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# Multiple choice question for engineering

## Set 1

1. With what component is the output equation of DC machines related to?
a) power
b) voltage
c) current
d) losses

Answer: a [Reason:] 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 [Reason:] 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 main dimensions of output equation of DC machine?
a) diameter
b) length
c) diameter and length
d) voltage

Answer: c [Reason:] 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 [Reason:] 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 [Reason:] 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 [Reason:] 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 in deriving the output equation of the DC machine?
c) thermal coefficient of machine

Answer: d [Reason:] 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 [Reason:] 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 [Reason:] 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 [Reason:] 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.

## Set 2

1. What does the copper factor in PMDC motors represent?
a) it represents the armature circular area for conductors
b) it represents the field circular area for conductors
c) it represents the fraction of the armature circular area for conductors
d) it represents the fraction of the field circular area for conductors

Answer: c [Reason:] The copper factor represents the fraction of the armature circular area for conductors. It is represented by the letter K.

2. What is the range of the copper factor in PMDC motors?
a) 0.1-0.3
b) 0.1-0.2
c) 0.1-0.4
d) 0.2-0.4

Answer: b [Reason:] The copper factor represents the fraction of the armature circular area for conductors. The range of the copper factor is between 0.1-0.2.

3. What is the formula for the armature resistance in PMDC motor?
a) armature resistance = (Diameter + length)*total number of armature conductors/1.2 * 104 * number of parallel paths in the armature2
b) armature resistance = (Diameter + length)*total number of armature conductors*1.2 * 104 * number of parallel paths in the armature2
c) armature resistance = (Diameter + length)*total number of armature conductors/1.2 * 104 + number of parallel paths in the armature2
d) armature resistance = (Diameter + length)+total number of armature conductors/1.2 * 104 * number of parallel paths in the armature2

Answer: a [Reason:] First the diameter, length and the total number of armature conductors are obtained. Next the number of parallel paths in the armature is calculated and on substitution it provides the armature resistance.

4. What happens to the diameter when the poles are more than 2?
a) diameter = 2 * diameter * (number of armature teeth embraced by one coil/total number of armature teeth)
b) diameter = 2.32 * diameter * (number of armature teeth embraced by one coil/total number of armature teeth)
c) diameter = 2.32 * diameter * (number of armature teeth embraced by one coil * total number of armature teeth)
d) diameter = 2 * diameter / (number of armature teeth embraced by one coil/total number of armature teeth)

Answer: b [Reason:] The diameter is the exact calculated value for 2 pole motors. But when the poles are more than 2, the above formula is made use of to calculate the armature resistance.

5. What factor does the permeance coefficient depend upon?
a) geometry of the magnet
b) geometry of the magnet, airgap, associated non-portions of the magnetic circuit
c) airgap
d) associated non-portions of the magnetic circuit

Answer: b [Reason:] The permeance coefficient depends upon the geometry of the magnet and the airgap. It also depends on the associated non-portions of the magnetic circuit.

6. What is the range of the permeance coefficient in the PMDC motors?
a) 3-5
b) 4-9
c) 4-8
d) 3-9

Answer: c [Reason:] The minimum value of the permeance coefficient used in the PMDC motors is 4. The maximum value of the permeance coefficient used in the PMDC motor is 8.

7. What is the usual value of the permeance coefficient of the PMDC motor?
a) 4
b) 5
c) 6
d) 7

Answer: c [Reason:] The range of the permeance coefficient for the PMDC motor is 4-8. The value is usually around 6 for most of the applications.

8. The field current flowing in the conductor’s acts as demagnetizing force on the fraction tips of the magnet?
a) true
b) false

Answer: b [Reason:] The armature current flowing in the conductors acts as demagnetizing force. Its acts on the fraction tips of the magnets present.

9. What is the value of the demagnetizing coefficient if the total number of teeth is greater than 107?
a) d = angle/360
b) d = angle/240
c) d = angle/540
d) d = angle/720

Answer: d [Reason:] If the total number of teeth is greater than 107 then the demagnetizing coefficient become the ratio of the angle and 720. Otherwise d is one half the ratio of the maximum number of teeth that can be situated within the angle to the total number of teeth.

10. What is the value of the reluctance factor in the calculation of the intensity of magnetic field?
a) 1
b) 2
c) 1.15
d) 1,45

Answer: c [Reason:] The reluctance factor is one of the factors made use of in the calculation of the intensity of magnetic field. The value of the reluctance factor is around 1.15 generally.

