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# Multiple choice question for engineering

## Set 1

1. How many losses are present in induction motors?
a) 4
b) 3
c) 5
d) 2

Answer: c [Reason:] There are 5 losses present in the induction motor. They are i) Stator copper losses, ii) Rotor copper losses, iii) Stator iron losses, iv) Friction and winding losses, v) Additional losses.

2. What is the formula for efficiency at full load?
a) efficiency at full load = output / output + losses
b) efficiency at full load = output / output – losses
c) efficiency at full load = output / output * losses
d) efficiency at full load = output * output + losses

Answer: a [Reason:] First the various losses are calculated for the machine. Then the output obtained is observed and the substitution of the values in the formula gives the efficiency at full load.

3. How many types of additional losses are present?
a) 1
b) 4
c) 2
d) 3

Answer: c [Reason:] The additional losses are divided in to 2 types. They are i) Additional copper loss ii) Additional iron losses.

4. What factor does the additional copper losses depend upon?
a) skin effect
b) mmf harmonics
c) machine design
d) mmf harmonics and skin effect

Answer: d [Reason:] With a sinusoidal voltage impressed over the terminal of the motor, the additional copper losses are caused. They are caused due to the higher order mmf harmonics and skin effect.

5. The additional losses owing to the higher order mmf harmonics occur mainly in windings of squirrel cage rotor?
a) true
b) false

Answer: a [Reason:] The additional losses are depending on the higher order mmf harmonics and skin effect. The losses occur mainly in the squirrel cage rotor.

6. How can the additional losses be decreased in the induction motor?
a) chording the stator winding
b) skewing the rotor
c) having a proper slot combination
d) chording the stator winding, skewing the rotor, having a proper slot combination.

Answer: d [Reason:] There are 3 methods to decrease the additional losses in induction motor. They are chording the stator winding, skewing the rotor, having a proper slot combination.

7. What is the use of skin effect in the induction motor?
a) it helps in improving the efficiency
b) it helps in improving the stopping characteristics
c) it helps in improving the starting characteristics
d) it helps in improving the running characteristics

Answer: c [Reason:] The skin effect phenomenon is observed in stator and rotor windings in the induction motor. The effect may be used for improving starting characteristics.

8. What should be the maximum permissible level for frequency in normal operating conditions?
a) < 2 Hz
b) > 3 Hz
c) < 4 Hz
d) > 3 Hz

Answer: b [Reason:] The normal condition operation depends upon the frequency levels in the machine. It should not exceed 3 Hz.

9. How many types are the additional losses in iron classified into?
a) 2
b) 3
c) 4
d) 5

Answer: a [Reason:] The additional iron losses are classified into 2 types. They are i) pulsation losses and ii) surface losses.

10. The pulsation losses are caused by the direct axis pulsation of magnetic flux?
a) true
b) false

Answer: a [Reason:] The pulsation losses are one type of additional iron losses produced. They are produced by the direct axis pulsation of magnetic flux due to the variation of permeance caused by the continuous change in mutual positions of rotor and stator teeth during rotation of rotor.

11. How much does the addition iron losses relate with the supplied power ?
a) additional iron losses = 0.5% of supplied power
b) additional iron losses = 0.6% of supplied power
c) additional iron losses = 0.8% of supplied power
d) additional iron losses = 0.9% of supplied power

Answer: a [Reason:] The additional iron losses are a small amount when compared with the supplied power. They are 0.5% of the supplied power.

## Set 2

1. What is the property of magnetic materials?
a) Resistivity
b) Conductivity
c) Permeability
d) Ductility

Answer: c [Reason:] There are many properties of magnetic materials, and permeability is one among them. The other 3 properties are related to other materials like conducting and insulating materials.

2. What is the property of permeability in magnetic materials?
a) how easily the magnetic flux is broken/clear
b) how easily the magnetic flux is set up
c) how long the magnetic flux takes to form
d) how long the magnetic flux takes to clear

Answer: b [Reason:] The basic operation of magnetic material is to form magnetic flux. Permeability is the ability of the material to determine how easily the magnetic flux is set up.

