UNIT-III

(Transformer and Induction Motor)

  1. A 50 kVA transformer has an efficiency of 98% at full – load 0.8 p.f. and 97% at ½ full – load 0.8 p.f. Determine the full – load Copper loss and Iron loss. Find the load at which maximum efficiency occurs and also maximum efficiency.
  2. A 25 KVA, single phase transformer has 500 turns on the primary and 40 turns on the secondary winding. The primary is connected to 3000V,50Hzsupply. Calculate (i) Primary and secondary currents on Full load (ii) The secondary e.m.f. (iii) The maximum flux in the core (8.33 A,240 V,0.027 Wb )
  3. A125 KVA transformer has a primary voltage of 2000V at 60 Hz Primary turns are 182 and the secondary turns are 40.Neglecting losses calculate : i) No load secondary emf (ii) Full load primary and secondary currents (iii)fiux in the core (62.5 A,439.6v,41.2mwb)
  4. A 10 KVA 50 Hz single phase transformer has a turn ratio 300/23.The primary is connected to 1500V, 50Hz supply. Find the secondary voltage on open circuit and maximum flux in the core (115V,22.52 wb)
  5. A 440/230 V, 50Hz single phase transformer is having 525 turns on L.V. .calculate (i)Number of turns on the H.V.side (ii) maximum flux density in the core, if theeffective cross sectional area of the core is 15cm2 (1005,1.3156wb/m2)
  6. A single phase transformer has turns ratio of 1:10 and a secondary windings of 1000 turns The primary winding is connected to a 25V sinusoidal supply. If the maximum core flux is 2.25 m Wb.Find (i) the frequency of the supply (ii) ihe number of primary turns 9iii) secondary voltage on open circuits (25Hz,100,250V)
  7. Following are the test figures for the 4 KVA,200V/400V,50Hz,single phase transformer oc test:200V,0.8A,70W SC test: 17.5V,9A,50W,CALCULATE (I)Full load efficiency at unity power factor (ii)Half load at 0.8 power factor (96.81%,94.93%)
  8. A 200 KVA transformer has an efficiency of 98% at full load. If the maximum efficiency occurs at three-quarters of ful load, calculate the efficiency at half load. Assume negligible magnetising current and power factor of 0.8 at all loads (97.92%)
  9. In a 25 KVA,2000/200 V transformer, the iron and the copper losses are 350 Watts and 400 watts respectively. Calculate the efficiency at full load and 0.8 power factor lagging. Determine the maximum efficiency and corresponding load (97.1%,23.38KVA)
  10. The required no-load ratio in a single phase 50Hz ,core type transformer is 6000/250 V.Find the number of turns in each winding, if the flux in the core is to be about 0.06wb (primary-450,Secondary -18)
  11. A 1 KVA single phase transformer has a core loss of 15 W and a full load copper loss of 20W.Deterine its efficiency (i) at full load ,unity power factor (ii) at 1/3 rd full load ,0.9p.f. (iii) at ¼ th full l0ad 0.75 p.f. (96.62%,94.57%,92.02%
  12. Find the number of turns required on the H.V and L.V sides of a 415/240 V,50 Hz, single phase transformer .If the net area of cross section of the core is 25 cm2 and the maximum flux density is 1.3 wb/m2 (575,333)
  13. A 100KVA lighting transformer has a full load loss of 3kW,the losses being equally divided between iron and copper. During a day , the transformer operates on full load for three hours, on half load 4 hours, the output being negligible for the remainder of the day calculate all day efficiency (92.25%)
  14. A11000/230 ,150 KVA ,1-Phase,50Hz transformer has core lossof1.4 kW and full load copper loss of 1.6 kW.Determine (i) KVA load for maximum efficiency and value of maximum efficiency at unity p.f. (ii) Efficiency at half full load0.8 p.f. leading (140 KVA,98.04% (ii)97%)
