111 + Interview Questions in Design Of Transformers in Design Of Electrical Machines Page 1 InterviewSolution

1.	The axial length of 16 coils = axial length of each coil * 16.(a) true(b) falseThe question was posed to me at a job interview.I would like to ask this question from Overall Dimensions topic in portion Design of Transformers of Design of Electrical Machines
Answer» Correct answer is (a) true The EXPLANATION is: The axial LENGTH of each coil is calculated INITIALLY from its corresponding formula. Then the VALUE is multiplied by the number of COILS present.

Discussion

2.	What is the formula for number of turns/coil axially?(a) number of turns/coil axially = axial length / diameter of the insulated conductor(b) number of turns/coil axially = axial length * diameter of the insulated conductor(c) number of turns/coil axially = axial length – diameter of the insulated conductor(d) number of turns/coil axially = axial length + diameter of the insulated conductorThe question was posed to me during an interview.My doubt stems from Overall Dimensions in division Design of Transformers of Design of Electrical Machines
Answer» Right choice is (a) number of turns/coil axially = axial length / DIAMETER of the insulated conductor To elaborate: FIRST the axial length is CALCULATED from its respective formula. Then the diameter of the insulated conductor is calculated, and the RATIO gives the number of turns/coil axially.

Discussion

3.	What is the relation of the height of the window with the winding height with respect to the rectangular conductors?(a) winding height = 60% * window height(b) winding height = 50% * window height(c) winding height = 80% * window height(d) winding height = 70% * window heightI have been asked this question in exam.My question is taken from Overall Dimensions topic in division Design of Transformers of Design of Electrical Machines
Answer» Correct ANSWER is (d) WINDING height = 70% * window height For explanation: In CASE of selection of the rectangular conductors, first the window height is OBTAINED. Next the 70% of the window height PROVIDES the winding height.

Discussion

4.	What is the range for the current density at HT side for a distribution transformer?(a) 2.4-3.5 Amp per mm^2(b) 2-2.5 Amp per mm^2(c) 1-3.5 Amp per mm^2(d) 2-3.5 Amp per mm^2I got this question by my school principal while I was bunking the class.My question is based upon Overall Dimensions topic in section Design of Transformers of Design of Electrical Machines
Answer»

Discussion

5.	What is the assumption for width of the largest stamping for the stepped core transformer?(a) 0.9d(b) 0.71d(c) 0.85d(d) 0.8dThe question was asked in an internship interview.My question is taken from Overall Dimensions in portion Design of Transformers of Design of Electrical Machines
Answer» The CORRECT CHOICE is (a) 0.9d The explanation: If the width of the largest stamping is not PROVIDED, then for stepped core a = 0.9d. For the cruciform it is a = 0.85d and for the square core it is a = 0.71d.

Discussion

6.	What is the formula for the inside diameter of the low voltage windings?(a) inside diameter = diameter of the circumscribing circle + pressboard thickness insulation between l.v winding and core(b) inside diameter = diameter of the circumscribing circle – pressboard thickness insulation between l.v winding and core(c) inside diameter = diameter of the circumscribing circle + 2pressboard thickness insulation between l.v winding and core(d) inside diameter = diameter of the circumscribing circle – 2 pressboard thickness insulation between l.v winding and coreI have been asked this question in a national level competition.I'm obligated to ask this question of Overall Dimensions in division Design of Transformers of Design of Electrical Machines
Answer» The correct answer is (C) inside diameter = diameter of the circumscribing circle + 2*pressboard THICKNESS insulation between l.v winding and core The best explanation: For calculating the inner diameter, FIRST the diameter of the circumscribing circle is obtained using the CORRESPONDING formula. Then the pressboard thickness insulation is calculated.

Discussion

7.	What is the formula for the window clearance of the transformer?(a) window clearance = (height of the window + axial depth)/2(b) window clearance = (height of the window – axial depth)(c) window clearance = (height of the window – axial depth)/2(d) window clearance = (height of the window + axial depth)I had been asked this question in homework.The doubt is from Overall Dimensions topic in portion Design of Transformers of Design of Electrical Machines
Answer» Correct ANSWER is (c) window CLEARANCE = (height of the window – axial DEPTH)/2 The best explanation: FIRST the height of the window is obtained. Then the axial depth is calculated using the formula axial depth = NUMBER of secondary turns * width of the conductor and substituting in the above formula provides the window clearance.

