Category: DC Machines Objective Questions

DC Machines Multiple Choice Questions on “Wave Winding”.

1. What will be the value of “Y_f + Y_b” for a wave winding?
a) Equal to Y_c
b) Half of the Y_c value
c) Double of the Y_c value
d) Four times Y_c value
Answer: c
Clarification: In the wave winding, as the number of coil-sides is double the number of segments, the top coil-side of the second coil will be numbered as (1+2*Y_c). After numbering other coil sides,
1 + 2*Y_c – Y_f = 1+ Y_b
So Y_f + Y_b = 2Y_c.

2. For a progressive wave winding Y_c = ______
a) 2C/P
b) 2(C+1)/P
c) 2(C-1)/P
d) 2C/(P+1)
Answer: b
Clarification: Starting at segment 1 and after going through P/2 coils or Yc (P/2) segments, the winding should end in segment 2 for progressive winding or segment (C) for retrogressive winding. That is mathematically,
Y_c (P/2) = (C+1)
Y_c = 2(C+1)/P

3. Number of parallel paths in wave winding are ______
a) Equal to P
b) Equal to P/2
c) 2
d) Depends on other parameters
Answer: c
Clarification: In wave winding all coils are divided into 2 groups- all coils carrying clockwise current are series connected and so are all coils with counter-clockwise current- and these 2 groups are in parallel because the winding is closed. Thus, a wave winding has 2 parallel paths irrespective of number of poles.

4. What is the spacing between the brushes for a wave winding when a machine is 6-pole DC armature with 16 slots having 2-coil sides per slot and single-turn coils.
a) 4 segments
b) 8 segments
c) 16 segments
d) 12 segments
Answer: b
Clarification: Only 2 brushes are required in this case as the number of poles in wave winding is equal to 2. So, spacing between the brushes is equal to total number of segments i.e. total slots divided by 2. Spacing between brushes = C/A = 16/2 = 8 segments.

5. What is the relation between conductor current and armature current in wave winding?
a) Ic = Ia
b) Ic = 2Ia
c) Ic = 4Ia
d) Ic = Ia/2
Answer: d
Clarification: the number of parallel paths in the in a wave winding is equal to 2. So, armature current will get divided equally into total number of conductors/paths. Conductor current in a wave wounded machine is half of the Ia.

6. For a conductor current equal to 4mA, Current carried by a particular brush in a 2-pole machine will be _____
a) 16mA
b) 8mA
c) 2mA
d) 10mA
Answer: b
Clarification: Conductor current in a wave wounded machine is half of the Ia. So, Ia= 8mA. All positive and all negative brushes are respectively connected in parallel to feed the external circuit. Thus, IBRUSH = Ia /(P/2). Solving we get Brush current = 8mA.

7. Equalizer rings are needed in the wave winding.
a) True
b) False
Answer: b
Clarification: The armature coil forms 2 parallel paths 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 wave winding.

8. For a wave winding when a machine is 6-pole DC armature with 16 slots having 2-coil sides per slot and single-turn coil, Yf value is ____
a) 5
b) 3
c) 2
d) 7
Answer: a
Clarification: Y_cs = 16/6 = 2 slots (nearest lower integral value)
Y_b= 2*2+1 = 5
Y_c= 2(16-1)/6 = 5 segments
Y_f = 2Y_c – Yb = 5.

9. Wave winding machines are used in ______ currents applications.
a) High
b) Moderate
c) Low
d) Can be used anywhere
Answer: c
Clarification: Lap winding machine has the advantage of large number of parallel paths and lower conductor current and is therefore used in low voltage and high current applications. Wave winding has fixed number of parallel paths so, wave wounded machine is used in low currents application.

10. For a wave wounded machine number of brushes for small, large machines respectively is ________ _________
a) 2, 2
b) 4, 2
c) 2, P
d) Both values depend on the given conditions
Answer: c
Clarification: For a small wave wounded machine number of parallel paths are 2, thus 2 brushes are used. For a large machine total number of brushes is equal to the total number of poles. The spacing between adjacent brushes is C/P commutator segments.

