DYNAMO 



2751 



DYNAMO 



current induced in the coil can be 

 changed into a direct or contin- 

 uous current travelling always in 

 one direction. Fig. 2 shows the es- 

 sential principle of a commutator. 

 The ends of the coil are joined to 

 two halves of a split tube, which is 

 fastened to a spindle on which the 



Dynamo. Fig. 2. Diagram show- 

 ing essential principle of a com- 

 mutator. N and S are the opposite 

 poles of a magnet between which 

 the wire coil is made to revolve 



coil revolves, in such a way that 

 the two halves are insulated from 

 one another. Two metallic plates, 

 or brushes, are arranged so that 

 the induced currents pass through 

 them to the external circuit from 

 the segments of the coil. The 

 segments change brushes as the 

 coil takes up the position where 

 the induced current is zero. 



The intensity of the current is 

 increased if a flat coil of many 

 turns of wire be substituted for 

 the simple coil, but even then a 

 cycle occurs only once per revo- 

 lution ^To increase the number 

 of cycles and so make the current 

 more constant, a large number of 

 coils are used, arranged at angles 

 to one another in external grooves 

 cut upon the surface of a drum of 

 soft iron plates, which has the 

 effect of concentrating the lines 

 of magnetic forces into the path 

 of the coils. An intense magnetic 

 field is obtained by using electric 

 magnets, excited by a current 

 through the coils encircling them, 

 in place of the permanent magnets 

 employed in the early machines. 

 These two chief parts of a dynamo 

 are the Armature and Field Mag- 

 net System or Field. 



In direct current machines the 

 armature is the revolving part, but 

 in alternating current machines it 

 is usually found expedient to 

 employ stationary armatures and 

 revolving field-magnets. 



DIRECT CURRENT DYNAMOS. 

 The armature of a direct current 

 dynamo consists of a number of 

 very thin circular sheets or lam- 

 inations of charcoal iron pressed 

 together on a shaft or spindle so as 

 to form a cylinder or drum. Each 

 disk is insulated from its neighbour, 

 in order to reduce the so-called 

 Foucault currents which would 

 cause a solid mass of iron to heat 

 excessively. The soft iron core of 

 an armature, whether revolving or 

 fixed, concentrates the magnetic 

 field. The surface of the drum has 



Dynamo. Fig. 2a. 

 Diagram of a split- 

 ring commutator 



longitudinal slots (Fig. 3) in which 

 the conductors or windings are 

 embedded. The three openings 

 surrounding the central hole com- 

 municate with air ducts for the 

 purpose of ventilating and cooling 

 the armature. Very large arma- 

 tures have a hollow or ring core. 

 The ring is carried on radial spokes 

 from a hub keyed on to the shaft, 

 the arrangement being termed a 

 spider. At one end of the armature 

 is the commutator, composed of 

 a number of 

 segmental cop- 

 per bars insu- 

 lated from each 

 other, and from 

 the shaft. A 

 complete drum 

 armature 

 shown in Fig. 4, 

 the commutator 

 being the smal- 

 ler of the two 

 cylinders. 



There are a number of ways of 

 winding the wires on the armatures, 

 and the most easily grasped prin- 

 ciple is that of the ring winding 

 of the Gramme and other early 

 machines, as 

 shown dia - 

 grammatically 

 in Fig. 5. 



A d j acen t 

 j coils are con- 

 j nected to the 

 | commutator 

 bars in the 

 manner shown , 

 which, by re- 

 versing the 

 connexion o f 

 the outer cir- 

 cuit when the 

 E.M.F. in the coil is reversed, 

 causes the current to flow in one 

 direction. If, instead of being 

 connected to the commutator bars 

 the beginning and the end of the 

 complete spiral were joined to a 

 separate collecting ring, the cur- 

 rent would not be rectified, but 

 delivered as alternating current. 



Dynamo. Fig. 3. 

 Diagram showing 

 longitudinal slots 

 in which the con- 

 ductors of an ar- 

 mature are em- 



Dynamo. 



Fig. 4. Complete drum 

 armature 



Nearly all modern drum armatures 

 are wound upon what are known as 

 the lap and the wave principles, the 

 first being adapted to large currents 

 of low voltage and the second to 

 small currents of high voltage. 



In lap winding each winding 

 forms a loop, lapping over other 

 loops, and each end is joined to 

 a commutator segment. Wave 



Dynamo. Fig. 5. 

 Ring winding on 

 an armature, 

 shown diagram- 

 matical^. N and 

 S, north and south 

 poles of the mag- 

 net 



winding follows a zigzag line round 

 the core, the successive coils being 

 connected in series. 



The term brush used here is 

 derived from the bundle of copper 

 wires, thin sheets or gauze formerly 

 employed ; but 

 carbon is now 

 used except for 

 the collection of 

 currents o'f very 

 low tension. 

 Contact with 

 the c o m m u - 

 tator is main- 

 tained by the 

 pressure of a 

 light spring 



The field 

 magnet system 

 of a dynamo, originally in the form 

 of a hardened steel permanent 

 horseshoe magnet, or assemblage 

 of magnets, as in Fig. 6, usually 

 consists of two, four or more cores, 

 contained within a circular yoke. 

 For many years the horseshoe form 

 was retained, but in this the loss of 

 strayed magnetism is greater. It 

 was early discovered that the re- 

 sidual magnetism present in soft 

 iron was sufficient to provide for 

 the generation of an initial current 

 by which the magnetism could be 

 " built up ; " the final result being 

 a field far more powerful than 

 could be obtained from permanent 

 magnets. Soft wrought iron and 

 special kinds of soft steel have a 

 higher " permeability " or mag- 

 netic conductivity than cast-iron, 

 and the magnet cores are therefore 

 of this material, except in the case 

 of very small machines. In quite 

 small machines the low residual 

 magnetism of wrought iron leads 

 to excitation difficulties, besides 

 which the cast-iron construction 

 is cheaper. 



Magneto machines are still used 

 for special purposes where instant 

 generation of small currents is 

 required. 



The direction of the winding of 

 the magnet coils is such that the 

 cores become N. and S. poles al- 

 ternately, and the winding may be 

 in " series," " shunt," or a combi- 

 nation of both ("compound") 

 with the external circuit. 



In series winding (see Fig. 7) 

 the whole of the current passes 

 through the 

 magnet coils, 

 which in this 

 case consist of a 

 small number of 

 turns of thick 

 wire. In other 

 words, the arma- 

 ture, field coils, 

 and the external 



of magneto series. A series 