Discussion

8.	What is the formula to calculate the radial depth of low voltage windings?(a) radial depth of the lv windings = number of layers * radial depth of the conductors * insulation between layers(b) radial depth of the lv windings = number of layers * radial depth of the conductors – insulation between layers(c) radial depth of the lv windings = number of layers / radial depth of the conductors + insulation between layers(d) radial depth of the lv windings = number of layers * radial depth of the conductors + insulation between layersI got this question during an interview.I would like to ask this question from Overall Dimensions topic in section Design of Transformers of Design of Electrical Machines
Answer» CORRECT OPTION is (d) radial DEPTH of the LV windings = number of layers * radial depth of the conductors + insulation between layers To explain I would say: The number of layers is first taken note of. Then the radial depth of the conductors is calculated along with the insulation between layers. On substituting the values in the above FORMULA the radial depth of the low voltage windings is obtained.

Discussion

9.	What is the formula for axial depth of low voltage winding?(a) axial depth = number of secondary turns / width of the conductor(b) axial depth = number of secondary turns * width of the conductor(c) axial depth = number of secondary turns + width of the conductor(d) axial depth = number of secondary turns – width of the conductorThe question was posed to me during an interview for a job.Question is from Overall Dimensions topic in chapter Design of Transformers of Design of Electrical Machines
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Discussion

10.	What is the formula for the conductor dimensions in transformer?(a) conductor dimensions = conductor width * conductor thickness + 0.5 mm(b) conductor dimensions = conductor width / conductor thickness + 0.5 mm(c) conductor dimensions = conductor width + conductor thickness + 0.5 mm(d) conductor dimensions = conductor width – conductor thickness + 0.5 mmI have been asked this question by my school teacher while I was bunking the class.Question is taken from Overall Dimensions in division Design of Transformers of Design of Electrical Machines
Answer» CORRECT answer is (a) CONDUCTOR dimensions = conductor WIDTH * conductor THICKNESS + 0.5 mm The explanation: The width of the conductor is FIRST calculated. Next, the thickness of the conductor is calculated. On obtaining these data the conductor dimensions can be obtained.

Discussion

11.	What is the formula for the cross sectional area of the secondary conductor of the transformer?(a) cross sectional area = secondary current * current density(b) cross sectional area = secondary current + current density(c) cross sectional area = secondary current / current density(d) cross sectional area = secondary current – current densityThe question was posed to me during an online exam.Question is taken from Overall Dimensions topic in section Design of Transformers of Design of Electrical Machines
Answer» Right choice is (c) cross SECTIONAL area = SECONDARY current / current density For explanation: The current flowing through the secondary winding of the TRANSFORMER is calculated. Next the current density is calculated and the RATIO gives the cross sectional area of the secondary CONDUCTOR.

Discussion

12.	What is the formula to calculate the number of turns/phase?(a) number of turns = secondary voltage * voltage per turn(b) number of turns = secondary voltage / voltage per turn(c) number of turns = secondary voltage + voltage per turn(d) number of turns = secondary voltage – voltage per turnI had been asked this question in an interview for internship.Query is from Overall Dimensions in section Design of Transformers of Design of Electrical Machines
Answer» Right OPTION is (b) NUMBER of turns = secondary voltage / voltage per turn Explanation: First the voltage across the secondary winding of the TRANSFORMER is OBTAINED. Next, the voltage across each turn is obtained. On substituting we get the number of turns.

Discussion

13.	What is the formula for the height and width of the single phase shell transformer?(a) width = 2width of the window + 4width of the largest stamping, height = height of the window + 2width of the largest stamping(b) width = 2width of the window – 4width of the largest stamping, height = height of the window + 2width of the largest stamping(c) width = 2width of the window + 4width of the largest stamping, height = height of the window – 2width of the largest stamping(d) width = 2width of the window – 4width of the largest stamping, height = height of the window -2width of the largest stampingI have been asked this question in examination.Question is from Overall Dimensions topic in division Design of Transformers of Design of Electrical Machines
Answer» The correct CHOICE is (a) width = 2width of the window + 4width of the LARGEST stamping, HEIGHT = height of the window + 2*width of the largest stamping For explanation I would say: First the width of the window is obtained. Next, the height of the window is obtained. Then, the width of the largest stamping is obtained and substituted in the above formula.

