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Right OPTION is (C) depth of armature core = flux / 2 * LENGTH of the iron core * flux density

Easiest explanation: The flux VALUE is calculated along with the length of the iron core. NEXT, the suitable flux density is chosen and the depth of armature core is calculated.

Right option is (c) outer diameter = INNER diameter + 2*(depth of the slots + depth of armature core)

The best I can EXPLAIN: The depth of the armature core is calculated and the depth of the slots is also calculated. The inner diameter is calculated and SUBSTITUTING the outer diameter is OBTAINED.

Correct option is (a) true

The explanation is: The value of depth of CORE can be CALCULATED by assuming a suitable value of FLUX density. The value of the flux density VARIES from 1-1.2 Wb PER m^2.

Correct choice is (a) LENGTH of mean TURN = 2*length + 2.5*pole pitch + 0.06 KV + 0.2

Explanation: The length of the slots is obtained along with the pole pitch. The OUTPUT kV is calculated and on substituting we get the length of mean turn.

The CORRECT CHOICE is (b) three times

The explanation: The DEPTH of the slot depends upon the width of the slot. The depth should not exceed three times the width of the slot.

The correct CHOICE is (b) to reduce the SHORT circuit current

To elaborate: The slots USED in the machine are basically deeper slots. The slots are made deeper even more to increase the LEAKAGE REACTANCE and to limit the short circuit current.

Right choice is (d) maximum permissible width = slot pitch – minimum width of the TEETH

For EXPLANATION: The slot pitch is first calculated with its RESPECTIVE formula. NEXT, the minimum width of the teeth is calculated and the difference between both gives the maximum permissible width.

The correct ANSWER is (a) parallel SIDED

Best EXPLANATION: The most commonly USED type of slots are parallel sided. The other type of slots may be used for the purposes REQUIRED.

Correct choice is (a) TEETH is tapered and minimum WIDTH is ACROSS the medium

For explanation: Parallel SIDED SLOTS are made use of. Hence, the teeth is tapered and their minimum width occurs at the air gap surface.

Correct option is (a) 1.7-1.8 WB per mm^2

For explanation I would SAY: The minimum value of the flux density in the teeth is given to be 1.7 Wb per mm^2. The MAXIMUM value of the flux density in the teeth is given to be 1.8 A per mm^2.

Right choice is (a) 20,000-40,000 A PER m

Explanation: The value of SPECIFIC ELECTRIC loading for the salient POLE alternators is 20,000-40,000 A per m. The value of specific electric loading for the turbo alternators is 50,000-75,000 A per m.

The correct option is (a) true

For EXPLANATION: High VALUE of SPECIFIC electric loading can be USED for low voltage machines. This is because the space required for INSULATION is small.

Correct answer is (a) high specific electric loading GIVES high copper losses and high TEMPERATURE rise

To ELABORATE: The specific electric loading is DIRECTLY PROPORTIONAL to the copper losses and the temperature rise. The high specific electric loading gives high copper losses and high temperature rise.

Right CHOICE is (a) 0.52-0.65 WB per m^2

Easiest explanation: The RANGE of air gap density for SALIENT POLE machines is 0.52-0.65 Wb per m^2. The range of air gap density for turbo-alternators is 0.54-0.65 Wb per m^2.

Right choice is (c) 4

The explanation is: There are 4 factors that influence the choice of SPECIFIC electric loading. They are a) Copper loss and temperature RISE, b) voltage, c) synchronous REACTANCE, d) STRAY load loss.

Right ANSWER is (B) false

Easiest explanation: The machines having high air gap density allows high amount of SYNCHRONIZING POWER. Thus the machines having high air gap density provides high SYNCHRONISM.

Correct choice is (a) AIR gap density is directly proportional to the steady STATE STABILITY

To elaborate: The air gap density is directly proportional to the steady state stability. The steady state stability is IMPROVED if the air gap density is high.

The correct choice is (a) air GAP density is DIRECTLY PROPORTIONAL to the SHORT circuit current

To explain: The air gap density is directly proportional to the short circuit current. The air gap density should be kept low in order to reduce the INITIAL electromagnetic forces under short circuit condition.

Right CHOICE is (B) air gap DENSITY is indirectly proportional to the VOLTAGE

To explain: The air gap density is indirectly proportional to the voltage. High voltage machine should have low air gap density, to AVOID excessive values of flux density in the teeth and core.

Correct choice is (a) choice of MAGNETIC LOADING is DIRECTLY proportional to the IRON loss

Easy EXPLANATION: The choice of specific magnetic loading is directly proportional to the iron loss. The iron loss increases with the increase in the air gap density.

Right choice is (c) 5

Explanation: The choice of specific magnetic LOADING depends upon 5 factors BASICALLY. They are a) IRON Loss, b) Voltage, c) Transient short circuit current, d) STABILITY, e) Parallel Operation.

Right option is (b) output = 1.11* specific magnetic loading * specific electrical loading * winding space factor * 10^-3 * peripheral speed^2 *LENGTH / SYNCHRONOUS speed

Explanation: The output equation with RESPECT to the peripheral speed depends on the square of the peripheral speed of the machine. It doesn’t CONSIST of the diameter TERM in the output equation.

