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Viva questions of transformer part 2

Viva questions of transformer part 2

 

Q: What is the angular displacement between the i⍵ and iμ currents?

Ans: 90°

 Q: if the I⍵ and Iμ are known, can you tell the formula for calculating the no-load current? 

Ans: I_o^2=I_\omega^2+I_\mu^2

Q: If Bm, I, and Np are given can you give the formula for finding out the magnetizing current?

Ans: I_\mu=\frac{B_m.I}{\mu_o.\mu_r.N_p}amp

Q: What is the utility of the magnetizing current? Can you imagine the transformer without the magnetizing source? 

Ans: The magnetizing current is used to produce the magnetic flux in the cores. The flux is the base of the magnetic induction and without induction, the secondary cannot induce any voltage, so without the magnetizing source, the transformer has meaning. 

Read: Viva questions of transformer part 1

Q: When do we call a transformer of load?

Ans: When the secondary winding is connected to any load, the transformer is said to be on load. 

Q: Will the current in the primary winding and secondary winding be the same?

Ans: No, the primary winding current is the sum of the no-load current and secondary transferred current on the primary side. 

Q: What about the power factor on the primary and secondary sides of a transformer? 

Ans: It is not the same but generally taken as the same. 

Q: Which side power factor is more? 

Ans: Generally primary side, but for all the practical purposes the power factor on primary and secondary winding is taken as same. 

Read: Viva questions of transformer part 1

Q: Which type of load is put on the transformer? 

Ans: All types, i.e. resistive, capacitive, and inductive, and mixed too. 

Q: What do you mean by the resistive drop and reactance drop? 

Ans: The drops in the resistance of the winding are known as resistive drop, i.e. I.R volts and if the drop is taken in reactance of the winding, I.X volts is known as the reactive drop. 

Q: Generally which drop resistive or reactive is more in value? 

Ans: Reactive voltage drop. 

Q: What is the voltage regulation? 

Ans: The voltage regulation is defined as the rise in voltage when the full load is thrown off shown in the percentage of no-load voltage. i.e.

\%Regulation=\frac{V_o-V_L}{V_o}\times100

Read: Viva questions of transformer part 1

Q: What are the methods for finding out the voltage regulation? 

Ans: 

  1. By actual loading,

     2.By calculations. 

 Q: If V_2,R_2,X_{2\;},f_2 are given how the voltage regulation is calculated?

Ans: \%Regulation=\frac{I_1R_1\cos\left(\varphi\right)+I_2X_2\sin\left(\varphi\right)}{V_2}\times100

Q: What do you understand by tap changing? 

Ans: Whenever the load is put on the transformer some voltage drops are inherent. For that reason, the transformation ratio is changed, that changing from one tap to another is termed tap changing.

Q: Generally which tap changing is preferred step up or step down? 

Ans: Both as required. 

Q: What are the classifications of tap changing? 

Ans: 

  1. On load tap changing.
  2. No-load changing

Read: Viva questions of transformer part 1

Q: What are the no load taps changing? 

Ans: If the tap changing is done, when there is no load on the secondary of the transformer it is called no-load tap changing. And if there is a load on the secondary and tap changing is down, then it is known as on-load tap changing. 

Q: What are the methods of no-load tap changing? 

Ans: Manual and Automatic. 

Q: What types of regulators are used for no load changing? 

Ans: Sliding contact type and fact plate type. In these cases, tapping is brought out. 

Q: Given some idea about the sliding contact type and faceplate type regulators

Ans: Sliding contact type: In this case, a number of tapping are brought out and contacts are slided manually.

Face plate type: In this case, the terminals are brought out and a D.C. motor is energized with the movement of the arm, the contacts are formed and the transformation ratio is changing. It is changed. It is generally having a sector or quadrant type shape.

Read: Viva questions of transformer part 1

Q: What are the methods of on load changing? 

Ans: It is done by the following methods:

  1. Parallel winding.
  2. Preventive reactor. 
  3. Regulation Transformer.
  4. Tertiary winding. 

Q: Why in case preventive regulator and parallel winding regulators the tapping are taken from the centre of the winding? 

Ans: To avoid any breaking or cause of any discontinuity in the combined circuit. In reactance two contacts of the reactor are joined to the same contact of the tapping, so that the current divide in the same quantity and that too in opposite direction to cancel out the reactance voltage. Thus smooth and undisturbed change is obtained. 

Q: What types of losses are there in a transformer? 

Ans: Copper losses, Iron losses, and stray losses.

Q: What are copper losses in a transformer? 

Ans: These are the losses in the primary and secondary winding because of the resistive component. These are I2R watts. These are variable losses. 

Read: Viva questions of transformer part 1

Q: Why do we call the iron losses as constant losses? 

Ans: These losses are constant from no load to full load. These losses are because of the magnetic flux, which remains constant through the working of the transformer. 

Q: What is efficiency?

 Ans: The efficiency is the ratio of output to input shown in percentage. 

Q: What is the all-day efficiency? 

