Category: DC Machines Objective Questions

DC Machines Problems on “External and Armature Characteristics of DC Generator”.

1. Armature characteristic is _____________
a) No-load characteristic
b) Load characteristic
c) OCC Current
d) Cannot be determined
Answer: b
Clarification: It is the presentation of I_a vs I_f with V_t held constant (at rated value) and generator run at constant n and load varied. It reveals the armature reaction effect on the flux/pole. It is also called regulation characteristic.

2. External characteristics is drawn with constant term/s ____________
a) Field current
b) Speed
c) Both Field current and Speed
d) Load
Answer: c
Clarification: External characteristic is a load characteristic. In this, variation of terminal voltage across armature current is drawn keeping speed and field current constant. Load is generally kept variable.

3.Armature characteristic is drawn across ________
a) I_a vs I_f
b) I_a vs V_t
c) I_f vs V_t
d) E_a vs I_a
Answer: a
Clarification: Armature characteristic is drawn across I_a vs I_f. This characteristic is on load characteristic, drawn when terminal voltage and speed of the generator is kept constant. Armature characteristic is regulation characteristic.

4. what is the first step in drawing the armature characteristic?
a) I_f is made 0
b) Field current is adjusted to give Voc equal to rated value
c) Switch connecting armature circuit with external circuit is made open
d) Cannot be determined
Answer: c
Clarification: While drawing armature characteristic, switch of armature circuit is made open so that armature current is set to 0. Then, field current is adjusted to give V_oc equal to rated value. Switch its then closed.

5. In armature characteristic, nature of If at lower values of armature current is ______
a) Very large
b) Almost constant
c) Decreasing rapidly
d) Increasing then decreasing
Answer: b
Clarification: It is seen from the characteristic that at low values of I_a, the increase in I_f is very small to provide for increasing Ia *Ra drop. At large values of Ia there is a sharp increase in If to compensate for voltage drop caused by armature reaction.

6. At some point If On OCC, the value of voltage obtained which is more than rated voltage is _______
a) I_a *R_a + I_f *R_f
b) I_a *R_a
c) V_d + I_a *R_a
d) Can’t tell
Answer: c
Clarification: The difference in the observed value and rated value of voltage can be obtained from the y-axis that is, from V_oc. This difference is equal to V_d + I_a *R_a. Thus, by subtracting voltage drop in armature V_d can be found.

7. Which of the following is the correct formula for cumulative compound generator?
a) N_se= N_f (ΔI_a/I_f)
b) N_se= N_f (ΔI_f/I_a)
c) N_se= 2N_f (ΔI_a/I_f)
d) N_se= N_f (ΔI_a/I_f)/2
Answer: b
Clarification: According to proportionality constant obtained by analysis of OCC, various value of armature current can be obtained. From ΔI_f values we can compute number of turns in series of armature in cumulative compound machine.

8. External Characteristic is a plot of _____________
a) Generated emf and load current
b) Terminal voltage and load current
c) Generated voltage and field current
d) Armature current and field current
Answer: b
Clarification: The load characteristic of a dc generator at a particular speed is the relationship between its terminal voltage and load current (line current) and is also termed as the external characteristic. The internal characteristic is the plot between the generated emf and load current.

9. External characteristics for all separately DC generators (except series generator) can be summarized as ____________
a) Parabola opening in +x direction
b) Parabola opening in -x direction
c) Parabola opening in +y direction
d) Parabola opening in -y direction
Answer: b
Clarification: All parabolas for separately excited DC generator, shunt generator, compound generator are parabolas which are opening in -x directions, only centres of all parabolas lie at different places depending on the type of generator.

DC Machines Problems, .

Basic DC Machines Questions on “Speed Control Using Field Control of Series Motor”.

1. Which of the following DC motor has the poorest speed control?
a) Differentially compounded motor
b) Cumulatively compounded motor
c) Shunt motor
d) Series motor
Answer: d
Clarification: DC series motor at no load condition gives infinite speed ideally. Practically it will damage all the armature circuit. Thus, as the load is reduced speed of the motor will go on increasing rapidly. So, speed control is very poor in series motor.

