474 ELEMENTS OF ELECTRICAL ENGINEERING, 



Art. 73, page 148, in order to check the above rating as claimed 

 by the dealer. Ans. 265 kilowatts. 



115. A certain enclosed motor is rated by the manufacturers 

 as a 22O-volt, 27. 5 -horse-power motor at a speed of 600 revolu- 

 tions per minute. The diameter of the armature is 1 5 inches and 

 the gross length of the armature core is 1 3 inches. Calculate its 

 rating according to Art. 73, page 148. Ans. 34 horse-power. 



116. A certain four-pole dynamo is rated as a 5 -horse-power 

 I lo-volt motor to run at a speed of 600 revolutions per minute. 

 This motor is connected to 22O-volt mains and the armature flux 

 <!> is increased 25 per cent, by the 100 per cent, increase of field 

 current, (a) At what speed would this motor run under these 

 conditions ? () The resistance of the shunt field winding is 100 

 ohms, how much resistance must be connected in series with the 

 field winding to increase the motor speed to 1,200 revolutions per 

 minute on 2 20- volt mains ? (c) What would the power rating of 

 this motor be at this speed of 1,200 revolutions per minute ? (d) 

 Would it be permissible to drive this motor at a speed of 1,200 

 revolutions per minute from 22O-volt mains, the diameter of the 

 armature being 10 inches and the number of commutator seg- 

 ments being 60? Ans. (a) 960 revolutions per minute ; (b) 100 

 ohms ; (r) 10 horse-power ; (d) the machine would in all prob- 

 ability operate satisfactorily. 



NOTE. In dynamos of medium current rating an average electromotive force of 

 20 volts per commutator segment is the greatest that can be used without danger of 

 sparking. By average electromotive force per segment is meant E\n where E is the 

 electromotive force between the brushes and n is equal to J(]p, where K is the total 

 number of commutator bars and/ is the number of field magnet poles. 



A well constructed armature can be expected to withstand a speed which corre- 

 sponds to a peripheral velocity of from 3,000 to 4,000 feet per minute. 



117. A certain bipolar drum-wound armature has 384 con- 

 ductors ; its pole faces have an angular breadth of 120 ; and its 

 full-load armature current is 200 amperes. Find : (a) per cent. 

 of armature periphery covered by poles ; (fr) the number of de- 

 magnetizing ampere-turns on the armature if the angle of lead of 

 the brushes is 30 ; (c) the cross-magnetizing ampere -turns. 