11. What is the formula of the magnetic to electrical boarding ratio?
a) magnetic to electrical boarding ratio = number of poles * permeance coefficient * flux per pole/number of conductors * armature current
b) magnetic to electrical boarding ratio = number of poles / permeance coefficient * flux per pole*number of conductors * armature current
c) magnetic to electrical boarding ratio = number of poles + permeance coefficient * flux per pole/number of conductors * armature current
d) magnetic to electrical boarding ratio = number of poles * permeance coefficient / flux per pole*number of conductors * armature current

Answer: a [Reason:] The permeance coefficient is first calculated along with the number of poles and the flux per pole. Then the number of conductors are noted and the armature current is calculated to give the magnetic to electrical boarding ratio.

12. How is the value of the magnetic to electrical boarding ratio related with the volume of iron and volume of copper?
a) high magnetic to electrical boarding ratio gives high copper volume and high iron volume
b) high magnetic to electrical boarding ratio gives low copper volume and high iron volume
c) low magnetic to electrical boarding ratio gives low copper volume and low iron volume
d) low magnetic to electrical boarding ratio gives low copper volume and high iron volume

Answer: b [Reason:] The high value of magnetic to electrical boarding ratio gives a high volume of iron. But the high value of magnetic to electrical boarding ratio gives low copper volume.

13. For good performance the small dc motor should have magnetic to electrical boarding ratio greater than 70?
a) true
b) false

Answer: b [Reason:] The performance of the small DC motor depends on the magnetic to electrical boarding ratio. The magnetic to electrical boarding ratio should be greater than 50 for good performance.

14.What is the formula for the flux density for the PM motors?
a) flux density = residual flux density / 1 + (1.11/permeance coefficient)
b) flux density = residual flux density * 1 + (1.11/permeance coefficient)
c) flux density = residual flux density / 1 + (1.11*permeance coefficient)
d) flux density = residual flux density * 1 + (1.11*permeance coefficient)

Answer: a [Reason:] The residual flux density is calculated first along with the permeance coefficient to obtain the flux density of the PMDC motor. The flux density is 0.85 times the residual flux density.

## Set 3

1. How many design principles are present in the current transformers?
a) 2
b) 3
c) 4
d) 5

Answer: d [Reason:] There are 5 design principles present in the current transformers. They are core design, secondary current rating, primary current rating, windings and behavior of the transformer under short circuit current.

2. What should be done in order to reduce the errors in the core?
a) armature mmf is to kept low
b) field mmf to be kept high
c) the exciting mmf is to be kept low
d) the field mmf is to be kept high

Answer: c [Reason:] The errors in the core are reduced by keeping the exciting mmf low. This can take place with the core having a low reluctance and low iron loss.

3. How many classifications are the magnetic alloys used in the current transformers classified into?
a) 3
b) 2
c) 4
d) 5

Answer: a [Reason:] The magnetic alloys used in the current transformers are divided into 3 types. They are hot rolled silicon steel, cold rolled grain oriented silicon steel and nickel iron alloys.

4. What is the material used in the transformer when the transformer errors should be small?
a) mumetal cores
b) steel cores
c) permender cores
d) presshamn cores

Answer: a [Reason:] The mumetal cores are commonly used when it is essential that transformer errors shall be small. Mumetal has the properties of high permeability, low loss and small retentivity.

5. What is the relation of the secondary winding leakage reactance and secondary circuit impedance?
a) secondary winding leakage reactance is directly proportional to the secondary circuit impedance
b) secondary winding leakage reactance is indirectly proportional to the secondary circuit impedance
c) secondary winding leakage reactance is directly proportional to the square of the secondary circuit impedance
d) secondary winding leakage reactance is indirectly proportional to the square of the secondary circuit impedance

Answer: a [Reason:] The secondary winding leakage reactance is directly proportional to the secondary circuit impedance. In secondary winding the leakage reactance is reduced and in turn the secondary circuit impedance is reduced.

6. The ring shaped cores are made use of in the reduction of the secondary winding leakage reactance and secondary impedance?
a) true
b) false

Answer: a [Reason:] The secondary winding leakage reactance is directly proportional to the secondary impedance. The ring shaped cores around which the toroidal secondary windings of one or more layers are uniformly distributed.

7. What type of core is employed when the performance standard required is not so high?
a) rectangular strips
b) c-shaped sections
c) rectangular strips or c-shaped sections
d) rectangular strips and c-shaped sections

Answer: c [Reason:] Ring core type is used only for the high performance operation. The rectangular strips or c-shaped sections are used when the standard of performance required is not so high.

