Design Of Electrical Machines MCQs – Thermal Design Aspects of Electrical Machines & D.C. Machines MCQs ( Design Of Electrical Machines ) MCQs
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Design Of Electrical Machines MCQs – Thermal Design Aspects of Electrical Machines & D.C. Machines MCQs ( Design Of Electrical Machines ) MCQs
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Rating of Machines
1. What is the concept of power rating of machines with respect to voltage?
a) the required supply voltage for smooth running of the machine
b) the required supply voltage for stopping the machine
c) the required supply voltage for speeding the machine
d) the required supply voltage for slowing up the machine
Answer: a
Explanation: Power rating is nothing but the standard value at which the machine is said to be safe in operation. Rating determines the voltage which allows the smooth running of the machine.
2. What is the concept of power rating of machines with respect to current?
a) Maximum permissible amount of current that can easily flow
b) Minimum permissible amount of current that can easily flow
c) Maximum permissible amount of current that can stop the machine
d) Maximum permissible amount of current that can stop the machine
Answer: a
Explanation: Power rating always deals with two variables, one is current and the other is voltage. When it comes to the current, it is the maximum permitted current that can be allowed into the machine.
3. What happens if there is insufficient rating of the machine?
a) The efficiency of the machine increases
b) The efficiency of the machine improves
c) Damage and shutdown occurs
d) Loading problems occur
Answer: c
Explanation: When there is insufficient rating, it can lead to the damage of the windings of the machine. It indirectly, leads to the shutdown of the machine to avoid more hazards.
4. What happens if the power ratings of the machine are decided liberally?
a) Damage occurs to the machine
b) Efficiency of the machine improves
c) Long life of the machine
d) Uneconomical usage of the machine
Answer: d
Explanation: If the power rating becomes very liberal, then it causes a high initial cost. Along with the high initial cost, loss of energy also occurs and leads to uneconomical usage.
5. If the power ratings are crossed, machine breakdown occurs.
a) True
b) False
Answer: a
Explanation: Every machine has a permissible limit for both voltage and current, for its efficient operation. If the limit is crossed, it will lead to the breakdown of the machine.
6. Which are the important criteria related to the power ratings of the machine?
a) Heat should be prevented from generation
b) Heat should be dissipated through power ventilation, irrespective of the time
c) Heat should be prevented through power ventilation within a short time period
d) Heat should be converted to some useful form
Answer: c
Explanation: Due to the components present in the machine, I2R losses occur in the machine. Due to this, heat is produced, and proper thermal ventilation should be provided to prevent the machine from breakdown.
7. What is the concept of thermal loading?
a) Output power is indirectly proportional to the temperature rise
b) Output power is indirectly proportional to the square of temperature rise
c) Output power is directly proportional to the temperature rise
d) Output power is directly proportional to the square of temperature rise
Answer: c
Explanation: Thermal loading is nothing, but the increase of the output power with respect to the temperature rise. It can lead to the power rating levels being crossed.
8. What is the ideal condition for thermal dissipation?
a) Heat generated > Heat Dissipated
b) Heat generated < Heat Dissipated
c) Heat generated = Heat Dissipated
d) Heat generated = 0
Answer: c
Explanation: For ideal thermal dissipation, the heat dissipated should be equal to the heat generated. In that case, there will be very less power loss and high efficiency.
9. What is the main objective of power ratings of machines?
a) helps in building a suitable thermal model of machines
b) helps in building a suitable physical model of machines
c) helps in classifying the machines into different types
d) helps to improve the machine efficiency
Answer: a
Explanation: The power ratings, mainly help in building the thermal model. It helps in reducing the heat losses and helps in bringing out a smooth and efficient operation of the machines.
10. Power ratings help in classifying machines to different classes of duties.
a) true
b) false
Answer: a
Explanation: The classification of the machines into different classes of duties depends on the power ratings. The ratings also helps in identifying the different types of machines available under different classes.
Design of Armature
1. How many type of armature windings are present in the DC machine armature?
a) 2
b) 3
c) 4
d) 5
Answer: a
Explanation: There are basically two types of armature windings present. They are known as lap winding and wave winding.