Right answer is (c) OUTPUT coefficient = 11 * SPECIFIC magnetic LOADING * specific electrical loading * winding SPACE factor * 10^-3

Easy explanation: The output coefficient is one of the terms required in the calculation of the output of the machine. The specific magnetic and electrical loading terms are first calculated along with the winding space factor.

Right choice is (a) KVA output = output coefficient * diameter^2 * length * SYNCHRONOUS speed

Explanation: The output equation is found out by FIRST calculating the output coefficient. Next, the diameter and length are OBTAINED, and the synchronous speed is calculated USING the tacho-generator to obtain the kVA output.

The correct answer is (d) hysteresis, eddy and heating losses

To elaborate: The HEAT to be DISSIPATED by the cooling surface of the armature core would consist of the hysteresis loss and the eddy current loss. It ALSO consists of the heating loss or the I2R in the active part of the armature.

The correct ANSWER is (a) true

Easy explanation: There are VARIOUS peripheral SPEEDS in various parts of the machine. In the stationary field coils the peripheral SPEED is nothing but the ARMATURE peripheral speed.

The correct answer is (c) SPEED of the MACHINE and configuration of the surface

To explain I would SAY: The cooling COEFFICIENT depends UPON the speed of the machine. The cooling coefficient also depends upon the configuration of the surface.

Correct OPTION is (c) temperature rise of the SURFACE = Surface AREA * cooling coefficient / dissipating surface

The explanation is: The surface area is first calculated from its FORMULA. Next, the cooling coefficient and the dissipating surface are obtained and on SUBSTITUTION gives the temperature rise of the surface.

Right answer is (c) 0.3-0.4 % of kVA rating

The best explanation: The FRICTION loss depends UPON the TYPE of construction, speed and ratings of the machines. The HYDROGEN cooling REDUCES the total friction loss by 0.3-0.4% of the kVA rating.

Correct answer is (d) construction, SPEED, rating of the machine

To elaborate: This loss consists of the bearing friction and rotor windage loss. The loss DEPENDS UPON the type of construction, speed and RATINGS of the machines.

Right option is (a) 1 V

For explanation: The voltage drop in the CARBON and graphite BRUSHES is 1 V. The voltage drop in the brushes containing METAL is 0.3 V.

Right ANSWER is (a) STRAY field

Explanation: The stray load loss OCCURS due to stray FIELDS. They are FORMED when the machine is being loaded.

The correct OPTION is (a) total copper loss = 3 * average value of the eddy current constant * current per phase^2 * dc resistance

To elaborate: The average value of the eddy current constant is obtained. Next the I^2R loss values are CALCULATED and MULTIPLYING with 3 gives the total copper loss.

Right choice is (c) copper loss PER phase = current per PHASE2 * dc resistance

For explanation I would SAY: First the current per phase is calculated and the value is squared. Next the dc resistance is calculated and the SUM of the SQUARE of the current per phase and dc resistance gives the copper loss per phase.

The correct ANSWER is (c) 25-70 % of IRON loss

The explanation is: The pole FACE loss has a minimum value of 25% of the iron loss. The pole face loss has a maximum value of 70% of the iron loss.

The CORRECT answer is (d) slot OPENING, air gap length, number of slots and SPEED of machines

To elaborate: The POLE FACE loss depends upon the slot opening and the air gap length. It also depends on the number of slots and speed of machines.

Correct answer is (d) HYSTERESIS loss and EDDY current loss

To explain I would say: The iron loss due to MAIN FIELD is due to the hysteresis loss. The eddy current loss ALSO contribute to the iron losses due to main field.

The correct option is (a) 7

To explain I WOULD say: There are 7 losses in the synchronous machines. They are i) IRON loss due to main field, ii) iron loss due to parasitic field, iii) I2R loss in the armature winding, iv) EDDY current loss in armature conductors, v) STRAY load loss, vi) loss in field windings, vii) friction and WINDAGE loss.

Right choice is (a) true

To explain: The pole SHOE surface can be DRAWN with the ARMATURE surface being fixed. The pole shoe DRAWING is completed by fixing the height of pole shoe.

The CORRECT choice is (a) true

For explanation I would say: The pole profile drawing is essential to obtain the design CHARACTERISTICS of the rotor. It HELPS in the PROCESS of OBTAINING the various dimensions of the pole.

Correct choice is (B) height of POLE SHOE = 2 * DIAMETER of damper bars

The EXPLANATION is: First the diameter of the damper bars is calculated. It is then multiplied by 2 to get the height of pole shoe.

Right choice is (a) 59 mm^2

Best explanation: AREA of each BAR = TOTAL area / NUMBER of bars

Area of each bar = 473/8 = 59 mm^2.

The correct answer is (b) AREA of each RING short-circuiting the bars = (0.8-1) * area of damper bar

Explanation: FIRST the area of damper bars is calculated first. NEXT the area of damper bars is multiplied by a value between 0.8-1 to obtain the area of each ring short-circuiting the bars.

Right choice is (C) area of cross section of each damper BAR = total area of bars per pole / number of damper bars per pole

To explain: The total area of bars per pole is first OBTAINED. NEXT the number of damper bars per pole is calculated and the RATIO gives the area of cross section of each damper bar.

Correct OPTION is (a) pole arc = number of bars PER pole * stator slot pitch * 0.8

To explain I would say: The number of bars per pole is CALCULATED along with the stator slot pitch. NEXT, all the values are MULTIPLIED by 0.8 and the pole arc value is obtained.