DC Machines MCQs – Commutation Process & Excitation Methods ( DC Machines ) MCQs
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DC Machines MCQs – Commutation Process & Excitation Methods ( DC Machines ) MCQs
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Commutation Process -1
1. Commutation is delayed due to __________
a) Leakage reactance
b) Effect of armature reaction
c) Leakage reactance and armature reaction
d) Because of other factors
Answer: c
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.
2. Why brushes shifting method is not employed in practical commutation?
a) Expensive
b) Construction problems
c) Causes demagnetization
d) Used practically
Answer: c
Explanation: 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.
3. Which are the methods for getting an ideal commutation?
a) Resistance commutation
b) Voltage commutation
c) Current commutation
d) Resistance and voltage commutation
Answer: d
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.
4. In resistance commutation method we add resistance between __________________
a) Brushes and external circuit
b) Commutator and armature winding
c) At field winding
d) Commutator and brush
Answer: d
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.
5. What is the effect on time constant of transient current, as a result of resistance commutation?
a) Remains same
b) Decreases
c) Increases
d) Increases then decreases
Answer: d
Explanation: 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.
6. Which voltage is neutralized in voltage commutation process?
a) Armature
b) Reactance
c) Field
d) Cannot be determined
Answer: b
Explanation: To speed up the commutation process, the reactance voltage must be neutralized by injecting a suitable polarity dynamical (speed) voltage into the commutating coil. In order that this injection is restricted to commutating coils, narrow interpoles are provided in the interpolar region.
7. Interpoles are excited with ____________
a) Armature current
b) Field current
c) Separate supply
d) Mains current
Answer: a
Explanation: 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.
8. Interpoles are excited by keeping them in ____________ with armature.
a) Series
b) Parallel
c) Anywhere
d) Not kept with armature
Answer: a
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.
9. Polarity of interpoles is one pole ahead in the direction of armature rotation in _____________
a) Motor
b) Generator
c) Always ahead
d) Always behind
Answer: b
Explanation: 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.
10. What is the size of interpolar air gap compare to main pole air gap?
a) Small
b) Same
c) More
d) Can be of any size
Answer: c
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.
11. Formula for cancellation of reactance voltage on an average basis using interpoles ________
a) 2[Bi(av)liva] Nc = Lc (dic/dt) = Lc (2lc/tc)
b) [Bi(av)liva] Nc = Lc (dic/dt) = Lc (2lc/tc)
c) 2[Bi(av)liva] Nc = Lc (dic/dt) = Lc (lc/tc)
d) Cannot be determined
Answer: a
Explanation: 2[Bi(av)liva] Nc = Lc (dic/dt) = Lc (2lc/tc).
With Bi determined by above equation, the ampere-turns needed to cancel the armature reaction ampere-turns and then to create the necessary flux density are given by, ATi = ATa (peak)+(Bi/µ0 )lgi
12. For a given 4-pole machine, carrying armature current 56.82 A, with 846 conductors. The mean flux density in the air gap under the interpoles is 0.5 Wb/m2 on full load and radial gap length is 0.3 cm. Ampere-turns required for an interpole is ____________
a) 3198
b) 2099
c) 4198
d) 6297
Answer: c
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.
13. A 440 V, 4-pole, 25 kW, dc generator has a wave-connected armature winding with 846 conductors. The mean flux density in the air-gap under the interpoles is 0.5 Wb/m2 on full load and the radial gap length is 0.3 cm. Number of turns required on each interpole is _________
a) 74
b) 84
c) 64
d) 54
Answer: a
Explanation: Corresponding value of ampere-turns for an interpole
ATi = ATa (peak) +(Bi/µ0) lgi.
= Ia* Z/2*A*P + (Bi/µ0) lgi.
Assuming Ia = Iline Ia = 25*103/440= 56.82 A.
ATi = [(56.82*846)/ (2*2*4)] +(0.5/4π*10-7 )* 0.3*10-2 =4198.
Ni= 4198/56.82= 74.
Commutation Process – 2
1. To achieve spark less commutation brushes of a DC generator are placed ________
a) just ahead of magnetic neutral axis
b) in magnetic neutral axis
c) just behind the magnetic neutral axis
d) can be placed anywhere
Answer: a
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.
2. If in the DC machine, the reversal of current in the coil is faster than ideal or linear commutation then the commutation is said to be __________ commutation.
a) Retarded
b) Curvilinear
c) Accelerated
d) Under
Answer: c
Explanation: 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.
3. For a DC machine, in a commutator ________
a) Copper is harder than mica
b) Mica and copper are equally hard
c) Mica is harder than copper
d) Cannot be determined
Answer: c
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.
4. The insulating material used between the commutator segments is normally _______
a) Graphite
b) Paper
c) Air gap
d) Mica
Answer: d
Explanations: 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.