2. In variable speed motor ____________
a) Stronger commutating field is needed at low speed than at high speed
b) Weaker commutating field is needed at low speed than at high speed
c) Same commutating field is needed at low speed and at high speed
d) Cannot be determined
Answer: b
Clarification: According to commutating fields of machine, speed and overall performance of a machine is determined. For low speeds of motor in a variable speed motor we need weaker commutating field.

3. The speed of a motor falls from 1200 rpm at no-load to 1050 rpm at rated load. The speed regulation of the motor is ____________
a) 12.36%
b) 14.28%
c) 16.77%
d) 18.84%
Answer: b
Clarification: Speed of regulation of DC series motor is given by ratio of difference of no-load speed and full load speed with full load speed. Thus, in this case, speed regulation = (1200-1050)/1050= 14.28%.

4. Which of the following is not the method of speed control in DC series motor?
a) Diverter
b) Tapped-field control
c) Variable resistance in series with armature
d) Series- parallel control
Answer: c
Clarification: Diverter means adding a variable resistance in parallel with field winding. Tapped field control is the method where field ampere turns are adjusted in steps by varying the number of turns included in the circuit. In series parallel method field winding is connected so that it will form two parts.

5. For speed reversal, field control method is suitable.
a) True
b) False
Answer: b
Clarification: Field control method is not suited to applications needing speed reversal; since the only way to reverse speed is to disconnect the motor from the source and reverse the field/armature polarity. The field circuit being highly inductive, it is normally the armature which is reversed.

6. For large motors what is the ratio of compensating winding is required for increasing the speed of the motor?
a) 2:1
b) 4:1
c) 6:1
d) 8:1
Answer: a
Clarification: For motors requiring a wide range of speed control, the field ampere-turns are much smaller than the armature ampere-turns at high speeds causing extreme distortion of the flux density in the air-gap. This leads to poor commutation. Compensating winding can be used to increase the speed range which can be 2 to 1 for large motors.

7. In diverter resistor field control method of DC series motor, variable resistor is added ________
a) In parallel with field
b) In series with field
c) In parallel with armature
d) In parallel with load
Answer: a
Clarification: Diverter is a small variable resistor, which is connected in parallel with field winding. By varying this resistance value, the field current and consequently the field ampere-turns can be varied and speed can be controlled.

8. Why it is advisable to use inductively wound diverter resistor?
a) To make speed control on more range
b) For long-life of machine
c) Cost efficient
d) To avoid oscillations in speed
Answer: d
Clarification: One precaution to be taken in this method in order to avoid oscillations in speed initiated by load changes is to use an inductively wound diverter resistor. Thus, speed controlled output will vary smoothly when load is changed.

9. For higher diverter resistance, speed-torque characteristic will lie ____________
a) Above speed-torque characteristic of lower resistance
b) Below speed-torque characteristic of lower resistance
c) On the speed-torque characteristic of lower resistance
d) Cannot say
Answer: b
Clarification: Speed-torque characteristic for decreasing values of diverter resistance or decreasing value of k_d will lie above of the previous or lower value of diverter resistance. Thus, for higher diverter value curve will lie below the previous one.

10. In tapped field control method ____________
a) A variable resistor is connected in parallel
b) A variable resistor is connected in series
c) Field winding is made with more out pins
d) Another field winding is added with the previous one
Answer: c
Clarification: In tapped field control of the DC series motor a field winding is simply tapped at various points and many out pins taken, so that any out pin can be connected and particular part of field winding will be selected in the circuit, while other part is made open.

11. Which of the following method will not give many speed values for a DC series motor?
a) Diverter
b) Series-parallel
c) Field tapped
d) All field control methods will give many speed values
Answer: b
Clarification: In series-parallel method, field winding is equally divided in two parts which are connected in series and parallel with respect to each other. Thus, in series connection we’ll get one speed and in parallel mode we’ll get one speed. These 2 speeds are only possible.

