^TIONS AND PROBLEMS 



493 



123. What is the ratio of the break-down TOPJU. milar induction 

 motors, one of 25-cycles and the other of 60-cycles if the rotor inductance, 

 the flux and the currents are the same in each? 



124. A 10 lip . _':( >-v lit. three-phase, 60-cyclc induction motor, when 

 conn. .Mo-volt, three-phase mains, takes 125 amp. the 

 instant that the circuit is closed. What current will the motor take and 

 what will l.e the line current if a compensator similar to the one in Fig. 

 238, page 251, is used and the motor is connected to the 50 per cent, tape? 



125. .ml.lern 124 for the 60 per cent. taps. What will l.e tin- 

 ratio <>f starting torques in the three cases, of full voltage, 50 per cent, and 

 60 per cent, taps? 



126. A >i\-pole. tiO-cyde. wound-rotor, induction motor is taking 10,000 

 watts from the line. The core loss plus friction losses is 700 watts and the 

 stator I*R loss is 300 watts. The rotor slip is 4 per cent, and the rotor 

 I*R loss is 400 watts. What is the motor efficiency under these condition-'.' 

 At what speed does it run and what torque does it develop? 



127. Find the motor efficiency in problem 126 if resistance is introduced 

 in the rotor circuit so that the motor runs at (a) 900 r.p.m. (6) 600 r.p.m. 

 (Hint : The ratio of the rotor I*R loss to the rotor input is proportional to 



the slip.) 



128. A 10 hp., 230-volt, six-pole, 60-cycle, 1,140 r.p.m.. induction motor 

 is tested I iy means of a prony brake. The data are as follows: 



i is 2 ft. loim .-Hid t' 



follo\\ inn : 

 torque; per cei, 1 



1 Mot the above data with -T as abscissas. \\ ! 



increase to a maximum and thru decrease'.' Why doos the power 



h tlioM- ..' ity the 



volt-am perea. 