DC Machines, here is complete set of
1000+ Multiple Choice Questions and Answers.

DC Machines Multiple Choice Questions on “Parallel Operation of DC Generator”.

1. For connecting two generators in parallel, they should have __________________
a) Same current rating
b) Same ohmic rating
c) Same voltage rating
d) All ratings must be same
Answer: c
Clarification: For connecting two DC generators in parallel we equal voltage ratings. As the generator voltage is easily adjustable in a range, so the condition stated above is not a must. But, it is desirable condition.

2. While connecting two DC generators in parallel, which of the following is not a desirable condition?
a) Same voltage rating
b) Same percentage voltage regulation
c) Same percentage speed regulation of the prime movers
d) Same current rating
Answer: d
Clarification: Same voltage rating, same percentage voltage regulation, same percentage speed regulation are the desirable conditions for connecting two generators in parallel, though these conditions are not a must.

3. We connect two generators in parallel _____________
a) For large DC load
b) For small DC load
c) For any DC load
d) For any AC or DC load
Clarification: a
Answer: We connect two generators in parallel for supplying large DC load. It is desirable to use more than one generator in parallel. This arrangement provides the security that if one generator gives way, the other(s) can feed part load.

4. Load sharing of two generators connected in parallel is determined by __________
a) Internal characteristics
b) External characteristics
c) Both internal and external characteristics
d) It doesn’t depend on load sharing
Answer: b
Clarification: Two generators are connected in parallel such that, summation of current carried by both generators is equal to load current. In such cases we need to see external characteristics on loaded condition.

5. For a parallel operation of 2 DC shunt generators, we get net external characteristics ___________
a) Starting from the same no-load point and between generator 1 and generator 2
b) Starting from the same no-load point and below generator 1 and generator 2
c) Starting from the same no-load point and above generator 1 and generator 2
d) Can’t be determined
Answer: c
Clarification: The load sharing by these generators is determined by addition of external characteristics of both of generators. Thus, at no load generators will have common point, on y- axis so parallel characteristic will start from same point and will lie above of both.

6. When two compound generators are connected in parallel, when load current in generator 1 is increased __________
a) Both generator will share same load
b) Generator 1 will start running as motor
c) Generator 2 will be overloaded
d) Generator 2 will start running as motor
Answer: d
Clarification: If load current in G1 increases, load current in G2 decreases. Series excitation and internal voltage in G1 increases and for G2, these quantities decrease. Finally, all load shifts, which may turn to run G2 as a motor. All this leads to heavy overloading of G1.

7. While running two compound motors in parallel, we connect equalizer ring between ___________
a) Two armatures
b) Two fields
c) Two load points
d) Anywhere
Answer: a
Clarification: A low-resistance equalizer connection is made directly between the two armatures before the series fields. Any emf variations of the armatures causes equalizing circulating current which do not affect the current through the series windings. Thereby the parallel operation is stabilized.

8. When two DC series motors are connected in parallel, the resultant speed is __________
a) More than the normal speed
b) Loss than the normal speed
c) Normal speed
d) Zero
Answer: a
Clarification: When two DC series motor are connected in parallel then the resultant speed is increase the normal speed. If the voltage across each motor is assumed to be V, then current through each motor in parallel connection will be I/2. So, speed α (voltage/current) α (V/Ia/2). Hence the resultant speed is more than normal speed.

DC Machines, .

DC Machines Question Bank on “Braking of DC Motors – 2”.

1. Regenerative braking is used when duty cycle ____________
a) Requires braking of machine
b) Requires accelerating of machine
c) Requires constancy of machine
d) Cannot comment on duty cycle
Answer: a
Clarification: Regenerative braking is used specially where the duty cycle requires the braking or slowing of the machine more frequently and is most useful in holding a descending load of high potential energy at a constant speed.