12. In series-parallel speed control method ______________
a) Speed given by parallel connection is more
b) Speed given by series connection is more
c) Both speeds can be equal
d) Will depend on other parameters
Answer: a
Clarification: In parallel case, total ampere turns will be exactly half of the ampere turns of the series case. Since speed of any DC motor is inversely proportional to the MMF or flux or ampere turns developed, speed of the parallel control method will always large than the series one.

13. Where diverters are used?
a) In shunt motors
b) In series motors
c) In both motors
d) All other motors except shunt and series motors
Answer: b
Clarification: Diverters are used in series motors for speed control, they are not used in shunt motors. Since, shunt field winding resistance is very high, if we connect a diverter across shunt field winding, total current will flow through diverter and it will almost short circuit the shunt field winding. This will increase the motor speed to a very high value.

14. What will happen to the speed of a series motor if the temperature of armature resistance is increased?
a) Not change
b) Decreases
c) Increases
d) Cannot be determined
Answer: b
Clarification: In a DC series motor, when the resistance increases due to rise in temperature, the IR drop in the circuit will increase, which reduces the effective voltage applied to the armature, hence speed decreases due to the direct proportionality.

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DC Machines Multiple Choice Questions on “PMDC Motors”.

1. In PMDC motors we use magnetic material with ___________
a) Low residual flux
b) Low coercivity
c) High residual flux
d) All magnetic materials
Answer: c
Clarification: In market magnetic materials with various properties are available, but for DC motor application we need magnetic materials with high residual flux in order to get high no load voltage. Along with residual flux, coercivity of material should be high.

2. In PMDC magnetic material is placed instead of _____________
a) Armature
b) Rotor
c) Stator
d) Can be placed anywhere
Answer: c
Clarification: In PMDC motors we keep magnetic material or magnetic poles in the place of field winding of original DC machine. That is we are changing stator windings with two permanent magnetic poles bounded on casing.

3. PMDC motors are _________________
a) Smaller than corresponding rated field wound motor
b) Larger than corresponding rated field wound motor
c) Almost same size that of corresponding rated field wound motor
d) Can be smaller or greater than corresponding rated field wound motor
Answer: a
Clarification: PMDC motors are always of smaller size compare to field wound motors of same rating. Here, field winding is replaced by small block of magnet so that power output remains same. PMDC motors are costly compare to DC motor of same rating.

4. PMDC motor offers ___________
a) Cumulative compound type characteristic
b) Differential compound type characteristic
c) Series characteristic
d) Shunt characteristic
Answer: d
Clarification: The stator is made of permanent magnet in the PMDC motor hence, there is no need for separate field excitation for stator. So, PMDC acts as a shunt motor with fixed field. This can be seen from the construction also.

5. Which speed control method is employed in PMDC?
a) Field control method
b) Armature control method
c) Armature Field control method
d) Cannot be determined
Answer: b
Clarification: Unlike in DC motors, field is provided by permanent magnets in PMDC. This implies that field provided in PMDC motor is permanent and can’t be altered unless whole construction is modified. Thus, armature voltage control method is employed here.

6. What is the air gap flux density for PMDC motor?
a) B_g= µ₀(t_m/l_g) H_m
b) B_g= – µ₀H_m
c) B_g= – µ₀(t_m/l_g) H_m
d) B_g= – µ₀(lg/t_m) H_m
Answer: c
Clarification: Since, flux crosses the air gap length lg two times a thickness of permanent magnet two times tm. We get, 2*l_g* H_g + 2*t_m H_m = 0 and we know that air gap flux density is equal to µ times the magnetic field intensity.

7. Which of the following is most favoured choice for PMDC motor?
a) Neodymium-iron-boron
b) Iron-nickel
c) Nickel-neodymium
d) Cannot be given
Answer: a
Clarification: Obvious choice of PMDC motor is neodymium-iron-boron which has high coercivity and high retentivity. Its characteristic is almost a straight line. Hence, neodymium-iron-boron combination is preferred over the other materials.

8. Speed-torque characteristic of PMDC motors is _______________
a) Starting from zero increasing continuously
b) Starting from some positive value and remaining constant
c) Starting from some positive value and decreasing exponentially
d) Starting from some positive value and decreasing on straight line
Answer: d
Clarification: The material chosen for PMDC motor is such that speed torque characteristic of a given motor remains straight line for wider range of torque, for wider range of armature voltage. The straight line is such that it makes positive intercepts on both axes.

