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Correct ANSWER is (a) ROUND

The EXPLANATION is: Round conductors are used for the small motors. Bar conductors are being used for the LARGE motors.

Right answer is (a) true

Best EXPLANATION: There occurs a LOT of excessive copper loss in the rotors. The value of CURRENT density of ROTOR is almost equal to that in the STATOR.

Correct ANSWER is (a) rotor turns per PHASE = (winding factor for stator/winding factor for rotor) * (Rotor voltage per phase/Stator voltage per phase) * Number of turns per phase for stator

To ELABORATE: Firstly, the winding factor for stator is obtained ALONG with the winding factor for stator. Next the ratio of the rotor voltage per phase to the stator voltage per phase. Finally, the number of turns per phase for stator is also calculated to obtain the rotor turns per phase.

The CORRECT OPTION is (d) 1000-2000 V

The BEST I can explain: The minimum value of the rotor voltage should be 1000 V. The maximum value of the rotor voltage should not exceed 2000 V.

The CORRECT option is (a) true

Easy explanation: The rotor voltage on open circuit between slip rings should not EXCEED 500 V for the small machines. The voltage is limited to a small VALUE in order to protect persons working the motor if the brush gear is not PERFECTLY protected.

The correct option is (c) THREE phase windings

To EXPLAIN: The rotor windings for wound rotor motors are 3 phase windings. The number of rotor SLOTS should be such that a balanced winding is obtained.

The correct answer is (c) MULTIPLE of phases and pair of poles

For EXPLANATION: Fractional slot windings can be used in the wound rotor design. It is preferable to USE a number of SLOTS which are a multiple of phases and pair of poles in the case of fractional slot windings.

The correct option is (c) 4

For explanation I WOULD say: They are 4 types of design of wound rotors. They are NUMBER of Rotor Slots, Number of Rotor TURNS, Area of Rotor Conductors, Rotor Windings.

Right OPTION is (d) AREA of the RING = depth of the end ring * thickness of the end ring

The best explanation: For CALCULATION of the area of end rings, first the depth of end rings is calculated. NEXT, the thickness of the end ring is calculated and substituting in the above formula gives the area of the ring.

Correct choice is (a) CURRENT DENSITY of ROTOR bars < current density of end rings

For explanation: The ventilation is generally BETTER for the end rings. Thus, the current density of the end rings should be greater than that of the rotor bars.

The correct choice is (a) true

Explanation: The VALUE of CURRENT density is CHOSEN for the END rings should be chosen such that the desired value of the rotor resistance is obtained. If not it can lead to the starting problems in the machine.

Right answer is (c) maximum current in END RING= (2*3.14) * (bars per pole/2*no of poles) * current per bar

Easy explanation: The bars per pole is first OBTAINED. Then the no of poles is calculated ALONG with the current per bar. On substituting in the formula the maximum current in the end ring with the current through all the bars under one pole is not maximum.

Correct answer is (b) maximum value of the CURRENT through each bar = (2 * current through each bar)^1/2

To explain I would say: Firstly, the current through each bar is calculated. Then it is multiplied by 2 and its square ROOT provides the maximum value of current through each bar is obtained.

Right OPTION is (a) 60 A

Best explanation: maximum current in the end ring= bars PER pole / 2 * current per bar

Maximum current in the end ring = (6/2)*20 = 3 * 20 = 60 A.

The CORRECT option is (b) maximum current in the END ring= (bars PER pole / 2) * current per bar

To EXPLAIN: First the bars per pole are obtained. Then the current per bar is calculated. Then substituting in the above formula, the maximum current in the end RINGS.

The correct answer is (c) resistance COMING in each current path is resistance of two bars

For explanation I would say: If the resistance of END RINGS is negligible then the resistance of combined bars are taken into account. GENERALLY the resistance of two bars are taken into account.

The correct choice is (B) instantaneous emfs, POSITION of BARS in magnetic FIELD

For explanation I would say: The current that the bars carry are proportional to their instantaneous emfs. The instantaneous emfs are proportional to the position of the bars in the magnetic field.

Right OPTION is (a) revolving FIELD produces EMF of fundamental frequency in the bars

Explanation: The stator winding is 3 PHASE distributed winding and thus produces a revolving field. This revolving field produces emfs of fundamental frequency in the bars.

Correct option is (d) high CLEARANCES are PROVIDED for skewed SLOTS

For explanation I would SAY: When the skewed slots are being USED, higher clearances are provided. High clearances can lead to the smooth and efficient operation of the machine for skewed slots.