2. Regeneration is not easily possible for ____________
a) DC shunt motor
b) Separately excited motor
c) Compounding motor with weak series compounding
d) DC series motor
Answer: d
Clarification: Regeneration is possible with a shunt and separately excited motors and with compound motors with weak series compounding. Series motors need a reversal of either the field or the armature connections.

3. Which of the following method is not used for regeneration?
a) Increasing field current
b) Increasing armature speed
c) Increasing supply voltage
d) Reducing supply voltage
Answer: c
Clarification: Regeneration is achieved by either increasing field current, increasing armature speed, or by reducing supply voltage. Increasing supply voltage is not the method which is employed in regeneration process.

4. If the terminals of armature of DC motor are interchanged, this action will offer following kind of electrical braking ______________
a) Regenerative
b) Plugging
c) Dynamic braking
d) Depends on other parameters
Answer: b
Clarification: Plugging is electrical braking method, where field or armature connections are reversed technically. But field reversal is not employed as results obtained from field reversal are not good compare to armature reversal.

5. The plugging braking gives the _____________
a) Zero torque braking
b) Smallest torque braking
c) Highest torque braking
d) Variable torque braking
Answer: c
Clarification: In electrical braking called plugging direct reversal of connections is done, which causes maximum torque to act on shaft but in opposite direction. As the speed decreases this torque also starts decreasing.

6. Regenerative method of braking is based on ___________
a) Back emf is less than the applied voltage
b) Back emf is equal to the applied voltage
c) Back emf of rotor is more than the applied voltage
d) Cannot be determined
Answer: c
Clarification: The condition for regeneration is that the rotational emf is more than the applied voltage so that the current is reversed and the mode of operation changes from motoring to generating.

7. During regenerative braking of DC motors ____________
a) Motor will run as a generator
b) Motor will reverse in direction
c) Motor will run at reduced speed
d) Motor will run as free rotating shaft
Answer: a
Clarification: In regenerative method of electrical braking, motor is suddenly forced to act as a generator, all the energy then obtained is pushed back into the supply unlike in dynamic braking this energy is wasted.

8. Where dynamic braking is used?
a) Shunt motors
b) Series motors
c) Compound motors
d) All DC motors
Answer: d
Clarification: Dynamic braking is used in all DC motors though its implantation in series DC motor requires one more additional step of reversal of connections. Only care taken is, addition of braking resistance, armature resistance and series field resistance is lower than the critical resistance at that speed.

9. Which method of braking is generally used in elevators?
a) Plugging
b) Regenerative braking
c) Rheostatic braking
d) Mechanical braking
Answer: a
Clarification: Plugging braking provides maximum torque in opposite direction at the instant of braking, this characteristic of braking suits perfectly with the application that is in elevators. If switch is kept ON, we get reverse rotation also.

10. For which of the following motor dynamic braking is very effective?
a) Shunt motors
b) Separately excited motors
c) Series motors
d) Differential compound motors
Answer: b
Clarification: Dynamic braking is very effective for separately excited DC motors. As in separately excited motors the direction of field can be very easily altered by altering the terminals of the field, which is the condition in dynamic braking.

11. When is the dynamic braking is employed?
a) Non-reversing drive
b) Reversing drive
c) Both Reversing and Non-reversing
d) Cannot tell
Answer: c
Clarification: Dynamic braking is employed to brake both reversing drives and non-reversing drives.
In dynamic braking, the electrical energy generated during stopping action is released as heat through a voltage regulated transistor and resistor.

DC Machines Question Bank, .

DC Machines Multiple Choice Questions on “EMF and Torque Production – 1”.

1. In which mode machine is operating, given that conductor current is in the same direction of conductor emf?
a) Motoring
b) Generating
c) Can’t be determined using directions
d) In both modes for different cycles
Answer: b
Clarification: If the conductor current is in the same direction of conductor emf then machine outputs electrical power and absorbs mechanical power. So, when mechanical power is absorbed machine is said to be in a generating mode. When conductor emf and conductor current are in opposite directions then machine is said to be in a motoring mode.