Discussion

14.	The height and the width of the single phase and three phase core type transformers are equal.(a) true(b) falseThe question was asked by my school principal while I was bunking the class.I would like to ask this question from Overall Dimensions in chapter Design of Transformers of Design of Electrical Machines
Answer» The correct option is (b) false For EXPLANATION I WOULD SAY: The height of both the SINGLE phase and three phase core type transformers are equal. The WIDTH of the single phase and three phase core type are not same.

Discussion

15.	What is the formula for the depth and height of the yoke for stepped core?(a) depth = width of largest stamping, height = 2* width of largest stamping(b) depth = 2width of largest stamping, height = width of largest stamping(c) depth = width of largest stamping, height = width of largest stamping(d) depth = 2width of largest stamping, height = 2* width of largest stampingThe question was asked by my school teacher while I was bunking the class.My doubt is from Overall Dimensions in portion Design of Transformers of Design of Electrical Machines
Answer» Correct option is (C) depth = width of largest STAMPING, height = width of largest stamping To explain I would say: The depth of the yoke of STEPPED CORE is equal to the width of the largest stamping. The height of the yoke for the stepped core is also equal to the width of the largest stamping.

Discussion

16.	The range of the ratio of the height of the window to the width of the window is 2-4.(a) true(b) falseThe question was asked by my school principal while I was bunking the class.My enquiry is from Overall Dimensions topic in chapter Design of Transformers of Design of Electrical Machines
Answer» The CORRECT OPTION is (a) true Easiest explanation: The RATIO of the HEIGHT of the window to the width of the window should be adjusted such that it is above 2. The ratio of the height of the window to the width of the window should be adjusted such that it is below 4.

Discussion

17.	What is the formula for the height of the window?(a) height of window = area of window * width of the window(b) height of window = area of window + width of the window(c) height of window = area of window – width of the window(d) height of window = area of window / width of the windowI had been asked this question in homework.This is a very interesting question from Overall Dimensions topic in section Design of Transformers of Design of Electrical Machines
Answer» Right answer is (d) height of WINDOW = AREA of window / width of the window To explain I would say: First the area of the window is obtained. Next the window width is obtained. The RATIO of both gives the height of the window.

Discussion

18.	What is the formula for window area of the transformer?(a) window area = output power * 2.22 * frequency * magnetic field * window space factor * current density * area of cross section of the core 10^3(b) window area = output power / 2.22 frequency * magnetic field * window space factor * current density * area of cross section of the core 10^3(c) window area = output power / 3.33 frequency * magnetic field * window space factor * current density * area of cross section of the core 10^3(d) window area = output power 3.33 * frequency * magnetic field * window space factor * current density * area of cross section of the core*10^3The question was asked in semester exam.I need to ask this question from Overall Dimensions in division Design of Transformers of Design of Electrical Machines
Answer» The correct choice is (b) WINDOW area = output power / 2.22 * frequency * magnetic field * window SPACE factor * current density * area of cross SECTION of the CORE *10^3 To explain I would say: The window space factor, the current density and the core cross sectional area are obtained by their respective FORMULA. The frequency is 50Hz and then the magnetic field and the output power is calculated to obtain the window space factor.

Discussion

19.	What is the formula for the width of the window of the transformer?(a) width of the window = distance between core centers + diameter of the circumscribing circle(b) width of the window = distance between core centers – diameter of the circumscribing circle(c) width of the window = distance between core centers * diameter of the circumscribing circle(d) width of the window = distance between core centers / diameter of the circumscribing circleThis question was addressed to me in an interview for internship.The query is from Overall Dimensions topic in division Design of Transformers of Design of Electrical Machines
Answer» Correct CHOICE is (b) width of the window = distance between core centers – diameter of the circumscribing circle Best EXPLANATION: The diameter of the circumscribing circle is obtained from the formula, diameter of the circumscribing circle = square root of RATIO of CROSS sectional AREA of the core to the space factor. After obtaining the distance between core centers, the width of the window is obtained.

Discussion

20.	What is the formula for the diameter of the circumscribing circle of the transformer?(a) diameter of the circumscribing circle = 2square root of ratio of cross sectional area of the core to the space factor(b) diameter of the circumscribing circle = 3square root of ratio of cross sectional area of the core to the space factor(c) diameter of the circumscribing circle = square root of ratio of cross sectional area of the core to the space factor(d) diameter of the circumscribing circle = 4*square root of ratio of cross sectional area of the core to the space factorI got this question in an online quiz.The doubt is from Overall Dimensions in chapter Design of Transformers of Design of Electrical Machines
Answer» RIGHT choice is (c) diameter of the circumscribing circle = square root of ratio of CROSS sectional area of the core to the SPACE factor To elaborate: First the cross sectional area of the core is obtained by the formula cross sectional area = VOLTAGE per turn / 4.44 * frequency * magnetic FIELD. Next the space factor is obtained. Substituting in the formula provides the diameter of the circumscribing circle.

