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Right option is (b) SHORT shunt compound GENERATING mode

The explanation is: In a DC MACHINE, for a long shunt compound motor Rse is CONNECTED in series with ARMATURE, while in short shunt compound motors Rse is connected in series with terminal voltage. IL is supplied to the terminals which is equal to Ia – If.

Correct option is (a) Series

The best I can EXPLAIN: In a compound excited machine, both series and parallel WINDINGS are connected with armature circuit. The series WINDING is specially designed for increasing or DECREASING flux per pole in DC machine up to certain extent.

Right ANSWER is (a) True

To explain I would say: If a dc compound machine connected as a generator is run as a motor, the series field connections must be reversed as the ARMATURE CURRENT reverses. The motoring action as cumulative/ differential would then be preserved (same as in the generator). This EQUALLY applies vice VERSA – motor to generator.

The correct choice is (b) Decreases

To explain: If the two fields OPPOSE each other, the excitation is called differential COMPOUND. The AIR gap flux/pole decreases with armature current. The series field is so designed that the increase or DECREASE in flux/pole is to a LIMITED extent.

Correct ANSWER is (b) TAPPINGS

Explanation: For a given field current, control of series field is achieved by means of a diverter, a low resistance connected in PARALLEL to series winding. A more practical way of a series field control is CHANGING the number of turns of the winding by SUITABLE tappings which are brought out for control purpose.

Correct answer is (c) Both

For EXPLANATION I WOULD SAY: In compound excitation, both series and shunt windings are CONNECTED to the armature. For compound excitation both of these windings are excited. Two types are cumulative excitation and differential excitation.

The correct option is (a) Shunt field

For explanation: The shunt field winding is provided with a large NUMBER (hundreds or EVEN THOUSANDS) of turns of thin wire and is excited from a voltage source. The field winding, therefore, has a high resistance and carries a small amount of current. It is USUALLY excited in PARALLEL with armature circuit.

Right choice is (b) Series field

The explanation: Series field WINDING is used when wire is thick. Thus, by MAKING minimum number of TURNS it can be used to excite a DC machine. For a given field CURRENT, control of this field is achieved by means of a DIVERTER, a low resistance connected in parallel to series winding.

Right answer is (b) 2

Easy explanation: Commutation takes place simultaneously for P COILS in a lap-wound machine (it has P BRUSHES) and two COIL sets of P/2 coils each in a wave-wound machine (ELECTRICALLY it has two brushes independent of P).

Right choice is (c) Coil LYING in an INTERPOLAR REGION

The best I can explain: During the commutation period, the coil is shorted VIA the commutator segments by the brush. These brushes are LOCATED in interpolar regions electrically, and magnetically in neutral region. Thus, interpolar coil gets shorted.

The CORRECT answer is (b) P/2

Easy explanation: One coil each under an adjoining pole-pair is CONNECTED between adjacent commutator SEGMENTS in a lap wound DC armature, while in a wave-wound armature the only DIFFERENCE is that P/2 coils under the influence of P/2 pole-pairs are connected between adjacent segments.

Correct OPTION is (d) Compensating winding

The explanation: Interpoles are located in interpolar REGION in a DC machine, CALLED as interpoles. They raise up the SPEED of voltage commutation. So, they are ALSO called as commutating poles. Compensating winding though used for reducing armature reaction, performs different function compare to interpoles.

The CORRECT option is (d) Mica

To elaborate: s: Each CONDUCTING segment of the commutator is insulated from adjacent segments. Mica is a GOOD electric insulator and good thermal conductor as well. Its applications in electric fields are derived from its unique mechanical properties. Thus, it allows mica to be ductile enough for its APPROPRIATE SPACE of application.

The CORRECT CHOICE is (c) Mica is harder than copper

Explanation: Due to its MECHANICAL strength and insulating properties mica is a satisfactory material. However, mica is much harder than the copper SEGMENTS, so during manufacturing it requires to under-cut the mica by sawing slots between the adjacent segments of the commutator.

The correct ANSWER is (a) just AHEAD of magnetic neutral axis

Best explanation: Brushes collect the current due to the induced emf in the armature COILS. When a brush is at any particular commutator segment, it shorts out that particular coil and DRAWS current from the REST of the coils and fed to the commutator. To achieve all positive outcomes, we place them just ahead of MNS.

The correct ANSWER is (c) 4198

Easy explanation: Required ampere-TURNS are given by ATi = ATA (peak) +(Bi/µ0 )lgi . Substituting the

 ATi =[(56.82*846)/ (2*2*4 )] +(0.5/4π*10^-7 )* 0.3*10^-2 =4198. Turns can be found by DIVIDING with armature CURRENT.