2. Which factor determines the difference between the types of armature windings?
a) brush connection
b) slip ring connection
c) commutator connection
d) pole connection
Answer: c
Explanation: The armature windings are all connected to the commutator in the case of DC machines. Commutator is nothing but device which converts dc voltage to ac voltage.
3. What is the meaning of pole pitch?
a) type of armature slot
b) number of armature slots/pole
c) number of poles/armature slot
d) number of poles/number of armature slots
Answer: b
Explanation: Pole pitch is the concept which is used in the armature slot design. It is nothing but the ratio between the number of armature slots present to the number of poles used in the machine design.
4. A 4 pole DC machine has 36 number of armature conductors. What is the pole pitch?
a) 9
b) 1/9
c) 92
d) 1/92
Answer: a
Explanation: Pole pitch = Number of armature slots/pole
Pole Pitch = 36/4 = 9.
5. The choosing of the slot values for armature depends on the type of windings.
a) true
b) false
Answer: a
Explanation: The slot values are chosen based on the type of windings. If wave winding is used, then multiples of pole pair is chosen as slot values and if lap windings are used the slot values are continuous and not multiples of pole pair.
6. What is the condition for choosing the armature current/parallel path in armature design?
a) > 200 ampere for both lap and wave windings
b) >200 ampere for lap winding, <200 ampere for wave winding
c) < 200 ampere for both lap and wave windings
d) <200 ampere for lap winding, >200 ampere for wave winding
Answer: c
Explanation: While designing the armature slots, the armature current should be less than 200 ampere. The armature current should also never cross 200 ampere for the lap winding.
7. What is the range of armature slot pitch in the armature slot design?
a) 20mm – 25mm
b) 25mm – 35mm
c) 30mm – 35mm
d) > 35mm
Answer: b
Explanation: For the armature design, the lowest value of slot pitch is generally chosen as 25mm. The highest value of slot pitch is generally chosen as 35mm.
8. For a DC machine, the armature slot pitch is 35mm and the diameter is 0.2m. What is the number of armature slots for the machine?
a) 17
b) 18
c) 19
d) 20
Answer: b
Explanation: Number of armature slots = (3.14*D)/Number of armature slot pitch
Number of armature slots = (3.14*0.2)/35*10-3
Number of armature slots = 17.94 = 18(approx).
9. What is the formula to reduce flux pulsations?
a) armature slot/number of poles = integer
b) armature slot/number of poles = integer + 0.5
c) armature slot/number of poles = integer – 0.5
d) armature slot/number of poles = integer +-0.5
Answer: d
Explanation: First the number of armature slots is calculated. Then the ratio of the number of armature slots to the number of poles is approximately equal to integer+-0.5.
10. The number of coils chosen should be minimum in number.
a) true
b) false
Answer: a
Explanation: The number of coils chosen for the armature slot design should always be minimum in number. This is because the machine cost reduces and it becomes cost efficient.
11. What should be the range of the integer value while calculating the formula to reduce the flux pulsations?
a) 8-14
b) 9-15
c) 9-16
d) 9-17
Answer: c
Explanation: While calculating the formula to reduce the flux pulsations, the lowest value is generally chosen as 9. The highest value of integer chosen is generally 16.
12. What is ‘Z/Sa’ calculation during design of armatures?
a) armature poles/slots
b) armature conductors/slots
c) armature slots/poles
d) armature slots/conductors
Answer: b
Explanation: This is the second step in the armature design. Firstly, the number of armature conductors is found out. Then the number of slots used is taken and the ratio of both gives the value of ‘Z/Sa’.
13. What is the formula for calculating the minimum number of coils?
a) Cmin = Voltage
b) Cmin = Voltage/5
c) Cmin = Voltage/10
d) Cmin = Voltage/15
Answer: d
Explanation: The voltage through the armature is first calculated from the data given. Then the value of voltage is divided by 15, to get the value of minimum number of coils.