Discussion

21.	What is the formula for the net cross sectional area of the core of the transformer?(a) cross sectional area = voltage per turn * 4.44 * frequency * magnetic field(b) cross sectional area = voltage per turn / 4.44 * frequency * magnetic field(c) cross sectional area = voltage per turn * 4.44 / frequency * magnetic field(d) cross sectional area = voltage per turn * 4.44 * frequency / magnetic fieldI got this question by my college professor while I was bunking the class.My question is from Overall Dimensions in division Design of Transformers of Design of Electrical Machines
Answer» Correct option is (b) cross sectional area = voltage per TURN / 4.44 * frequency * magnetic field Best explanation: For obtaining the cross sectional area, the voltage per turn is obtained. The frequency is ALWAYS 50 Hz. Then the magnetic field is obtained and SUBSTITUTED in the above FORMULA.

Discussion

22.	What is the height of the single phase shell type transformer?(a) height = height of window + width of the largest stamping(b) height = 2height of window + width of the largest stamping(c) height = height of window + 2 width of the largest stamping(d) height = height of window – width of the largest stampingThe question was asked in unit test.My question comes from Overall Dimensions in chapter Design of Transformers of Design of Electrical Machines
Answer» Correct choice is (C) height = height of window + 2* width of the largest stamping To explain: First the height of the window is obtained. Then the width of the largest stamping is CALCULATED and SUBSTITUTING in the above FORMULA provides the height of the single phase SHELL type transformer.

Discussion

23.	What is the formula to calculate the voltage per turn of the transformer?(a) voltage per turn = space factor * square root of output power(b) voltage per turn = space factor / square root of output power(c) voltage per turn = space factor / square root of output power(d) voltage per turn = space factor * 2*square root of output powerThis question was addressed to me in an online interview.Asked question is from Overall Dimensions in portion Design of Transformers of Design of Electrical Machines
Answer» RIGHT choice is (a) voltage per turn = space FACTOR * SQUARE root of output power For explanation: The corresponding space factor is OBTAINED using the formula. Then the output power is obtained and square root of the output power is taken and SUBSTITUTED in the above formula to obtain the voltage per turn.

Discussion

24.	What is the formula for the width of the single phase shell type transformer?(a) width = 2Width of the window + width of the largest stamping(b) width = Width of the window + 4width of the largest stamping(c) width = Width of the window + width of the largest stamping(d) width = 2Width of the window + 4width of the largest stampingI have been asked this question in quiz.I need to ask this question from Overall Dimensions in chapter Design of Transformers of Design of Electrical Machines
Answer» Right option is (d) width = 2Width of the WINDOW + 4width of the LARGEST stamping Explanation: FIRST the width of the window is obtained. Next the width of the largest stamping is obtained. Substituting in the above formula PROVIDES the width of the single-phase shell type transformer.

Discussion

25.	What is the formula for the width over one limb?(a) width over one limb = outer diameter of hv winding(b) width over one limb = 2Diameter – outer diameter of hv winding(c) width over one limb = 2Diameter + outer diameter of hv winding(d) width over one limb = Diameter + outer diameter of hv windingThe question was posed to me in a national level competition.The query is from Overall Dimensions topic in portion Design of Transformers of Design of Electrical Machines
Answer» Correct answer is (a) WIDTH over one limb = outer diameter of hv winding Easiest explanation: Width over one limb = 2*Diameter + outer diameter of hv winding is the FORMULA for the width over 3 limbs. For one limb the width is EQUAL to the outer diameter of hv winding.

Discussion

26.	The formula for single phase core type and three phase core type diameter and height are same.(a) true(b) falseI got this question during an interview.My enquiry is from Overall Dimensions in chapter Design of Transformers of Design of Electrical Machines
Answer» The correct ANSWER is (a) true Easy EXPLANATION: D = diameter of circumscribing circle + Width of window is the diameter of single phase and 3 phase core TYPE TRANSFORMERS. height = height of the window + (2*height of the yoke) is the height of the single and three phase core type transformers.

