This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Two Reaction Theory of Salient Pole Machines”.
1. In salient-pole machines, the armature mmf cannot be accounted for by introducing one equivalent reactance due to
a) non-uniform airgap
b) variable resistance
c) non-uniform air gap
d) any of the mentioned
Explanation: Due to saliency of the of the rotor poles, the air gap varies and the so one reactance is not sufficient to express the armature mmf.
2. If the internal power factor angle of synchronous motor is 30 degree. Then the direct-axis component of the armature current will be
a) 0.5 pu
b) 0.866 pu
c) 1.73 pu
Explanation: Id = Ia*sinψ = 1*sin30
= 0.5 pu.
3. If the internal pf angle of a synchronous motor is 30 degree. Then the quadrature axis component of the armature is
a) 0.866 pu
b) 0.5 pu
c) 1.73 pu
Explanation: Id = Ia*cosψ = 1*cos(3*0).
4. A salient pole synchronous generator has following per-unit parameter Xd = 1.2 pu, Xq = 0.8pu, ra = 0.25 ohms.
Then the armature current at 0.8 lagging is
b) 1, 36.9°
c) 1, 73°
d) 1, 27°
Explanation: In per unit, armature current will be lagging by 36.9°.
5. The internal power factor angle is given for a lagging load of a 3- phase alternator.
a) ψ = δ – θ
b) ψ = δ + θ
c) ψ = -δ -θ
d) ψ = δ -θ
6. Mark the correct expression for alternator working at a lagging power factor
a) tanψ = (IaXq – Vtsinθ)/(Vtcosθ – Ia*ra)
b) tanψ = (IaXq + Vtsinθ)/(Vtcosθ – Ia*ra)
c) tanψ = (IaXq + Vtsinθ)/(Vtcosθ + Ia*ra)
d) tanψ = (IaXq – Vtsecθ)/(Vtcosecθ – Ia*ra)
Explanation: The internal power factor angle is given as tanψ = (IaXq – Vtsinθ)/(Vtcosθ – Iara) for an alternator.
7. For a 400 V, 3 phase alternator gives an open circuit voltage of 380V and armature current of 38A at a field current of 20A. Then the synchronous reactance of the machine is
a) 10 ohm
c) 19 ohm
d) 1.9 ohm
Explanation: Z = Vt/Ia at the same field current = 380/38 = 38 ohms.
8. While finding the parameters for an alternator, its voltage regulation is found to be negative. Then the load connected to it must be
a) capacitive nature
b) inductive nature
c) resistive nature
d) can not be judged based on voltage regulation
Explanation: The given voltage regulation is mentioned as negative. So it must have a load which is a source of reactive power and hence it must be a capacitor.
9. for a synchronous motor, its power factor
a) improves with increase in excitation and may even become leading at higher excitations
b) is independent of its excitation
c) decreases with increase in excitation
d) increases with loading for a given excitation
Explanation: A per the inverted V- curves it can be easily concluded that as the excitation increases synchronous motor becomes leading.
10. For obtaining maximum current when we conduct the ‘Slip Test’ on a synchronous machine, its armature field will align along
c) 45° to d-axis
d) 45° to q-axis
Explanation: The maximum current is observed at low reluctance which is along q-axis.
Sanfoundry Global Education & Learning Series – Electrical Machines.
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