146 ELEMENTS OF ELECTRICAL ENGINEERING. 



A shunt dynamo has an inherently proper speed. Thus a 

 shunt generator should be driven at that speed which will de- 

 velop sufficient electromotive force in the armature to produce 

 enough current through the shunt field winding to magnetize the 

 field magnet nearly to saturation. If a shunt generator is driven 

 at a speed much lower than this, its field excitation will be low, 

 and its maximum permissible output of power will be consid- 

 erably less than it would be if its field winding were redesigned 

 to give full field excitation at this reduced speed. 



A shunt motor should have its field winding connected to 

 supply mains giving the proper electromotive force to fully excite 

 its field magnet, and under these conditions the armature when 

 connected to the same supply mains, will run at a certain definite 

 speed. 



A series dynamo does not have an inherently proper speed. 

 Thus, a series generator may be driven at any desired speed, and, 

 if the receiving circuit has a suitable resistance, the full rated 

 current will be delivered by the machine, its field will be fully 

 excited, and the electromagnetic action of the machine will be at 

 its best. Driven at high speed a series generator will develop a 

 high electromotive force, and driven at low speed the machine 

 wilPdevelop a low electromotive force, when the current is at its 

 full load value. 



A series motor does not have an inherently proper speed. 

 Such a motor may be run at full current intake (and consequently 

 at full field excitation) at any speed, although, of course, a high 

 electromotive force will be needed to drive the machine at high 

 speed with full current intake. 



Correlation of speed and voltage ratings of shunt dynamos. 

 From what has been said above, it is evident that a given dynamo, 

 even a given shunt dynamo, may be run more or less satisfactorily 

 at a speed very considerably above or below the rated speed of the 

 machine, but the use in commercial work of certain widely sepa- 

 rated standard voltages of supply, 1 10 volts, 220 volts, and 500 

 volts, leads to very definite speed ratings of generators and motors. 



