POLYPHASE INDUCTION MOTORS 



101 



connected primary and Y-connected secondary. The full-load 

 efficiency is 92% and the power factor is 85%. There are 432 

 stator and 338 rotor conductors. Neglecting losses, what will be 

 the full-load currents in stator and rotor, and the e.m.f. per phase 

 (to neutral) in the rotor when at standstill? The resistance per 

 phase being 0.022 and the reactance neglected, what will be the 

 rotor e.m.f. and the slip at rated load? (10 min.) 



9. If the reactance at standstill of the rotor of problem 8 is 

 0.20 ohm, and if an outside resistance of 0.218 ohm be connected 

 by the slip rings into each phase, at what speed will the motor run 

 when taking full-load current? (9 min.) 



10. Find the equivalent single-phase value of the rotor resist- 

 ance and of the rotor and stator currents at rated load in problem 

 8. (5 min.) 



11. A 25-cycle, 200-h.p., 1000-volt, Y-connected induction 

 motor has a wound Y-connected rotor with slip rings and a trans- 

 formation ratio of 3.6. The resistance per phase is 0.01. If the 

 inductance per phase is 0.00064 henry, what would be the starting 

 current with short-circuited slip rings? Also what would be its 

 phase angle with the flux? , The full-load slip being 3%, what will 

 be the current and its angle? What resistance per phase would 

 have to be inserted to get 300-amperes starting current in the 

 primary, and what would be the phase angle of the current in the 

 secondary? Neglect the primary losses. (25 min.) 



12. The conductors of an induction motor are divided into 

 groups numbered from 1 to 48, all wound in the same direction and 

 having the ends marked + and . Show to what each end of 

 each section must be connected (that is to a main or the + or 

 end of another coil) to give: (a) a 4-pole, 2-phase motor; (b) a 

 12-pole, 2-phase motor; (c) a 4-pole, 3-phase motor, delta-con- 

 nected; (d) an 8-pole, 3-phase motor, Y-connected. (25 min.) 



13. The total no-load losses in a 381-volt, 45-h.p., Y-connected 

 motor are 1416 watts. The magnetizing ampere-turns necessary 

 for the air gap are 184, and for the rest of the circuit 36, maximum 

 per pole per phase. The turns per pole per phase are 10. What 

 is the effective no-load current and what is its power factor? 

 Assuming the magnetizing current constant and neglecting leak- 

 age, what would be the power factor with an input of 20 kw.? 

 Note that, in this case, leakage reduces the power factor about 

 5%. (25 min.) 



