Q41: A synchronous generator is feeding a zero power factor (lagging) load at rated current. The armature reaction is (2006)
(a) magnetizing
(b) demagnetizing
(c) cross-magnetizing
(d) ineffective
Ans: (b)
Sol: 
Ef > Vt, the generator is overexcited. Therefore, armature-reaction is demagnetizing in nature i.e. ϕar opposes ϕf.
Q42: A 1000 kVA , 6.6 kV, 3-phase star connected cylindrical pole synchronous generator has a synchronous reactance of 20 Ω. Neglect the armature resistance and consider operation at full load and unity power factor.
The power (or torque) angle is close to (2005)
(a) 13.9°
(b) 18.3°
(c) 24.6°
(d) 33.0°
Ans: (c)
Sol: From previous solution, δ = 24.6°
Q43: A 1000 kVA , 6.6 kV, 3-phase star connected cylindrical pole synchronous generator has a synchronous reactance of 20 Ω. Neglect the armature resistance and consider operation at full load and unity power factor.
The induced emf (line-to-line) is close to (2005)
(a) 5.5 kV
(b) 7.2 kV
(c) 9.6 kV
(d) 12.5 kV
Ans: (b)
Sol: Rated output power = S = 1000kVA
Rated line to line voltage = Vl−l = 6.6kV
Rated current,
As generator is star connected.
Phase to neutral voltage
Taking Vt as the reference
Induced emf(line to line)
Q44: In relation to the synchronous machines, which on of the following statements is false? (2005)
(a) In salient pole machines, the direct-axis synchronous reactance is greater than the quadrature-axis synchronous reactance.
(b) The damper bars help the synchronous motor self start.
(c) Short circuit ratio is the ratio of the field current required to produces the rated voltage on open circuit to the rated armature current.
(d) The V-cure of a synchronous motor represents the variation in the armature current with field excitation, at a given output power.
Ans: (c)
Sol: (i) Pd, Pq= permeance of pole-arc oriented along the d-axis/q-axis.
(ii) Damper bars not only minimize hunting to a tolerable level but also help the synchronous motor self-start.
(iii) SCR is defined as the ratio of the field current required to rpoduce rated voltage on open-circuit to the field current required to produce rated armature current with the armature terminals shorted while the machine is mechanically run at synchronous speed.
Q45: A hydraulic turbine having rated speed of 250 rpm is connected to a synchronous generator. In order to produce power at 50 Hz, the number of poles required in the generator are (2004)
(a) 6
(b) 12
(c) 16
(d) 24
Ans: (d)
Sol: P = 120 f/N
where,
Q46: A 500 MW, 3-phase, Y-connected synchronous generator has a rated voltage of 21.5 kV at 0.85 p.f. The line current when operating at full load rated conditions will be (2004)
(a) 13.43 kA
(b) 15.79 kA
(c) 23.25 kA
(d) 27.36 kA
Ans: (b)
Sol:
Q47: A 400 V, 50 kVA, 0.8 p.f. leading △-connected, 50 Hz synchronous machine has a synchronous reactance of 2 Ω and negligible armature resistance. The friction and windage losses are 2 kW and the core loss is 0.8 kW. The shaft is supplying 9 kW load at a power factor of 0.8 leading. The line current drawn is (2004)
(a) 12.29A
(b) 16.24 A
(c) 21.29 A
(d) 36.88 A
Ans: (c)
Sol: Power available at shaft = Pshaft = 9kW
Losses = Core loss + Friction and windage loss + Copper loss
Core loss = 0.8 kW
Friction and windage loss = 2kW
Armature resistance is negligibl, so copper loss is neglected.