Discussion

27.	What is the formula for the width over 2 limbs?(a) width = Width of largest stamping + Diameter of the transformer(b) width = Diameter + outer diameter of hv windings(c) width = Diameter – outer diameter of hv windings(d) width = outer diameter of hv windingsThis question was posed to me by my college professor while I was bunking the class.I need to ask this question from Overall Dimensions topic in section Design of Transformers of Design of Electrical Machines
Answer» CORRECT option is (b) width = Diameter + outer diameter of hv WINDINGS Best explanation: width = outer diameter of hv windings is the width over one limb.width = Width of largest stamping + Diameter of the TRANSFORMER is the FORMULA for width of the transformer.

Discussion

28.	What is the formula for width of the single phase core type transformer?(a) width = Width of largest stamping / Diameter of the transformer(b) width = Width of largest stamping + Diameter of the transformer(c) width = Width of largest stamping – Diameter of the transformer(d) width = Width of largest stamping * Diameter of the transformerThis question was posed to me in unit test.I want to ask this question from Overall Dimensions in section Design of Transformers of Design of Electrical Machines
Answer» CORRECT answer is (b) width = Width of LARGEST STAMPING + Diameter of the transformer Easiest explanation: Firstly, the width of the largest stamping is calculated. Next, the diameter of the transformer is calculated and the sum of both the values gives the width of the transformer.

Discussion

29.	What is the formula for height of the single phase core type transformer?(a) height = height of the window – height of the yoke(b) height = height of the window + height of the yoke(c) height = height of the window – (2height of the yoke)(d) height = height of the window + (2height of the yoke)I had been asked this question in quiz.The above asked question is from Overall Dimensions in chapter Design of Transformers of Design of Electrical Machines
Answer» Correct option is (d) height = height of the WINDOW + (2*height of the YOKE) Explanation: The height of the window is first obtained. NEXT, the height of the yoke is calculated and it is multiplied by 2. ADDITION of both the values gives the height of the SINGLE phase core type transformer.

Discussion

30.	What is the formula for the diameter of the single phase core type transformer?(a) D = diameter of circumscribing circle + Width of window(b) D = diameter of circumscribing circle – Width of window(c) D = diameter of circumscribing circle * Width of window(d) D = diameter of circumscribing circle / Width of windowI had been asked this question by my school teacher while I was bunking the class.This intriguing question comes from Overall Dimensions in division Design of Transformers of Design of Electrical Machines
Answer» RIGHT choice is (a) D = diameter of CIRCUMSCRIBING circle + Width of window Best explanation: First the diameter of the circumscribing circle is obtained. Next, the width of the window is CALCULATED, and the sum of both the data provides the diameter of the single PHASE CORE transformer.

Discussion

31.	What is the formula for the length of the tank?(a) length of the tank = external diameter of h.v winding + clearance on each side between the winding and tank along the width(b) length of the tank = external diameter of h.v winding * clearance on each side between the winding and tank along the width(c) length of the tank = external diameter of h.v winding + 2clearance on each side between the winding and tank along the width(d) length of the tank = external diameter of h.v winding / 2clearance on each side between the winding and tank along the widthThe question was posed to me in examination.This is a very interesting question from Methods of Cooling of Transformers in portion Design of Transformers of Design of Electrical Machines
Answer» Correct option is (c) LENGTH of the tank = EXTERNAL diameter of h.v winding + 2*CLEARANCE on each side between the winding and tank along the width The explanation: The external diameter of h.v winding is obtained. NEXT, the clearance on each side between the winding and tank along the width is calculated and is SUBSTITUTED in the above formula.

Discussion

32.	What is the formula for the height of transformer tank?(a) height of transformer tank = Height of transformer frame + clearance height between the assembled transformer and tank(b) height of transformer tank = Height of transformer frame * clearance height between the assembled transformer and tank(c) height of transformer tank = Height of transformer frame/clearance height between the assembled transformer and tank(d) height of transformer tank = Height of transformer frame – clearance height between the assembled transformer and tankThis question was posed to me in an interview for job.I'm obligated to ask this question of Methods of Cooling of Transformers topic in section Design of Transformers of Design of Electrical Machines
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Discussion

