Saturday , October 23 2021
Recent Post

# Transformers viva questions part 1

### Transformers viva questions part 1

Q: What is a transformer?

Ans: The transformer is a static device that is used to transfers the electrical power from one volt to another volt, without changing the frequency

Q: What do you understand by step up and step down transformers?

Ans: The transformer which changes high voltage into low voltage is known as step down transformer and the transformer which changes low voltage into high voltage is known as a step-up transformer.

Q: Why there is no change in the frequency of transformation of the voltage in the transformer?

Ans: The flux which is the base of inducing a voltage in the transformer is the same for both the winding of the transformer or says the flux in the cores has got definite frequency and it will be the same on the primary and secondary sides hence the question of changing the frequency does not arise.

Q: What is the working principle of a transformer?

Ans: The transformer works on the principle of “Faraday’s laws of electromagnetic induction.” According to that, whenever a conductor, this e.m.f. is also proportional to the rate of change of flux and the number of conductors.

Q: Which type of induction takes place in the two winding transformers?

Ans: Mutual induction.

Q: What are the different types of windings?

Ans: Low tension (L.T.) and high tension (H.T.)

Q: Name the winding which is connected across the supply?

Ans: Primary winding.

Q: Where the load is connected?

Ans: On the secondary winding.

Q: If the winding has more number of turns the winding is called H.T. or L.T.?

Ans: H.T.

Q: Is there any relationship between the flux linkage and e.m.f. induced?

Ans: Yes, e.m.f. induced is proportional to flux linkage.

Q: What are the different parts of the transformer?

Ans: Cores, winding, oil, tank, conservator and breather, and cooling arrangement

Q: What types of cores are used?

Ans: Core type, shell type, and berry type.

Q: What is the function of the cores?

Ans: The main factor is to offer an easy path for the lines of force and to reduce the eddy current losses etc.

Q: How many magnetic paths are there in core-type cores?

Ans: One path.

Q: Name the cores which have two magnetic paths?

Ans: Shell type.

Q: If The distributed magnetic circuit is required which cores are used?

Ans: Berry type construction.

Q: Which material is used for cores?

Ans: Laminations of silicon steel

Q: Why a laminated solid block is used instead of a solid homogeneous block?

Ans: To reduce eddy current losses.

Q: What are the types of winding according to the construction?

Ans: Sandwich type and cylindrical type.

Q: According to the voltage, the winding are named as?

Ans: H.T. and LT winding.

Q: What are the cylindrical types winding?

Ans: In the type of coil, the length of coil is equal to the length of the core limb. The primary and secondary winding may be placed one over the other.

Q: What is the sandwich type winding?

Ans: If the high tension and low tension windings are placed one over the other like the sandwiches limb-length-wish, this type is known as sandwich type winding.

Q: While selecting the method of placement of the winding, which winding is placed near the core, in sandwich type winding?

Ans: L.T. and then H.T.

Q: Why L.T. winding is placed near the cores?

Ans: To reduce the dielectric strength of the dielectric or insulation provided. In this case the total dielectric strength of the insulation will be less than if H.T. would be near the cores.

Q: Is there any insulation between the laminations?

Ans: Yes, insulating varnish paper.

Q: What do you mean by the limb?

Ans: It is the vertical portion where the winding is placed.

Q: How the central limb and side limbs of the E shape are designed:

Ans: Central limb has double the width than the side limbs.

Q: Why double width?

Ans: As the flux has two magnetic paths and total flux will pass through the central limb.

Q: What is the transformer oil?

Ans: It is a mineral oil. It is filled in the tank of the transformer.

Q: What are the functions of the transformer oil?

Ans: It is use for heat dissipation, insulation and spark quenching medium also.

Q: In which units the dielectric strength of the transformer oil is measured?

Ans: kV/mm.

Q: What are the factors that affect the dielectric strength of the transformer oil?

Ans: The moisture, dust, and temperature.

Q: Will the increase of decrease the dielectric strength?

Ans: Decrease.

Q: How the moisture contained is removed or dehydration is performed?

Ans: It is cleaned and passed through the dehydration plant. The moisture is removed and dehydration increases the dielectric strength of the transformer oil.

Q: How the tank of a transformer is made?

Ans: The tank of a small transformer is made of the iron sheet welded and for a big transformer, it is made from plain boiler plates.

Q: What is a conservator?

Ans: conservator’s small tank mounted on the top of the transformer tank. The transformer oil is filled in it There is a provision to check the color and level of the oil. The conservators are required to bear the expansion and contraction of the oil. It is generally of cylindrical construction

Q: What is the breather?

