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Correct choice is (a) Less

The explanation: The ampere-turns REQUIRED for compensating winding in a DC machine is ATcw /POLE = ATA (peak) *(pole arc/pole pitch) = [IaZ/(AP2)] * (pole arc/pole pitch). THUS, if compensating winding ampere turns are less then, pole arc is smaller than pole pitch and vice-versa.

The correct answer is (c) Chamfering the pole shoes

The BEST explanation: By chamfering the pole-shoes which increases the air-gap at the pole TIPS. This METHOD increases the reluctance to the path of main flux in a DC machine but its INFLUENCE on the cross-flux is MUCH greater. This is because the cross flux has to cross the air-gap twice.

Right choice is (d) Open coil in armature, defective interpoles and INCORRECT BRUSH spring pressure

The EXPLANATION: Brushes are the point of contacts between rotating and non-rotating PARTS. So, if point of contact is rough then brushes will face damage and will lead to sparking in them. Defective interpoles will contribute in IRREGULAR commutation ultimately it will lead to sparking.

The correct choice is (c) To NEUTRALISE the cross-magnetising effect of the armature reaction

For explanation I would SAY: Armature reaction can be reduced with the help of compensating winding. To neutralize the cross-magnetizing effect of armature reaction in a DC machine, a compensating winding is used. The compensating windings CONSIST of a SERIES of COILS embedded in slots of the pole faces.

The correct option is (c) EQUALIZER rings

To elaborate: The armature coil in lap windings have equal to or more than 2 parallel PATHS. Under the influence of pole pairs there MAY arise asymmetry in various paths resulting in different parallel path voltages. So, equalizer rings are connected in ORDER to ACHIEVE symmetry again.

The correct choice is (c) Compensating winding and INTERPOLES

To explain I WOULD say: To neutralize the CROSS magnetizing effect of armature reaction, a compensating winding is used. The compensating windings consist of a series of COILS embedded in SLOTS of the pole faces. Interpoles are designed to overcome the effects of the armature reactance and the self-induction of the machine.

Correct choice is (a) Only wave WINDING

Explanation: The armature coils forming each of the TWO parallel paths are under the INFLUENCE of all pole-pairs so that the effect of the magnetic circuit asymmetry is equally present in both the parallel paths resulting in EQUAL parallel-path voltages. Thus, equalizer rings are not needed in a wave winding.

Correct answer is (b) EQUAL to the total armature ampere conductors under the pole shoe

To explain: The number of ampere-turns required for compensating winding is ATcw /pole = ATA (peak) *(pole arc/pole pitch) = [IaZ/(AP2)]*(pole arc/pole pitch). Here, pole arc= pole pitch, is given. Thus ampere-conductors required are equal to the total armature ampere conductors under the pole shoe.

Correct OPTION is (a) 6252 AT/pole

To EXPLAIN: For a given machine number of parallel paths is equal to 6. So, conductor CURRENT will be equal to ARMATURE current divide by no. of parallel paths i.e. 625/6. Conductor current = 104.2 A. Total armature ampere-turns, ATa = ½(720*104.2/6)= 6252 AT/pole.

The correct option is (b) 9000

To explain: Ampere-conductors/pole =ZIc/P= Zla/AP. Ampere TURNS PER pole is CALCULATED by multiplying total no. of conductors with the current CARRIED by them divided by the twice the total no. of poles.

Ampere-turns/pole = 600*120/8 =18000/2= 9000 .

Right OPTION is (d) Increase in iron losses, commutation PROBLEMS and commutator sparking

Explanation: Armature reaction in a DC machine is a result of distortion of MAIN field flux distribution by armature CURRENT, which produces its own mmf called armature mmf. Directly or indirectly armature reaction is the problem occurring in DC machine as it causes various effects, which reduce machine efficiency.

The correct choice is (d) Load CURRENT in armature

Easy explanation: When the armature of a dc MACHINE carries current i.e. load current in armature, the distributed armature winding produces its own mmf (distributed) KNOWN as armature reaction. Thetothe field ampere-turns (ATf) and armature ampere-turns (ATA).

Right OPTION is (d) Trapezoidal

Easiest explanation: Flux density waveform is SYMMETRICAL and square wave with distortion at the zero POINTS, causing wave to be in trapezoidal shape. The wave is flat topped, which get DISTRIBUTED due to armature mmf distribution, giving rise to the resultant flux distribution wave.

Right choice is (a) AXES of main poles

To explain I would say: DIRECT axes is simply defined as the line passing through the axes of main poles. Maximum flux passes through this line. It’s also CALLED as Direct AXIS. Direct axes is always perpendicular to the geometrical NEUTRAL axis of machine.

Right answer is (a) 6

The EXPLANATION: The NUMBER of ampere-turns required for compensating winding is ATcw /pole = ATa (peak) *(pole arc/pole pitch) = [IaZ/(AP2)] * (pole arc/pole pitch).

Ncw/pole = (Z/2AP) * (pole arc/pole pitch) = [286 / (2*6*6)] 0.7 = 2.78.

Compensating conductors/pole = 2 * 2.78 = 6 (nearest integer).

Right choice is (d) To provide mechanical BALANCE for the rotor

Explanation: Dummy coils are connected in armature winding of type wave winding. In a lap winding Yc= +/-1 irrespective of the NUMBER of armature coils so that coils can always be CHOSEN to completely fill all the slots (C =1/2 US). In a wave winding the number of coils must fulfil the condition C =P/2 Yc +- 1 while at the same time C must also be governed by C =1/2 US.

Correct OPTION is (c) Neutralise both the armature reaction flux as WELL as commutating EMF induced in the coil

The explanation is: INTERPOLES are designed to overcome the EFFECTS of the armature reactance and the self-induction of the machine i.e. to neutralise both the armature reaction flux as well as commutating emf induced in the coil.