The correct choice is (a) smooth SURFACE leads to the QUIET operation

The explanation is: For the design of the rotor BARS of the three phase INDUCTION machine, smooth surface is preferred. Smooth surface HELPS in the silent operation.

Correct choice is (a) true

Easy explanation: RECTANGULAR SHAPED bars and slots are PREFERRED to circular bars and slots. This is because while using the rectangular shaped bars, ROTOR resistance increases and this leads to the IMPROVEMENT of starting torque.

Right answer is (b) CLOSED slots GIVE high leakage REACTANCE, and decreases the OVERLOAD capacity

Explanation: The closed slots have the main advantage of giving high leakage reactance. A high leakage reactance gives the advantage that the current at the starting can be limited.

The correct OPTION is (a) true

Explanation: Closed SLOTS are preferred for SMALL machines. It is because the reluctance of the AIR gap is not large owing to absence of slot OPENINGS.

Right answer is (b) 4-6 A per mm^2

For explanation: The MINIMUM VALUE of the current DENSITY in the ROTOR bars is 4 A per mm^2. The maximum value of the current density in the rotor bars is 6 A per mm^2.

The CORRECT choice is (a) rotor resistance = resistance of the BARS + resistance of END rings

Easiest explanation: First the resistance of the bars are obtained. NEXT, the resistance of the end rings are calculated and the SUM gives the rotor resistance.

Right choice is (B) as rotor resistance, LOSSES increase, efficiency decreases

The best I can explain: As the rotor resistance INCREASES, the I^2R losses increases and cause HEATING EFFECTS. This increase in losses decreases efficiency.

The correct answer is (c) HIGHER the current DENSITY, LOWER the conductor area, higher the RESISTANCE

The BEST explanation: Higher the current density leads to lower conductor area, as current density is the ratio of current per area. As the conductor area decreases, resistance increases.

Correct OPTION is (B) ROTOR resistance is DIRECTLY proportional to the starting torque

Explanation: The rotor resistance is directly proportional to the starting TORQUES. High resistance leads to high starting torque.

Right answer is (a) rotor mmf = 0.85 * STATOR mmf

To EXPLAIN I would say: FIRST the stator mmf is CALCULATED. Then it is MULTIPLIED by 0.85 to obtain the rotor mmf.

The correct option is (c) CURRENT in rotor bar = 2 * slot pitch * window space factor * stator torque * stator current * power factor / rotor slots

The explanation is: For the CALCULATION of the current through each rotor bar, firstly we find out slot pitch, window space factor and rotor slots. Then the stator torque and stator current are OBTAINED with RESPECT to stator side.

The correct answer is (d) 100-125 MM

The explanation: The STATOR is provided with the ventilating ducts if the core length EXCEEDS 100-125 mm. If the value is less than 100 mm, then no ventilating ducts are provided.

The CORRECT choice is (C) <=30 m per s

The explanation is: For the normal DESIGN, the DIAMETER value is chosen such that the peripheral speed value doesn’t EXCEED 30 m per s. The diameter value is directly proportional to the peripheral speed.

Right CHOICE is (a) 60-75 m per s

Explanation: For the standard rotor CONSTRUCTION can generally be used for peripheral speeds UPTO 60 m per s. For special rotor construction, the peripheral speeds upto 75 m per s are permissible.

Right OPTION is (c) pole pitch = (0.18 * core length)^1/2

Easiest explanation: For the BEST power factor, the pole pitch is equal to the SQUARE root of the PRODUCT of 0.18 and core length. This value however is not dimensionally correct and is valid for the VALUES in meters.

The correct OPTION is (d) size of machine, minimum cost, good efficiency

The best I can explain: The value of core length to pole pitch varies from 0.6 to 2.It DEPENDS on the cost, efficiency, power factor, OVERALL DESIGN.

The correct option is (C) for small motors small ratio is preferred

The best I can explain: For small motors, high values of the ratio of core LENGTH to pole pitch which may not be able to accommodate EVEN a small amount of slots. THUS small motors prefer only small ratio to accommodate slots.

The CORRECT option is (b) 1.5

Easy explanation: 1 is the value of the ratio of the core LENGTH to POLE pitch for GOOD overall design. 1.5 is the value of the ratio of the core length to pole pitch for good efficiency.

Right option is (a) 1.5-2

The BEST explanation: 1.0-1.25 is the RANGE of the ratio of the CORE length to pole PITCH for good power FACTOR. For minimum cost, 1.5 is the minimum range for the ratio and 2 is the maximum range for the ratio.

Right CHOICE is (c) diameter and LENGTH

Easy EXPLANATION: The main dimensions entirely deal with the calculation of the diameter and length. They are obtained from the OUTPUT EQUATION of the machine.