# Electrical Machines Questions and Answers – Phasor Diagram of a Cylindrical Rotor Alternator

This set of Electrical Machines Quiz focuses on “Phasor Diagram of a Cylindrical Rotor Alternator”.

1. The flux addition is due to ______________

```(i) sinusoidal distribution of mmfs
(ii) zero relative velocity between two mmfs
(iii) spatial distribution of three phases by 120°
```

a) (i), (ii)
b) (i), (ii), (iii)
c) (iii)
d) (ii), (iii)

Explanation: Flux addition in ac machines is due to sinusoidal flux distribution and they must be stationary with respect to each other.

2. If the resultant flux will not be in phase with the resultant mmf Fr. This is due to __________
a) hysteresis effect
b) armature reaction
c) hysteresis effect and armature reaction
d) eddy currents

Explanation: Hysteresis effect causes the flux to be lagging in nature.

3. The internal power factor angle Ψ is between __________
a) Ef and Ia
b) Ia and field flux
c) Ef and Vt
d) Vt and Ia

Explanation:

4. The load angle δ is between __________
a) Ef and Ia
b) Ia and field flux
c) Ef and Vt
d) Vt and Ia

Explanation: It is the angle between excitation voltage and the terminal voltage.
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5. If it is assumed that the iron part of magnetic circuit requires zero mmf, relation between Ef and If will be __________
a)
b)
c)
d)

Explanation: There will be linear relation in Ef and Ia as mmf required is zero.

6. Phasor diagram below is best describing the __________

a) Open circuit characteristic of alternator
b) Short circuit characteristic of alternator
c) External circuit characteristic of alternator
d) Internal circuit characteristic of alternator

Explanation: Graph of Ef vs Ff is the open circuit characteristic of the machine.

7. The short circuit characteristic of alternator is a straight line due to __________
a) no saturation
b) no armature reaction
c) no eddy current
d) all of the mentioned

Explanation: At the short circuit of the alternator is having the linear relation as the armature does not enter the saturation mode due to demagnetizing nature of armature reaction.

8. From the OCC and SCC curves, what does the intercept AB represent?

a) Armature reaction
b) Leakage reactance
c) Armature copper losses
d) Sum of armature reaction and leakage reactance

Explanation: It is armature reaction which is calculated based on Potier triangle.

9. From the OCC and SCC curves, what does the intercept BC represent?

a) Armature reaction
b) Leakage reactance
c) Armature copper losses
d) Sum of armature reaction and leakage reactance

Explanation: It is leakage reactance which is calculated based on potier triangle.

10. The leakage reactance can be determined with lesser accuracy by using Potier triangle.
a) True
b) False

Explanation: The leakage reactance can be determined with better accuracy by using Potier triangle.

11. Potier method is very accurate due to __________
a) emfs are handled as voltage and mmf as AT
b) emf and mmf are handled as AT
c) emf and mmf are handled as voltage
d) none of the mentioned

Explanation: The potier’s method is very accurate because emfs are handled as voltage and mmf as AT.

12. Choose the correct representation for the time phasor below.
a)
b)
c)
d) all of the mentioned

Explanation: It is the angle between excitation voltage and the armature current.

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