DYNAMO 



2752 



DYNAMO 



machine generates, within limits, 

 a current of constant quantity: 

 increase of output being repre- 

 sented by a rise in voltage. Series 

 machines were formerly much used 

 for series arc lighting. With 

 " shunt " (Fig. 8) winding only a 

 fraction of the current delivered to 

 the circuit passes through the field 



Dynamo. Fig. 7. 

 Series winding 



Dynamo. Fig. 

 Shunt winding 



coils, which contain many turns of 

 fine wire. 



In the compound- wound dynamo 

 (Fig. 9) the series winding compen- 

 sates for the loss of voltage in the 

 armature due to increased output, 

 and a compound machine may be 

 made self-regulating for varying 

 loads. By " over-compounding," 

 that is, increasing the proportion 

 of the series winding, it is possible 

 to provide for an increase of voltage 

 with increase of load. In this way a 

 loss of pressure in distributing 

 mains, owing to the extra work put 

 upon them, may be made good. 

 Compound dynamos or generators 

 are employed for direct electric 

 lighting and for power and trac- 

 tion services. 



Figure 10 represents a modern 

 direct-current dynamo. Small 

 pole-pieces between the main pole- 

 pieces will be noticed ; these are 

 known as " interpoles," or " corn- 

 mutating " poles. They are excited 

 by a few turns of series winding, 

 and their function is to reduce the 

 effect of armature reaction, and 

 so prevent sparking between the 

 brushes and the commutator when 

 a change of load takes place. A 

 change of load causes a change of 

 flux, and this formerly necessitated 

 an adjustment of the position or 

 " lead " of the brushes which re- 

 quired constant watchfulness*. 



ALTERNATING CURRENT DYNA- 

 MOS. In the early days of electrical 

 engineering, motors, arc lamps and 

 other appliances had not been 

 developed for alternating-current, 

 therefore all dynamos were pro- 

 vided with commutators, which 

 changed the alternating into direct, 

 currents. One special kind of ma- 

 chine the "uni -polar" or "homo- 

 polar " actually generates a di- 

 rect current, but although it has 

 occupied the attention of inventors 

 for many years past, it is still it 

 the experimental stage. Improve- ' 



ments in static transformers (which 

 can only be employed with alter- 

 nating currents) and a recognition 

 of the value of high voltage for 

 long-distance transmission of elec- 

 trical energy, directed attention to 

 the " alternator." 



Direct current dynamos are not 

 adapted to the production of high 

 voltage cur- 

 rents, owing, 

 mainly, to the 

 practical diffi- 

 culty in col- 

 lecting such 

 currents from 

 the commuta- 

 tors,and partly 

 to thedifficulty 



Dynamo. Fig. 9. J? insulating 

 Compound winding the coils on the 

 revolving arm- 

 ature. With alternators these diffi- 

 culties can be overcome and ma- 

 chines have been constructed to 

 supply current at 15,000 volts. A 

 lower pressure from 12,000 to 

 13,000 volts is, however, generally 

 regarded as the advisable limit. 



If a higher voltage is required, 

 the usual practice is to raise it by 

 means of a transformer. The high 

 voltage generally associated with 

 the alternators has led to a funda- 

 mental change in their design, viz. 

 the moving of the magnetic^field in 



Dynamo. Fig. 10. Modern direct- 

 current dynamo 



relation to the armature wires, in- 

 stead of the armature in relation 

 to the field. The field-magnets are, 

 except in the case of small machines 

 of low voltage, included in the 

 revolving parts (see Fig. 11), 

 whilst the armature (Fig. 12) forms 

 the stationary part. Fig. 13 shows 

 the machine' assembled with the 

 field-magnets inside the armature. 



Dynamo. Fig. 11. Field magnet 

 oi alternator 



Dynamo. Fig. 12. Armature of 

 alternator 



In this way the current from the 

 armature can be led off without the 

 interposition of any moving col- 

 lector, while the insulation is not 

 cramped or subjected to mechani- 

 cal stress. The current necessary 

 to excite the field magnets is of 

 low voltage, and is led to them by 

 carbon brushes bearing upon two 

 " slip " rings revolving with the 

 field-magnet system. In another 

 kind of machine (the " Inductor " 

 Alternator) both field magnets and 

 armature are stationary, but have 

 an annular space between them. 

 The annular space is traversed by 

 bare poles ("inductors"), attach- 

 ed to a revolving spider. The 

 poles complete the magnetic cir- 

 cuit between the field-magnets and 

 the armature coils as they pass 

 between them, and, in doing so, 

 cause the necessary variation in 

 magnetic flux. To avoid confusion, 

 the revolving portion of an alter- 

 nator is called the " rotor " and the 

 stationary part the " stator "- 

 names which do not necessarily 

 distinguish between armature and 

 field -magnets. It has been pointed 

 out that to increase the number of 

 alternations per revolution, a large 

 number of coils are employed. One 

 complete impulse outwards and 

 then back is produced by the 

 revolution of two pole-pieces past 

 a conductor (or vice versa), so that 

 the number of alternations per 

 second ("periodicity" or "fre- 

 quency " ) is governed by the speed 

 of revolution and the number of 

 pairs of polos. 



Formerly machines giving as 

 many as 100 or more cycles per 

 second were employed ; to-day the 

 frequency varies from 25 to 60 

 cycles, but, except for special pur- 

 poses, the present tendency is in 

 favour of 50 cycles. If the e'xoiting 

 current be direct (non-alternat- 

 ing), the generator is said to he of 

 the " synchronous " type, since at 

 a given speed the frequency will 

 always be the same, depending, 

 as before mentioned, upon the 

 number of poles and angular 