9. PMDC produces high torque on ______________
a) At low speeds
b) Only at maximum speed
c) Torque is constant
d) Cannot say
Answer: a
Clarification: PMDC produces high torque even at very low speeds. In DC shunt motor torque produced at very low speeds is lower than the torque produced at the same speeds in PMDC motor.

10. PMDC gives better speed regulation than DC shunt motor.
a) True
b) False
Answer: a
Clarification: PMDC motor exhibits better speed regulation and efficiency than dc shunt motor. The main problem of dc shunt motor is going to run-away when the field terminals are opened. But in PMDC there is no run-away problem, so it gives practical benefit to the industry applications.

11. If higher terminal voltage is applied to PMDC what will happen?
a) Speed at 0 torque will be reduced
b) Torque at 0 speed will be reduced
c) Torque at 0 speed will be increased
d) Torque will remain constant
Answer: c
Clarification: The straight-line characteristic of PMDC is such that it produces positive intercepts on both axes. If terminal voltage is increased both of these intercepts increases, slope remaining constant. Thus, speed and torque at initial conditions both will increase.

DC Machines, .

DC Machines Multiple Choice Questions on “Characteristics of Self Excited Generator”.

1. What is Self-excitation in DC shunt generator?
a) Field winding is connected in series of armature
b) Field winding is connected in parallel of armature
c) Field winding is not connected to the armature
d) Field Winding is not excited
Answer: b
Clarification: Rather than arranging a separate dc source for excitation purposes, practical generators are always excited from their own armature terminals, this method of excitation being known as self-excitation. A self-excited generator with such connection known as a shunt generator.

2. For a self-excited DC shunt generator I_a = _____________
a) I_L – I_f
b) I_L + I_L
c) – I_L – I_L
d) – I_L + I_L
Answer: b
Clarification: For a self-excited DC shunt generator field winding is connected in parallel with armature winding so that, armature current splits into load current and field current. Load current will be given out as the machine is working as a generator.

3. For self-excited DC generator, field circuit curve in the OCC will be __________
a) Straight line not passing through the origin with +ve slope
b) Straight line passing through the origin
c) Straight line not passing through the origin with -ve slope
d) Increasing curve
Answer: b
Clarification: Field is connected such that this current increases the field mmf and therefore the induced emf, the machine will continuously build up. For the field circuit V = I_f *R_f, which is a straight-line relationship, called the Rf -line, in V-I_f plot.

4. Intersection of Rf line and magnetization curve will give _____________
a) No load terminal voltage
b) Field current at no load
c) Both a and b
d) Not specific data
Answer: c
Clarification: When OCC is plotted against terminal voltage and field current where, field resistance line is also drawn, an intersection point will arise when OCC almost becomes constant due to saturation effect. So, corresponding value field current and no-load voltage can be observed from x and y axis resp.

5. What will happen if field resistance is decreased?
a) No load voltage will decrease
b) No load voltage will increase
c) No change in no load voltage
d) Field current will decrease
Answer: b
Clarification: When field resistance is decreased, it will increase field current by Ohm’s law. Thus, field resistance line will lie below the older line of field resistance. Eventually, it’ll intersect OCC afterwards causing increase in no-load voltage.

6. No-load voltage can’t be defined where _____________
a) Field resistance line lies below the magnetization linear line
b) Field resistance line intersects the magnetization linear line
c) Field resistance line coincides the magnetization linear line
d) Defined everywhere
Answer: c
Clarification: We get no-load voltage when magnetization curve intersects with field resistance line. If that intersection is not occurring, we’ll not get no load voltage. Thus, when both lines coincide we get undefined value of no-load voltage.

7. Machine does not excite to an appreciable value when ___________
a) Field resistance is less than critical resistance
b) Field resistance is more than critical resistance
c) In both cases a and b
d) Depends on other parameters
Answer: b
Clarification: The no-load voltage is undefined for a field resistance whose line coincides with the linear portion of the magnetization curve. With field resistance even slightly more than this value, the machine does not excite to any appreciable value.

