RATINGS AND GUARANTEES. 147 



Thus a motor which has a field winding designed to give full 

 field excitation from 22O-volt mains, and an armature that is well 

 enough insulated to withstand a 22O-volt supply, and strong 

 enough mechanically to run at the speed given by a 22O-volt 

 supply, will run at a definite speed, say 1,200 revolutions per 

 minute, under these conditions. Such a motor would be rated as 

 a 220- volt motor at a speed of 1,200 revolutions per minute, and 

 although it would hardly be permissible to run this motor from 

 1 10- or 5OO-volt mains, still the motor would perhaps run satis- 

 factorily from, say, 25o-volt mains at a speed, say, of 1,3 50 revo- 

 lutions per minute. A shunt dynamo has a fairly definite, in- 

 herently proper speed, which fact, taken in conjunction with the 

 widely separate standard voltages used in practice, limits a shunt 

 machine (generator or motor) to a certain very definite speed. 



73. Limitation of output of a dynamo as a generator or as a 

 motor. The output of a motor or generator is limited in practice 

 by three distinct things : (a) By overheating and consequent 

 destruction of the insulating materials used in the construction of 

 the machine ; () by excessive sparking at the brushes and the 

 consequent rapid wear, unsatisfactory running, and excessive 

 heating of the commutator ; and (c) by excessive drop in voltage 

 of a generator or excessive drop in speed of a motor, at excessive 

 loads. That is to say, a dynamo is rated for that load for which 

 it neither heats nor sparks excessively, and for which neither its 

 voltage, if it is to be used as a " constant voltage" generator, 

 nor its speed, if it is to be used as a " constant speed" motor, 

 falls off more than a certain small amount. Heating rather than 

 sparking is the thing that usually limits the output of a machine, 

 especially in the case of completely enclosed machines like the 

 enclosed motors shown in Figs. 60 and 91 ; while closeness of 

 regulation (voltage regulation in case of a generator and speed 

 regulation in case of a motor) sets a limit to the permissible rating 

 of a machine only in rare special cases where very close regula- 

 tion is demanded. 



