This set of Design of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Construction of Hydro-Generators – 1”.
1. How many factors are involved in the construction of hydro-generators?
a) 11
b) 10
c) 9
d) 8
View Answer
Explanation: The hydro-generators are the synchronous generators that are driven by the water turbines. They are chosen based on 10 different categories.
2. Which factor does the constructional feature of the hydro-generators depend on?
a) speed
b) voltage
c) power
d) current
View Answer
Explanation: The constructional feature of hydro-generators are basically dependent upon the mechanical considerations. The main mechanical consideration is speed of the machine.
3. What factors does the speed of the machines depend upon?
a) head
b) blades
c) type of turbine used
d) head and type of turbine used
View Answer
Explanation: The hydro-generators are generally low speed machines. The speed depends upon the head and the type of turbines used.
4. Why is the stator core built up of laminations?
a) to reduce core loss
b) to reduce copper loss
c) to reduce iron loss
d) to reduce eddy current loss
View Answer
Explanation: The stator core is built up of laminations in order to reduce the eddy current loss. The loss in the laminated core is usually the single largest loss and hence the choice of type and grade of steel is of utmost importance.
5. The modern synchronous machines make use of non-directional cold rolled steel.
a) true
b) false
View Answer
Explanation: The modern synchronous machines make use of the non-directional cold rolled steel which has electrical characteristics similar to that of the hot rolled steel. But the non-directional cold rolled steel has much improved mechanical characteristics.
6. What is the thickness of the most commonly used grade for stator laminations?
a) 0.5 mm
b) 1 mm
c) 1.5 mm
d) 2 mm
View Answer
Explanation: The most commonly used grade for the stator laminations is grade 230. The thickness of this most commonly used grade for the stator laminations is 0.5 mm.
7. What is the range of the outside diameter of the stator frame of the large hydro-generator?
a) 3-18 m
b) 2-18 m
c) 3.5-18 m
d) 4-18 m
View Answer
Explanation: The minimum value for the outside diameter of the stator frame of the large hydro-generators is 3.5 m. The maximum value for the outside diameter of the stator frame of the large hydro-generators is 18 m.
8. How are the stator windings of all synchronous generator connected?
a) star-delta connection
b) star connection
c) star connection with neutral earthed
d) delta connected
View Answer
Explanation: The stator windings of all the synchronous generator are connected in the star connection with neutral earthing. The main advantage that the winding has to be insulated to earth for the phase voltage and not the line voltage.
9. What among the following is the advantages of the star connection?
a) eliminates single frequency harmonics
b) eliminates double frequency harmonics
c) eliminates triple frequency harmonics
d) eliminates sinusoidal harmonics
View Answer
Explanation: The hydro-generators stator windings are connected in the star connected with the neutral earthed. It eliminates the triple frequency harmonics from the line voltage.
10. The capacity of the pull out machines used for making the coils limits the pole pitch to less than 0.8 m.
a) true
b) false
View Answer
Explanation: The capacity of the pull out machines used for making the coils limits the pole pitch to less than 0.8 m. The capacity of the pull out machines used for making coils limits the length of slot portion to about 3 m.
11. What is the main advantage of a winding with multi-turns coils?
a) reduce the choosing of the value of stator slots
b) allows greater flexibility in selecting the value of stator slots
c) increases the flexibility in selecting the number of stator slots
d) has no effect on the number of stator slots
View Answer
Explanation: The main advantage of a winding with multi-turns coils is that it allows a greater flexibility in selecting the value of stator slots. It in turn helps in choosing the required number of turns per phase.
12. What happens to the current in the windings during the sudden short circuits at the line terminals?
a) the current reduces to 15 times the full load current
b) the current increases to 15 times the full load current
c) the current reduces to 10 times the full load current
d) the current increases to 10 times the full load current
View Answer
Explanation: The current in the windings during short circuit at the line terminals increases to about 15 times the full load current. It can also increase to higher level depending on the direct axis sub-transient reactance.
13. What happens to the electromagnetic forces during the sudden short circuits at the line terminals?
a) the electromagnetic forces get increased by 250 times the force under normal full load condition
b) the electromagnetic forces get decreased by 250 times the force under normal full load condition
c) the electromagnetic forces get increased by 200 times the force under normal full load condition
d) the electromagnetic forces get decreased by 200 times the force under normal full load condition
View Answer
Explanation: The electromagnetic forces are directly proportional to the square of the current. It increases by 250 times the force under normal full load conditions.
14. What should be done to the conductors in the overhang of the stator?
a) bracing
b) shaving
c) punching
d) compressing
View Answer
Explanation: The conductors in the overhang of the stator must be braced. Bracing is the process of raising the mechanical strength of the conductors.
15. How many steel brackets are used along with the support to steel rings?
a) 5
b) 7
c) 4-6
d) 5-9
View Answer
Explanation: During the process of bracing one or two circular steel rings are used to support the overhang. Along with the steel rings 4-6 steel brackets are also used.
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