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Viva questions for transformer part 3

Viva questions for transformer part 3


Q: What is a P.T?

 Ans: It is the potential transformer and is used for measuring high voltage. It is a step-down transformer.

Q: Can you tell the amperes for which the C.T. secondary is generally designed?

Ans: 5 A. 

Q: The secondary of the P.T. is designed for how much voltage? 

 Ans: Generally 110 V. 

Q: Can a wattmeter be connected with C.T. and P.T. for measuring power on H.T. fine? 

Ans: Yes. 

Q: Is there any change in the core flux of a transformer, underloaded and without load Conditions?  

Ans: No, it remains the same under all conditions of load. 

Read: Viva questions for transformer part 1

Q: Should the secondary’s of both C.T. and P.T. be earthed, if yes, why? 

Ans: Yes, The secondaries should be earthed, to protect against electric shock. 

Q: What precautions should be taken while replacing the ammeter of C.T. circuit? 

Ans: The secondary terminals must be short circuited before taking out the ammeter. Replace the meter and open the terminals. It is essential that in no case the secondary of the C.T. be left open. 

Q: What is an autotransformer? 

Ans: The transformer which has only one winding, both primary and secondary is permanently connected electrically, magnetically and mechanically. A portion of the winding works,as the primary and other as secondary, such transformer is known as autotransformer. 

Q: What is the working principle of an autotransformer? 

Ans: Self-induction. 

Read: Viva questions for transformer part 2

Q: How the secondary voltage is calculated? 

Ans: In a particular transformer, the voltage per turn is constant. So, the voltage can be calculated, multiplying the voltage per turn to the number of secondary turns. 

Q: Are the number of turns per voltage constant for a single winding transformer or two winding transformers? 

Ans: Constant for the particular transformer whether it is a single phase or three-phase, single winding or two winding transformers. 

Q: For maximum efficiency can you tell the transformation ratio of an autotransformer? 

Ans: Yes. 

Q: What are the advantages of an autotransformer?

Ans: Copper saving, only one winding is sufficient, light in weight, labour cost is less. 

Read: Viva questions for transformer part 1

Q: What are the common uses of an autotransformer? 

Ans: Used as a starter, stabilizer for T.V. and refrigerators, and in long-distance line as a booster also.

Q: What are the disadvantages of an autotransformer? 

Ans: There is only one winding, so the secondary portion is also energized and may give shock even when the load is not connected. 

Q: Is the current in both primary and secondary portions sane or different? 

Ans: Different in the common portion the current is the difference of primary and secondary


Q: What safety devices are used for a transformer? 

Ans: Air circuit breaker, lightning arrester, relays, and earth, etc.

Read: Viva questions for transformer part 2

Q: What are the advantages of using star connections in the distribution transformer? 

Ass: Because for distribution purposes, the three-phase four-wire systems is generally used. The lighting load and power load can be put on the same supply. Power load on three phases and lighting load on one phase and one neutral. 

Q: What is the utility of delta connections? 

Ans: For transmission purposes, where the power is to be a far distance, only three lines will solve the purpose. Thus the cost of transmission is saved, secondly, in an open delta, the supply will be continued but with 60% efficiency only. 

Q: How many ways are generally used for three phase transformation? 

Ans: Single unit of three-phase and three units of single-phase transformers. 

Read: Viva questions for transformer part 1

Q: Why are the utilities of using a three phase single transformer? 

Ans: It requires less space, less cost, and less maintenance also 

Q: Is there any disadvantage of using a single unit of three phases? 

Ans: Yes, in case of fault the whole unit is required to be repaired or replaced. 

Q: What are the drawbacks of using three single-phase transformers? 

Ans: More space is required, the cost is more and the maintenance cost is also more. 

Read: Viva questions for transformer part 2

Q: Can a three phase supply be transformed into a two-phase? 

Ans: Yes, be using Tee or Scott connections. 

Q: What are the uses of the Scott connection? 

Ans: In this method, the three-phase supply can be converted into two-phase supply for welding purposes. 

Q: Which is the ideal transformer? 

Ans: The transformer which has 100% efficiency. 

Q: What is the leakage reactance? 

Ans: Whenever the flux is induced by the winding some flux links only this particular winding and causes some self-induced voltage in it. This flux is called the leakage flux and reactance because this flux is the leakage reactance of that particular winding. 

Q: What is the equivalent resistance of the transformer referred to as primary winding? 

Ans: R_{01}=R_1+\frac1{\rho^2}R_{1\;}ohm

Where, Ro1= equivalent resistance referred to primary winding;

R1 = resistance of primary winding; And

R2=resistance of the secondary winding and ρ is the transformation ratio of the transformer. 

Read: Viva questions for transformer part 1

Q: What is the equivalent reactance of the transformer referred to as secondary winding? 

Ans: X_{o2}=X_2+X_1\;\rho^{2\;}\;ohm

Q: In which units the capacity of a transformer is shown? 

Ans: kVA. 

Q: Why it is shown in kVA and not in kW? 

Ans: Because the load power factor is not given. If it is in kW the power factor is fixed, which is not always applicable. 

Q: What are the instrument transformers? 

Ans: The current transformer (C.T.) and potential transformer (P.T.) are known as instrument transformers. 

Q: Where C.T. is used and what is the type of this transformer? 

Ans: It is a current transformer and is used to measure high current. It is a step-up transformer.

Read: Viva questions for transformer part 2

Q: If a transformer is connected to D.C. what will happen? 

Ans: It will not induce any voltage (except the rise and decay time) and finally because of less winding resistance it will burn out. 

Q: What will be the direction of current in primary and secondary windings/portions? 

Ans: Opposite to others. 

Q: What is tertiary winding? 

Ans: The transformers are sometimes designed with three windings, two main windings and third are the tertiary winding, which is used to make a closed mesh connection to stabilize the neutrals of both primary and secondary circuits.

Read: Viva questions for transformer part 1

Q: What do you understand about the transformer bushings? 

Ans: The bushings are provided for insulating purposes and bringing the terminals of the winding out for connections to the external circuit. 

Q: Which harmonic is dangerous and which winding is placed to carry it? 

Ans: The third harmonic and the winding is tertiary winding.


Read: EEE viva questions bangla part 1

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