This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Design of End Rings”.
1. How does the revolving field produce emf in the bars?
a) revolving field produces emf of fundamental frequency in the bars
b) revolving field produces emf of third frequency in the bars
c) revolving field produces emf of no frequency in the bars
d) revolving field produces emf of sinusoidal frequency in the bars
View Answer
Explanation: The stator winding is 3 phase distributed winding and thus produces a revolving field. This revolving field produces emfs of fundamental frequency in the bars.
2. What happens if the resistance of the end rings is negligible?
a) resistance coming in each current path is resistance of three bars
b) resistance coming in each current path is resistance of four bars
c) resistance coming in each current path is resistance of two bars
d) resistance coming in each current path is resistance of five bars
View Answer
Explanation: If the resistance of end rings is negligible then the resistance of combined bars are taken into account. Generally the resistance of two bars are taken into account.
3. What factors does the current in the bars depend on?
a) emfs, position of bars in magnetic field
b) instantaneous emfs, position of bars in magnetic field
c) emf
d) instantaneous emf
View Answer
Explanation: The current that the bars carry are proportional to their instantaneous emfs. The instantaneous emfs are proportional to the position of the bars in the magnetic field.
4. The end resistance, if not negligible, will tend to distort the bar current distribution from being sinusoidal.
a) true
b) false
View Answer
Explanation: The end resistance is proportional to the current distribution. If it is not negligible then the resistance will distort the bar current distribution.
5. What is the formula for the maximum current in end ring, if the current in all bars are maximum at the same time?
a) maximum current in the end ring= bars per pole * 2 * current per bar
b) maximum current in the end ring= (bars per pole / 2) * current per bar
c) maximum current in the end ring= bars per pole / 2 / current per bar
d) maximum current in the end ring= bars per pole * 2 / current per bar
View Answer
Explanation: First the bars per pole are obtained. Then the current per bar is calculated. Then substituting in the above formula, the maximum current in the end rings.
6. Given the bars per pole is 6 and the current per bar is 20 A, what is the value of the maximum current in the end rings?
a) 60 A
b) 80 A
c) 90 A
d) 70 A
View Answer
Explanation: maximum current in the end ring= bars per pole / 2 * current per bar
Maximum current in the end ring = (6/2)*20 = 3 * 20 = 60 A.
7. What is the formula for the maximum value of current through end ring, when the current is not maximum in all the bars under one pole at the same time?
a) maximum current in end ring= (2*3.14) / (bars per pole/2*no of poles) * current per bar
b) maximum current in end ring= (2/3.14) * (bars per pole/2*no of poles) / current per bar
c) maximum current in end ring= (2*3.14) * (bars per pole/2*no of poles) * current per bar
d) maximum current in end ring= (2*3.14) / (bars per pole/2*no of poles) / current per bar
View Answer
Explanation: The bars per pole is first obtained. Then the no of poles is calculated along with the current per bar. On substituting in the formula the maximum current in the end ring with the current through all the bars under one pole is not maximum.
8. What is the formula for the maximum current through each bar?
a) maximum value of the current through each bar = 2 * current through each bar
b) maximum value of the current through each bar = (2 * current through each bar)1/2
c) maximum value of the current through each bar = (2 * current through each bar)2
d) maximum value of the current through each bar = (2 * current through each bar)1/3
View Answer
Explanation: Firstly, the current through each bar is calculated. Then it is multiplied by 2 and its square root provides the maximum value of current through each bar is obtained.
9. What is the formula for the rms value of the end ring current?
a) rms value of end ring current = (bars per pole * current per bar) / (3.14*no of poles)
b) rms value of end ring current = (bars per pole * current per bar) * (3.14*no of poles)
c) rms value of end ring current = (bars per pole * current per bar) / (no of poles)
d) rms value of end ring current = (bars per pole * current per bar) / (3.14+no of poles)
View Answer
Explanation: First the bars per pole is obtained. Then the current per bar is calculated and the no of poles is calculated. Substituting in the above formula, the rms value of the end ring current is obtained.
10. The value of the current density is chosen for the end rings such that the desired value of rotor resistance is obtained.
a) true
b) false
View Answer
Explanation: The value of current density is chosen for the end rings should be chosen such that the desired value of the rotor resistance is obtained. If not it can lead to the starting problems in the machine.
11. How is the current density of the rotor bars chosen with respect to the end rings?
a) current density of rotor bars < current density of end rings
b) current density of rotor bars > current density of end rings
c) current density of rotor bars = current density of end rings
d) current density of rotor bars <= current density of end rings
View Answer
Explanation: The ventilation is generally better for the end rings. Thus, the current density of the end rings should be greater than that of the rotor bars.
12. What is the formula for the area of the ring?
a) area of the ring = depth of the end ring + thickness of the end ring
b) area of the ring = depth of the end ring – thickness of the end ring
c) area of the ring = depth of the end ring / thickness of the end ring
d) area of the ring = depth of the end ring * thickness of the end ring
View Answer
Explanation: For calculation of the area of end rings, first the depth of end rings is calculated. Next, the thickness of the end ring is calculated and substituting in the above formula gives the area of the ring.
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