Alternators, 1
Page 70
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HOUGH in recent years dynelos have been produced with a reater output in relation to their ize, there is a limit to what can
e achieved in this connection. II the time there is a demand )r more electrical power on lotor vehicles while at the same me increased traffic congestion asults in vehicles having to perate at low road speeds. A charging system had to be esigned to produce larger curants at lower engine speeds nd the alternator has been deeloped to meet these needs. The rotating parts of an alterator are more robust than on a C machine so that a higher peed of rotation can be used rhich provides a higher maxium output and a useful charg1g rate at the lower engine peeds, when a vehicle is crawlig in heavy traffic or when it is landing at traffic signals.
In the alternator, the magnet ?valves and the conductors relain stationary, the exact oppoite to the layout of the DC dyamo (see diagram). The lectro-magnet in the alternator omprises a coil mounted on a haft; the coil is fed with direct urrent. Iron pole pieces surmind the coil which is connecad to two copper slip rings. Brush holders, mounted on le fixed frame, hole brushes fhich press on to the slip rings nd feed direct current to the Dils, causing the coil and the ole pieces to become a rotating lectromag net.
The fixed frame or stator asambly consists of a laminated nd slotted iron core. Three sets f insulated windings are fitted ) the iron core. The stator asambly fits over and very close ) the rotor so that the magnetic eld of the rotor cuts across the tator windings as the rotor reolves. This induces a voltage in ach of the three windings, each oltage alternating in strength om maximum to zero, giving a Iree-phase alternating current utput.
Alternating current can't be used for battery charging so rectification to DC is necessary and this is done by the use of diodes. A diode is a form of an electrical one-way valve, it will only allow current to flow in one direction through it. Diodes take the form of a button and are usually made from silicon.
Six are used in an alternator to rectify the current, "heat sinks" are incorporated in their construction to help cool them and a fan, fitted behind the drive pulley, ensures a flow of air over them.
Because the diodes only allow current to flow one way, the cutout fitted to dynamo charging systems is not needed. On modern alternators the output is controlled electronically by a special type of diode and transistors within the machine itself.
More about alternators in the next article and especially the special care needed to make sure they aren't damaged.
by Preceptor