Ans: It is also the ratio of output to input in a fixed time shown in percentage but the duration is taken 24 hrs. So,

\%All\;day\;efficiency=\frac{kWh\;output\;in\;24\;hrs}{kWh\;input\;in\;24\;hrs}\times100

Read: Viva questions of transformer part 1

Q: How core losses are found out? 

Ans: By means of an open-circuit test. 

Q: In the case of the open-circuit test, which losses are shown by the reading of the wattmeter?

 Ans: Iron losses. 

Q: In the open-circuit test, how the secondary winding is connected? 

And: It is left open. 

Q: Generally while side L.T. or H. T. is left open? 

Ans: H.T. 

Q: Why H.T. is opened? 

Ans: It is because metering on H.T. winding is somewhat difficult. 

Q: Why the reading shown by wattmeter is core losses? 

Ans: Because the current flowing in the winding is very small in comparison to the full load current, but the cores of the transformer are fully energized i.e. having the normal working flux. 

Q: Why short circuit test is carried out? 

Ans: To find out the copper losses. 

Read: Viva questions of transformer part 1

Q: Which side is short-circuited? 

Ans: L.T. side. 

Q: Can you short circuit it through an ammeter? 

Ans: Yes, there is no problem but the ammeter should be suitably selected. 

Q: How the primary winding of both transformers are connected? 

Ans: In parallel across the supply. 

Q: How that injecting transformer circuits the current? 

Ans: As both secondaries are connected in parallel so there is no resultant voltage working across, with the help of the injecting transformer, we are injecting some voltage in the circuit which, depending upon their impedance, circuits the current in the secondary and the transformer is loaded.

Q: Is there any instrument connected in the secondary winding? 

Ans: Yes, the wattmeter and the ammeter can also be connected. 

Read: Viva questions of transformer part 1

Q: What this Wattmeter shows? 

Ans: The copper losses. 

Q: While injecting the voltage, a circulating starts flowing in the secondary’s, it will also cause some current in the primary circuit, it is going to affect the line current or wattmeter which is connected in primary circuit? 

Ans: No, the circulating current will only flow through the winding and their leads, but will not contribute anything to the wattmeters.

 Q: Why an impulse test is performed? 

Ans: This test is perfume to determine the ability of the transformer to withstand the effect of high unidirectional voltages resembling the sky lightning or surge voltage. 

Q: What is the insulation test?

Ans: This test is carried out to find out the insulation resistance between the winding, between cores and copper.

Q: How the core test is performed and how much voltage and frequency is kept for testing? 

Ans: In this test, the H.J. winding is left open and L.T. winding is connected to the normal voltage and frequency through ammeter and wattmeter.

Read: Viva questions of transformer part 1

Q: What do you mean by the parallel operation of the transformer? 

Ans: When the increasing demand of the load is not suitably fed by one transformer, another transformer is brought into operation to share the load. This is called the parallel operation 

Q: How are these connected? 

Ans: In parallel. 

Q: What are the conditions to be fulfilled before parallel operation of the single-phase transformers? 

Ans: 

  1. The voltage transformation ratio must be the same.
  2. The percentage impedance drop should be the same.
  3. The polarity should be the same. 

Read: Viva questions of transformer part 1

Q: What are the conditions to be fulfilled for a 3 phase transformer for parallel operation?

 Ans: 

  1. The voltage transformation ratio should be the same
  2. The phase sequence should be the same. 
  3. Phase rotation must be the same.
  4. Percentage impedance drop must be the same. 

Q: Can you tell the necessity of parallel operation? 

Ans: 

  1. To meet the increasing demand of the load.
  2. To continue the supply in case of any fault and regular maintenance also. 
  3. For period overhauling of the transformer.
  4. For maximum operational efficiency. 

Q: What do you know about the transmission transformer? 

Ans: It is a transformer which changes the voltage from low voltage to high voltage in generating stations for onward transmission. These are high-capacity transformers.

Read: Viva questions of transformer part 1

Q: What do you know about the distribution transformer? 

Ans: The transformer which is used to distribute the electrical power to the consumers is known as a distribution transformer. The primary of the transformer is connected in delta and secondary in star to facilitate three phases four wires distribution. These are designed to work for 24 hours. 

Q: What do you understand from the power transformer? 

Ans: The transformers which are installed in big industries and are used for any specific ans particular purpose. These are designed for maximum efficiency and less duration. These are energized for a short period only for the need, i.e. operation. 

Q: How is the current shared in parallel operation?

 Ans: According to their capacity and characteristics of the transformer. 

Q: What are the types of the transformer according to the number of phases? 

Ans: Single phase, two-phase, three-phase, six-phase, etc. 

Q: How are the transformers classified according to the core construction? 

Ans: Core type, shell type, and distributed core type. 

Read: Viva questions of transformer part 1

Q: How many limbs are there in a core type stamping used for a three-phase transformer? 

Ans: Three of equal length and area. 

Q: How many limbs are there in a stamping used for a three-phase shell type transformer?

Ans: Five. 

Q: What types of connections are there for three-phase transformers? 

Ans: Star/star, delta/delta, star/delta, delta/star, open delta and scott connections are also possible.

 

Read: Comprehensive viva questions for eee Electrical Measuring Instruments part-1

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