DYNAMO LOSSES AND EFFICIENCIES 37 



Plot a curve of efficiency and output and find from it the effi- 

 ciency for 2.2-h.p. output. (15 min.) 



15. A 120-volt shunt motor is giving, as determined by a brake, 

 an output of 23 h.p. It is then absorbing 20.2 kw.; the field 

 current is 5.3 amperes and the armature resistance is 0.031 ohm. 

 What is the efficiency and power intake when the motor is giving 

 10 h.p.? (10 min.} 



16.. The following data is given with regard to the armature 

 of the 500-kw. generator of problem 2. 



Volume of iron in body of core, cu. cm 97,800 



Volume of iron in teeth 26,050 



Density in body of core 17,000 



Density of teeth, mean 21,000 



Speed, r.p.m 330 



Poles 8 



Hysteresis constant, for watts 2 X 10~ 10 



Eddy loss 9350 



Determine the total iron loss for the armature. (8 min.) 



17. The shunt field of the 500-kw. generator of problem 16 

 has a hot resistance of 89 ohms, the series field of 0.0022 and the 

 interpole winding of 0.0012, the brush drop at full load is 1.6 volts, 

 and the brush friction 2280 watts. The bearing and windage 

 friction loss is 3500 watts. Required the one-half load and full- 

 load efficiencies. (10 min.) 



18. A 105-volt, 4-pole dynamo gives the following data: 

 r.p.m., 750; gap density, 13,000; full-load current, 300 amperes; 

 length of armature core, 50 cm.; outside diameter, 25 cm.; and 

 inside diameter, 7.5 cm. The temperature at which the machine 

 is to run is 64 C. The shunt field is wound with 2.25 kilometers 

 of No. 11 wire. The armature resistance is 0.005 ohm, at 14 C. 

 The drop through the brushes and connections at full load is 

 J volt. The hysteresis loss in the armature is 880 watts per cubic 

 meter, at one cycle per second. The friction loss is 400 watts. 

 The eddy current loss is 2 watts per cubic meter, at one cycl- 

 second. Required the iron loss, copper loss, total loss and effi- 

 ciency at full load. (15 min.) 



19. In a brake test upon a 30-kw., 500-volt motor, 52 amperes 

 are supplied, the motor is making 1000 r.p.m., and the load on the 

 3-foot brake arm is 55 Ibs. Required the efficiency of the motor. 



