Design of Electrical Machines Questions and Answers – Armature Design – 2

This set of Tough Design of Electrical Machines Questions and Answers focuses on “Armature Design – 2”.

1. What is the range for the stator slot pitch for the large hydro-electric generators?
a) 50-60 mm
b) 50-70 mm
c) 50-80 mm
d) 50-90 mm

Explanation: The minimum value for the stator slot pitch for the large hydro-electric generators is 50 mm. The maximum value for the stator slot pitch for the large hydro-electric generators is 90 mm.

2. When is the range of the number of slots per pole per phase in the salient pole machines?
a) 2-3
b) 3-4
c) 2-4
d) 2-6

Explanation: The minimum value of the number of slots per pole per phase is 2. The maximum value of the number of slots per pole per phase is 4.

3. Fractional windings are invariably used in synchronous machines.
a) true
b) false

Explanation: Fractional slot windings reduces the distribution factor for higher harmonics thus reducing their corresponding generated emfs and making the voltage nearly sinusoidally. Fractional slot windings are invariably used in synchronous machines.

4. What is the relation between coil span and harmonics?
a) low coil span decreases harmonics to less amount
b) low coil span decreases the harmonics drastically
c) high coil span decreases the harmonics drastically
d) high coil span decreases the harmonics by small amount

Explanation: The coil span is kept low in order to decreases the harmonics. The advantage of having lower coil spans is that it reduces the harmonics drastically.

5. The coil span should be 8.33 percent of pole pitch to obtain the maximum reduction of harmonics.
a) true
b) false

Explanation: The coil span adjustment indirectly affects the harmonics reduction. The coil span should be minimum of about 8.33 percent of pole pitch to obtain the maximum reduction of harmonics.
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6. When is the formula for the flux per pole?
a) flux per pole = average magnetic field * pole pitch * length of the core
b) flux per pole = average magnetic field / pole pitch * length of the core
c) flux per pole = average magnetic field * pole pitch / length of the core
d) flux per pole = 1 / average magnetic field * pole pitch * length of the core

Explanation: To obtain the value of flux per pole first the average magnetic field is obtained. Then the pole pitch and the length of the core is obtained to obtain the flux per pole.

7. What is the formula for the turns per phase in the armature design?
a) turns per phase = voltage per phase * parallel paths per phase / 4.44 * flux density * frequency * winding space factor
b) turns per phase = voltage per phase / parallel paths per phase * 4.44 * flux density * frequency * winding space factor
c) turns per phase = voltage per phase * parallel paths per phase * 4.44 * flux density * frequency * winding space factor
d) turns per phase = voltage per phase * parallel paths per phase * 4.44 * flux density / frequency * winding space factor

Explanation: For obtaining the turns per phase, the voltage per phase is obtained along with the parallel paths per phase. Next the winding space factor is calculated and the substitution in the formula gives the turns per phase.

8. What is the formula for current in each conductor?
a) current in each conductor = kVA * 103 * 3 * voltage per phase
b) current in each conductor = kVA / 103 * 3 * voltage per phase
c) current in each conductor = kVA * 103 / 3 * voltage per phase
d) current in each conductor = kVA * 103 * 3 / voltage per phase

Explanation: The kVA output is first obtained from the operation of the machine. Next the voltage per phase is calculated to obtain the current in each conductor.

9. What is the permissible current density in the armature conductors?
a) 3-4 A per mm2
b) 3-6 A per mm2
c) 4-6 A per mm2
d) 3-5 A per mm2

Explanation: The minimum permissible value of the current density in the armature conductors is 3 A per mm2. The maximum allowed value of the current density in the armature conductors is 6 A per mm2.

10. What is the formula for the area of cross section of armature conductors?
a) area of cross section = current per conductor * current density in the armature conductors
b) area of cross section = current per conductor + current density in the armature conductors
c) area of cross section = current per conductor – current density in the armature conductors
d) area of cross section = current per conductor / current density in the armature conductors

Explanation: For obtaining the area of the cross section the current per conductor is calculated. Next the current density is calculated and the ratio of both gives the current density of the area of cross section.

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