8. What should the magnetic path be in order to reduce the core reluctance?
a) length of the magnetic path in core should be low
b) length of the magnetic path in core should be medium
c) length of the magnetic path in core should be high
d) length of the magnetic path in core should be very high

Answer: a [Reason:] The length of the magnetic path in core should be as small as possible. This reduces the core reluctance of the current transformer.

9. What is the value of the rated secondary current?
a) 1 A
b) 2 A
c) 3 A
d) 5 A

Answer: d [Reason:] The rating of the secondary current is the maximum current that can be passed through the secondary windings. It is 5 A for the current transformer.

10. What are the disadvantages of the low rated secondary current transformer?
a) high cost
b) high voltages
c) high voltages or high cost
d) high voltages and high cost

Answer: d [Reason:] When there is a low secondary current rating in the current transformers they produces high voltages if the secondary is left open. It is also costly to produce the windings because of the extra time involved in the making.

11. What is the ideal condition with respect to the primary current rating?
a) ratio of secondary mmf to primary mmf should be high
b) ratio of secondary mmf to primary mmf should be low
c) ratio of excitation mmf to primary mmf should be high
d) ratio of excitation mmf to primary mmf should be low

Answer: d [Reason:] The primary current rating depends on exciting mmf and primary mmf. The ratio of the exciting mmf to the primary mmf should be low.

12. What is the rating of the primary current in the current transformer?
a) 200 A
b) 300 A
c) 400 A
d) 500 A

Answer: d [Reason:] The rating of the primary current is minimum 500 A. If the rating is less than 500 A, then multiturn primary windings and rating is above than 500 A, then single turn winding is enough.

## Set 4

1. How many design steps are available for the design of rotor?
a) 5
b) 6
c) 7
d) 8

Answer: b [Reason:] There are 6 design steps involved in the design of the rotor. They are number of rotor slots, area of rotor bars, area of end rings, rotor resistance, rotor teeth, rotor core.

2. What is the main motive while choosing the number of rotor slots?
a) increasing the efficiency
b) decreasing the losses
c) no noise is produced
d) high output is produced

Answer: c [Reason:] There are basically 6 steps involved in the rotor design. The number of slots is chosen such that no noise is produced.

3. What is the formula for the harmonic poles due to slots?
a) harmonic poles due to slots = 2 * (number of slots ± number of poles / 2)
b) harmonic poles due to slots = 2 / (number of slots ± number of poles / 2)
c) harmonic poles due to slots = 2 * (number of slots ± number of poles * 2)
d) harmonic poles due to slots = 1/ 2 * (number of slots ± number of poles / 2)

Answer: a [Reason:] First the number of slots and number of poles are first calculated. On substitution we get the harmonic poles due to the slots.

4. What factors are used fixing the number of stator slots?
a) winding arrangement
b) number of poles
c) winding arrangement or number of poles
d) winding arrangement and number of poles

Answer: d [Reason:] The number of poles are fixed according to the winding arrangement. The number of poles are also fixed according to the number of poles.

5. Which condition satisfies the quiet operation in machines?
a) number of stator slots is divisible by number of pairs of poles
b) number of rotor slots differs from the number of stator slots by more than the number of poles
c) number of rotor slots is not divisible by number of pairs of poles
d) number of stator slots differs from the number of rotor slots by more than the number of poles

Answer: b [Reason:] The number of rotor slots are decided for quieter operation of the machine. The number of rotor slots differs from the number of stator slots by more than the number of poles.

6. What among the following are considered for the selection of number of rotor slots?
a) magnetic locking
b) cusps
c) magnetic locking or cusps
d) magnetic locking and cusps

Answer: d [Reason:] The selection of number of rotor slots depends on the magnetic locking. The selection of number of rotor slots depends on the cusps also.

7. What is the formula for the total stator copper section for main winding?
a) total stator copper section for main winding = number of turns in the running winding * area of the running winding conductor
b) total stator copper section for main winding = 2 * number of turns in the running winding * area of the running winding conductor
c) total stator copper section for main winding = number of turns in the running winding / area of the running winding conductor
d) total stator copper section for main winding = 2* number of turns in the running winding / area of the running winding conductor

Answer: b [Reason:] First the number of turns in the running winding is calculated along with the area of the running winding conductor. On substitution it gives the total stator copper section for main winding.