Electrical Engineering Materials MCQs
14. Given Z=228 conductors, number of coils C=38 for a DC machine. What is the turns/coils ratio?
a) 4
b) 3
c) 7
d) 3
Answer: d
Explanation: Turns/coils = Number of armature conductors/(2*Number of coils)
Turns/coils = 228/38*2 = 3.
15. For a DC generator, given the load current is 510 ampere, field current is 1.4 ampere. What is the value of armature current?
a) 508.6 ampere
b) 714 ampere
c) 511.4 ampere
d) 364.28 ampere
Answer: c
Explanation: For DC generator, Armature Current = Load Current + Field Current
Armature Current = 510 + 1.4 = 511.4 ampere.
Design of Commutator and Brushes
1. What is a commutator in DC machines?
a) electrical device which reverses the current direction between the rotor and external circuit
b) mechanical device which reverses the current direction between the rotor and external circuit
c) electrical device which allows the current flow between the rotor and external circuit
d) mechanical device which allows the current flow between the rotor and external circuit
Answer: a
Explanation: Commutator is an electrical device found only in DC machines. It is used to reverse the current direction between rotor and external circuit. It also changes dc voltage to alternating voltage.
2. What is the use of brushes in DC machines?
a) to connect the parts of the machine to the external circuit
b) to conduct current between moving parts
c) to conduct current between stationary wires and moving parts
d) used for smooth conduction of current
Answer: c
Explanation: Brushes are another important part in the construction of DC machine. They are connected in the lower end of the machine to allow the current flow between the moving parts and stationary wires.
3. Which material is commonly used in brushes?
a) copper
b) carbon
c) silicon
d) steel
Answer: b
Explanation: Carbon is the most commonly used material in the manufacture of brushes. It is because carbon has high melting point, and is also less prone to high temperatures.
4. What is the total number of design steps available for the commutators in DC machines?
a) 4
b) 3
c) 2
d) 5
Answer: a
Explanation: There are basically 4 design steps available for the commutators. They include the finding of the number of commutator segments, voltage across the commutator, width of the commutator and length of the commutator.
5. What is the total number of design steps for the brushes in DC machines?
a) 4
b) 3
c) 5
d) 6
Answer: c
Explanation: There are basically 5 steps in the brush design. They include the calculation of the brush current, number of brushes, thickness of the brush, width of the brush, total commutator losses.
6. What factor does the diameter of the commutator depend on?
a) length of the commutator
b) speed of the commutator
c) peripheral speed of the commutator
d) opening of the commutator
Answer: c
Explanation: Peripheral speed is a term which is used in the design of the commutators of DC machines. If the peripheral speed of the machine gets crossed, then the diameter should be reduced.
7. For a DC machine, given Diameter of the commutator= 0.48 m, Speed = 600 rpm. What is the voltage across the commutator?
a) 15 V
b) 15.5 V
c) 15.2 V
d) 15.1 V
Answer: d
Explanation: Voltage across the commutator = 3.14*(Diameter of the commutator)*Speed
Voltage = 3.14*0.48*600 = 15.1 V.
8. What is the other name for commutator segment pitch of DC machines?
a) width of the commutator
b) length of the commutator
c) breadth of the commutator
d) height of the commutator
Answer: a
Explanation: Commutator segment pitch is otherwise known as the width of the commutator. It is one of the commutator design steps.
9. What is the formula for finding out the width of the commutator of the DC machines?
a) width of the commutator = 3.14*(Diameter of the commutator)
b) width of the commutator = 3.14*(Diameter of the commutator)*(Number of armature coils)
c) width of the commutator = 3.14*(Diameter of the commutator) / (Number of armature coils)
d) width of the commutator = Number of armature coils / 3.14*(Diameter of the commutator)
Answer: c
Explanation: First by the design of the armature, the number of armature coils is found out. Then the diameter of the commutator is measured.
10. The diameter of the commutator should be 0.2-0.4 times the main diameter for a good design.
a) true
b) false
Answer: b
Explanation: For a good design of the commutator, the diameter of the commutator should be in the range of 0.6-0.8 times of the main diameter. If the value goes above or below this range, we should not choose that value.