Q48: Two 3-phase, Y-connected alternators are to be paralleled to a set of common bus bars. The armature has a per phase synchronous reactance of 1.7 Ω and negligible armature resistance. The line voltage of the first machine is adjusted to 3300 V and that of the second machine is adjusted to 3200 V. The machine voltages are in phase at the instant they are paralleled. Under this condition, the synchronizing current per phase will be (2004)
(a) 16.98 A
(b) 29.41 A
(c) 33.96 A
(d) 58.82 A
Ans: (a)
Sol: Per phase synchronizing current
We have Zs1 = Zs2
E1 and E2 must be phase quantities
Q49: A 4-pole, 3-phase, double-layer winding is housed in a 36-slot stator for an ac machine with 60c phase spread. Coil span is 7 short pitches. Number of slots in which top and bottom layers belong to different phases is (2003)
(a) 24
(b) 18
(c) 12
(d) 0
Ans: (a)
Sol: Pole pitch = 36/4 = 9 slots
Coil span = 7 slots, Slot/pole/ph = 3
So that 3 slots in one phase, if it is chorded by 2 slot, then
Out of 3 → 2 have different phase
Therefore out of 36 → 24 have diffferent phase.
Q50: When stator and rotor windings of a 2-pole rotating electrical machine are excited, each would produce a sinusoidal mmf distribution in the airgap with peal values Fs and Fr respectively. The rotor mmf lags stator mmf by a space angle δ at any instant as shown in figure. Thus, half of stator and rotor surfaces will form one pole with the other half forming the second pole. Further, the direction of torque acting on the rotor can be clockwise or counter-clockwise.
The following table gives four set of statement as regards poles and torque. Select the correct set corresponding to the mmf axes as shown in figure. (2003)
(a) a
(b) b
(c) c
(d) d
Ans: (c)
Sol: Inside a magnetic material, flux flows from S-pole to N-pole.
Outside a magnetic material flux flow from N-pole to S-pole
When stator winding is excited, it produces flux. If ABC is assumed to N-pole flux will link rotor. As we know, flux flows from N-pole to S-pole externally, abc from S-pole.
Inside rotor, flux flows from surface abc to cda
So, surface cda forms N-poles.
To identify direction of torque
The torque acts in a direction to reduce δ i.e. to align the two fields. So, torque on the rotor acrs in counter-clockwise direction to reduce δ.
Q51: A round rotor generator with internal voltage E1 = 2.0 p.u. and X = 1.1 p.u. is connected to a round rotor synchronous motor with internal voltage E2 = 1.3 p.u. and X = 1.2 p.u. The reactance of the line connecting the generator to the motor is 0.5 p.u. When the generator supplies 0.5 p.u. power, the rotor angle difference between the machines will be (2003)
(a) 57.42°
(b) 1°
(c) 32.58°
(d) 122.58°
Ans: (c)
Sol: Rotor angle difference = δ1−δ2
Power transferred from generator to motor
Q52: Curves X and Y in figure denote open circuit and full-load zero power factor(zpf) characteristics of a synchronous generator. Q is a point on the zpf characteristics at 1.0 p.u. voltage. The vertical distance PQ in figure gives the voltage drop across (2003)
(a) Synchronous reactance
(b) Magnetizing reactance
(c) Potier reactance
(d) Leakage reactance
Ans: (d)
Sol: The vertical distance PQ between O.C. (curve X) and ZPF characteristic (curve Y) in above characteristic is leakage reactance.
Q53: A stand alone engine driven synchronous generator is feeding a partly inductive load. A capacitor is now connected across the load to completely nullify the inductive current. For this operating condition. (2003)
(a) the field current and fuel input have to be reduced
(b) the field current and fuel input have to be increased
(c) the field current has to be increased and fuel input left unaltered
(d) the field current has to be reduced and fuel input left unaltered
Ans: (d)
Sol: Assuming resistance of the armature to be zero. In first case, the generator is feeding a partly inductive load. It means that generator is supplying lagging power. The generator supplies a lagging power factor current when it is overexcited which is represented by Ef1.
In second case, a capacitor is connected across the load to completely nullifythe inductive current. It means the generator supplies no reactive power and unity power factor current is drawn from the generator. The excitation (Ef2) corresponding to unity power factor is known as normal excitation.
From the phasor diagram Ef1 > Ef2 and as the field current approximately directly proportional to excitation, the field current has to be reduced. From phasor diagram Ef1 sin δ1 = Ef2 sin δ2 = constant
Pe (power delivered)=
Fuel input remains unaltered.