8. What is the formula for the total cross section of rotor bars?
a) total cross section of rotor bars = number of rotor slots * area of each bar
b) total cross section of rotor bars = number of rotor slots / area of each bar
c) total cross section of rotor bars = number of rotor slots + area of each bar
d) total cross section of rotor bars = number of rotor slots – area of each bar

Answer: a [Reason:] The number of rotor slots and area of each bar is first calculated. On substitution it gives the total cross section of rotor bars.

9. What is the range of the ratio of the total cross section of rotor bars to the total stator copper section for main winding for copper?
a) 0.4-0.8
b) 0.3-0.7
c) 0.5-0.8
d) 0.8-0.9

Answer: c [Reason:] The minimum value of range of the ratio of the total cross section of rotor bars to the total stator copper section for main winding is 0.5. The maximum value range of the ratio of the total cross section of rotor bars to the total stator copper section for main winding is 0.8.

10. What is the formula of the end ring current?
a) end ring current = number of rotor slots * bar current * 3.14 * number of poles
b) end ring current = number of rotor slots * bar current * 3.14 / number of poles
c) end ring current = number of rotor slots / bar current * 3.14 * number of poles
d) end ring current = number of rotor slots * bar current / 3.14 * number of poles

Answer: d [Reason:] The number of rotor slots and the bar current along with the number of poles is calculated. On substitution it gives the end ring current value.

11. What is the range of the ratio of the total cross section of rotor bars to the total stator copper section for main winding for aluminium?
a) 1-1.3
b) 1-1.4
c) 1-1.6
d) 1.2-1.5

Answer: c [Reason:] The minimum value of range of the ratio of the total cross section of rotor bars to the total stator copper section for main winding is 1. The maximum value range of the ratio of the total cross section of rotor bars to the total stator copper section for main winding is 1.6.

12. What is the formula for the area of each bar?
a) area of each bar = current through each bar / current density through each bar
b) area of each bar = current through each bar * current density through each bar
c) area of each bar = current density through each bar / current through each bar
d) area of each bar = current density through each bar * current through each bar

Answer: a [Reason:] The current through each bar and the current density through each bar is calculated. On substitution the area of each bar is obtained.

13. What is the formula of the area of each end ring?
a) area of each end ring = 0.32 * total cross section of rotor bars * number of poles
b) area of each end ring = 0.32 / total cross section of rotor bars * number of poles
c) area of each end ring = 0.32 * total cross section of rotor bars / number of poles
d) area of each end ring = 1/0.32 * total cross section of rotor bars * number of poles

Answer: c [Reason:] First the total cross section of rotor bars along with the number of poles are calculated. On substitution the area of each end ring is obtained.

14. What is the formula of the rotor teeth flux density?
a) flux density of rotor teeth = maximum flux / (number of rotor slots / number of poles) * length of the teeth * depth of rotor core
b) flux density of rotor teeth = maximum flux * (number of rotor slots / number of poles) * length of the teeth * depth of rotor core
c) flux density of rotor teeth = 1/maximum flux * (number of rotor slots / number of poles) * length of the teeth * depth of rotor core
d) flux density of rotor teeth = maximum flux / (number of rotor slots * number of poles) * length of the teeth * depth of rotor core

Answer: a [Reason:] The maximum flux, the number of rotor slots per pole and the length of teeth along with the depth of rotor core is calculated. On substitution the flux density of the rotor teeth is obtained.

15. What is the range for the ratio of the resistance to reactance in the split phase motors?
a) 0.40-0.55
b) 0.45-0.55
c) 0.45-0.8
d) 0.45-0.6

Answer: b [Reason:] The range for the ratio of the resistance to reactance in the split phase motors is 0.45-0.55. The range for the ratio of the resistance to reactance in the capacitor start motors is 0.45-0.8.

## Set 5

1. How many factors does the design of rotor of synchronous machines depend upon?
a) 2
b) 3
c) 4
d) 5

Answer: c [Reason:] There are 4 factors which are associated with the design of rotor in the synchronous machines. They are height of pole, design of damper windings, height of pole shoe, pole profile drawing.

2. What is the formula for the flux in pole body?
a) flux in pole body = leakage coefficient * useful flux per pole
b) flux in pole body = leakage coefficient / useful flux per pole
c) flux in pole body = leakage coefficient – useful flux per pole
d) flux in pole body = leakage coefficient + useful flux per pole

Answer: a [Reason:] The leakage coefficient is obtained first from its formula. Next the value of useful flux per pole is calculated and this gives the flux in pole body value.