5. Why interpoles are tapered in a DC machine?
a) Simpler design
b) Reduction in the weight
c) Increase in acceleration of commutation
d) Cannot be determined
Answer: c
Explanation: To speed up the commutation process, the reactance voltage must be neutralized by injecting a suitable polarity dynamical (speed) voltage into the commutating coil. In order that this injection is restricted to commutating coils, narrow interpoles are provided in the interpolar region.
6. The main function of interpoles in a loaded DC machine is to minimize _______between the brushes and the commutator.
a) Friction
b) Sparking
c) Current
d) Wear and tear
Answer: b
Explanation: Interpoles are introduced in a DC machine in order to speed up the commutation process, sp that sparking will be minimised. As, sparking arises at end of commutation period when commutation is not completed in given time.
7. Which of the following is different component?
a) Commutating poles
b) Compoles
c) Interpoles
d) Compensating winding
Answer: d
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.
8. How many coils under an adjoining pole pairs is/are connected between adjacent commutator segments in lap winding?
a) 1/2
b) 2
c) 1
d) 1/4
Answer: c
Explanation: 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.
9. In wave winding, P/2 coils are connected under the influence of “x” pole-pairs which connect adjacent segments. X is ____
a) P
b) P/2
c) 2P
d) P/4
Answer: b
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.
10. The process of current reversal takes place when the coil is passing through the interpolar region.
a) True
b) False
Answer: a
Explanation: 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.
11. How many coil sets undergo commutation simultaneously in a wave winding?
a) 1
b) 2
c) 4
d) 3
Answer: b
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).
12. Which coil is shorted in commutation process?
a) Coil under north pole
b) Coil under south pole
c) Coil lying in an interpolar region
d) Cannot be determined
Answer: c
Explanation: 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.
13. Ideal commutation can be shown graphically by _____________
a) Straight line passing through origin
b) Straight line not passing through origin but with +ve slope
c) Straight line not passing through origin but with -ve slope
d) Curve increasing towards +ve t axis
Answer: c
Explanation: 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.
Methods of Excitation
1. Which winding has large number of turns?
a) Shunt field
b) Series field
c) Both have same number of turns
d) Depends on requirement
Answer: a
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 series field winding has a few turns of thick wire and is excited from armature current by placing it in series with armature.
2. Which winding contains wire with higher thickness?
a) Shunt field
b) Series field
c) Both have same number of turns
d) Depends on requirement
Answer: b
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.
3. Which winding have higher resistance?
a) Shunt field
b) Series field
c) Both have same number of turns
d) Depends on requirement
Answer: a
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.
4. Which is more practical method used in control of series field?
a) Diverter
b) Tappings
c) Switch relay
d) Cannot be determined
Answer: b
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.
5. In compound excitation, which winding/s is/are excited?
a) Shunt
b) Series
c) Both
d) Cannot be determined
Answer: c
Explanation: 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.
6. In which type of excitation air gap flux increases with armature current?
a) Differential compound
b) Cumulative compound
c) Differential and Cumulative
d) Cannot be determined
Answer: b
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.
7. In differential compound excited machine, what is the variation in air gap flux per pole with respect to armature current?
a) Increases
b) Decreases
c) Remains constant
d) Always varies
Answer: b
Explanation: 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.
8. Which winding in compound excitation is responsible for change in air gap flux per pole?
a) Series
b) Parallel
c) Interconnected
d) No coil is responsible
Answer: a
Explanation: 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.
9. How short shunt and long shunt compound winding is selected?
a) Mechanical considerations
b) Switch reversal
c) Performance
d) Mechanical considerations and switch reversal
Answer: d
Explanation: In long shunt compound, the shunt field is connected across terminals. In short shunt compound, the shunt field is connected directly across the armature. There is no significant difference in machine performance for the two types of connections. The choice between them depends upon mechanical consideration or the reversing switches.
10. If a DC compound machine connected as a motor is about to use as a generator, we reverse the series field connections.
a) True
b) False
Answer: a
Explanation: 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.
11. For a long-shunt compound motoring, which of the following equation is correct?
a) Vt= Ea+ Ia(Ra+ Rse)
b) Vt= Ea– Ia(Ra+ Rse)
c) Vt= Ea+ Ia(Ra– Rse)
d) Vt= -Ea+ Ia(Ra+ Rse)
Answer: a
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.
12. Vt= Ea– Ia(Ra+ Rse) is the equation for _______________
a) Short shunt compound motoring mode
b) Short shunt compound generating mode
c) long shunt compound motoring mode
d) Cannot be determined
Answer: b
Explanation: 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.
13. How shunt field is controlled?
a) Diverter resistor in parallel
b) Tapped field winding
c) Series regulating resistance
d) Other methods
Answer: c
Explanation: 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.
14. The generator is called flat compounded if _____________
a) The series field ampere turns are such as to produce the same voltage at rated load as at no load
b) The series field turns are such as that the no load voltage is smaller than the rated load voltage
c) The rated voltage is less than the no load voltage
d) Cannot be determined
Answer: a
Explanation: 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.