3. What is the representation of permeability?
a) coercivity/retentivity
b) flux/flux density
c) magnetic force/magnetic flux density
d) magnetic flux density/magnetic force

Answer: d [Reason:] Permeability is the property which deals, with the relationship with magnetic flux density and magnetic force. Magnetic force/Magnetic flux density deals with the reciprocal of permeability. Coercivity/Retentivity deals with the terms of B-H curve.

4. How should the permeability and number of ampere turns for good magnetic materials be?
a) high permeability, high ampere turns
b) high permeability, low ampere turns
c) low permeability, low ampere turns
d) low permeability, high ampere turns

Answer: b [Reason:] High permeability is always required in magnetic materials for its good operation. At the same time high permeability leads to less ampere turns in the materials.

5. Is retentivity associated with B-H curve?
a) Yes
b) No

Answer: a [Reason:] B-H curve deals with the concepts of retentivity and coercivity. The property of retentivity can be shown in the B-H curve by an increasing curve in the curve.

6. What is the property of retentivity in magnetic materials?
a) After removal of external magnetic fields, magnetization exists
b) After removal of external magnetic fields, magnetization doesn’t exist
c) After removal of internal magnetic fields, magnetization exists
d) After removal of internal magnetic fields, magnetization doesn’t exist

Answer: a [Reason:] Magnetic materials have the property of retentivity in which the magnetic flux produced acts according to the external magnetic field. When the external field is removed, the magnetization in the materials doesn’t deform immediately.

7. What is coercivity force in magnetic materials?
a) The force required to add upon the existing magnetization
b) The force required to remove the existing magnetization
c) The force required to produce magnetic flux
d) The force required to break magnetic flux

Answer: b [Reason:] Magnetic materials generally have the property of retaining magnetization, even if the external magnetic field is removed. So, coercive force is the force which is required to reduce the magnetization.

8. What are magnetic hard materials?
a) High retentivity, low coercivity
b) High retentivity, high coercivity
c) Low retentivity, low coercivity
d) Low retentivity, high coercivity

Answer: b [Reason:] High retentivity is required for protecting the magnetic materials from losing its magnetic property. High coercivity is required to reduce the effect of retentivity to protect the material.

9. What is reluctance in magnetic materials?
a) Allows the buildup of magnetic flux
b) Reduces the buildup of magnetic flux
c) Resists the buildup of magnetic flux
d) Increases the buildup of magnetic flux

Answer: c [Reason:] Reluctance, as the name suggests, is something which is reluctant or hesitant to do. As per the magnetic terms it resists the building up of magnetic flux in the materials.

10. High Reluctance affects the performance of magnetic materials.
a) True
b) False

Answer: a [Reason:] High reluctance means the materials resist in building up the magnetic flux to a higher extent. So, for the proper functioning the reluctance values should be as low as possible.

11. What is the unit of reluctance in magnetic materials?
a) Henry/m
b) Weber/m2
c) Ampere-turns/Weber
d) Ampere-turns/m

Answer: c [Reason:] Henry/m deals with the unit of permeability. Weber/m2 deals with the unit of magnetic field. Reluctance is the opposite of permeance.

12. How many classification of magnetic materials are present?
a) 3
b) 4
c) 5
d) 6

Answer: c [Reason:] There are basically 4 properties in magnetic materials and 5 classifications. They are diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic, ferrimagnetic.

13. What is the property of ferromagnetic materials?
a) Negative magnetization
b) Magnetization slightly less than 1
c) Magnetization slightly greater than 1
d) Magnetization very much higher than 1

Answer: d [Reason:] Negative magnetization denotes the property of Diamagnetic materials. Magnetization slightly greater than 1 denotes the property of Paramagnetic materials. Ferromagnetic materials have magnetization in the range of 1000+.

14. What is the example of diamagnetic materials?
a) Quartz
b) Pyrite
c) Montmorillonite
d) Biotite

Answer: a [Reason:] The other 3 materials are paramagnetic in nature, which means magnetization is slightly above 1. Quartz is a diamagnetic material in which the magnetization is negative.

15. What is the example of ferromagnetic materials is?
a) Magnetite
b) Hematite
c) Nickel
d) Biotite

Answer: a [Reason:] Hematite denotes the example of antiferromagnetic materials. Nickel denotes an example of ferromagnetic materials. Biotite denotes the example of paramagnetic materials.