3. What is the range of the permissible values of the flux densities in pole body?
a) 1.4-1.7 Wb per m2
b) 1.5-1.7 Wb per m2
c) 1.4-1.6 Wb per m2
d) 1.5-1.6 Wb per m2

Answer: b [Reason:] The minimum value of the flux density in the pole body is given to be 1.5 Wb per m2.The maximum permissible value of the flux density in the pole body is given to be 1.7 Wb per m2.

4. What is the range of the leakage coefficient in the pole body?
a) 1.1 to 1.2
b) 1.00 to 1.5
c) 1.15 to 1.2
d) 0.75 to 2.3

Answer: c [Reason:] The minimum value of the leakage coefficient in the pole body is 1.15. The maximum value of the leakage coefficient in the pole body is 1.2.

5. What is the formula for the area of cross-section of pole body for rectangular poles?
a) area of cross section of pole body = 0.98 * axial length of the pole * breadth of the pole
b) area of cross section of pole body = 0.98 / axial length of the pole * breadth of the pole
c) area of cross section of pole body = 0.98 * axial length of the pole / breadth of the pole
d) area of cross section of pole body = 1/0.98 * axial length of the pole * breadth of the pole

Answer: a [Reason:] The axial length of the pole and the breadth of the pole are calculated. Next by multiplying the two values with the stacking factor, we get the area of cross section of pole body.

6. What is the formula for the copper area of the field windings?
a) copper area = full load field mmf * current density in the field winding
b) copper area = full load field mmf / current density in the field winding
c) copper area = full load field mmf + current density in the field winding
d) copper area = full load field mmf – current density in the field winding

Answer: b [Reason:] For the calculation of the copper area, first the current density in the field winding is calculated. Next the full load field mmf is calculated and the ratio gives the copper area of field windings.

7. What is the formula for the total space required for the winding?
a) total space = copper area + space factor
b) total space = copper area – space factor
c) total space = copper area / space factor
d) total space = copper area * space factor

Answer: c [Reason:] The copper area is calculated from its respective formula. Then the space factor is calculated and the ratio gives the value of total space.

8. What is the value of space factor for the strip on edge winding?
a) 0.8-0.9
b) 0.4
c) 0.65
d) 0.75

Answer: a [Reason:] The space factor for the strip on edge winding is 0.8-0.9. The space factor for small round wires is 0.4 and for large round wires it is 0.65. The space factor for large rectangular conductors is 0.75.

9. What is the formula for the height of winding?
a) height of winding = total winding area / depth of winding
b) height of winding = total winding area * depth of winding
c) height of winding = total winding area + depth of winding
d) height of winding = total winding area – depth of winding

Answer: a [Reason:] The total winding area is first calculated. Next the depth of the winding is calculated. The ratio of both gives the height of winding.

10. What is the formula for the radial length of the pole shoe?
a) radial length of the pole shoe = height of winding – height of pole shoe – 0.02
b) radial length of the pole shoe = height of winding + height of pole shoe – 0.02
c) radial length of the pole shoe = height of winding – height of pole shoe + 0.02
d) radial length of the pole shoe = height of winding + height of pole shoe + 0.02

Answer: d [Reason:] First the height of the winding is calculated from its formula. Next the height of pole shoe is calculated. Both the values are added with 0.02 to give the radial length of the pole shoe.

11. What is the formula for the height of pole body?
a) height of pole body = height of the winding + 0.02
b) height of pole body = height of the winding * 0.02
c) height of pole body = height of the winding – 0.02
d) height of pole body = height of the winding / 0.02

Answer: a [Reason:] The height of the pole body is one of the design factors in the design of rotor. It is obtained by adding the value of the height of winding with 0.02, which is the approximate space occupied by flanges.

12. What is the range of the ratio of radial length of pole to pole pitch?
a) 0.3-1
b) 0.3-1.5
c) 0.7-1
d) 0.7-1.5

Answer: b [Reason:] The minimum value of the ratio of radial length of pole to pole pitch is given to be 0.3. The maximum value of the ratio of radial length of pole to pole pitch is given to be 1.5.

13. The damper windings are made use of in synchronous generators to reduce the oscillations and to prevent hunting?
a) true
b) false

Answer: a [Reason:] The purpose of the damper windings is to reduce the oscillations and to prevent the hunting in synchronous generators. Next the damper windings are used to suppress the negative sequence field in the synchronous generator.

14. The mmf of the damper windings depends on the pole pitch value?
a) true
b) false