102 ELECTRICAL ENGINEERING PROBLEMS 



14. A 440- volt induction motor gives the following data on 

 test: No-load current, 4.0; power factor, 0.3; current for blocked 

 rotor, 50; and power factor, 0.5; primary resistance, 2.3 ohms. 

 Construct the circle diagram of the motor and determine the rotor 

 current, power factor, output, input, efficiency, per cent slip and 

 torque in synchronous watts for a stator current of 20 amperes. 

 Determine also the maximum power factor and torque with the 

 corresponding outputs. (60 ram.) 



15. A 220-volt, 20-h.p., delta-connected motor takes 20 am- 

 peres per phase line current at no load and 270 amperes with 

 blocked rotor. The corresponding watts per phase are 300 and 

 15,000. The stator resistance, as measured from line to line, 

 is 0.1 ohm. Construct curves of line current, input, efficiency, 

 per cent slip and power factor with horse-power output as ab- 

 scissae. Get four sets of points besides those for no load and 

 blocked rotor. Give the above items corresponding to 138.5 

 amperes, line current, input. (90 ram.) 



16. A 350-h.p., 500-volt, 3-phase motor, 300-r.p.m., 50-cycles 

 has both stator and rotor star-connected. It gives on test the 

 following data: With no load the current is 145 amperes and the 

 power 1.1 kilowatts. With blocked rotor the current is 1600 

 amperes and the power factor 0.28. The stator resistance per 

 phase is 0.015 ohm. Construct the circle diagram and by means 

 of it plot the speed-torque curve. Determine the resistance of 

 the rotor circuit and that which must be added to give maximum 

 torque at starting. (60 min.) 



T' r ' ( r "2 i X 2 S 2) 



17. Using the formula ^ = r // (r / 2 + X 2 S 2) ' 



where r' and r" are two secondary resistances and x is the second- 

 ary reactance, 0.047, plot the speed torque curve for the motor 

 of problem 16 which will give maximum torque at starting. Base 

 this upon the rotor resistance as found in problem 16. (60 min.) 