33.	What is the formula for width of the tank for single phase transformers used?(a) width of tank = 2distance between adjacent limbs + external diameter of h.v windings + 2clearance between h.v windings and tank(b) width of tank = distance between adjacent limbs + external diameter of h.v windings + 2clearance between h.v windings and tank(c) width of tank = 2distance between adjacent limbs * external diameter of h.v windings + 2clearance between h.v windings and tank(d) width of tank = distance between adjacent limbs external diameter of h.v windings + 2*clearance between h.v windings and tankThe question was asked in homework.Query is from Methods of Cooling of Transformers in division Design of Transformers of Design of Electrical Machines
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Discussion

34.	The temperature difference between the incoming and outgoing water is greater than 10°C.(a) true(b) falseI have been asked this question in a job interview.This is a very interesting question from Methods of Cooling of Transformers in portion Design of Transformers of Design of Electrical Machines
Answer» RIGHT option is (b) false The BEST explanation: The water flow rate is about 1.5 litres per MINUTE. The difference in TEMPERATURE between the incoming water and outgoing water is 10°C.

Discussion

35.	What is the range of the flow rate of circulating oil per KW of losses?(a) 6-7 liters per minute(b) 5-6 liters per minute(c) 6-8 liters per minute(d) 6-7 liters per minuteI got this question during an online interview.My doubt stems from Methods of Cooling of Transformers in chapter Design of Transformers of Design of Electrical Machines
Answer» Correct choice is (c) 6-8 LITERS PER minute Explanation: The minimum VALUE of the flow rate of circulating oil per KW of losses is derived to be 6 liters per minute. The maximum value of the flow rate of circulating oil per KW of losses is derived to be 8 liters per minute.

Discussion

36.	What is the range of the cooler surfaces per 1 KW of losses?(a) 0.1-0.25 m^2(b) 0.18-0.2 m^2(c) 0.1-0.2 m^2(d) 0.18-0.25 m^2This question was addressed to me in semester exam.Origin of the question is Methods of Cooling of Transformers topic in chapter Design of Transformers of Design of Electrical Machines
Answer» The correct choice is (d) 0.18-0.25 m^2 Explanation: The minimum VALUE of the COOLER SURFACES per 1 KW of LOSSES is 0.18 m^2. The MAXIMUM value of the cooler surfaces per 1 KW of losses is 0.25 m^2.

Discussion

37.	What is the relation of the increase of the oil circulation rate with energy losses?(a) increase of the oil circulation rate is not depending with energy losses(b) increase of the oil circulation rate is directly proportional to the energy losses(c) increase of the oil circulation rate is directly proportional to the square of energy losses(d) increase of the oil circulation rate is indirectly proportional to energy lossesI had been asked this question in semester exam.My doubt stems from Methods of Cooling of Transformers topic in portion Design of Transformers of Design of Electrical Machines
Answer» Right OPTION is (b) increase of the oil circulation rate is directly proportional to the energy losses For EXPLANATION I would SAY: The increase in the oil circulation rate is unsuitable because this increases the large energy losses In the pumping unit. To cool the oil, it is circulate through a special oil COOLER.

Discussion

38.	What is the flow rate of the circulating oil in an air cooler with natural air cooling?(a) 12.5 litre per minute per KW of losses(b) 12 litre per minute per KW of losses(c) 14 litre per minute per KW of losses(d) 13 litre per minute per KW of lossesThis question was addressed to me in an internship interview.My question is based upon Methods of Cooling of Transformers topic in division Design of Transformers of Design of Electrical Machines
Answer» Correct OPTION is (B) 12 litre PER minute per KW of losses Best explanation: When NATURAL air COOLING is used, the flow rate is 12 litres per minute per KW of losses. Even when the air blast cooling is used, the transformer output increases roughly to the same extent.

Discussion

39.	What type of cooling is being made use of in transformers having a capacity of less than 11MVA?(a) natural cooling(b) forced cooling(c) air blast cooling(d) forced cooling and air blast coolingI have been asked this question in an internship interview.My doubt stems from Methods of Cooling of Transformers in division Design of Transformers of Design of Electrical Machines
Answer» The correct answer is (a) natural COOLING The explanation is: For TRANSFORMERS having capacity less than 11MVA, natural cooling is made use of. For transformers having capacity more than 11MVA, air BLAST cooling is used.

Discussion

40.	Compared to the natural cooling, how much of heat dissipation is increased by air blast cooling?(a) 50-70%(b) 60-70%(c) 50-60%(d) 40-60%I got this question in an interview for internship.My question is from Methods of Cooling of Transformers in division Design of Transformers of Design of Electrical Machines
Answer» The correct option is (C) 50-60% EXPLANATION: Air blast cooling helps in INCREASED heat dissipation. The minimum value of increased heat dissipation is 50% and MAXIMUM value is 60%.

