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Electrical Machines MCQs – Magnetic Circuits MCQs ( Electrical Machines ) MCQs
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Principle of Energy Conversion
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
Answer: c
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
Answer: c
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
Answer: b
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
Answer: b
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)
Answer: d
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)
Answer: b
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
Answer: b
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)
Answer: a
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
Answer: a
Explanation: As the movable part is fixed, Wmech = 0, we know, Welec = Wmech + Wfld, therefore, Welec = Wfld.
Singly Excited Magnetic Systems
1. For a toroid to extract the energy from the supply system, the flux linkages of the magnetic field must be ________
a) zero
b) changing or varying
c) constant
d) any of the mentioned
Answer: b
Explanation: dWelec=idφ=eidt, where dWelec = differential electrical energy to coupling field, and if the flux linkages are either constant or zero, i.e, dφ=0, then dWelec=0.
2. Magnetic stored energy density for iron is given by ______
a) 1/2 B/μ
b) 1/2 B2 μ
c) 1/2 ∅2 Rl
d) 1/2 B2/μ
Answer: d
Explanation: Magnetic stored energy density for iron is given as
wfld=Wfld/((Length of the magnetic path through Iron)*(Iron area normal to the magnetic flux))=1/2 (F∅)/(length*Area)=1/2 F/length ∅/area=1/2 H*B
Also, H = B/μ,thus wfld=1/2 B2/μ.
3. The energy stored in a magnetic field is given by ____________ where L=self-inductance and Rl=reluctance.
a) 1/2 Li2
b) 1/2 (mmf*Rl)2
c) 1/2∅Rl
d) 1/2 φ2i
Answer: a
Explanation: We know that Wfld=1/2 φi and L=φ/i, thus Wfld=1/2 Li2.
Constructional Features of Rotating Electrical Machines
4. When a current of 5A flows through a coil of linear magnetic circuit, it has flux linkages of 2.4 wb-turns. What is the energy stored in the magnetic field of this coil in Joules?
a) 6
b) 12
c) 1.2
d) 2.4
Answer: a
Explanation: Wfld = 1/2 φ*i = 1/2*2.4*5 = 6 Joules.
5. For a linear electromagnetic circuit, which of the following statement is true?
a) Field energy is less than the Co-energy
b) Field energy is equal to the Co-energy
c) Field energy is greater than the Co-energy
d) Co-energy is zero
Answer: b
Explanation: Wfld=Wfld1=1/2 φ*i=1/2 F*∅.
6. The electromagnetic force and/or torque, developed in any physical system, acts in such a direction as to tend to ____________
a) decrease the magnetic stored energy at constant mmf
b) decrease the magnetic stored energy at constant flux
c) increase the magnetic stored energy at constant flux
d) increase the magnetic stored energy at constant current
Answer: b
Explanation: fe=-(∂Wfld (φ,x))/∂x = -(∂Wfld (∅,x))/∂x and Te = -(∂Wfld(φ,θ))/∂θ = -(∂Wfld (∅,θ))/∂θ
The negative sign before ∂Wfld indicates that fe acts in a direction as to tend to decrease the stored energy at constant mmf.
7. The electromagnetic force developed in any physical system acts in such a direction as to tend to _____________
a) decrease the co-energy at constant mmf
b) increase the co-energy at constant flux
c) decrease the co-energy at constant flux
d) increase the co-energy at constant mmf
Answer: d
Explanation: fe = (∂Wfld1 (i,x))/∂x = (∂Wfld1 (F,x))/∂x, the positive sign before ∂Wfld1 indicates that force fe acts in a direction as to tend to increase the co-energy at constant mmf.
8. Consider a magnetic relay with linear magnetization curve in both of its open and closed position. What happens to the electrical energy input to the relay, when the armature moves slowly from open position to closed position?
a) Welec=Wfld
b) Welec=Wmech
c) Welec=Wmech/2+Wfld/2
d) Welec=0
Answer: c
Explanation: For the above mentioned case, Wfld=Wmech and Wfld=Welec/2 hence, option “c” is the correct answer.
9. The electromagnetic torque developed in any physical system, and with magnetic saturation neglected, acts in such a direction as to tend to ____________
a) decrease both the reluctance and inductance
b) increase both the reluctance and inductance
c) decrease the reluctance and increase the inductance
d) increase the reluctance and decrease the inductance
Answer: c
Explanation: fe=1/2 ∅2 dRl/dx, Te=-1/2 ∅2 dRl/dθ = 1/2 i2 dL/dθ.
10. Electromagnetic force and/or torque developed in any physical system, acts in such a direction as to tend to ____________
a) increase both the field energy and co-energy at constant current
b) increase the field energy and decrease the co-energy at constant current
c) decrease both the field energy and co-energy at constant current
d) decrease the field energy and increase the co-energy at constant current
Answer: a
Explanation: fe = (∂Wfld1(i,x))/∂x = (∂Wfld(i,x))/∂x.