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The correct choice is (a) 8 W per m^2 – °C

The best I can explain: The value of the specific HEAT dissipation for convection due to air is 8 W per m^2 – °C. The value of specific heat dissipation will VARY for DIFFERENT medium.

The CORRECT OPTION is (d) 30.8 W per °c

To elaborate: LOSS DISSIPATED by tubes by convection = 8.8 * Area of tubes

Loss = 8.8 * 3.5 = 30.8 W per °c.

Correct answer is (a) true

The explanation: An addition of 35 percent should be made to TUBE area of the TRANSFORMER. This should be done in ORDER to take into account this improvement in dissipation of loss by CONVECTION.

Right OPTION is (d) it can be improved by USING more effective HEADS of pressure

To elaborate: The circulation of oil is improved in tanks with tubes. It takes PLACE with the HELP of using more effective heads of pressure.

Right OPTION is (d) transformer SURFACE has no CHANGE with respect to dissipation of heat

Explanation: When the tanks with tubes are provided, the dissipation of heat INCREASES. The dissipation of heat has no effect on the transformer surface.

Correct option is (c) increase, increase, increase

Best EXPLANATION: As the rating of the transformer increases, the losses ALSO increase. As the losses increase the heat dissipated increases and gives HIGH temperature RISE.

Correct option is (d) the plain walled tanks can accommodate for SMALL OUTPUTS but cannot accommodate for large outputs

To EXPLAIN: The plain walled tanks are large enough to accommodate the transformer and OIL has sufficient surface to keep the temperature rise within limits for small outputs. But the plain walled tanks cannot accommodate the transformers for large outputs.

Correct OPTION is (B) 6, 6.5 W per m^2 – °C

The explanation is: The heat DISSIPATED by the plain walled tanks of the transformer by RADIATION is 6 W per m^2 – °C. The heat dissipated by the plain walled tanks of the transformer by convection is 6.5 W per m^2 – °C.

The CORRECT answer is (B) convection

The explanation is: When heat flows from the outer surface of TRANSFORMER part to the oil which COOLS it, it is convection. In transformers all 3 types of heat dissipation occurs.

The correct choice is (B) CONVECTION

Explanation: When HEAT flows from oil to walls of the cooler, the heat dissipation type is convection. In TRANSFORMERS all 3 types of heat dissipation TAKES place.

Correct ANSWER is (d) the LOSSES are converted to thermal energy

The BEST I can EXPLAIN: In both the transformer and the rotating machines the losses are converted to thermal energy. This thermal energy causes heating of the transformer parts.

Correct choice is (b) upto 33 kV

The best explanation: Helical windings have a VOLTAGE of upto 15 kV. WHEREAS, the cylindrical WINDING with rectangular conductors have a voltage of upto 6 kV.

Correct answer is (a) true

To explain: There are PROBLEMS of temperature rise and cooling of transformers which DECREASES the efficiency of the transformers. The same problems are ALSO seen in the rotating machinery.

The correct choice is (a) 5000-10000 kVA

Easy explanation: 5000-8000 kVA is USED for RECTANGULAR CONDUCTORS with CYLINDRICAL winding. 160-10000 kVA is used for helical winding. 200-10000 kVA is used for continuous disc TYPE of winding.

Right option is (c) 3

For explanation I would say: There are 3 high VOLTAGE windings present. They are i) Cylindrical WINDING, ii) Cross-over winding iii) CONTINUOUS DISC type winding.

The correct ANSWER is (c) 2.2-3.2 A per mm^2

The explanation is: For large transformers with self oil COOLED type, the highest permissible VALUE of CURRENT density is 3.2. The MINIMUM current density value required is 2.2.

Correct choice is (d) 1.1-2.3 A PER mm^2

Easy explanation: In small and medium power transformers, the LOWEST value of current DENSITY is 1.1. The HIGHEST permissible value is 2.3 for small and medium power transformers.

The correct choice is (a) current DENSITY is directly proportional to the temperature

The EXPLANATION: As the current density increases, the temperature also increases. As the temperature increases, it can CAUSE DAMAGE to the insulation.

Right ANSWER is (d) local heating and efficiency

To ELABORATE: The PERMISSIBLE CURRENT density depends upon the local heating as the heating should not affect the output. It also depends on the efficiency of the transformer.

The CORRECT answer is (d) current DENSITY

For explanation: The area of the conductors is directly dependent on the current density. The area of the conductors are determined after CHOOSING a SUITABLE current density.

The CORRECT choice is (d) current in primary WINDING = KVA per phase * 10^3 / primary voltage

Easiest explanation: For OBTAINING the current in primary winding, the kVA output per phase is obtained. Then the primary voltage is calculated, and the ratio of both gives the current in primary WINDINGS.

Right answer is (b) NUMBER of turns of primary winding = Voltage of primary WINDINGS/voltage per turn

To elaborate: For calculating the number of turns of primary windings first we CALCULATE the voltage across the primary windings. Then the voltage per turn is CALCULATED and the ratio GIVES the number of turns.

The correct CHOICE is (c) 0.6

The BEST I can explain: 0.45 is the net CORE AREA for the square core transformers. 0.56 is the core area for cruciform or STEPPED core transformers.

The correct choice is (B) 0.79

To explain I WOULD say: 0.71 is the RATIO of NET core AREA to the area of circumscribing circle in stepped cores. The gross core area for stepped cores is 0.618 * d^2.

The correct answer is (a) true

Explanation: The laminations are manufactured in the standard size of width, 0.75m to 1 m. This is used to AVOID excessively wide ASSORTMENT of laminations and to minimize wastage of STEEL during punching of laminations.

The CORRECT answer is (a) 0.58

For explanation: 0.64 is the RATIO of the gross core area to the area of the circumscribing circle. Net core area is the PRODUCT of stacking factor and gross iron area.

The correct answer is (B) false

The explanation is: Circular coils are preferred because of their high mechanical STRENGTH. Their high mechanical strength allows them to be USED in high VOLTAGE distribution and power transformer.

Correct CHOICE is (c) when CIRCULAR coils are REQUIRED for high voltage distribution

Best explanation: Circular coils are required for high voltage distribution and power TRANSFORMER. When circular coils are required square and STEPPED cores are used.

The correct ANSWER is (c) 1.4-2

The explanation: In RECTANGULAR CORE, the ratio of the depth to core should be minimum 1.4. The MAXIMUM value of ratio of depth to core is 2.

Right answer is (a) rectangular

The best I can explain: Shell type transformers PREFER only rectangular core section. Shell type transformer are MODERATE and LOW VOLTAGE transformer which use only rectangular core section.

Right answer is (b) 3

Best explanation: There are BASICALLY 3 types of core section available for core TYPE of transformer. They are RECTANGULAR, square or stepped type of core sections.

The correct option is (b) core area increases and the volt/turns increases

The BEST I can explain: When the UTILIZATION factor increases the core area gets increased. This leads to the INCREASE in the volt/turn for any particular core diameter and specified FLUX density.

The correct OPTION is (c) 6, 15

For explanation: The optimum NUMBER of STEPS used for the large transformers is maximum of 15. The optimum number of steps for the SMALL transformers is maximum of 6.

The correct ANSWER is (a) UTILIZATION factor is directly proportional to the number of core steps

Easiest explanation: The utilization factor increases with the INCREASE in the number of core steps used. This eventually increases the MANUFACTURING COST.

Right choice is (d) utilization factor= cross sectional area / gross area of the CORE

The explanation: The utilization factor is EQUAL to the RATIO of the cross sectional area to the gross area of the core. The cross sectional area and the gross area of the core are first found out, and the ratio GIVES utilization factor.