Correct answer is (C) More

Explanation: The interpolar air-gap is kept LARGER than that of the main pole so that their magnetic circuit is LINEAR resulting in cancellation of the REACTANCE voltage (a linear derivative term) at all LOADS. Large air-gap results in greater amount of leakage flux which is accommodated by tapering the interpoles with a wider base.

The correct answer is (b) Generator

To elaborate: POLARITY of an interpole is that of the main pole ahead in the DIRECTION of armature ROTATION for the generating mode and that of the main pole left BEHIND with RESPECT to the direction of rotation for motoring mode.

The correct OPTION is (a) Series

The explanation: In VOLTAGE commutation METHOD we use interpoles to speed up the commutation PROCESS. For neutralization of reactance voltage at all loads, the interpoles must be excited by ARMATURE current by connecting them in series with armature.

Correct choice is (d) INCREASES then decreases

The explanation is: We add high contact resistance between commutator segments and brushes thus, it adds resistance to the CIRCUIT of the commutating coil THEREBY reducing the time constant (L/R) of the current transient (IC(t)), helping it to change faster in the desired DIRECTION.

Correct ANSWER is (d) Commutator and brush

The explanation: High contact resistance between commutator segments and BRUSHES, achieved by using carbon brushes, adds resistance to the CIRCUIT of the commutating coil thereby REDUCING the time constant (L/R) of the current transient (ic(t)), helping it to change faster in the desired direction.

Correct option is (d) Resistance and voltage commutation

Best explanation: ADDING resistance between commutator SEGMENTS and brushes, thus, reducing L/R and consequently getting faster commutation is one of the method. In voltage commutation we, introduce narrow poles CALLED as INTERPOLES to fasten the PROCESS.

The correct choice is (c) Causes demagnetization

To EXPLAIN: Brushes are located at GNA’s, a small VOLTAGE is induced in the commutating COIL. It opposes current commutation as the commutating coil is cutting the flux which has the same sign as that of the pole being LEFT behind. It could be partially remedied by shifting the brushes towards MNA but that causes direct demagnetization and is therefore not employed in practice.

The correct option is (B) Cumulative compound

Easiest EXPLANATION: In compound excitation both shunt and SERIES field are EXCITED. If the two field aid each other (their ampere-turn is additive), the excited is CALLED cumulative compound. The shunt field is much stronger than the series field. The air gap flux increases with armature current.

Correct choice is (c) 1

For explanation I would say: No. of PARALLEL paths in a lap winding are EQUAL to P (No. of poles). Thus for 1 pole pir there will be exact 1 coil, connected to ADJACENT COMMUTATOR segments in lap winding. In wave winding there will be half coils connected between adjacent commutator segments.

The correct option is (C) Leakage reactance and armature reaction

Explanation: The leakage inductance Lc of the COIL UNDERGOING commutation has INDUCED in it reactance voltage Lc (dic/dt) which opposes the change in current thereby delaying commutation. The effect of armature reaction CAUSES shift in MNA, delaying the whole commutation process ultimately.

The correct option is (c) Series regulating RESISTANCE

The explanation is: Control of Excitation: 1) Shunt field: by a series regulating resistance.2) Series field: For small armature by a diverter resistance connected in parallel with series field. For large armature by tapped field WINDING so the winding TURNS can be changed.

The correct option is (a) True

To EXPLAIN: The PROCESS of current reversal called commutation TAKES place when the coil is passing through the interpolar region (q-axis) and during this period the coil is shorted via the COMMUTATOR segments by the brush located (electrically) in the interpolar region.

Right choice is (C) Accelerated

The explanation is: Speed of the commutation is DEPENDABLE on change in an induced CURRENT DIRECTION. When reversal of current in coil is faster then, obviously change of coils are taking place at faster rate. Thus, commutation is said to be accelerated commutation.

The correct choice is (a) Vt= Ea+ IA(Ra+ Rse)

The explanation: For a long shunt compound motor Rse is connected in series with ARMATURE, while in short shunt compound motors Rse is connected in series with terminal VOLTAGE. IL is supplied through the TERMINALS which SPLIT into If and Ia.

Correct choice is (c) Straight LINE not PASSING through origin but with -ve slope

The EXPLANATION is: Ideal Commutation (also called straight-line commutation) is that in which the current of the commutating COILS CHANGES linearly from + Ic to – Ic in the commutation period. Thus, it will form a straight line with -ve slope.

The correct choice is (a) The series field ampere turns are such as to produce the same voltage at rated load as at no load

To explain I would say: According to the OPERATING characteristics of a DC compound generator, if series field mmf PRODUCES same voltage at rated load as that of no load then it is CALLED as flat compounded generator.

Correct OPTION is (a) Armature current

The best I can explain: These interpoles apply a local CORRECTION to the air-gap flux density wave such that a pip of appropriate flux density EXISTS over the commutating coil to induce in it a voltage of the same sign as that of coil current after commutation. HENCE, they are excited with armature current.