Electrical Machines Questions and Answers – Two Reaction Theory of Salient Pole Machines

«
»

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
View Answer

Answer: a
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.
advertisement

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
d) 0
View Answer

Answer: a
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
d) 0
View Answer

Answer: a
Explanation: Id = Ia*cosψ = 1*cos(3*0).
Sanfoundry Certification Contest of the Month is Live. 100+ Subjects. Participate Now!
advertisement
advertisement

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?
a) 1,-36.9°
b) 1, 36.9°
c) 1, 73°
d) 1, 27°
View Answer

Answer: a
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
View Answer

Answer: b
Explanation: The internal power factor angle is for lagging load of a 3- phase alternator
advertisement

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)
View Answer

Answer: a
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
b) 38/√3
c) 19 ohm
d) 1.9 ohm
View Answer

Answer: a
Explanation: Z = Vt/Ia at the same field current = 380/38 = 38 ohms.
advertisement

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
View Answer

Answer: a
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
View Answer

Answer: a
Explanation: A per the inverted V-curves it can be easily concluded that as the excitation increases synchronous motor becomes leading.
advertisement

10. For obtaining maximum current when we conduct the ‘Slip Test’ on a synchronous machine, its armature field will align along __________
a) d-axis
b) q-axis
c) 45° to d-axis
d) 45° to q-axis
View Answer

Answer: b
Explanation: The maximum current is observed at low reluctance which is along q-axis.

Sanfoundry Global Education & Learning Series – Electrical Machines.

To practice all areas of Electrical Machines, here is complete set of 1000+ Multiple Choice Questions and Answers.

advertisement
advertisement
Subscribe to our Newsletters (Subject-wise). Participate in the Sanfoundry Certification contest to get free Certificate of Merit. Join our social networks below and stay updated with latest contests, videos, internships and jobs!

Youtube | Telegram | LinkedIn | Instagram | Facebook | Twitter | Pinterest
Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

Subscribe to his free Masterclasses at Youtube & technical discussions at Telegram SanfoundryClasses.