11. What should be the range for the width of the commutator in a good design?
a) < 4 mm
b) > 4 mm
c) 3-4 mm
d) < 3 mm
Answer: b
Explanation: For a good design of the commutator, the width of the commutator should be greater than 4 mm. If the value goes below 4 mm, we should not choose that value.
12. The design of commutator and the brushes of DC machines are interconnected.
a) true
b) false
Answer: a
Explanation: The design of brushes and commutators are interconnected, because in the calculation of the length of the commutator, the width of the brush, number of brushes are used. In the same way, brush values are also used in calculation of commutator loss.
13. What is the formula for calculation of brush current of DC machine for wave winding?
a) brush Current = 2*(Armature Current) / P
b) brush Current = (Armature Current) / P
c) brush Current = Armature Current
d) brush Current = P/2*(Armature Current)
Answer: c
Explanation: Brush Current = 2*(Armature Current) / P is the formula for the calculation of brush current of DC machine for lap winding.
14. What is the formula for brush contact loss of DC machines?
a) brush contact loss = armature current + brush contact voltage
b) brush contact loss = armature current – brush contact voltage
c) brush contact loss = armature current * brush contact voltage
d) brush contact loss = (armature current + brush contact voltage) / 2
Answer: c
Explanation: For calculation of the brush contact loss, first the armature current is obtained. Then the voltage through the brush contacts is calculated and the product gives the brush contact loss.
15. What is the formula for total commutator loss for DC machine?
a) brush contact loss + brush friction loss
b) brush contact loss – brush friction loss
c) brush contact loss * brush friction loss
d) brush contact loss / brush friction loss
Answer: a
Explanation: Brush contact loss = brush contact loss = armature current * brush contact voltage
Brush friction loss = (Coefficient f friction) * Brush Pressure * Area of the brush*Commutator voltage.
Design of Output Equations
1. With what component is the output equation of DC machines related to?
a) power
b) voltage
c) current
d) losses
Answer: a
Explanation: The output equation generally deals with the power generated in the machine. Power of the machine relates the voltage and current flowing though the machine.
2. What can be found out using the output equation of the DC machine?
a) main dimensions
b) angle of rotation
c) losses
d) efficiency
Answer: a
Explanation: The output equation of DC machine is mainly used to obtain the main dimensions of the machine. Main dimensions are very important in calculation of various factors related to the machine.
3. What are the components of the main dimensions of output equation of DC machine?
a) diameter
b) length
c) diameter and length
d) voltage
Answer: c
Explanation: Main dimensions generally deal with the diameter of the machine. It also deals with the length of the machine.
4. What is the starting equation for deriving the output equation of DC Machines?
a) P = Generated Emf + Armature Current
b) P = Generated Emf – Armature Current
c) P = Generated Emf * Armature Current
d) P = Generated Emf / Armature Current
Answer: c
Explanation: The first equation for deriving the output equation of DC machines starts from this equation. The generated emf is multiplied along with the armature current.
5. The output equation of the DC machines can be used to calculate the speed of the machine.
a) true
b) false
Answer: a
Explanation: The output equation can be used to calculate the speed of the machine. The output coefficient of the DC machine, diameter and length of the conductors must be provided.
6. What is the output equation of DC machine?
a) Output power = Output Coefficient of the machine* Diameter2 * Length * Speed in rpm
b) Output power = Output Coefficient of the machine* Diameter2 * Length * Speed in rps
c) Output power = Output Coefficient of the machine* Diameter2 * Length / Speed in rps
d) Output power = Output Coefficient of the machine* Diameter2 * Length / Speed in rpm
Answer: b
Explanation: While calculating the output power of the DC machine, the coefficient of output equation, the diameter of the conductor, the length of the conductor and the speed is required. Speed should be in rotations per second only.