8. No-load voltage observed at two different field resistance values, is V1 < V2. What will be the relation in field resistance values?
a) Rf₁ < Rf₂
b) Rf₁ = Rf₁
c) Rf₁ > Rf₁
d) Can’t determine from no-load voltage
Answer: c
Clarification: When field resistance value increases the line of field resistance shifts towards more +ve slope, proving that field current decreases. This line will now intersect OCC at lower regions indicating lower values of terminal voltage.

9. No-load voltage observed at two different field resistance values, is V1 < V2. What will be the relation in speed if field resistance is kept same for both cases?
a) N₁ < N₂
b) N₁ = N₁
c) N₁ > N₁
d) Can’t determine from no-load voltage
Answer: a
Clarification: For Field resistance kept constant, field resistance line will intersect OCC at different points as speed variation may shift OCC up or down. When speed is increased OCC shifts up indicating that no-load voltage will also rise up.

10. DC generator will not self-excite in __________
a) Positive feedback
b) Negative feedback
c) Zero Feedback
d) Doesn’t depend on feedback
Answer: b
Clarification: The field connection to the armature is such that the induced emf due to the residual magnetism tends to destroy the residual magnetism, is called negative feedback. In the absence of residual magnetism machine will fail to self-excite.

11. Which of the following is not a reason, behind failing of self-exciting machine?
a) Residual magnetism is absent
b) Negative feedback
c) Field resistance value is more than critical resistance value
d) Armature resistance is non-zero
Answer: d
Clarification: In case a and b, machine will tend to destroy the residual magnetism present in a core, which will not excite machine. When field resistance value is more than critical value as seen from OCC, machine will not excite to appropriate value.

12. By changing the polarity of the armature of failed self-exciting machine, machine can be started.
a) True
b) False
Answer: a
Clarification: By changing the polarity of field connections to the armature negative feedback can be turned into positive feedback, thus it will help to build up the voltage and machine will act good in self-excitation mode.

13. Which of the following will be useful to generate positive feedback?
a) Increasing the field resistance
b) Increasing the armature resistance
c) Decreasing the armature resistance
d) Changing the shaft direction
Answer: d
Clarification: Negative feedback can be remedied simply by reversing the field connection to the armature or reversing the direction of rotation. For large dc generators with permanent connections and a fixed direction of rotation, the problem is overcome by temporarily exciting the field from a battery source.

DC Machines, .

DC Machines Questions and Answers for Aptitude test on “Speed Control Using Armature Control in DC Motor”.

1. For which speed control method we get minimum efficiency?
a) Voltage control method
b) Field control method
c) Armature control method
d) Cannot be determined
Answer: c
Clarification: In armature control method, we vary armature voltage by adjusting variable resistance value. So, when we define armature efficiency, it is equal to η = [ (V_t-I_aR_e) I_a] / V_tI_a= 1-I_aR_e / V_t. Thus, efficiency is reduced drastically for large speed reductions.

2. The speed of a DC motor can be varied by changing ______________
a) Field current
b) Applied voltage
c) Resistance in series with armature
d) Field current, applied voltage or resistance in series with armature any method will work
Answer: d
Clarification: In field control method we change the flux produced by machine with the help of field current. If applied voltage is changed, definitely speed will change. By adding resistance in series with armature voltage of the armature can be varied to change the speed.

3. For constant torque drive which of the following speed control method is preferred?
a) Field control
b) Armature voltage control
c) Shunt armature control
d) Voltage control
Answer: b
Clarification: Armature control method provides a constant-torque drive. In the shunt motor case by keeping the field current at maximum value full motor torque can be obtained at full-load armature current at all speeds.

4. When the armature of a DC motor rotates, emf induced is in machine is called as ___________
a) Self-induced emf
b) Mutually induced emf
c) Back emf
d) Cannot be determined
Answer: c
Clarification: According to terminology used in DC machines, emf induced in a machine when armature rotates is called as back emf. This back emf value is directly proportional to the speed of the motor.

