a change in
flux density caused by changing the current flowing in the field coil or by using a variable resistor; or a
change in the size of the field or the size and turns of the rotating coil.
Since the current in the armature is alternating, some method must be used to change it to direct
current before it enters the outside circuit. This can be done by using a commutator. The commutator
reverses the connections of the coil to the external circuit at the same time that the emf reverses in the
coil, and the alternating current in the generator becomes a pulsating direct current when it reaches the
outside circuit.
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