Multiple choice question for engineering
Set 1
1. What is the rotor windings for wound rotor motors?
a) single phase windings
b) double phase windings
c) three phase windings
d) concentrated phase windings
Answer
Answer: c [Reason:] The rotor windings for wound rotor motors are 3 phase windings. The number of rotor slots should be such that a balanced winding is obtained.
2.How should the number of slots be in the case of fractional slot windings?
a) multiples of slots
b) multiple of phases
c) multiple of phases and pair of poles
d) multiples of pair of poles
Answer
Answer: c [Reason:] Fractional slot windings can be used in the wound rotor design. It is preferable to use a number of slots which are a multiple of phases and pair of poles in the case of fractional slot windings.
3.How many types of design of wound rotors are available?
a) 2
b) 3
c) 4
d) 5
Answer
Answer: c [Reason:] They are 4 types of design of wound rotors. They are Number of Rotor Slots, Number of Rotor Turns, Area of Rotor Conductors, Rotor Windings.
4. What should be done to keep the rotor voltage to an acceptable level?
a) rotor to stator turns must be properly adjusted
b) stator to rotor turns must be properly adjusted
c) stator turns must be adjusted
d) rotor turns must be adjusted
Answer
Answer: b [Reason:] The effective ratio of stator to rotor turns should be adjusted to keep the rotor voltage to an acceptable level. The choice of this turns ratio is arbitrary and is controllable by the designer.
5. The rotor voltage on open circuit between slip rings should not exceed 500 V for small machines?
a) true
b) false
Answer
Answer: a [Reason:] The rotor voltage on open circuit between slip rings should not exceed 500 V for the small machines. The voltage is limited to a small value in order to protect persons working the motor if the brush gear is not perfectly protected.
6. How should the rotor voltage be with respect to the high voltage and large machines?
a) low
b) moderate
c) high
d) very high
Answer
Answer: c [Reason:] The high voltage and large machines should have high rotor voltage. If the rotor voltage is kept low, the rotor current becomes large, involving use of large conductor sections.
7. What is the range of the rotor voltage for the large machines?
a) 1000-1500 V
b) 1000-1750 V
c) 500-1500 V
d) 1000-2000 V
Answer
Answer: d [Reason:] The minimum value of the rotor voltage should be 1000 V. The maximum value of the rotor voltage should not exceed 2000 V.
8. What is the formula for rotor turns per phase?
a) rotor turns per phase = (winding factor for stator/winding factor for rotor) * (Rotor voltage per phase/Stator voltage per phase) * Number of turns per phase for stator
b) rotor turns per phase = (winding factor for stator/winding factor for rotor) / (Rotor voltage per phase/Stator voltage per phase) * Number of turns per phase for stator
c) rotor turns per phase = (winding factor for stator/winding factor for rotor) * (Rotor voltage per phase/Stator voltage per phase) / Number of turns per phase for stator
d) rotor turns per phase = (winding factor for stator/winding factor for rotor) / (Rotor voltage per phase/Stator voltage per phase) / Number of turns per phase for stator
Answer
Answer: a [Reason:] Firstly, the winding factor for stator is obtained along with the winding factor for stator. Next the ratio of the rotor voltage per phase to the stator voltage per phase. Finally the number of turns per phase for stator is also calculated to obtain the rotor turns per phase.
9. What is the formula for the full load rotor mmf?
a) 65% of stator mmf
b) 75% of stator mmf
c) 85% of stator mmf
d) 90% of stator mmf
Answer
Answer: c [Reason:] The full load rotor mmf is taken as 0.85 of stator mmf. Full load rotor mmf = 0.85 *
(stator current * no of stator turns) / no of rotor turns.
10. The value of the current density of rotor is chosen almost equal to that in the stator?
a) true
b) false
Answer
Answer: a [Reason:] There occurs a lot of excessive copper loss in the rotors. The value of current density of rotor is almost equal to that in the stator.
11. What type of conductor is chosen for the small motors?
a) round
b) bar
c) skewed
d) rectangular
Answer
Answer: a [Reason:] Round conductors are used for the small motors. Bar conductors are being used for the large motors.
