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dynamo armature and the driving shaft coupled to the source of power. At all normal speeds the tension of the governor spring keeps the two plates of the clutch in contact and the armature is turned at the same speed as the driving shaft.
Should the driving shaft speed exceed a certain predetermined limit the governor weights will fly out by centrifugal force and the
governor spring will be compressed so the driving and driven plates of the clutch are separated and the driving shaft revolves independently of the armature. As soon as the armature speed becomes reduced sufficiently to allow the governor spring to overcome the centrifugal force and draw back the governor weights, the clutch
plates are again brought into contact and the armature is again joined to the driving shaft.
A current of air is kept circulating through the casing by means of the fan action of the reënforcing webs of the clutch plate, the object being to absorb any heat which may be produced while
the dynamo is in action. An appliance of this nature may be driven from the engine by belt, chain, or gear connection (Fig. 15). It will deliver low voltage current which must be transformed by means of an inducction coil to current of higher value in order that it may be successfully utilized to produce the spark in the combustion chambers of the engine.
A very ingenious application of the dynamo is shown at Figs. 16 and 17. The electric generator is built in such a manner that it forms an integral part of the power plant. The magneto field is produced by a series of revolving magnets which are joined to and turn with the fly wheel of the motor. The armature coils are carried by a fixed plate which is attached to the engine base. This apparatus is really a magneto having a revolving field and a fixed armature, and as the magnets are driven from the fly wheel there is no driving connection to get out of order and cause trouble.
As the coils in which the current is generated are stationary, no commutator or brushes are needed to collect the current because the electricity may be easily taken from the fixed coils by direct connection. It has been advanced that this form of magneto is not as efficient as the conventional patterns, because more metal and wire are needed to produce the current required. As the magnets which form the heavier portion of the apparatus are joined to the fly wheel, which can be correspondingly lighter, this disadvantage is not one that can be considered seriously because the magnet weight is added to that of the motor fly wheel, the combined weight of the two being that of an ordinary balance member used on any other engine of equal power.
Methods of Winding Dynamos.—The reader not versed in electrical science is apt to be puzzled by the designation of the various windings used on dynamos and motors. The armature windings and field coils may be connected together in a number of ways, as outlined at Fig. 18. The simple machine shown at A uses a permanent magnet to produce the field and therefore has only one set of windings to be considered, i. e., those on the armature. When the field magnet is an electro magnet another set of windings must be considered, i. e., those of the field magnet. When the current generated in the armature must first pass through the field windings before it reaches the external circuit the machine is said to be a series wound machine as shown at B because the armature and field windings are joined together in series. If only a portion of the current generated by the armature is directed to the field magnet windings the machine is said to be shunt wound, as shown at C. A compound wound dynamo is shown at D. In this two sets of field windings are used, one connected in shunt, the other coils in series. The shunt winding provides an initial excitation sufficient to generate full voltage at no load. The series coils provide an excitation that increases as the load increases and thereby strengthen the field so as to prevent the falling off in voltage that would otherwise occur. If the series coils are sufficiently powerful to make the voltage rise as the load increases the machine is said to be over-compounded.
The compound wound dynamo is the type used almost universally for direct current production. In stationary applications, compound wound motors are used where the load varies considerably under which conditions the extreme speed variation of series motors would be objectionable and where increased torque or turning power would be needed that shunt motors could not give. A compound wound dynamo is, to a certain extent, self-regulating, as the two coils counteract each other and bring about a more regular action for varying currents than that of the ordinary shunt or series wound dynamo. The extent of the regulation possible depends upon the proportions of the different windings though a compound