7. What are the terms related to deriving the output equation of the DC machine?
a) specific electric loading
b) specific magnetic loading
c) thermal coefficient of machine
d) specific electric and magnetic loading
Answer: d
Explanation: For deriving the output equation, both the specific magnetic and electric loading formulas are made use of. By substituting the 2 formulas, the output equation is derived.
8. For a DC generator, what is the output power equation?
a) Output power = Generated Power * efficiency
b) Output power = Generated Power / efficiency
c) Output power = Generated Power – efficiency
d) Output power = efficiency / generated power
Answer: b
Explanation: For DC generator, the efficiency also taken into account, while calculating the final output power value. Whereas, the same is not considered while calculating for motor.
9. For a DC motor, what is the output power equation?
a) Output power = Generated Power / efficiency
b) Output power = Generated Power * efficiency
c) Output power = Generated Power
d) Output power = Generated Power + efficiency
Answer: c
Explanation: Output power = Generated Power / efficiency – For DC Generator
Output power = Generated Power – For DC Motor.
10. For a DC generator, given D = 0.35 m, L = 0.21 m, Coefficient of output = 108.5, efficiency = 0.9, speed = 1400 rpm. What is the output power of the DC generator?
a) 65.12 W
b) 72.35 KW
c) 72.35 W
d) 65.12 KW
Answer: b
Explanation: Generated power = 108.5 * 0.35 * 0.35 * 0.21 * (1400/60) = 65.12 KW
Output Power = Generated Power / Efficiency = 65.12 / 0.9 = 72.35 KW.
Selection of Number of Poles
1. How is the selection of number of poles made in AC and DC machines?
a) any number of poles can be used for both AC and DC machines
b) fixed number of poles in both AC and DC machines
c) fixed number of poles in DC machines and any number of poles in AC machines
d) fixed number of poles in AC machines and any number of poles in DC machines
Answer: d
Explanation: When it comes to DC machines, any number of poles can be made use of, but it should be within a range. Whereas in AC machines the number of poles is fixed by supply frequency and speed.
2. How many considerations are present in the selection of number of poles?
a) 4
b) 5
c) 6
d) 7
Answer: d
Explanation: There are 7 consideration in selection of number of poles. They are frequency, weight of iron parts, weight of copper, length of commutator, labor charges, flash over, distortion of field form.
3. What is the formula for frequency of flux reversals?
a) f = p*n
b) f = p/n
c) f = n/p
d) f = (p*n)/2
Answer: d
Explanation: Frequency is directly proportional to the number of poles (p). It is also proportional to the speed of the machine as well.
4. What is the range of frequency during the selection of number of poles?
a) 20-50 Hz
b) 25-40 Hz
c) 25-50 Hz
d) >50 Hz
Answer: c
Explanation: While selecting the number of poles, the lowest value of frequency should be minimum 25 Hz. The highest value of frequency should be limited to 50 Hz.
5. What is the relation of hysteresis loss and weight of iron parts with respect to increase of number of poles?
a) decrease in hysteresis loss, increase in weight
b) decrease in hysteresis loss, decrease in weight
c) increase in hysteresis loss, increase in weight
d) increase in hysteresis loss, decrease in weight
Answer: b
Explanation: With larger number of poles, the area of cross section can be reduced, henceforth decreasing the hysteresis loss. Also by increasing pole number, weight of iron parts is reduced.
6. What happens to the weight of copper in both armature and field windings when the number of poles increase?
a) weight of copper in armature winding decreases and weight of copper in field winding increases
b) weight of copper in armature winding increases and weight of copper in field winding decreases
c) weight of copper in armature winding and field winding decreases
d) weight of copper in armature winding and field winding increases
Answer: c
Explanation: The weight of copper is indirectly proportional to the number of poles. As the number of poles increases, the weight of the copper decreases.
7. What happens to the length of the commutators with the increase in number of poles?
a) The length of commutators are increased
b) The length of commutators are decreased
c) The length of commutators are stable
d) The length of commutators are higher
Answer: b
Explanation: The area of the brushes decreases if the number of poles are being increased. As the area of the brushes are decreased, the length of the commutators also decrease.