Correct answer is (a) (pole arc / pole pitch) * armature ampere turns per pole

The best EXPLANATION: The number of ampere-turns required for this purpose is ATcw /pole = ATa (peak) *(pole arc/pole pitch) = [IaZ/(AP2)]*(pole arc/pole pitch). The COMPENSATING winding neutralizes the armature mmf directly under the pole while in the INTERPOLAR region, there is INCOMPLETE neutralization.

Correct answer is (d) 0.6

For explanation I would say: The ampere-turns REQUIRED for compensating winding in a DC MACHINE is ATcw /POLE = ATA (peak) *(pole ARC/pole pitch). If compensating winding of 360AT is used the, 360/1960 will give ratio of pole pitch /pole arc, equal to 0.6.

Right option is (b) 5731

For explanation: For calculations, from given mech. DEGREES SHIFT we NEED to find electrical degrees shift. Electrical shift= mechanical shift*(P/2). Thus, electrical shift is EQUAL to 7.50. Cross-magnetizing ampere-turns is given by 6250*(1-2*7.5/180) = 5731 AT/Pole.

The correct option is (d) ATresultant(∅) = ATf (∅) + ATa(∅)

EXPLANATION: The exact WAY to find the flux density owing to the simultaneous action of field and ARMATURE ampere-turns is to find the resultant ampere-turn distribution ATresultant(∅) = ATf (∅) + ATa(∅), where ∅ is the ELECTRICAL SPACE angle.

The correct answer is (b) Large air gap

Easy explanation: The exact way to find the flux DENSITY owing to the simultaneous action of field and armature ampere-turns is to find the resultant ampere-turn distribution ATresultant(∅) = ATF(∅) + ATA(∅). The flux density of ATa(∅) which, because of large air-gap in the interpolar REGION, has a strong dip ALONG the q-axis even though ATa(peak) is oriented along it.

Correct ANSWER is (a) True

The best I can explain: BRUSHES are generally LOCATED at 900 to direct axis. The axis 900 to the direct axis is called as quadrature axis (q-axis). Generally, q-axis is along the geometric neutral axis (GNA) of machine. The brushes in a DC machine are NORMALLY located along the q-axis.

Right answer is (a) Demagnetisation of leading pole tip and MAGNETISATION of trailing pole tip

The best explanation: Leading pole tip (LPT) and trailing pole tip (TPT) are the two EDGES of the pole, they depend upon the DIRECTION of motion of the armature (in case of DC). While performing a motion the armature first SAWS an edge of the pole, that edge is called leading pole tip. THUS, at leading pole tip there will be demagnetization.

The correct answer is (b) MNA

To elaborate: Apart from DISTORTION of the resultant flux density wave, its MNA also gets shifted from its GNA by a SMALL angle α so that the BRUSHES placed in GNA are no longer in MNA as is the CASE in the absence of armature CURRENT.

The correct choice is (a) Demagnetizing mmf

The EXPLANATION: If the main pole EXCITATION is such that iron is in the saturated region of magnetization (this is the case in a PRACTICAL machine), the increase in flux density at one END of the poles caused by armature reaction is less than the decrease at the other end, so that there is a net reduction in the flux/pole, a demagnetizing effect.

The CORRECT option is (a) Cross-magnetizing

To ELABORATE: INITIALLY at unsaturated CONDITION in a DC machine armature reaction LIES along the q-axis. It will cause no change in flux/pole if iron is unsaturated. Now, when iron gets saturated axis gets shifted which will cause reduction in flux/pole.

Right choice is (b) Pole arc > Pole pitch

Easy explanation: The NUMBER of ampere-turns required for compensating winding in a DC MACHINE is ATcw /pole = ATA (peak) *(pole arc/pole pitch) = [IaZ/(AP2)] * (pole arc/pole pitch). Thus, if compensating winding ampere turns are more then, pole arc is definitely greater than pole pitch.

The correct choice is (b) Decreasing the length of air gap

To elaborate: Decreasing the air gap is simple to say but HARD to achieve, DUE to VARIOUS other limitations. But it is one of the way to reduce dropping down of armature mmf FLUX DENSITY distribution, which further reduces the distortion of resultant flux density.

Right choice is (a) 625A

To explain: Armature current multiplied by the armature voltage is called as rating of a DC generator. THUS, 250 KW is the given rating while 400 V is the armature voltage. So, armature current is EQUAL to 250*1000/400 = 625A.

Correct CHOICE is (a) Machine is designed with iron which is slightly saturated

For EXPLANATION I would say: The ARMATURE REACTION in a DC machine is cross-magnetizing causing distortion in the flux density wave shape and a slight shift in MNA. It also CAUSES demagnetization because a machine is normally designed with iron slightly saturated.

The CORRECT option is (C) Anti-MAGNETIZING mmf

Explanation: The armature reaction FLUX strengthens each main pole at one end and weakens it at the other end (cross magnetizing effect). Armature reaction with axis at 90° to the main field axis is known as cross-magnetizing mmf.

Correct OPTION is (d) Reduce and distort it

To explain I would say: When non-zero load current is passed through the armature winding, the DISTRIBUTED armature winding produces its own mmf known as armature reaction. ACCORDING to its nature cross-magnetizing and demagnetizing, it will distort or reduce the main FLUX distribution.

The CORRECT ANSWER is (a) ATa (peak) = ATa (total) /P

To elaborate: Peak flux density is defined as ratio of total flux density given to the NUMBER of poles for a given DC machine. Peak flux density in terms of total flux density is given by ATa (peak) = ATa (total) /P.