## Set 3

1. What are the parameters which come under the term “Main Dimensions”?
a) diameter
b) length
c) diameter and length
d) area

Answer: c [Reason:] The main dimensions entirely deal with the calculation of the diameter and length. They are obtained from the output equation of the machine.

2. What is the range of the ratio of the core length to pole pitch for minimum cost?
a) 1.5-2
b) 1-2
c) 1.0-1.25
d) 2-2.5

Answer: a [Reason:] 1.0-1.25 is the range of the ratio of the core length to pole pitch for good power factor. For minimum cost, 1.5 is the minimum range for the ratio and 2 is the maximum range for the ratio.

3. What is the value of the ratio of the core length to pole pitch for good efficiency?
a) 1
b) 1.5
c) 2
d) 3

Answer: b [Reason:] 1 is the value of the ratio of the core length to pole pitch for good overall design. 1.5 is the value of the ratio of the core length to pole pitch for good efficiency.

4. What is the relation between motors and ratio of core length to pole pitch?
a) for small motors high ratio is preferred
b) for big motors high ratio is preferred
c) for small motors small ratio is preferred
d) for big motors small ratio is preferred

Answer: c [Reason:] For small motors, high values of the ratio of core length to pole pitch which may not be able to accommodate even a small amount of slots. Thus small motors prefer only small ratio to accommodate slots.

5. What are the factors the value of core length to pole pitch depend upon?
a) area of the slots
b) size of machine
c) size of conductors
d) size of machine, minimum cost, good efficiency

Answer: d [Reason:] The value of core length to pole pitch varies from 0.6 to 2. It depends on the cost, efficiency, power factor, overall design.

6. What is the Relation between pole pitch and the core length in terms of the best power factor?
a) pole pitch = (0.18 * core length)3
b) pole pitch = (0.18 * core length)2
c) pole pitch = (0.18 * core length)1/2
d) pole pitch = (0.18 * core length)1/3

Answer: c [Reason:] For the best power factor, the pole pitch is equal to the square root of the product of 0.18 and core length. This value however is not dimensionally correct and is valid for the values in meters.

7. What is the range of the permissible peripheral speeds in the 3 phase induction machine?
a) 60-75 m per s
b) 60-70 m per s
c) 40-70 m per s
d) 50-70 m per s

Answer: a [Reason:] For the standard rotor construction can generally be used for peripheral speeds upto 60 m per s. For special rotor construction, the peripheral speeds upto 75 m per s are permissible.

8. What is the maximum permissible level for the peripheral speed for a normal design?
a) < 30 m per s
b) > 30 m per s
c) <=30 m per s
d) >=30 m per s

Answer: c [Reason:] For the normal design, the diameter value is chosen such that the peripheral speed value doesn’t exceed 30 m per s. The diameter value is directly proportional to the peripheral speed.

9. What is the range of the core length for which the stator is provided with the ventilating ducts?
a) 105-120 mm
b) 100-120 mm
c) 100-150 mm
d) 100-125 mm

Answer: d [Reason:] The stator is provided with the ventilating ducts if the core length exceeds 100-125 mm. If the value is less than 100 mm, then no ventilating ducts are provided.

10. The width of each duct is about 8 to 10 mm?
a) true
b) false

Answer: a [Reason:] The width of the duct is chosen such that, the minimum value of the duct is 8 mm. The maximum value of the duct is chosen to be less than 10 mm.

## Set 4

1. What are the subjects to which the design of electrical machines is compared to?
a) science and maths
b) maths and art
c) maths
d) science and art

Answer: d [Reason:] The design of electrical machines is basically compared to as a science. It is also compared to as an art because of the designing.

2. How many design problems are present according to the modern trends in design of electrical machines?
a) 2
b) 3
c) 4
d) 5

Answer: b [Reason:] There are 3 design problems involved in the modern trends in the design. They are electromagnetic design, mechanical design and thermal design.

3. What is one of the major aspects in the modern day design?
a) design machines to form a group
b) design machines to provided integrated systems
c) design machines to provide a single system
d) design machine to form multiple group with interconnection

Answer: c [Reason:] The major aspect of the modern trends is that the design of machines should be many in number. At the same time, all the machines should form a part of a single system.

