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Right answer is (d) north and south pole of a MACHINE are similar, no harmonics, no COSINE terms

Explanation: If the field form is SYMMETRICAL about the pole AXIS the north and south pole of a machine are similar. As the POLES are similar, no harmonics and no cosine terms and the constant term is zero.

Correct option is (C) induction motors

Best explanation: The closed slots are being made use often in the small induction motors. The closed slots are used to increase the LEAKAGE REACTANCE which in turn decreases the starting CURRENT.

Right answer is (b) fourier series

The explanation is: The field FORM of a salient pole is rectangular is the air gap under pole arc is uniform and if fringing effects are NEGLECTED. The field form can be ANALYZED for its harmonic contents with the HELP of Fourier series.

Correct CHOICE is (a) amplitude of the fundamental CURVE = 1.27 * flux density in the air gap * cosine (phase angle/2)

To explain: The flux density in the air gap is CALCULATED along with the cosine of the phase angle divided by 2. On SUBSTITUTION, the amplitude of the fundamental curve is calculated.

Correct choice is (a) sinusoidal

To explain: The flux distribution in the CASE of ac MACHINES should be sinusoidal. The flux distribution in the case of dc machines should be RECTANGULAR.

The CORRECT answer is (b) 3

The explanation is: The flux distribution in the rotating machines depends UPON 3 factors. They are shape of pole, the distribution of field windings and the LOAD CONDITION.

Right CHOICE is (b) 3

The BEST I can explain: The flux lines leave and enter from surfaces, bounding the gap, at right angles if it is assumed that iron has infinite PERMEABILITY as compared with air and then the flux and equipotential lines intersect at right angles. The flux and the equipotential lines are so DRAWN that each flux is divided into equal number of curvilinear squares.

Correct option is (a) true

To elaborate: CARTER’s fringe CURVES is one of the techniques used in the PLOTTING of the field form in the salient pole machines. The plot between carter’s coefficient and the relative flux density is CALLED as carter’s fringe curve.

Right choice is (c) permeance of the flux tube = PERMEABILITY in the air * mean width of flux tube / mean length of flux tube

Explanation: The permeability in the air, mean width of the flux tube and the mean length of flux tube is calculated FIRST. On SUBSTITUTION the permeance of the flux tube is obtained.

Correct choice is (B) flux density in the gap at a distance ‘X’ from the CENTRE of the pole = length of air gap at the centre of pole / length of air gap at a distance ‘x’ from the centre of the pole * maximum flux density in air gap

For explanation: The length of air gap at the centre of pole, the length of air gap at a distance ‘x’ from the centre of the pole and the maximum flux density in air gap is calculated. On SUBSTITUTION the flux density in the gap at a distance ‘x’ from the centre of the pole.

Correct choice is (a) 2

To explain: There are 2 techniques which are used to PLOT the field form in salient pole machines. They are CARTER’s fringe curves and the flux plotting by method of CURVILINEAR squares.

Correct answer is (a) WAVESHAPE of voltage

To EXPLAIN: The FLUX distribution CURVE in the ac machine determines the waveshape of the voltage. In the DC machine the flux distribution curve determines the commutation conditions.

Correct answer is (a) true

Explanation: The flux DENSITY DISTRIBUTION CURVE is used to calculate the magnetizing current in the non sinusoidal flux distribution of the distributed windings. The curve is between the flux density and the PHASE ANGLE.

Correct OPTION is (d) to keep the total reluctance low

For explanation I would SAY: The flux in passing from poles into the armature, does not confine itself over the pole ARC but spreads out over the entire pole pitch. The flux will distribute itself in the air GAP in such a way that the total reluctance is minimum.

Correct option is (C) magnetizing current is INDIRECTLY proportional to the turns per phase

Easy explanation: The magnetizing current is ACTUALLY calculated for the CONCENTRATED windings and the distributed windings. In the distributed windings, magnetizing current is indirectly proportional to the turns per phase.

Correct option is (a) in DISTRIBUTED WINDINGS the FLUX does not link with all the TURNS

The explanation is: The MAGNETIZING current is actually calculated for the concentrated windings and the distributed windings. In the distributed windings, the flux does not link with all the turns.

Right answer is (a) RMS VALUE of the magnetizing current = MAXIMUM magnetizing current / peak FACTOR

Best explanation: The maximum value of the magnetizing current and the peak factor are calculated first. On substitution, the rms value of the magnetizing current is obtained.

Right choice is (b) 3

For explanation I would SAY: There are 3 factors UPON which the magnetizing current depends upon. They are total mmf required, number of turns in the EXCITING winding and upon the way in which the winding is DISTRIBUTED.

Right CHOICE is (b) specific permeance of a differential path is INDIRECTLY proportional to the LENGTH

For explanation I WOULD say: The specific permeance is DEFINED as the permeance per unit length. It is indirectly proportional to the length.

The correct OPTION is (B) flux DIVIDING into infinitesimal parts = MMF producing the flux * permeance of infinitesimal part

Easiest explanation: The mmf producing the flux and the permeance of infinitesimal part is CALCULATED. On substitution the flux dividing into infinitesimal parts is calculated.

