Electrical Machines Questions and Answers – Efficiency of Synchronous Machines

This set of Electrical Machines Questions and Answers for Entrance exams focuses on “Efficiency of Synchronous Machines”.

1. The short circuit load losses is/are ______________
a) direct load loss and stray load losses
b) direct load loss
c) stray load losses
d) field current loss
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2. The open circuit voltage and open circuit core loss variation is?
a)
b)
c)
d)
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3. Rotational losses vary as follows with respect to field current.
a)
b)
c)
d)
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4. A 100 KVA, 400V, 3-phase, star connected alternator due to the following data:

Friction and windage losses = 340W
Open circuit core loss = 480W
Rf = 180ohms, ra = 0.02ohms.

The voltage applied to the field winding is 220V. The short circuit load loss at half full load is?
a) 258W
b) 268W
c) 480w
d) 340W
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5. A 100 KVA, 400V, 3-phase, star connected alternator due to following data:

Friction and windage losses = 340W
Open circuit core loss = 480W
Rf = 180ohms, ra = 0.02ohms.

The voltage applied to the field winding is 220V. The field current loss will be?
a) 270W
b) 258W
c) 480W
d) 250W
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6. A 100 KVA, 400V, 3-phase, star connected alternator due to following data:

Friction and windage losses = 340W
Open circuit core loss = 480W
Rf = 180ohms, ra = 0.02ohms.

The voltage applied to the field winding is 220V. The efficiency of the machine at half load is?
a) 96.7%
b) 95%
c) 94.2%
d) 97
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7. A 100 KVA, 400V, 3-phase, star connected alternator due to following data:

Friction and windage losses = 340W
Open circuit core loss = 480W
Rf = 180ohms, ra = 0.02ohms.

The voltage applied to the field winding is 220V. The load losses at full load is?
a) 1033W
b) 500W
c) 540W
d) 940W
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8. A 100 KVA, 400V, 3-phase, star connected alternator due to following data

Friction and windage losses = 340W
Open circuit core loss = 480W
Rf = 180ohms, ra = 0.02ohms.

The voltage applied to the field winding is 220V. Efficiency at full load is?
a) 97%
b) 96.5%
c) 92%
d) 95%
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9. 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?
a) 20.4A
b) 10.2A
c) 40A
d) 26A
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10. 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 full load armature current per phase.
a) 26.24 A
b) 20.41 A
c) 79 A
d) 40 A
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11. 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 

a) I, II, III
b) I
c) II, III
d) II, I
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12. 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?
a) rotating at synchronous speed w.r.t. stator
b) stationary w.r.t. rotor
c) rotating in the direction of the rotor rotor rotation
d) all of the mentioned
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13. 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 mmf lags the field flux by (90°+ internal p.f. angle)

a) A, B, D
b) A, C, D
C) B, D
D) C, D
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14. In a 3-phase cylindrical-rotor alternator, synchronous reactance is sum of _____________
a) mutual and leakage reactance
b) magnetizing and leakage reactance
c) magnetizing and mutual reactance
d) mutual, magnetizing and leakage reactance
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15. The reactive power output of a synchronous generator is limited by _________
a) armature current and field current
b) field current and load angle
c) load angle and excitation
d) armature current only
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Sanfoundry Global Education & Learning Series – Electrical Machines.

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