486 FARM MOTORS 



653. By poor regulation of voltage. An overload will 

 cause an excessive drop of the E.M.F. at the machine. 

 This will decrease the potential difference at the brushes 

 and cause a weak current over the line. 



By excessive heating. The heat from an armature 

 increases four times for each doubling of the cur- 

 rent. At this rate the armature would soon become red 

 hot. It would work at a little less than red heat, but even 

 this much heat would break down the insulation. TJie 

 armature should not become warmer than 212 F., and 

 the general custom is not to run it at a higher tem- 

 perature than 70 above the surrounding air. 



654. Commercial rating of dynamos. Dynamos are 

 rated according to the number of kilowatts they will carry 

 in the external circuit without excessive heating. For 

 example, a person calls for a 60 K. W. i lo-volt generator. 

 This means that he desires a machine which will deliver 

 60 K. W. to the external circuit and maintain a potential 

 difference of no volts across the brushes. Owing to 

 losses in the machine such a machine may develop 

 63 K. W. and still have only 60 K. W. available for use 

 in the external circuit. 



655. Efficiency of dynamos. The efficiency of a dyna- 

 mo is the ratio of its electrical output to the mechanical 

 energy exerted upon it. For a I K. W. machine it is only 

 about 50 per cent, and in generation of several thousand 

 kilowatts it is about 95 per cent. 



656. Sparking at the commutator. Sparking at the 

 commutator is the most serious trouble the attendant will 

 have with a dynamo, provided he keeps all other parts 

 clean, and the insulation does not break down or the 

 machine become short-circuited. There are several 

 causes for a dynamo to spark, some of which are : 



I. Brushes not set at neutral point. This can be remedied by 



