This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Design Principles”.
1. How many design principles are present in the current transformers?
a) 2
b) 3
c) 4
d) 5
View Answer
Explanation: There are 5 design principles present in the current transformers. They are core design, secondary current rating, primary current rating, windings and behavior of the transformer under short circuit current.
2. What should be done in order to reduce the errors in the core?
a) armature mmf is to kept low
b) field mmf to be kept high
c) the exciting mmf is to be kept low
d) the field mmf is to be kept high
View Answer
Explanation: The errors in the core are reduced by keeping the exciting mmf low. This can take place with the core having a low reluctance and low iron loss.
3. How many classifications are the magnetic alloys used in the current transformers classified into?
a) 3
b) 2
c) 4
d) 5
View Answer
Explanation: The magnetic alloys used in the current transformers are divided into 3 types. They are hot rolled silicon steel, cold rolled grain oriented silicon steel and nickel iron alloys.
4. What is the material used in the transformer when the transformer errors should be small?
a) mumetal cores
b) steel cores
c) permender cores
d) presshamn cores
View Answer
Explanation: The mumetal cores are commonly used when it is essential that transformer errors shall be small. Mumetal has the properties of high permeability, low loss and small retentivity.
5. What is the relation of the secondary winding leakage reactance and secondary circuit impedance?
a) secondary winding leakage reactance is directly proportional to the secondary circuit impedance
b) secondary winding leakage reactance is indirectly proportional to the secondary circuit impedance
c) secondary winding leakage reactance is directly proportional to the square of the secondary circuit impedance
d) secondary winding leakage reactance is indirectly proportional to the square of the secondary circuit impedance
View Answer
Explanation: The secondary winding leakage reactance is directly proportional to the secondary circuit impedance. In secondary winding the leakage reactance is reduced and in turn the secondary circuit impedance is reduced.
6. The ring shaped cores are made use of in the reduction of the secondary winding leakage reactance and secondary impedance.
a) true
b) false
View Answer
Explanation: The secondary winding leakage reactance is directly proportional to the secondary impedance. The ring shaped cores around which the toroidal secondary windings of one or more layers are uniformly distributed.
7. What type of core is employed when the performance standard required is not so high?
a) rectangular strips
b) c-shaped sections
c) rectangular strips or c-shaped sections
d) rectangular strips and c-shaped sections
View Answer
Explanation: Ring core type is used only for the high performance operation. The rectangular strips or c-shaped sections are used when the standard of performance required is not so high.
8. What should the magnetic path be in order to reduce the core reluctance?
a) length of the magnetic path in core should be low
b) length of the magnetic path in core should be medium
c) length of the magnetic path in core should be high
d) length of the magnetic path in core should be very high
View Answer
Explanation: The length of the magnetic path in core should be as small as possible. This reduces the core reluctance of the current transformer.
9. What is the value of the rated secondary current?
a) 1 A
b) 2 A
c) 3 A
d) 5 A
View Answer
Explanation: The rating of the secondary current is the maximum current that can be passed through the secondary windings. It is 5 A for the current transformer.
10. What are the disadvantages of the low rated secondary current transformer?
a) high cost
b) high voltages
c) high voltages or high cost
d) high voltages and high cost
View Answer
Explanation: When there is a low secondary current rating in the current transformers they produces high voltages if the secondary is left open. It is also costly to produce the windings because of the extra time involved in the making.
11. What is the ideal condition with respect to the primary current rating?
a) ratio of secondary mmf to primary mmf should be high
b) ratio of secondary mmf to primary mmf should be low
c) ratio of excitation mmf to primary mmf should be high
d) ratio of excitation mmf to primary mmf should be low
View Answer
Explanation: The primary current rating depends on exciting mmf and primary mmf. The ratio of the exciting mmf to the primary mmf should be low.
12. What is the rating of the primary current in the current transformer?
a) 200 A
b) 300 A
c) 400 A
d) 500 A
View Answer
Explanation: The rating of the primary current is minimum 500 A. If the rating is less than 500 A, then multiturn primary windings and rating is above than 500 A, then single turn winding is enough.
Sanfoundry Global Education & Learning Series – Design of Electrical Machines.
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