2. Nature of the flux density wave in the air gap is__________ (for armature current equal to 0)
a) Flat topped with quarter wave symmetry
b) Point topped with quarter wave symmetry
c) Flat topped with half wave symmetry
d) Point topped with half wave symmetry
Answer: a
Clarification: In a DC machine magnetic structure is such that the flux density wave in the air gap is flat topped with quarter wave symmetry as long as armature current is equal to 0. For non-zero value of armature current, this quarter wave symmetry is disturbed because of armature reaction.

3. In a DC machine, average energy stored in the magnetic field remains constant independent of the armature rotation.
a) True
b) False
Answer: True
Clarification: In a DC machine, barring the irrecoverable losses of both electric and magnetic origin, there is balance between electrical and mechanical powers of the machine; the average energy stored in the magnetic field remains constant irrespective of armature rotation.

4. Emf produced by DC machine, for zero armature current (E1) and non-zero armature current (E2) can be related as__________
a) E1 = E2
b) E1 > E2
c) E1 < E2
d) Can’t be determined
Answer: a
Clarification: In a DC machine flux density wave in the air gap is flat topped with quarter wave symmetry as long as armature current is equal to 0. For non-zero value of armature current, this quarter wave symmetry is disturbed because of armature reaction. Emf produced is independent of B-wave shape, thus we will get same value for both cases.

5. Average coil emf for 20 coil turns (E1) and 40 coil turns (E2), will have ratio E1/E2=____ (assuming all other parameters same for both machines).
a) 1/2
b) 2/1
c) 1/4
d) 4/1
Answer: a
Clarification: Emf generated in a DC machine is directly proportional to number of coil turns, Flux per pole, number of poles and armature speed in rad/s. Thus, ratio E1/E2= 20/40 (assuming all other parameters same for both machines).

6. What is the average coil emf generated in a 4-pole DC machine having flux/pole equal to 0.1 wb rotating at 1500 rpm? (No. of coil sides = 100)
a) 19 kV
b) 1.9 kV
c) 190 V
d) 19 V
Answer: a
Clarification: Average coil emf generated= ∅ωNP/π.
E= 0.1*1500*100*4/3.14
E= 60000/3.14
E≅ 19 Kv.

7. Emf and torque produced in a DC machine are proportional to ________ and _________ respectively.
a) Armature speed and armature emf
b) Armature emf and armature speed
c) Armature current and armature emf
d) Armature speed and armature current
Answer: d
Clarification: Average coil emf generated= ∅ωNP/π. Machine torque = ka*∅*Ia. Thus, average coil emf generated can also be represented as ka*∅*ω. So, average coil emf is directly proportional to ω (armature speed) and average torque is directly proportional to Ia (armature current).

8. What is the value of Np in an average coil emf equation, for 10 armature conductors with 2 parallel paths?
a) 2
b) 3
c) 2.5
d) 4
Answer: c
Clarification: In an emf equation Nc= Cp * Np. Here, Cp= coils/ parallel path. Np is defined as number of turns per parallel paths which is also called as ratio of total armature conductors to the twice of number of parallel paths. Np= 10/(2*2)= 10/4= 2.5.

9.What is the torque equation in terms of B, Ic, l, Zr (r= mean air gap radius)?
a) Bav*Ic*l*Zr
b) Bav*Ic*l/Zr
c) Bav*Ic*Zr/l
d) Can’t be expressed
Answer: a
Clarification: Avg. conductor force f= Bav*l*Ic. Here, Bav= Average flux density over pole, l= acyive conductor length. Thus, torque T= Z*f = Bav*l*Ia*Z. This torque is constant because both the flux density wave and current distribution is fixed in space at all times.
T developed= Bav*Ic*l*Zr (Here, r= mean air gap radius).