8. What happens to the labor charges when there is an increase in number of poles?
a) labor charges are reduced
b) labor charges are increased
c) labor charges are fixed always
d) labor charges vary
Answer: b
Explanation: With increase in the number of poles, the armature windings increase, and more work increases to insulate. The commutator segments also increase, and the work increases.
9. What is the effect of the distortion of field form with respect to the small number of poles?
a) small number of poles cause no distortions
b) small number of poles clears all distortions
c) small number of poles reduces distortions
d) small number of poles increases distortions
Answer: d
Explanation: When there is small number of poles, that time the armature mmf per pole increases. As the armature mmf increases, it results in increase of distortion.
10. Large number of poles lead to large flashover between brushes.
a) true
b) false
Answer: a
Explanation: The number of brushes is equal to number of poles. For the same diameter of the commutator, the distance between the adjacent brush arms decreases and this increases the possibility of flashover.
11. What is the dependency of the cost of the armature and field windings with respect to large number of poles?
a) high cost for armature windings, low cost for field windings
b) high cost for armature windings, high cost for field windings
c) low cost for armature windings, high cost for field windings
d) low cost for armature windings, low cost for field windings
Answer: d
Explanation: With large number of poles the armature and the field windings reduce in number. Thus the cost of the field and armature windings also decrease.
12. Lower values of frequency are used for small machines.
a) true
b) false
Answer: b
Explanation: Lower values of frequency are actually used for the large machines. Also, higher values of frequency are actually used for small machines.
13. What is the range of the current per parallel path for the choice of number of poles?
a) limited to 100 A
b) limited to 150 A
c) limited to 200 A
d) limited to 250 A
Answer: c
Explanation: The current per parallel path should be limited to maximum of 200 A. If the limit gets exceeded then there occurs damage to the machine.
14. What should be the range of the current per brush arm?
a) limited to 400 A
b) limited to 200 A
c) limited to 100 A
d) limited to 300 A
Answer: a
Explanation: The current per brush arm should be limited to maximum of 400 A. If the limit gets exceeded then there occurs damage to the machine.
15. What should be the armature mmf per pole for output over 1500 kW?
a) 5000 A
b) 5000-7500 A
c) 7500-10000 A
d) upto 12500
Answer: d
Explanation: 5000 A is for output of about 100 kW. 5000 to 7500 A for output voltage of 100 to 500 kW. 7500 to 10000 A is for the output of 500 to 1500 kW.
Pole Design
1. What are the factors the design of poles of the DC machine depends on?
a) length, breadth, height of the conductors
b) area of cross section of poles
c) area of cross section of poles and height of the poles
d) area of cross section of poles and height of the poles and the design of field windings
Answer: d
Explanation: For designing the poles, first the area of cross section of the poles and the height of the poles should be obtained. Then the field winding design details are also required during the pole design.
2. What is the relationship between flux in the pole body and the useful flux per pole?
a) flux in the pole body is directly proportional to useful flux per pole
b) flux in the pole body is indirectly proportional to the useful flux per pole
c) flux in the pole body is directly proportional to the square of useful flux per pole
d) flux in the pole body is indirectly proportional to the square of useful flux per pole
Answer: a
Explanation: According to the flux in the pole body formula the flux in the pole body is directly proportional to the useful flux per pole. It is also proportional to the leakage coefficient.
3. What is the flux in the pole body, given leakage coefficient = 1.2 and the useful flux per pole is 10 weber?
a) 12 weber
b) 11.2 weber
c) 8.2 weber
d) 20 weber
Answer: a
Explanation: Flux in the pole body = leakage coefficient * useful flux per pole
Flux in the pole body = 1.2 * 10 = 12 weber.
4. What is the meaning of useful flux?
a) the flux which is being created in the machine
b) the flux which can be used
c) the flux which can produce the output
d) the flux that is wasted
Answer: c
Explanation: Total flux is the maximum amount of flux that is being generated by the current flowing in the circuit. Useful flux is nothing but the flux which can produce the output in the machine.
