Determination of Moment of Inertia of Electrical Machines MCQ

1. What is the formula for the moment of inertia? (m – a mass of the body, r – distance from the axis of the rotation)

A. ∑miri2
B. ∑miri
C. ∑miri4
D. ∑miri3

Answer: A

The moment of inertia is the property by the virtue of which the body withstands the effect of angular acceleration. It depends on the shape and mass distribution of the body. 

 

2. The generated e.m.f from 50-pole armature having 400 conductors driven at 20 rev/sec having flux per pole as 30 mWb, with lap winding is ___________

A. 230 V
B. 140 V
C. 240 V
D. 250 V

Answer: C

The generated can be calculated using the formula

Eb = Φ×Z×N×P÷60×A

Where

Φ represents flux per pole
Z represents the total number of conductors
P represents the number of poles
A represents the number of parallel paths
N represents speed in rpm.

In lap winding number of parallel paths are equal to the number of poles.

Eb = .03×50×400×1200÷60×50= 240 V. 

 

3. The unit of the moment of inertia is Kgm2.

A. True
B. False

Answer: A

The moment of inertia is taken as the sum of the product of the mass of each particle with the square of their distance from the axis of the rotation. The unit of the moment of inertia is kg×m2=kgm2

 

4. Calculate the moment of inertia of the egg having a mass of 7 kg and a radius of 44 cm.

A. .968 kgm2
B. 1.454 kgm2
C. 1.545 kgm2
D. 1.552 kgm2

Answer: D

The moment of inertia of the egg can be calculated using the formula

I=∑miri2.

The mass of the egg and radius is given

I=(7)×(.44)2=1.552 kgm2.

It depends upon the orientation of the rotational axis. 

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5. Which of the theorems helps in the calculation of the moment of inertia?

A. The theorem of Parallel and Perpendicular Axes
B. The theorem of Horizontal and Perpendicular axes
C. The theorem of Vertical and Perpendicular axes
D. The theorem of Parallel and Tilted axes

Answer: A

The theorem of Parallel and Perpendicular axes helps in the calculation of the moment of inertia. The moment of inertia of the complex bodies can be easily calculated with the help of these theorems. 

 

6. What is the unit of resistance?

A. ohm
B. ohm-1
C. ohm2
D. ohm5

Answer: A

The resistance is the opposition offered by the body to the flow of current. It is the ratio of voltage and current. It is given in ohms. 

 

7. Calculate the value of the frequency if the time period of the signal is 20 sec.

A. 0.05 Hz
B. 0.04 Hz
C. 0.02 Hz
D. 0.03 Hz

Answer: A

The frequency is defined as the number of oscillations per second. It is reciprocal to the time period. It is expressed in Hz.

F = 1÷T=1÷20=.05 Hz. 

 

8. The slope of the V-I curve is 60°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.

A. 1.732 Ω
B. 1.608 Ω
C. 1.543 Ω
D. 1.648 Ω

Answer: A

The slope of the V-I curve is resistance. The slope given is 60° so

R=tan(60°)=1.732 Ω.

The slope of the V-I curve is resistance. 

 

9. Calculate mark to space ratio if the system is on for 5 sec and off for 10 sec.

A. .5
B. .4
C. .2
D. .6

Answer: C

Mark to space is Ton÷Toff. It is the ratio of the time for which the system is active and the time for which is inactive.

M = Ton÷Toff=5÷10=2 

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