## Electrical Machines Questions and Answers Part-1

1. An electro-mechanical energy conversion device is one which converts _______
a) Electrical energy to mechanical energy only
b) Mechanical energy to electrical energy only
c) Electrical to mechanical and mechanical to electrical
d) None of the mentioned

Explanation: The operating principles of electrical to mechanical and mechanical to electrical conversion devices are similar, hence, the common name electro-mechanical device. However, their structural details differ depending on their function.

2. What is the coupling field used between the electrical and mechanical systems in energy conversion devices?
a) Magnetic field
b) Electric field
c) Magnetic field or Electric field
d) None of the mentioned

Explanation: Either electric field or magnetic field can be used, however most commonly we use magnetic field because of its greater energy storage capacity

3. The energy storing capacity of magnetic field is about ________ times greater than that of electric field.
a) 50,000
b) 25,000
c) 10,000
d) 40,000

Explanation: As the energy storage capacity of the magnetic field is higher, it is most commonly used as coupling medium in electro-mechanical energy conversion devices

4. The formula for energy stored in the mechanical system of linear motion type is ______
a) 1/2 Jwr2
b) 1/2 mv2
c) 1/2 mv
d) Jwr2

Explanation: Energy stored is kinetic energy, since the system is of linear motion.

5. In an electro-mechanical energy conversion device, which of the following statements are correct regarding the coupling field?
(i) electrical side is associated with emf and current
(ii) electrical side is associated with torque and speed
(iii) mechanical side is associated with emf and current
(iv) mechanical side is associated with torque and speed
a) (i) & (ii)
b) (ii) & (iii)
c) (iii) & (iv)
d) (i) and (iv)

Explanation: The coupling field will be associated with the electrical quantities on electrical side and vice versa.

6. A coupling magnetic field must react with which of the following statements?
(i) electrical system in order to extract energy from mechanical system
(ii) mechanical system in order to extract energy from mechanical system
(iii) electrical system in order to extract energy from electrical system
(iv) mechanical system in order to extract energy from electrical system
(v) electrical or mechanical system for electro-mechanical energy conversion
a) (i), (ii) & (iii)
b) (ii), (iii) & (v)
c) (ii), (iii) & (iv)
d) (ii), (iii) & (v)

Explanation: To convert electrical to mechanical, the coupling magnetic field must take energy from input, the electrical system and vice versa

7. The developed electromagnetic force and/or torque in electromechanical energy conversion system, acts in such a direction that tends to ___________
a) increase the stored energy at constant mmf
b) decrease the stored energy at constant mmf
c) decrease the co-energy at constant mmf
d) increase the stored energy at constant flux

Explanation: fe = -(∂Wfld (φ,x))/∂x = -(∂Wfld (∅,x))/∂x and Te = -(∂Wfld (φ,θ))/∂θ = -(∂Wfld(∅,θ))/∂θ.

8. The developed electromagnetic force and/or torque in electromechanical energy conversion systems, acts in a direction that tends to ___________
(i) increase the co-energy at constant flux
(ii) increase the co-energy at constant mmf
(iii) decrease the stored energy at constant mmf
(iv) decrease the stored energy at constant flux

Which of the above statements are correct?
a) (ii), (iv)
b) (i), (iii)
c) (ii), (iii)
d) (i), (iv)

Explanation: fe = -(∂Wfld (φ,x))/∂x and fe = (∂Wfld1(F,x))/∂x.

9. A physical system of electromechanical energy conversion, consists of a stationary part creating a magnetic field with electric energy input, and a moving part giving mechanical energy output. If the movable part is kept fixed, the entire electrical energy input will be _______
a) stored in the magnetic field
b) stored in the electric field
c) divided equally between the magnetic and electric fields
d) zero