10. What is the value of pole pitch (in SI unit) for mean air gap radius= 0.5mm and P=4?
a) 0.785* 10^-6
b) 0.785* 10^-3
c) 0.785* 10^-2
d) 0.785* 10^-4
Answer: b
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. Pole pitch can be calculated as ratio of 2πr/P.
Pole pitch= 2*3.14* 0.5* 10^-3 / 4= 0.785* 10^-3 m.

DC Machines, .

DC Machines Multiple Choice Questions on “Operating Characteristics of DC Shunt Motor -1”.

1. For machine tools, which DC motor can be used?
a) DC Series motor
b) DC Shunt motor
c) DC cumulative compound motor
d) DC differential compound motor
Answer: b
Clarification: Shunt characteristics is the speciality of DC shunt motor. The speed of the motor almost remains constant on various loads, thus it suits perfectly to the application, where speed requirement is constant like in machine tools.

2. In a DC shunt motor, speed is related to armature current as _____________
a) Directly proportional to the armature current
b) Proportional to the square of the current
c) Independent of armature current
d) Inversely proportional to the armature current
Answer: d
Clarification: When armature reaction is ignored in a DC shunt motor, flux almost remains constant but the speed of the motor decreases according to the increase armature current, increasing Ia*Ra drop. Hence, inverse proportionality.

3. In a DC shunt motor for zero armature current we get speed ______
a) Non-zero and minimum
b) Zero
c) Non-zero and maximum
d) Doesn’t depend on armature current
Answer: c
Clarification: For zero armature current we get some non-zero value, indicated by positive intercept on speed characteristics. As armature current is increased speed of DC shunt motor starts decreasing due to increase in voltage drop at armature resistance.

4. What will be the effect of opening of field of a DC shunt motor while motor is running?
a) The speed of motor will be reduced
b) The armature current will reduce
c) The motor will attain dangerously high speed
d) The motor will continue to constant speed
Answer: c
Clarification: In a DC shunt motor if supply for the field winding is cut down, the speed would dangerously increase in order to maintain the back emf of the motor. For a constant back emf, flux is inversely proportional to the speed of DC shunt motor. So, if flux drops to zero theoretically speed will tend to infinity.

5. What will be the effect of reducing load on DC shunt motor?
a) Speed will increase abruptly
b) Speed will increase in proportion to reduction in load
c) Speed will remain almost constant
d) Speed will reduce
Answer: c
Clarification: For DC shunt motor, speed-armature current characteristics is called as a shunt characteristic as speed almost remains constant. Thus, by reducing load speed will increase negligibly, thus remaining almost constant.

6. How speed of the DC shunt motor can be increased?
a) Decreasing the field current
b) Increasing the field current
c) Decreasing the load current
d) Increasing the armature current
Answer: b
Clarification: For a constant load, load current will remain constant. Decreasing armature current will help in increasing speed for DC shunt motor. Since load current is addition of armature current and field current we’ll get less armature current for more field current.

7. Practical reason behind speed of DC shunt motor is proportional to back emf only is ____________
a) Back emf is equal to armature drop
b) Flux is proportional to field current
c) Flux is proportional to armature current
d) Flux is practically constant in DC shunt motors
Answer: d
Clarification: The field winding in DC shunt motor is connected in parallel to the armature winding and the supply. If we assume that the supply voltage for motor is constant then flux also becomes constant. At the rated speed the back emf also becomes almost constant if the load is same.

8. In a DC shunt motor, what will be the armature current at maximum load?
a) Almost negligible
b) Rated full-load current
c) Less than full-load current
d) More than full-load current
Answer: d
Clarification: As the load will increase in rapid manner, speed change in DC shunt motor will be visible. As load increase, speed will decrease, though by some small value. From, current armature characteristic armature current will be more than the earlier case.

