Electrical Machines Questions and Answers_pag4

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d) 1/2 B​2​/μ 
View Answer 
Answer: d 
Explanation: Magnetic stored energy density for iron is given as 
w​fld​=W​fld​/((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 w​fld​=1/2 B​2​/μ. 
3. The energy stored in a magnetic field is given by ____________ where L=self-inductance 
and Rl=reluctance. 
a) 1/2 Li​2 
b) 1/2 (mmf*Rl)​2 
c) 1/2∅Rl 
d) 1/2 φ​2​i 
View Answer 
Answer: a 
Explanation: We know that W​fld​=1/2 φi and L=φ/i , thus W​fld​=1/2 Li​2​. 
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 
View Answer 
Answer: a 
Explanation: W​fld​=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 
View Answer 
Answer: b 
Explanation: W​fld​=W​fld​
1​=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 
View Answer 
Answer: b 
Explanation: f​e​=-(∂W​fld​ (φ,x))/∂x= -(∂W​fld​ (∅,x))/∂x and T​e​=-(∂W​fld​(φ,θ))/∂θ=-(∂W​fld​ (∅,θ))/∂θ