38 ELECTRICAL ENGINEERING PROBLEMS 



What change in the length of the brake arm could be made to 

 facilitate the computation of the horse-power? (2 min.) 



20. A rated motor is being used as a dynamometer to measure 

 the power absorbed by a printing press. The iron losses are 450 

 watts, the field resistance 493 ohms and the armature resistance 

 0.53 ohm. The voltmeter reads 512 volts and the ammeter in 

 circuit with the machine 21.3 amperes. What power is the motor 

 furnishing to the press? (5 min.) 



21. The motor of problem 13 is to be used to drive a group of 

 tools in a machine shop. All the tools, including a punch press, 

 are in operation one hour per day and require 10 h.p. Shutting 

 down the press reduces the load to 5 h.p., at which it runs 6 hours. 

 During the remaining 2 hours of the working day only 2 h.p. are 

 required, and for an hour at noon only 1 h.p. is used. Plot a curve 

 of total loss with output for the motor and hence determine its 

 all-day or energy efficiency. (15 min.) 



22. Two 550-volt, 1600-kw. compound dynamos are coupled 

 together, and are being tested by the Knapp method, that is the 

 power for the losses is supplied electrically. The shunt-field 

 currents of the motor and generator are 26.1 and 26.9 amperes. 

 The armature resistance of each machine is 0.0037 ohm, and the 

 average of the armature currents is the rated full-load current. 

 The stray losses are assumed equal in the two machines. 322 

 amperes are being supplied from outside. What is the efficiency 

 of each machine? (10 min.) 