Discussion

41.	Increase in the velocity of oil circulation increases the transformer output.(a) true(b) falseThis question was addressed to me in unit test.Query is from Methods of Cooling of Transformers in division Design of Transformers of Design of Electrical Machines
Answer» The correct ANSWER is (a) true To EXPLAIN: The increases in velocity of the AIR CIRCULATION increases the temperature. The temperature RISE increases the transformer output.

Discussion

42.	How are the radiators cooled in the present time?(a) by natural cooling(b) by forced cooling using small fans(c) by forced cooling using large fans(d) by using external airI had been asked this question during an interview.This intriguing question originated from Methods of Cooling of Transformers topic in section Design of Transformers of Design of Electrical Machines
Answer» CORRECT option is (b) by forced cooling using small FANS Easy EXPLANATION: At present time the radiators are COOLED using forced cooling. The forced cooling takes place with the help of the small fans mounted on each RADIATOR.

Discussion

43.	How many types of cooling methods are available for the transformer?(a) 3(b) 2(c) 1(d) 4The question was asked at a job interview.Query is from Methods of Cooling of Transformers topic in division Design of Transformers of Design of Electrical Machines
Answer» Right choice is (a) 3 The best EXPLANATION: There are 3 TYPES of cooling methods available for transformers. They are natural cooling, AIR blast cooling, FORCED oil circulation.

Discussion

44.	What is the rating of the transformer for the voltage of above 11 kV upto 33 kV?(a) 1000-5000 kVA(b) less than 1000 kVA(c) above 1000 kVA(d) 100-500 kVAThe question was posed to me during an online interview.This is a very interesting question from Design of Tank in portion Design of Transformers of Design of Electrical Machines
Answer» RIGHT choice is (b) less than 1000 kVA Best EXPLANATION: 1000-5000 kVA is the RATING of the transformer for the voltage of about 11 kV. When the voltage rating is about 11-33 kV, then the rating of the transformer is less than 1000 kVA.

Discussion

45.	What is the rating of the transformer for the voltage of about 11 kV?(a) 1000-2000 kVA(b) 100-3000 kVA(c) 1000-5000 kVA(d) 100-500 kVAI had been asked this question in unit test.I'd like to ask this question from Design of Tank in division Design of Transformers of Design of Electrical Machines
Answer» Correct choice is (c) 1000-5000 kVA To EXPLAIN: The minimum VALUE of the rating of the transformer for a voltage of about 11 kV should be 1000 kVA. The MAXIMUM value of the rating of the transformer for a voltage of about 11 kV should be about 5000 kVA.

Discussion

46.	What is the formula for height of transformer tank?(a) height of transformer tank = Height of transformer frame + clearance height between the assembled transformer and tank(b) height of transformer tank = Height of transformer frame * clearance height between the assembled transformer and tank(c) height of transformer tank = Height of transformer frame/clearance height between the assembled transformer and tank(d) height of transformer tank = Height of transformer frame – clearance height between the assembled transformer and tankThis question was posed to me during an online interview.My doubt stems from Design of Tank topic in division Design of Transformers of Design of Electrical Machines
Answer» Correct CHOICE is (a) height of TRANSFORMER tank = Height of transformer frame + CLEARANCE height between the assembled transformer and tank To explain I would say: Firstly, the height of the transformer frame is CALCULATED. Next, the clearance height between the assembled transformer and tank is also calculated. SUBSTITUTE the values to obtain the height of transformer tank.

Discussion

47.	Elliptical tubes with pressed radiators are increasingly been used.(a) true(b) falseI had been asked this question in a job interview.I would like to ask this question from Design of Tank in section Design of Transformers of Design of Electrical Machines
Answer» The correct option is (a) true Explanation: Elliptical tubes with pressed radiators are on high demand now a days. This is because they GIVE a GREATER DISSIPATING surface for the small VOLUME of oil.

Discussion

48.	What is the range of the diameter of the tubes used?(a) 50-60 mm(b) 60-70 mm(c) 70-80 mm(d) 50-70 mmI have been asked this question during an interview for a job.My question is from Design of Tank topic in chapter Design of Transformers of Design of Electrical Machines
Answer» Correct OPTION is (d) 50-70 MM To explain: The MINIMUM value of the DIAMETER of tubes is derived to be around 50 mm. The maximum value of the diameter of tubes should be less than 70 mm.