4. How are the machines sometimes designed with respect to ratings?
a) design 2 machines with different rating
b) design a series of machines with different ratings to fit into a single frame size
c) design a series of machines with same rating to fit into a single frame size
d) design 2 machines with the same rating

Answer: b [Reason:] The machines are sometimes being designed as a series of machines with different ratings. They are done so to fit into a single frame size.

5. What are the factors which are considered when the optimal solution involves iterations wherein the values of variables are changed?
a) performance
b) cost constraint
c) performance or cost constraint
d) performance and cost constraint

Answer: d [Reason:] The optimal solution involves iteration wherein the values of variables are changed. This is done to satisfy both the performance and cost constraints.

6. The computer aided design is one of the modern techniques which is used to provide accurate and comprehensive design?
a) true
b) false

Answer: a [Reason:] The computer aided design is one of the most modern techniques of designing various machines. It is very easy, accurate, easily changeable and a comprehensive design procedure.

7. What is the relation between reluctance, flux and mmf of the machine?
a) low reluctance, less flux leakage, high mmf
b) low reluctance, high flux leakage, high mmf
c) high reluctance, high flux leakage, low mmf
d) high reluctance, less flux linkage, high mmf

Answer: a [Reason:] The designer’s problem is to provide a path of low reluctance to that of comparatively little flux leaks away. In order to compensate for the flux leakage, high mmf is maintained.

8. How should the air gaps be present in the magnetic circuit according to length and cross section?
a) low length, low cross section
b) high length, high cross section
c) high length, low cross section
d) low length, high cross section

Answer: d [Reason:] The air gaps should be present in the magnetic circuit in the low length. The air gap should have maximum cross section to reduce the reluctance.

9. What is the function of the leakage flux?
a) contributes to the transfer of energy
b) contributes to the conversion of energy
c) contributes to both the conversion of energy and transfer of energy
d) does not contributes to the conversion or transfer of energy

Answer: d [Reason:] The leakage flux produced due to the low reluctance path does not transfer the energy produced. The leakage flux also does not convert the energy in the system.

10. How many factors does the leakage flux affect?
a) 6
b) 7
c) 8
d) 9

Answer: c [Reason:] There are 8 factors which are affected by the leakage flux. They are performance of rotating machines and transformers, excitation demands of salient pole machines, leakage reactance of windings, forces between windings, voltage regulation of ac generator and transformers, commutation condition in dc machines, stray load losses, circulating current in transformer tank walls.

11. In the B-H magnetization curve, the flux density occupies the x axis.
a) true
b) false

Answer: b [Reason:] In B-H curve, the flux density occupies the y axis always. In the magnetization curve, magnetic field intensity is plotted.

12. What is the formula of the leakage coefficient?
a) leakage coefficient = total flux * useful flux
b) leakage coefficient = total flux / useful flux
c) leakage coefficient = useful flux / total flux
d) leakage coefficient = total flux + useful flux

Answer: b [Reason:] The total flux is the addition of the useful flux and the leakage flux is first calculated. On substitution the leakage coefficient can be obtained.

## Set 5

1. How many methods are present to obtain all the machine performance characteristics?
a) 3
b) 2
c) 1
d) 4

Answer: b [Reason:] There are 2 methods in obtaining all the open circuit characteristics. They are no load characteristics and short circuit characteristics.

2. How many components does the no load current characteristics comprise of?
a) 2
b) 3
c) 4
d) 1

Answer: a [Reason:] There are 2 main components under the no load current. They are Magnetizing current and Loss component of current.

3. How is the Magnetizing component with respect to the voltage?
a) the magnetizing component is in phase with the voltage
b) the magnetizing component is 900 leading the voltage
c) the magnetizing component is 900 lagging the voltage
d) the magnetizing component is 900 out of phase with the voltage

Answer: d [Reason:] The magnetizing current component is 900 out of phase with the voltage. The loss component is in phase with the voltage.

4. How many parts does the flux produced by stator mmf passes through?
a) 3
b) 4
c) 5
d) 6

Answer: c [Reason:] The flux produced by stator mmf passes through 5 parts. They are air gap, rotor teeth, rotor core, stator teeth, stator core.

