Electrical Machines Online Test

1. When a constant power output is desired to maintain __________

2. If the pu impedance of a single phase transformer is 0.01+j0.05, then its regulation at p.f. of 0.8 lagging will be?

3. A synchronous machine with its field winding on stator and polyphase armature winding on rotor. At steady state, its air gap field is?

I. stationary w.r.t. stator 
II. rotating at double the speed Ns w.r.t. rotor
III. rotating in direction opposite to rotor 

4. While the operation of dc motor, there are sparkings at the commutation observed. These can be happening due to ________

5. A 10 kVA, 400/200 V, 1-phase transformer with 2% resistance and 2% leakage reactance. It draws steady short circuit current at which angle?

6. The rotor mmf per pole in the 3-phase, 4-pole induction motor varied with slip is?

7. A synchronous machine with its field winding on stator and polyphase armature winding on rotor. At steady state, its air gap field is?

8. At steady state operation in a 3-phase synchronous generator. (where Tem is developed electromagnetic torque developed in the machine and Tpm is the prime mover torque.)

9. With the alternator running at rated terminal voltage Vt, at rated current Ia = 1 per unit. What happens if load current is reduced to zero, if it was operating at lagging power factor?

10. The rotor power input of 3-phase induction motor is 15 kW at a slip of 5%. The mechanical power developed at the shaft is?

11. Voltage induce in the induction motor is highest at __________

12. For a 4 pole 200 V, 20 A dc generator running at the rated rpm with no load will be ________

13. I. Pulse transformers use soft ferrites.
II. The transformers used in the radio recievers use air core.

14. Which of the given losses are directly proportional to square of speed?

15. In the following circle diagram, what is CC'?

16. Three phase voltages are applied to the three windings of an electrical machine. If any two supply terminals are interchanged, then the rotating MMF wave _____________

17. A synchronous machine with its field winding on rotor and polyphase armature winding on stator. At steady state running condition, its air gap field is?

I. stationary w.r.t. stator
II. rotating at double the Ns w.r.t. rotor
III. rotating at -Ns w.r.t. rotor
IV. stationary w.r.t. rotor.

18. Which of the following statements are correct regarding individual phase MMF in rotating machines?

19. If the torque is in the direction of rotation, the DC machine acts as ________

20. Which of the following is a shortcoming for using parallel connected transformers?

I. Reliable loading
II. Increased capacity of power system
III. Reducing the capacity of substation
 

21. The EMF equation e = Nω_rΦsinω_rt is applicable to ____________

22. For 3-phase induction motor, as load increases from no load towards the full load, torque _______ slip.

23. Consider a 3-phase cylindrical-rotor alternator.

A. E.m.f. generated by armature reaction lags armature current by 90°.
B. Air gap voltage leads the field flux by 90°
C. Air gap voltage lags the field flux by 90°    
D. Armature reaction due to intermediate lagging p.f is partly cross magnetizing & partly magnetizing

24. The magnitude of rotating flux _____________ at all instants of time.

25. The given variable losses are 5kW fr a 500 KVA, 11 kV, 3-phase star connected alternator having armature resistance of 4 ohms. Calculate the current at which maximum efficiency occurs?

26. When a transformer winding suffers a short circuit, then inter turns of the same winding experience ______ forces.

27. In case of a power transformer, the no load current in terms of rated correct is?

28. Mark the correct order for the external characteristics of compound generators from the below graph.

29. The voltage drop at brush-commutator contact is variable (1-2V) and dependent of armature current.

30. In a DC machine, z, p, I_a and a are respectively the number of conductors, number of poles, armature current and number of parallel paths. The peak value of fundamental component of armature MMF wave is ____________

31. Alternator compounding characteristic is obtained by _________

32. Which of the following statements support the ideal transformer features?

I. Zero winding resistance
II. Constant permeability
III. Constant core losses

33. While doing regular checks on the dc machine with the lap connected winding, it is reported to have ammeter fluctuations, this can be due to _________

34. The brushless excitation and conventional DC excitation systems are comparable but one significant change is _______

35. If the frequency is reduced by 50% for a 3-phase, 50 Hz induction motor having 4 poles. The slip at which the maximum torque occurs becomes _________

36. Select the possible dc magnetization characteristics of permanent magnetic materials.

37. In a synchronous generator, the frequency of the generated EMF depends on ___________

38. Consider the below remarks for an alternator.

(i) An over excited machine is more stiff
(ii) Machine with longer air-gap is less stable than one with smaller air-gap

39. When an induction generator is connected to an infinite bus, it _________

40. Which type of slots are used in the construction of large size and small size induction motors respectively?

41. The reactance vs slip is given by?

42. What is the amplitude of rotating MMF produced as a result of m-phase currents flowing in m-phase windings? (where F_m is maximum MMF in any one phase, when current is maximum in that phase.)

43. Which of the following informations are not obtained from short-circuit test?

I. Ohmic losses at rated current
II. Equivalent resistance and leakage reactance
III. Core losses
IV. Voltage regulation

44. If the synchronous machine is loaded from floating conditions, the alternator begins to operate at __________

45. Identify the armature characteristic of the dc generator.

46. A 6-pole alternator with 36 slots carries a 2-phase distributed winding. Each coil is short pitched by one slot. The winding factor is given by _____________

47. Mark the most appropriate phasor for the induction machine operation as motor.

48. A dc motor is connected in the short-shunt configuration, but the series and the shunt windings get interchanged by mistake and the motor is connected to the rated voltage?

49. Separately excited dc generators are used in __________

50. A three phase induction motor is not self starting.