Electrical Machines Questions and Answers_pag5

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The negative sign before ∂W​fld​ indicates that f​e​ 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 
View Answer 
Answer: d 
Explanation: f​e​=(∂W​fld​
1​ (i,x))/∂x=(∂W​fld​
1​ (F,x))/∂x, the positive sign before ∂W​fld​
1​ indicates that 
force f​e​ 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) W​elec​=W​fld 
b) W​elec​=W​mech 
c) W​elec​=W​mech​/2+W​fld​/2 
d) W​elec​=0 
View Answer 
Answer: c 
Explanation: For the above mentioned case, W​fld​=W​mech​ and W​fld​=W​elec​/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 
View Answer 
Answer: c 
Explanation: f​e​=1/2 ∅​2​ dRl/dx , T​e​=-1/2 ∅​2​ dRl/dθ= 1/2 i​2​ 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 
View Answer 
Answer: a 
Explanation: f​e​= (∂W​fld​
1​(i,x))/∂x=(∂W​fld​(i,x))/∂x.