The correct option is (a) EFFECTIVE PERMEANCE = effective flux/TOTAL mmf

Easiest explanation: The effective flux and the total mmf is first calculated. On SUBSTITUTION of the values the effective permeance is calculated.

The correct answer is (b) specific permeance = permeability in air * ∫SMALL CHANGE in width/length

For EXPLANATION I would say: The permeability in air, small change in width and length is calculated first. On SUBSTITUTION the specific permeance is calculated.

The correct answer is (C) SPECIFIC permeance is the permeance PER unit length

For explanation I would SAY: Specific permeance is defined as permeance per unit length. It is also known as the depth of field.

Correct choice is (b) overhang SPECIFIC permeance is indirectly PROPORTIONAL to the slot pitch

For explanation I WOULD say: The overhang specific permeance relation OBTAINED is an empirical relation. In the empirical relation, the overhang specific permeance is indirectly proportional to the slot pitch.

The correct choice is (b) ZIGZAG specific permeance = AVERAGE width of the rotor TOOTH / length * (1/2 * mmf PER slot^2)

Best EXPLANATION: The average width of the rotor tooth along with the length and mmf per slot is calculated. On substitution the zigzag specific permeance is obtained.

The correct option is (d) length of the overhang ALONG with the shape of the overhang

The best I can EXPLAIN: The overhang LEAKAGE reactance DEPENDS upon the length of the overhang. It ALSO depends upon the shape of the overhang.

Correct choice is (c) RELATIVE POSITION of STATOR and rotor

Explanation: The PERMEANCE for the path of zigzag leakage will depend upon the relative position of stator. The permeance for the path of zigzag leakage will depend upon the relative position of rotor.

Right answer is (C) stored energy at any position = 1/2 * mmf per SLOT^2 * permeance in a particular position

To elaborate: FIRST the mmf per slot along with the permeance in a particular position is calculated. On SUBSTITUTION, the stored energy at any position is calculated.

The correct choice is (b) induction motors

To EXPLAIN I would say: The pulsation losses are aggravated if the length of air gap is SMALL than the SLOT openings. The pulsation losses ALTHOUGH are CONSIDERABLE in the induction motors.

Right option is (a) true

Explanation: The pulsation LOSSES are aggravated if the LENGTH of air gap is small than the slot openings. The slotting produces HARMONIC fields which cause high frequency losses NEAR the gap surface.

The CORRECT answer is (a) if the air gap is small COMPARED with slot OPENINGS

Easy explanation: The PULSATION losses occurs in the POLE faces and teeth. The pulsation losses are aggravated if the air gap is small compared with slot openings.

The CORRECT CHOICE is (d) teeth and POLE faces

To elaborate: The pulsation losses OCCURS in the teeth of the machine. The pulsation losses ALSO occur in the pole faces.

The correct option is (b) armatures are SLOTTED and RESULTS in the movement of rotor

Easy EXPLANATION: In the ROTATING ELECTRICAL machine, the armatures are slotted and there is movement of rotor. Due to the movement of the rotor there are rapid changes of local gap reluctance.

The correct option is (d) flux pulsations OCCUR due to the change in reluctance

The explanation: In the ROTATING electrical machine, the armatures are slotted and there is movement of ROTOR which leads to changes of local gap reluctance. This change leads to flux pulsations which causes additional LOSSES called pulsating losses.

Correct choice is (a) true

Easiest EXPLANATION: The leakage FLUX on the top of the slot is through IRON, which is called ‘bridge’. Therefore the specific permeance of this PATH depends upon the permeance of iron which in turn depends upon the degree of saturation and HENCE upon the relative permeability of iron.

The correct option is (c) Laplace and Poisson equation

Explanation: The flux patterns MUST satisfy the Poisson equation WITHIN the SECTION of the conductors. The flux patterns must satisfy the Laplace equation in the other PARTS.

The correct answer is (b) iron loss OBTAINED from iron loss curve is multiplied by 1.4-1.6

To explain I would say: In ORDER to take additional LOSSES into account the iron loss obtained from the iron loss CURVES is multiplied into 1.4-1.6 in dc machines and SYNCHRONOUS machines. The iron loss obtained from the iron loss curves is multiplied into 1.2-1.4 in induction motors.

Correct ANSWER is (a) 4.7

Easiest explanation: The VALUE of CONSTANT ‘a’ in the core part of the ac machines is 4.7 and that in the TEETH part is 6.5. The value of constant ‘a’ in the core part of the dc machines is 6.5 and that in the teeth part is 2.3.

The correct choice is (C) hysteresis loss = hysteresis coefficient * frequency of MAGNETIZATION * (MAXIMUM flux density)^steinmetz coefficient

Easy EXPLANATION: The hysteresis coefficient along with the frequency of magnetization and maximum flux density is CALCULATED first. Next the Steinmetz coefficient is calculated and on substitution it gives the hysteresis loss.