# Electric Drives Questions and Answers – Motors Rating and Heating – Load Inertia Effect

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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Motors Rating and Heating – Load Inertia Effect”.

1. Hooke’s law is applicable within the elastic limit.
a) True
b) False

Explanation: Hooke’s law is only applicable within the elastic limit. It states that within the elastic limit the strain is directly proportional to the stress.

2. Calculate the load stress if the value of force is 20 N and the area of cross-section is 2 m2.
a) 10 N/m2
b) 20 N/m2
c) 30 N/m2
d) 40 N/m2

Explanation: The stress is defined as the ratio of the internal restoring force and area of the cross-section. Stress=Fres÷A=20÷2=10 N/m2.

3. What is the effect of impurities on elasticity?
a) It will increase
b) Remains the same
c) It will decrease
d) It will slightly increase

Explanation: The Young modulus of elasticity is slightly increased by the addition of impurities. The intermolecular forces inside the wire effectively increase by impurity due to this external force can be easily opposed.

4. Which of the following are the types of Stress?
a) Longitudinal stress, Volume stress, Tangential stress
b) Longitudinal stress, Volume stress
c) Longitudinal stress, Lateral stress, Tangential stress
d) Longitudinal stress, Shear stress

Explanation: There are three types of stress that are Longitudinal stress, Volume stress, Tangential or Shear stress. Stress is the ratio of restoring force and cross-section area.

5. The relationship between Young’s modulus (Y), Bulk modulus (K) and Poisson’s ratio (μ) is _____________
a) Y=K(1-2μ)
b) Y=3K(1-2μ)
c) Y=5K(1-2μ)
d) Y=4K(1-8μ)

Explanation: The relationship between Young’s modulus (Y), Bulk modulus (K) and Poisson’s ratio (μ) is Y=3K(1-2μ). The Young modulus of elasticity is defined as the ratio of Longitudinal stress and Longitudinal strain.

6. Calculate the work done in stretching the wire of volume=22 m3 and the values of stress and strain are 2 N/m2 and 4.
a) 66 J
b) 44 J
c) 22 J
d) 88 J

Explanation: The work done in stretching a wire is .5×Stress×Strain×Volume. The value of the work done is .5×22×4×2=88 J.

7. Plasticity is the ability of the material to deform without breaking.
a) True
b) False

Explanation: Plasticity is the ability of the material to deform without breaking. Plastic deformation occurs in metal forming processes.

8. _________ motor operates at low power factor.
a) Capacitor run
b) Capacitor start
d) Reluctance

Explanation: Shaded pole motors operate at a low power factor. There is a weak magnetic field develops in the case of Shaded pole motor. It is a single-phase AC Induction motor.

9. Calculate the compensator rating required for cos(Φ)=.7.
a) 0.71 P.U
b) 0.21 P.U
c) 0.47 P.U
d) 0.12 P.U

Explanation: The compensator rating can be calculated using the relation QP.U=√1-cos2(∅)=√1-.49=.71. This per unit value VAR compensator is required to improve the power factor of the system.

10. Full form of SPWM.
a) Sinusoidal pulse width modulation
b) State per width modulation
c) State pulse width modulation
d) Sinusoidal per width modulation

Explanation: SPWM stands for sinusoidal pulse width modulation. SPWM is a high-frequency PWM technique. This technique is used to achieve variable voltage level.

11. Which harmonic is not present in 3-Φ fully controlled rectifier?
a) 3rd harmonic
b) 5th harmonic
c) 7th harmonic
d) 11th harmonic

Explanation: Triplen harmonics are absent in the case of 3-phase fully controlled rectifier. Because of the absence of triplen harmonics THD of the rectifier 31%.

12. Lowest order harmonic present in 3-Φ fully controlled rectifier is __________
a) 5th
b) 7th
c) 3rd
d) 2nd

Explanation: Only 6k±1 order harmonics are present in 3-Φ fully controlled rectifier. The lowest order harmonic is 5th harmonic for k=1. Third order harmonics are absent in 3-Φ fully controlled rectifier.

13. Calculate the moment of inertia of the circular hollow disk having a mass of 2 kg and an internal radius of 5 m and an external radius of 10 m.
a) 125 kgm2
b) 127 kgm2
c) 122 kgm2
d) 123 kgm2

Explanation: The moment of inertia of the circular hollow disk can be calculated using the formula I=M(R12+R22)÷2. The mass of circular hollow disk and radius is given. I=(1)×{(5)2+(10)2}=125 kgm2. It depends upon the orientation of the rotational axis.

14. Calculate the mass of the solid cylinder having a mass of 4 kgm2 and radius of 4 m & length of 12 m.
a) 64 kgm2
b) 68 kgm2
c) 67 kgm2
d) 69 kgm2

Explanation: The moment of inertia of the solid cylinder can be calculated using the formula I= M(l2÷12+R22÷4). The mass of solid cylinder and radius, the length is given. I=(4){(16)}= 64 kgm2. It depends upon the orientation of the rotational axis.

Sanfoundry Global Education & Learning Series – Electric Drives.

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