5. Which of the following method is used for DC motor with 12+ HP requiring frequent start, stop, speed reversal?
a) Drum type controller is used
b) Three-point starter is used
c) Four-point starter is used
d) Cannot be determined
Answer: a
Clarification: Drum controllers are used when an operator is controlling the motor directly/manually. The drum controller is used to start, stop, reverse, and vary the speed of a motor, by its mechanism. This type of controller is used on crane motors, elevators, machine tools, and other heavy applications.

6. Flux density distribution is distorted by armature control method.
a) True
b) False
Answer: b
Clarification: Unlike field control method, the main field ampere-turns are maintained at a large value, flux density distortion caused by armature reaction is limited. Unlike field control scheme, speed reversal can be easily implemented here.

7. In rheostatic series control method of armature we add ____________
a) Variable resistor in parallel with armature
b) Variable resistor in series with armature
a) Fixed resistor in parallel with armature
b) Fixed resistor in series with armature
Answer: b
Clarification: In rheostatic series control method, we add variable resistance in series with the armature so that armature voltage can be varied, by varying the voltage drop at series resistor. By increasing value of series resistor speed can be decreased.

8. By series armature resistance method, we can get _________________________
a) Speed above rated speed
b) Speed equal to rated speed
c) Speed below rated speed
d) All speeds are possible
Answer: c
Clarification: The drawback of armature speed control method is we get only speeds which are below the rated speed. As the value of series resistance increases, back emf decreases which result in lowering of the speed.

9. Consider the armature control method, where 10 Ω resistance is connected in series with armature in first case. For second case resistance value is changed 100 Ω. Then ____________
a) Speed-torque characteristic of both will start from same point
b) For first case speed-torque characteristic will start below to the case 2
c) For first case speed-torque characteristic will start above to the case 2
d) Can’t say because other parameters are unavailable
Answer: a
Clarification: Speed-torque characteristic will start from same point because at zero torque presence of series resistance is not taken into account. Afterwards speed torque characteristic with higher series resistance value will lie below the other.

10. Speed regulation of armature series control method is __________
a) Very good
b) Zero
c) Poor
d) Cannot comment
Answer: c
Clarification: The speed regulation of the method is poor as for a fixed value of series armature resistance, the speed varies directly with load, being dependent upon the resistance voltage drop. In general, rheostatic control is economically feasible only for very small motors (fractional kW) or for short-time, intermittent show-downs for medium-sized motors.

11. In shunted rheostatic armature control method ______________
a) Variable resistor is added in parallel with armature
b) Variable resistor is added in series with armature
c) Variable resistor is added in parallel with armature and another variable resistor is added in series
d) Variable resistor is not added in whole circuit
Answer: c
Clarification: In shunted armature speed control method we add one variable resistance in series with armature and one variable resistance in parallel with armature, so by varying the ratio of this resistances we can get various different speeds.

12. Which of the following is correct statement?
a) Speed regulation of rheostatic armature control method is better than that of shunted armature control
a) Speed regulation of rheostatic armature control method is worse than that of shunted armature control
a) Speed regulation of rheostatic armature control method is almost equal to that of shunted armature control
a) Speed regulation of rheostatic armature control method and of shunted armature control are equally worst.
Answer: b
Clarification: Speed regulation of shunted armature control method is better than that of rheostatic armature control. External power loss in shunted armature control method is very high compare to that of rheostatic armature control method.

13. By series parallel method of armature control how many different speeds are possible?
a) 4
b) 8
c) 2
d) Infinite
Answer: c
Clarification: Here two identical motors are coupled together mechanically to a common load. Two speeds at constant torque are possible in this method—one by connecting the motors armatures in series and the other by connecting them in parallel.

14. Which method is best for series traction motors?
a) Rheostatic armature control
b) Shunted armature control
c) Series parallel control method
d) Any method can be employed
Answer: c
Clarification: Series-parallel control method is superior to the rheostatic control insofar as efficiency is concerned. However, it is limited to two speed steps. The method is commonly employed for speed control of series traction motors.