Discussion

49.	How do the walls of the transformer tank dissipate heat?(a) by radiation(b) by convection(c) by conduction(d) by convection and radiationThis question was posed to me during an online interview.I would like to ask this question from Temperature Rise in Transformers topic in portion Design of Transformers of Design of Electrical Machines
Answer» CORRECT option is (d) by convection and radiation To EXPLAIN: The plain walled tanks of the transformer also DISSIPATE heat through convection and radiation. The property is SIMILAR to that of the rotating MACHINERY.

Discussion

50.	What is the formula for number of tubes?(a) number of tubes = (1/ 8 * area of each tube) * (total loss / temperature rise with tubes – 12.5 * dissipating surface)(b) number of tubes = (1* 8 * area of each tube) * (total loss / temperature rise with tubes – 12.5 * dissipating surface)(c) number of tubes = (1/ 8 * area of each tube) / (total loss / temperature rise with tubes – 12.5 * dissipating surface)(d) number of tubes = (1/ 8 * area of each tube) + (total loss / temperature rise with tubes – 12.5 * dissipating surface)The question was posed to me in an international level competition.The question is from Design of Tank in portion Design of Transformers of Design of Electrical Machines
Answer» The CORRECT ANSWER is (a) number of TUBES = (1/ 8 * area of each tube) * (total loss / temperature rise with tubes – 12.5 * dissipating surface) Easiest explanation: First the temperature rise with tubes is obtained. Then the iron loss and copper loss are obtained and added. Area of each tube is also obtained. Substituting all the values in the above formula provides the number of tubes.

Discussion

Explore topic-wise InterviewSolutions in .

The axial length of 16 coils = axial length of each coil * 16.(a) true(b) falseThe question was posed to me at a job interview.I would like to ask this question from Overall Dimensions topic in portion Design of Transformers of Design of Electrical Machines

What is the assumption for width of the largest stamping for the stepped core transformer?(a) 0.9*d(b) 0.71*d(c) 0.85*d(d) 0.8*dThe question was asked in an internship interview.My question is taken from Overall Dimensions in portion Design of Transformers of Design of Electrical Machines

The range of the ratio of the height of the window to the width of the window is 2-4.(a) true(b) falseThe question was asked by my school principal while I was bunking the class.My enquiry is from Overall Dimensions topic in chapter Design of Transformers of Design of Electrical Machines

The formula for single phase core type and three phase core type diameter and height are same.(a) true(b) falseI got this question during an interview.My enquiry is from Overall Dimensions in chapter Design of Transformers of Design of Electrical Machines

The temperature difference between the incoming and outgoing water is greater than 10°C.(a) true(b) falseI have been asked this question in a job interview.This is a very interesting question from Methods of Cooling of Transformers in portion Design of Transformers of Design of Electrical Machines

Increase in the velocity of oil circulation increases the transformer output.(a) true(b) falseThis question was addressed to me in unit test.Query is from Methods of Cooling of Transformers in division Design of Transformers of Design of Electrical Machines

How many types of cooling methods are available for the transformer?(a) 3(b) 2(c) 1(d) 4The question was asked at a job interview.Query is from Methods of Cooling of Transformers topic in division Design of Transformers of Design of Electrical Machines

What is the rating of the transformer for the voltage of about 11 kV?(a) 1000-2000 kVA(b) 100-3000 kVA(c) 1000-5000 kVA(d) 100-500 kVAI had been asked this question in unit test.I'd like to ask this question from Design of Tank in division Design of Transformers of Design of Electrical Machines

Elliptical tubes with pressed radiators are increasingly been used.(a) true(b) falseI had been asked this question in a job interview.I would like to ask this question from Design of Tank in section Design of Transformers of Design of Electrical Machines

What is the range of the diameter of the tubes used?(a) 50-60 mm(b) 60-70 mm(c) 70-80 mm(d) 50-70 mmI have been asked this question during an interview for a job.My question is from Design of Tank topic in chapter Design of Transformers of Design of Electrical Machines

What is the assumption for width of the largest stamping for the stepped core transformer?(a) 0.9d(b) 0.71d(c) 0.85d(d) 0.8dThe question was asked in an internship interview.My question is taken from Overall Dimensions in portion Design of Transformers of Design of Electrical Machines