5. The flux is distributed sinusoidally and the mmf varies sinusoidally in a DC Machine?
a) true
b) false

Answer: b [Reason:] In a DC Machine, the flux is assumed to be uniform over any cross section and the same mmf for all the paths. But in an induction machine, the flux is distributed sinusoidally, and the mmf varies sinusoidally.

6. What factors does the value of magnetizing current depend on?
a) flux tube
b) output power
c) mean mmf
d) mean mmf and flux tube

Answer: d [Reason:] If the permeability of iron were constant this would cause no difficulty. The value of magnetizing current would be accurately obtained by considering the mean mmf and the flux tube where this mean occurs.

7. When maximum values of the design factors are considered, what is the relation between flux and the magnetizing current?
a) flux is directly proportional to the magnetizing current
b) flux is indirectly proportional to the magnetizing current
c) flux is directly proportional to square of the magnetizing current
d) flux is indirectly proportional to square of the magnetizing current

Answer: b [Reason:] The flux value is indirectly proportional to the magnetizing current. The flux is too small or rather the magnetizing current becomes high.

8. At what angle with respect to the interpolar axis does the flux tube gives a good approximation?
a) 300
b) 450
c) 600
d) 900

Answer: c [Reason:] The flux tube crossing the air gap at 600 from the interpolar axis will always give a good approximation. The calculation of the magnetizing mmf should be based upon the value of the flux density at 600 from the interpolar axis.

9. What is the formula for mmf for air gap?
a) mmf for air gap = 800000 * air gap flux density * air gap factor * length of air gap
b) mmf for air gap = 800000 / air gap flux density * air gap factor * length of air gap
c) mmf for air gap = 800000 * air gap flux density / air gap factor * length of air gap
d) mmf for air gap = 800000 * air gap flux density * air gap factor / length of air gap

Answer: a [Reason:] For calculating the mmf for air gap, the air gap flux density is first calculated. Next the air gap factor is calculated along with the length of air gap.

10. What is the formula for the mmf required for stator teeth?
a) mmf required for stator teeth = mmf per metre + depth of stator slots
b) mmf required for stator teeth = mmf per metre * depth of stator slots
c) mmf required for stator teeth = mmf per metre / depth of stator slots
d) mmf required for stator teeth = mmf per metre – depth of stator slots

Answer: b [Reason:] First the mmf per meter is obtained separately from its design equation. Then the depth of the stator slots is obtained and the product of both gives mmf required for stator teeth.

11. What is the formula for the mmf required for stator teeth?
a) stator teeth mmf = mmf per metre / length of flux path in rotor core
b) stator teeth mmf = mmf per metre + length of flux path in rotor core
c) stator teeth mmf = mmf per metre * length of flux path in rotor core
d) stator teeth mmf = mmf per metre – length of flux path in rotor core

Answer: c [Reason:] First the mmf per meter of stator slots is calculated by its equation. Then the length of the flux path in rotor core is obtained and the product of both gives the stator teeth mmf value.

12. What is the formula for the magnetizing current per phase?
a) magnetizing current per phase =( 0.427 * no. of poles * total magnetizing mmf per pole ) / stator winding factor * no of turns of stator slots
b) magnetizing current per phase =( 0.427 / no. of poles * total magnetizing mmf per pole ) / stator winding factor * no of turns of stator slots
c) magnetizing current per phase =( 0.427 * no. of poles / total magnetizing mmf per pole ) / stator winding factor * no of turns of stator slots
d) magnetizing current per phase =( 0.427 * no. of poles * total magnetizing mmf per pole ) * stator winding factor * no of turns of stator slots

Answer: a [Reason:] Firstly the total magnetizing mmf per pole is calculated. Then the no of poles and the stator winding factor is calculated. Next the no. of turns of stator slots is calculated and the magnetizing current per phase can be obtained.

13. What is the no load current percent of the full load current for the output of 0.75 KW?
a) 50%
b) 40%
c) 33%
d) 90%

Answer: a [Reason:] For output of 3 kW, the no load current is 40% of full load current. For output of 15 kW, the no load current is 33% of the full load current.

14. What is the no load current percent of the full load current for the output of 37 KW?
a) 50%
b) 30%
c) 27%
d) 67%