  15. Calculate the regulation of the transformer in which the ohmic loss is 15 of the output and the reactance drop 5%of the voltage ,when the power factor is (i)0.8 lagging (ii)unity (i)0.8 leading ((i) 3.8% ,(ii) 0.8% , 9 (iii) -2.2 %)
  16. An 8 pole alternator runs at 750 rpm and supplies power to a 6-pole induction motor which has a full load slip of 3 %. Find the full load speed of the motor and the frequency of the rotor emf (970 rpm,1.5 Hz )
  17. A 5 H.P. 400V, 6-pole,50 Hz 3-phaseinduction motor operating atFull load draws line current of 7 amperes at 0.866p.f. with 2% slip Find the rotor speed and the efficiency of motor
  18. A 50 kW,3-phase,400V,50Hz induction motor has a speed of 950 rpm on Full load .The motor has 6 poles. calculate (i) slip (ii) Rotor current frequency (iii)Number of alternation by rotor voltage per minute ((i) 5% (ii)2.5Hz (iii)150 )
  19. A12 pole induction motor is fed from a 50 hz supply If the frequency of the rotor emf at full load is 2 Hz Find the full load slip and the speed (4% , 480 rpm)
  20. A 3 phase 50 Hz ,induction motor is wound for 8 poles .If the full load slip is 2.5 % calculate (i) synchronous speed (ii)The slip speed (iii)the rotor speed (iv) Rotor frequency ( (i) 750 rpm (ii) 18.75 rpm (iii)731 rpm (iv)1.25 Hz )
  21. A 4 pole 50 Hz induction motor has an emf in the rotor, the frequency of which is 2 Hz. Determine (i) the synchronous speed (ii)the slip (iii)the speed of the motor ( (i) 1500 rpm (ii)4 % (iii)1440 rpm )
  22. The electromotive force in the stator of an 8-pole induction motor has a frequency of 50 Hz and that in the rotor is 1.5 Hz At what speed the motor is running and what is slip ( 727.5 rpm ,3 % )
  23. A 3-phase , 50 Hz induction motor has full load speed of 960 rpm calculate (i0 slip (ii) number of poles (iii)frequency of the rotor induced emf (iv) speed of the rotor with respect to rotor structure (v)speed of the rotor with respect to stator structure ( (i)0.04 (ii)6 (iii)2 Hz (iv)40 rpm (v)1000 rpm
  24. A 3-phase induction motor has 2 poles and is connected to 400 V , 50 Hz supply Calculate the actual rotor speed and rotor frequency when the slip is 4 % (2880rpm ,2 Hz )
  25. If a 6-pole induction motor supplied from a three phase ,50 Hz supply has a rotor frequency of 2.3 Hz , calculate (i) the percentage slip (ii)the speed of the rotor in rpm (4.6 %,954 rpm )
  26. Determine the percentage slip of a 3-phase induction motor running at 960 rpm The number of poles is 6 and supply frequency is 50Hz. (4 % )
  27. A 6-pole squirrel cage induction motor is connected to a supply of 400 V ,50 Hz. It runs at 960 rpm calculate (i) synchronous speed (ii) slip ( 1000 rpm,4 % )
  28. A 3-phase induction motor has 6 poles and runs at 960 rpm on full load it is supplied from an alternator having 4 poles and running at 1500 rpm Calculate (i)Full load slip of the motor ,(ii)frequency of the rotor current (4%,2Hz )
  29. The rotor e.m.f. of a 3 phase 6 pole,400V ,50 Hz ,induction motor alternates at 3 Hz, compute the speed and percentage slip of the motor. Find the rotor copper loss per phase, if the full load input to the rotor is 111.9 kW ( 940 rpm,6%,2.238 kW)
  30. The power input to the rotor of a 400 V, 50 Hz ,6-pole 3-phase induction motor is 80 kW .The rotor electromotive force observed to make 100 complete alternations per minute calculate (i) the slip (ii) the rotor spee 9iii)mechanical power developed ( (3.33 %,967 rpm,77.33kW )