Q54: The flux per pole in a synchronous motor with the field circuit ON and the stator disconnected from the supply is found to be 25 mWb. When the stator is connected to the rated supply with the field excitation unchanged, the flux per pole in the machine is found to be 20 mWb while the motor is running on no load. Assuming no load losses to be zero, the no load current down by the motor from the supply (2002)
(a) lags the supply voltage
(b) leads the supply voltage
(c) is in phase with the supply voltage
(d) is zero
Ans: (b)
Sol: Armature reaction is trying to deteriorate the main flux. So motor must be operating with leading power factor.
Q55: A power system has two synchronous generators. The Governor-turbine characteristics corresponding to the generators are
P1 = 50(50 − f), P2 = 100(51 − f)
Where f denotes the system frequency in Hz, and P1 and P2 are, respectively, the power outputs (in MW) of turbines 1 and 2. Assuming the generators and transmission network to be lossless, the system frequency for a total load of 400 MW is (2001)
(a) 47.5 Hz
(b) 48 Hz
(c) 48.5 Hz
(d) 49 Hz
Ans: (b)
Q56: A star-connected 440 V, 50 Hz alternators has per phase synchronous reactance of 10Ω. It supplies a balanced capacitive load current of 20 A, as shown in the per phase equivalent circuit of figure. It is desirable to have zero voltage regulation. The load power factor should be (2001)
(a) 0.82
(b) 0.47
(c) 0.39
(d) 0.92
Ans: (d)
Sol: Squaring both side, we get,
Q57: are steady state d-axis synchronous reactance, transient d-axis reactance and sub-transient d-axis reactance of a synchronous machine respectively. Which of the following statements is true? (2001)
(a) Xd > X'd > X"d
(b) X′′d > X'd > Xd
(c) X'd > X′′d > Xd
(d) Xd > X′′d > X'd
Ans: (a)
Q58: Figure shows the magnetization curves of an alternator at rated armature current, unity power factor and also at no load. The magnetization curve for rated armature current, 0.8 power factor leading is given by (2001)
(a) curve A
(b) curve B
(c) curve C
(d) curve D
Ans: (c)
Q59: It is desirable to eliminate 5th harmonic voltage from the phase voltage of an alternator. The coils should be short-pitched by an electrical angle of (2001)
(a) 30°
(b) 36°
(c) 72°
(d) 18°
Ans: (b)
Sol: For fifth harmonics, 5β = 180°
⇒ β = 36°.
Ef > Vt, the generator is overexcited. Therefore, armature-reaction is demagnetizing in nature i.e. ϕar opposes ϕf.
As generator is star connected.
Taking Vt as the reference
Induced emf(line to line)
(ii) Damper bars not only minimize hunting to a tolerable level but also help the synchronous motor self-start.
The following table gives four set of statement as regards poles and torque. Select the correct set corresponding to the mmf axes as shown in figure. (2003)
(a) a
Inside a magnetic material, flux flows from S-pole to N-pole.
The torque acts in a direction to reduce δ i.e. to align the two fields. So, torque on the rotor acrs in counter-clockwise direction to reduce δ.
Rotor angle difference = δ1−δ2
(a) Synchronous reactance
The vertical distance PQ between O.C. (curve X) and ZPF characteristic (curve Y) in above characteristic is leakage reactance.
Assuming resistance of the armature to be zero. In first case, the generator is feeding a partly inductive load. It means that generator is supplying lagging power. The generator supplies a lagging power factor current when it is overexcited which is represented by Ef1.
From the phasor diagram Ef1 > Ef2 and as the field current approximately directly proportional to excitation, the field current has to be reduced. From phasor diagram Ef1 sin δ1 = Ef2 sin δ2 = constant 
(a) 0.82
Squaring both side, we get,
are steady state d-axis synchronous reactance, transient d-axis reactance and sub-transient d-axis reactance of a synchronous machine respectively. Which of the following statements is true? (2001)
(a) curve A