Ans: It is pipe attached with the conservator. It contains the pot at its extreme and which has CaCl2, or silica get to extract the moisture from the atmospheric air.

Q: Why it is necessary to have CaCl2 or silica get in the breather?

Ans: While the temperature of the oil increases, the oil expands causing more volume to be occupied, thus releasing the gas or air to the external atmosphere through the breather. Now when temperature decreases, the volume decreases causing an empty space, which is refilled by the air from the atmosphere, sucked via a breather. The air may have the moisture which will affect the dielectric strength of the oil, so to extract moisture CaCl2 or silica gel is placed.

Q: Is there any arrangement provided to check the temperature of the oil?

Ans: Yes, the temperature gauge.

Q: Can you fill the conservator completely with oil and if not why?

Ans: It should never be completely filled because with the increase in temperature, the oil expands which may cause serious trouble for example tank may burst out,

Q: What is the flash point of the transformer oil?

Ans: The temperature at which the vapors above an oil surface ignite is known as flash point. For safety reasons, the oil not less than 160°C flashpoints is used in the tank.

Q: What do you understand by sledging in the transformer oil?

Ans: It is an important characteristic of the oil, which has to be watched very carefully. Sledging means the slow formation of semisolid hydrocarbons on the windings and tank walls.

Q: Why are corrugated sheets provided?

Ans: To increase the heat dissipating area of the tank.

Q: Where actually the heat is developed, in the cores or winding?

Ans: In both, in the winding because of the current following, in the cores because of eddy currents.

Q: What are the different methods of cooling?

Ans:

1. Natural air cooling,
2. Oil immersed natural air cooling.
3. Oil immersed forced water cooling,
4. Oil immersed forced air cooling,
5. Air blast cooling.

Q: What is meant by the air blast cooling?

Ans: In this case the stream of the forced air is forced to the winding and cores by means of the blowers or high speed fans. The air passes from bottom to top.

Q: Why forced water cooling is not preferred?

Ans: Because, if by chance, the tube leaks, water will come out in the oil and will decrease its dielectric strength,

Q: Up-to 500kVA. Which type of cooling is better?

Ans: Oil immersed natural air cooling.

Q: What are the different methods of increasing the dissipating area for better cooling?

Ans: By providing:

1. The corrugated sheets
2. Fins welded vertically to the sides of tank.
3. Elliptical or round tubes.

Q: What is the Bocholt’s relay?

Ans: It is the protective relay which is meant for the protection of oil immersed transformed from insulation failure, cores excess heating or any other internal faults which cause heating of the oil beyond the limit and maintain transformer oil level etc.

Q: Is there any possibility of overflowing the transformer oil?

Ans: No, A relay is also provided to alarm about the level.

Q: Is there any protection from the gases or fumes in the tank?

Ans: yes, Bocholz’s relay.

Q: What is the e.m.f equation of the transformer?

Ans: E = 4.44 Φm. f. N volts.

Q: Tell the e.m.f. equation in terms of flux density?

Ans: E= 4.44 Bm. A. f. N volts.

Q: how the number of turns of primary winding is calculated if the voltage, frequency and flux are given?

Ans: N_p=\frac E{4.44\;\varphi_m.f}

Q: What is the voltage transformation ratio?

Ans: The voltage transformation ratio is the ratio of secondary voltage to primary voltage.i.e. \frac{V_s}{V_p}

Q: What are the current and turn transformation ratios?

\frac{N_s}{N_p} is the turn transformation ratio,

\frac{I_s}{I_p} current transformation ratio,

Q: For an ideal transformer, what is the condition?

Ans: V_p.I_p=V_s.I_s

Q: Is there any relationship between the voltage, turns and current transformation ratios?

Ans: Yes, \frac{V_s}{V_p}=\frac{N_s}{N_p}=\frac{I_p}{I_s}=K

Read: Transformers viva questions part 2

Q: What do you mean by the no load current of the transformer?

Ans: When the primary winding is connected across the supply and secondary winding is open-circuited, then the transformer is said to be on no load and current taken is called the no load current of the transformer.

Q: What are the no load current’s component?

Ans: The wattful componenti.e. I_o\cos\left(\varphi_o\right) and the wattles or magnetizing component i.e. I_o\sin\left(\varphi_o\right)

Q: What is the wattful component of no load current?

Ans: The current I_o\cos\left(\varphi_o\right) is used to overcome the hysteresis and eddy current losses, and is known as wattful component.

Q: What is a wattles or magnetizing component of current?

Ans: It is the current which is used to produce the magnetic flux in the cores and is known as the magnetizing component of the no load current, i.e. I_\mu=I_o\sin\left(\varphi_o\right)