9. In which of the following motor, ratio of starting torque to full-load torque will be least?
a) DC series motors
b) DC shunt motors
c) DC compound motors
d) Synchronous motors
Answer: b
Clarification: From torque current characteristic, we get that torque at no load or starting torque is equal to zero. As load increases, speed decreases and armature current increases so as torque also increases linearly, if effect of armature reaction is neglected.

DC Machines, .

DC Machines Multiple Choice Questions on “Efficiency of DC Machine- 1”.

1. The efficiency of the DC motor at maximum power will be ___________________
a) 100%
b) Around 90%
c) Anywhere between 75% and 90%
d) Less than 50%
Answer: d
Clarification: For getting maximum power, derivative of power with respect to current is equal to 0. This is practically impossible to achieve as, current required is much more than its normal rated value. Large heat will be produced in a machine and efficiency of motor will be less than 50 %.

2. The hysteresis loss in a DC machine least depends on _____________
a) Frequency of magnetic reversals
b) Maximum value of flux density
c) Volume and grade of iron
d) Rate of flow of ventilating air
Answer: d
Clarification: As iron core of the armature is rotating in magnetic field, some losses occurs in the core which is called core losses. These losses are categorized as Hysteresis loss and Eddy current loss. They depend on all quantities listed above.

3. Which of the following is not the effect of iron loss?
a) Loss of efficiency
b) Excessive heating of core
c) Increase in terminal voltage
d) Rise in temperature of ventilating air
Answer: c
Clarification: Iron loss causes excessive heat production in the core of a machine, which will rise the temperature of ventilating air, as it acts as heat exchanger. Thus, terminal voltage rise is not an effect of any loss.

4. Which of the following loss is likely to have highest proportion at rated load of the DC generator?
a) Hysteresis loss
b) Field copper loss
c) Armature copper loss
d) Eddy current loss
Answer: c
Clarification: Armature copper loss is directly proportional to the square of armature current multiplied by the armature resistance and also the series field resistance if present any. As, at loaded condition armature current is very high.

5. Which of the following loss in a DC generator varies significantly with the load current?
a) Field copper loss
b) Windage loss
c) Armature copper loss
d) Cannot be determined
Answer: c
Clarification: Armature copper loss is directly proportional to the square of armature current, as load current varies armature current varies, which is reflected significantly in loss as a square of it. Thus, armature copper loss can be detected.

6. Which of the following methods is likely to result in reduction of hysteresis loss in a DC generator?
a) Providing laminations in armature core
b) Providing laminations in stator
c) Using non-magnetic material for frame
d) Using material of low hysteresis co-efficient for armature core material
Answer: d
Clarification: Providing laminations will work for reducing eddy current losses, but hysteresis loss is dependent on the material chosen. Thus, using different material for core of armature will definitely work.

7. Which of the following loss/losses in a DC generator is dissipated in the form of heat?
a) Mechanical loss
b) Core loss
c) Copper loss
d) Mechanical, Copper and Core
Answer: d
Clarification: All the losses listed above dissipate the heat. This, dissipated heat due to various losses results in increasing the temperature of ventilating air. These losses are dangerous in long running of a machine, can reduce efficiency also.

8. Which of the following losses are significantly reduced by laminating the core of a DC generator?
a) Hysteresis losses
b) Eddy current losses
c) Copper losses
d) Windage losses
Answer: b
Clarification: Hysteresis losses can be minimized by using material with lower hysteresis coefficient. Eddy current losses can be minimized by using laminated sheets of core structed together. To reduce other losses, current should be minimized, can’t be reduced as it also has lower limit.

9. The total losses in a well-designed DC generator of 10 kW will be nearly equal to ________
a) 100 W
b) 500 W
c) 1000 W
d) 1500 W
Answer: b
Clarification: Total losses in a DC machine can be approximated to 4-5% of its rating from the experimental observations. Thus, 5% of 10 kW is equal to 500 W. It’s an approximation formed on various observations.

DC Machines, .