12. What type of winding is made use of for the small motors?
a) mush windings
b) cross windings
c) interconnected windings
d) rounded windings
Answer
Answer: a [Reason:] For the small motors, it is a normal practice to use mush windings. The mush windings should be housed in the semi-closed slots.
13. What type of winding is made use of for the large motors?
a) mush windings
b) bar type windings
c) cross windings
d) rounded windings
Answer
Answer: b [Reason:] For the small motors mush windings is made used of. For the large motors, a double layer bar type winding is made use of.
Set 2
1. How many factors influence the power factor of an induction motor?
a) 3
b) 2
c) 1
d) 4
Answer
Answer: b [Reason:] There are 2 factors which influence the power factor of an induction motor. They are magnetizing current and ideal short circuit current.
2. What is the relation between the magnetizing current and power factor?
a) magnetizing current is directly proportional to the power factor
b) magnetizing current is indirectly proportional to the power factor
c) magnetizing current is directly proportional to the square of the power factor
d) magnetizing current is indirectly proportional to the square of the power factor
Answer
Answer: a [Reason:] Magnetizing current is indirectly proportional to the power factor. As the magnetizing current is large, the power factor is poor.
3. What is the relation between the leakage current and power factor?
a) leakage current is directly proportional to the power factor
b) leakage current is indirectly proportional to the power factor
c) leakage current is directly proportional to the square of the power factor
d) leakage current is indirectly proportional to the square of the power factor
Answer
Answer: b [Reason:] Leakage current is indirectly proportional to the power factor. A small leakage current means a very good power factor.
4. What is the formula for dispersion coefficient?
a) dispersion coefficient = magnetizing current / ideal short circuit current
b) dispersion coefficient = magnetizing current * ideal short circuit current
c) dispersion coefficient = magnetizing current + ideal short circuit current
d) dispersion coefficient = magnetizing current – ideal short circuit current
Answer
Answer: a [Reason:] First the magnetizing current is calculated. Next the ideal short circuit current is calculated. The ratio of both gives the value of dispersion coefficient.
5. What is the formula for dispersion coefficient?
a) dispersion coefficient = 0.838 * 106 * 3.14 / air gap length * effective specific permeance / pole pitch * (window space factor)2 * number of slots per pole per phase
b) dispersion coefficient = 0.838 * 106 * 3.14 * air gap length / effective specific permeance / pole pitch * (window space factor)2 * number of slots per pole per phase
c) dispersion coefficient = 0.838 * 106 * 3.14 * air gap length * effective specific permeance * pole pitch * (window space factor)2 * number of slots per pole per phase
d) dispersion coefficient = 0.838 * 106 * 3.14 * air gap length * effective specific permeance / pole pitch * (window space factor)2 * number of slots per pole per phase
Answer
Answer: d [Reason:] For the calculation of dispersion coefficient, first the air gap length, effective specific permeance is calculated. Next the pole pitch, window space factor and the number of slots per pole per phase.
6. The increase in number of poles, the dispersion coefficient increases and this gives a low power factor?
a) true
b) false
Answer
Answer: a [Reason:] The increase in number of poles increases the dispersion coefficient. The increases in dispersion coefficient gives a low power factor.
7. What is the relation between the number of poles and pole pitch with power factor?
a) number of poles increases, pole pitch increases, bad power factor
b) number of poles increases, pole pitch decreases, good power factor
c) number of poles increases, pole pitch decreases, good power factor
d) number of poles increases, pole pitch increases, bad power factor
Answer
Answer: c [Reason:] As the number of poles increases, the pole pitch decreases and the number of slots per pole per phase also decreases. This increases the dispersion coefficient and it leads to poor power factor.
8. What is the relation between the power factor and the air gap length?
a) small air gap length, dispersion coefficient increases, good power factor
b) small air gap length, dispersion coefficient decreases, bad power factor
c) small air gap length, dispersion coefficient increases, bad power factor
d) small air gap length, dispersion coefficient decreases, good power factor
Answer
Answer: d [Reason:] If the air gap length is small, the dispersion coefficient decreases. As the dispersion coefficient decreases, the power factor increases.