DC Machines for Aptitude test, .

DC Machines Multiple Choice Questions on “Armature Winding and Commutator”.

1. Armature winding is mounted on a __________
a) Stator
b) Rotor
c) Can be mounted anywhere on stator or rotor
d) Not required
Answer: b
Clarification: A DC machine is a heteropolar structure with stationary poles and the rotating armature. Field coil is mounted on stator. In an AC machine armature winding is located at rotor while field coil is located at stator. An ‘armature’ is a moving part as it a ‘rotor’.

2. In a DC machine, how coil-side emf varies towards the outer side of poles?
a) Decreases
b) Remains same
c) Increases
d) First increases the decreases
Answer: a
Clarification: Coil side current pattern is the same as the emf pattern. Only difference is that while the coil-side emf reduces towards the outer side of poles, the current remains the same in all the coil-sides except for alterations from pole to pole, while the coil side current reverses, the current exchanged with external circuit must be unidirectional.

3. Commutator performs rectification so that output of the machine is unidirectional.
a) True
b) False
Answer: a
Clarification: Since it is a DC machine the generated output must be DC. Any electrical machine works on induced emf concept which is AC in nature. Commutator and brush assembly of the DC machine performs the mechanical rectification process so; induced AC is converted into DC(Unidirectional).

4. What is the difference of DC voltages in the adjoining Brushes?
a) Depends on the Shaft speed
b) Zero
c) Non-zero
d) Depends on the various other parameters
Answer: b
Clarification: Brushes are located electrically in the magnetically neutral region. Due to their location adjoining brushes are at constant DC voltage and the coil in series between the constitute one parallel path.

5. What is the effect of armature coils at points where brushes are located?
a) Induces positive emf
b) Induces negative emf
c) Induces zero emf
d) Depends on the speed of rotor
Answer: c
Clarification: Brushes are at magnetically neutral region hence, induced emf due to armature coils at brushes will be equal to zero. As in the magnetically neutral region change in flux will be equal to the zero, emf will not be induced (Faraday’s law).

6. As the armature rotates, the number of coils in series tapped by the brush pairs_________
a) Remains same
b) Increases
c) Decreases
d) Depends on rotor speed and direction of torque
Answer: a
Clarification: Brushes are located electrically in magnetically neutral region. Adjoining brush pairs are at constant DC voltage and the number of coils tapped by brush pairs also remain constant. Their disposition relative to the poles is the same.

7. Coil span for 4-pole, 12-slot armature winding is_______
a) 24
b) 48
c) 8
d) 3
Answer: d
Clarification: Coil span is defined as a ratio of number of slots in the armature winding which are also equal to the number of commutator segments to the number of poles. Here, Slots in the armature winding= 12, Number of poles= 4.
Y_CS= 12/4= 3.

8. What is the nature of the coils when Y_CS value is non-integral?
a) Long-pitched
b) Medium-pitched
c) Short-pitched
d) Can’t be determined by Y_CS value
Answer: c
Clarification: Coil-side voltages around the coil are additive most of the time (except when coil-sides lie near the magnetic neutral region). Thus Y_CS= Nearest lower integer, which means that for non-integral S/P, the coils are short-pitched.

9. For a 2-pole DC machine with coil span equal to 6, what are the number of commutator segments?
a) 3
b) 12
c) 4
d) 8
Answer: b
Clarification: Coil span is defined as a ratio of number of slots in the armature winding which are also equal to the number of commutator segments to the number of poles. Here, Number of poles= 2, Slots in the armature winding= S= P* Y_CS. Here, Y_CS= 6, P=2. Thus S=12. C=S=Number of commutator segments.

10. When coil sides are pole pitch apart, the DC armature winding is called as ___________
a) Multiplex
b) Fractional-pitch
c) Full-pitch
d) Pole-pitch
Answer: c
Clarification: Pole pitch is called as center to center distance between two adjacent poles. When measured in electrical degrees one pole itch is equal to 1800. Coil span is simply a peripheral distance between two sides of a coil. If the coil span is equal to the pole pitch, then the armature winding is said to be full pitched coil.

DC Machines, .