5. What is the range of leakage coefficient for output of 100kW?
a) 1.12-1.25
b) 1.11-1.22
c) 1.10-1.20
d) 1.11-1.15
Answer: b
Explanation: 1.12-1.25 is the leakage coefficient when the output is 50kW. 1.10-1.20 is the leakage coefficient when the output is 200kW.
6. What is the range of leakage coefficient for output of 1000kW?
a) 1.12-1.25
b) 1.11-1.22
c) 1.09-1.18
d) 1.08-1.16
Answer: d
Explanation: 1.12-1.25 is the leakage coefficient when the output is 50kW. 1.11-1.22 is the leakage coefficient when the output is 100kW. 1.09-1.18 is the leakage coefficient when the output is 500kW.
7. What is the range of the flux density in the pole shrank for laminated poles?
a) 1.1-1.7 Wb per m2
b) 1.2-1.6 Wb per m2
c) 1.3-1.7 Wb per m2
d) 1.2-1.7 Wb per m2
Answer: d
Explanation: The flux density in the pole shrank of laminated poles should have a minimum value of 1.2. The flux density in the pole shrank of the laminated poles should not exceed 1.7 at the same time.
8. What is the formula for the area of the poles shrank of the laminated poles?
a) area of the pole shrank = flux in the pole body * magnetic field
b) area of the pole shrank = flux in the pole body + magnetic field
c) area of the pole shrank = flux in the pole body – magnetic field
d) area of the pole shrank = flux in the pole body / magnetic field
Answer: a
Explanation: For finding out the area of the pole shrank first the flux in the pole body is found out using the product of the leakage coefficient and the useful flux in the pole. Next, the magnetic field is measured and the product gives the area.
9. What should be the length of pole with respect to the length of the armature and what should be the range of the length of pole?
a) length of pole < length of armature, 10-15 m
b) length of pole > length of armature, 10-15 mm
c) length of pole > length of armature, 10-15 cm
d) length of pole < length of armature, 10-15 mm
Answer: d
Explanation: The length of the pole should be very much less than the length of the armature in order to permit the end play and t avoid magnetic centering. It should be in the range of 10-15 mm.
10. The formula for length of pole is L = Total length of armature – (0.001 to 0.005).
a) true
b) false
Answer: a
Explanation: First the total length of armature is calculated. Then the value is subtracted by 10-15 mm in order to obtain the length of the pole.
11. What is the formula for the width of pole of DC machines?
a) width of pole body = area of the pole * length of the pole
b) width of pole body = area of the pole + length of the pole
c) width of pole body = area of the pole – length of the pole
d) width of pole body = area of the pole / length of the pole
Answer: d
Explanation: For obtaining the width of the pole, the area of the pole is first obtained. Then the length of the pole is calculated. The ratio of area of the pole to the length of the pole gives the width.
12. Height of the pole depends on the mmf to be provided on the pole at full load.
a) true
b) false
Answer: a
Explanation: The height of the pole totally depends on the mmf provided to the poles. The mmf provided at full load is only taken into consideration for the height measurement.
13. How is the mmf required at full load obtained for the calculation of height of poles?
a) using closed circuit characteristics
b) using open circuit characteristics
c) using formula
d) using equivalent circuit
Answer: b
Explanation: The mmf at full load is calculated using the magnetization curve. The open circuit characteristics are obtained which help in finding out the mmf at full load.
14. How should the field mmf be with respect to armature mmf to reduce the armature reaction?
a) armature mmf > field mmf
b) armature mmf >= field mmf
c) armature mmf < field mmf
d) armature mmf = field mmf
Answer: c
Explanation: To reduce the armature reaction, the field system should be designed such that the field mmf should be dominant over the armature mmf. If the armature mmf becomes low, the armature reaction reduces.
15. What should be the range of the field mmf to armature mmf ratio at full load?
a) 1.0-1.2
b) 1.1-1.3
c) 1.3-1.5
d) 1.1-1.25
Answer: d
Explanation: The minimum value of the ratio should be atleast 1.1. The maximum value of the ratio should be not greater than 1.25.