9. What is the relation between the dispersion coefficient and maximum power factor?
a) dispersion coefficient is directly proportional to the power factor
b) dispersion coefficient is indirectly proportional to the power factor
c) dispersion coefficient is directly proportional to the square of the power factor
d) dispersion coefficient is indirectly proportional to the square of the power factor
Answer
Answer: b [Reason:] The dispersion coefficient is indirectly proportional to the maximum power factor. As the dispersion coefficient increases, the power factor reduces drastically.
10. What is the value of the no. of poles for obtaining a dispersion coefficient = 0.5?
a) 5
b) 7
c) 10
d) 6
Answer
Answer: d [Reason:] The machines with 6 poles can result in a dispersion coefficient of 0.5. The dispersion coefficient of 0.5 can be obtained for 2 pole and 4 pole machines also.
11. What is the relation between the overload capacity and dispersion coefficient?
a) overload capacity is directly proportional to the dispersion coefficient
b) overload capacity is indirectly proportional to the dispersion coefficient
c) overload capacity is directly proportional to the square of the dispersion coefficient
d) overload capacity is indirectly proportional to the square of the dispersion coefficient
Answer
Answer: b [Reason:] Overload capacity is indirectly proportional to the dispersion coefficient. The overload capacity of induction motors decreases with increase In dispersion coefficient.
12. What is the relation between the overload capacity and magnetizing current?
a) overload capacity is directly proportional to the magnetizing current
b) overload capacity is indirectly proportional to the magnetizing current
c) overload capacity is directly proportional to the square of the magnetizing current
d) overload capacity is indirectly proportional to the square of the magnetizing current
Answer
Answer: a [Reason:] Overload capacity is directly proportional to the magnetizing current. Overload capacity increases the magnetizing current and this increases the dispersion coefficient and this gives a poor power factor.
13. What is the relation between ideal short circuit current and the number of poles?
a) short circuit current is directly proportional to the number of poles.
b) short circuit current is directly proportional to the square of the number of poles.
c) short circuit current is indirectly proportional to the number of poles.
d) short circuit current is indirectly proportional to the square of the number of poles.
Answer
Answer: c [Reason:] The ideal short circuit current is indirectly proportional to the number of poles. As the number of poles increases, the ideal short circuit current decreases.
14. What is the relation between maximum power and the number of poles?
a) maximum power factor is directly proportional to the number of poles.
b) maximum power factor is directly proportional to the square of the number of poles.
c) maximum power factor is indirectly proportional to the number of poles.
d) maximum power factor is indirectly proportional to the square of the number of poles.
Answer
Answer: c [Reason:] Short circuit current is indirectly proportional to the number of poles. The short circuit current increases the dispersion coefficient. As the dispersion coefficient increases, the maximum power factor decreases.
15. The magnetizing current decreases as the number of poles is decreased?
a) true
b) false
Answer
Answer: a [Reason:] As the number of poles is reduced the magnetizing current is reduced. As the magnetizing current is reduced, the dispersion coefficient decreases and the power factor increases.
Set 3
1. What is the basic property of electrical conducting materials?
a) allows the passage of current through the materials
b) blocks the passage of current through the materials
c) leaks the current through the materials
d) reverses the direction of current in the materials
Answer
Answer: a [Reason:] The basic property of conducting materials is to allow the flow of charges, and align them in a particular direction. The process is nothing but the flow of current in the materials.
2. What is the correct classification of the conducting materials?
a) low resistivity, low conductivity
b) low resistivity, high conductivity
c) high resistivity, high conductivity
d) medium resistivity, medium conductivity
Answer
Answer: b [Reason:] Resistivity is inversely proportional to the conductivity. So, if the material consists of high resistivity then it will obviously have low conductivity and vice versa.
3. Example of low resistivity material is _____________
a) silver
b) manganese
c) magnesium
d) tungsten
Answer
Answer: a [Reason:] Silver is the low resistivity material of all given materials. Tungsten is a part of high resistivity materials. The other two materials do not have a fixed resistivity and they vary with temperature.
4. Example of high resistivity material is ________________
a) copper
b) gold
c) aluminum
d) carbon
Answer
Answer: d [Reason:] Carbon is the highly resistivity material of all the materials. Whereas the other 3 materials are associated with low resistivity property in nature.
5. Is high resistivity material used in making the filaments of incandescent lamp?
a) true
b) false
Answer
Answer: a [Reason:] It is because to protect the lamps from getting over-heated. If the filaments get over-heated, it can lead to the bursting of the lamps.
6. What materials are used as conductors in the Transmission and Distribution sector?
a) copper
b) silver
c) tungsten
d) carbon
Answer
Answer: a [Reason:] Conductors in power system require less resistivity, highly malleable, highly ductile and less cost. Silver has all the above properties, but it is highly costly. So that makes copper highly suitable.
7. What are the properties of Conducting Materials with respect to temperature coefficient of resistance and tensile strength?
a) low temperature coefficient, low tensile strength
b) low temperature coefficient, high tensile strength
c) high temperature coefficient, low tensile strength
d) high temperature coefficient, high tensile strength
Answer
Answer: b [Reason:] The resistance of the material should not increase with temperature rise. This can lead to the loss of conduction property. High tensile strength allows in withstanding external disturbances, for smooth functioning.
8. What are the conditions of the conducting materials with respect to melting point and resistance to corrosion?
a) high melting point, low resistance to corrosion
b) low melting point, low resistance to corrosion
c) high melting point, high resistance to corrosion
d) low melting point, high resistance to corrosion
Answer
Answer: c [Reason:] High melting point, allows the materials to withstand low temperatures. High resistance to corrosion allows the material to avoid corrosion, to conduct effectively.
9. How should the conducting materials be in terms of malleability and ductility?
a) highly malleable, less ductile
b) less malleable, less ductile
c) highly malleable, highly ductile
d) less malleable, highly ductile
Answer
Answer: c [Reason:] The materials, having high malleability allow smooth conduction in transmission and distribution. The materials having high ductility help in producing wires flexibly for conduction.
10. Aluminum has high conductivity than Copper.
a) true
b) false
Answer
Answer: b [Reason:] Copper has high conductivity than Aluminum. The conductivity of Copper is 58.14*106 s/m and the conductivity of aluminum is 37.2*106 s/m.
Set 4
1. What is the formula of the core length?
a) core length = pole length * pole pitch
b) core length = pole length / pole pitch
c) core length = pole length + pole pitch
d) core length = pole length – pole pitch
Answer
Answer: b [Reason:] Core length is one of the factors used in the separation of D and L of the DC machines. It is the ratio of the pole length to the pole pitch.
2. What is the range of the ratio of the pole length to pole pitch?
a) 0.60-0.70
b) 0.64-0.72
c) 0.65-0.75
d) 0.70-0.80
Answer
Answer: b [Reason:] The minimum value of the ratio of the pole length to pole pitch is 0.64. The maximum value of the ratio of the pole length to pole pitch is 0.72.
3. What is the maximum value of the peripheral speed that should not exceed?
a) 25 m per sec
b) 20 m per sec
c) 30 m per sec
d) 35 m per sec
Answer
Answer: c [Reason:] The peripheral speed of the machine should not exceed 30 m per sec. If it exceeds it leads to the damage of the windings. The banding wires on the overhang have to be made so strong if the peripheral speed exceeds 30 m per sec.
4. What is the range of the pole length to pole pitch ratio for obtaining good power factor in induction motors?
a) 1.5-2
b) 1.3–1.8
c) 1.0-1.25
d) 1.1-1.6
Answer
Answer: c [Reason:] The range of the pole length to pole pitch ratio should be between 1.0-1.25 for the good power factor. The range of the pole length to pole pitch ratio should be between 1.5-2 for minimum cost.
5. What is the relationship between the diameter and number of poles?
a) diameter is directly proportional to the number of poles
b) diameter is indirectly proportional to the number of poles
c) diameter is directly proportional to the square of the number of poles
d) diameter is indirectly proportional to the square of the number of poles
Answer
Answer: a [Reason:] The number of poles is one of the factors involved in the separation of D and L in the synchronous machines. The number of poles is directly proportional to the diameter.
6. The core length should be high to obtain high short circuit ratio?
a) true
b) false
Answer
Answer: a [Reason:] The core length is directly proportional to the short circuit ratio. If high short circuit ratio is required, high core length should be used.
7. What is the factor which is by modern motors which are into a series of standard frames?
a) current rating
b) voltage rating
c) power rating
d) output rating
Answer
Answer: c [Reason:] The modern motors for industrial applications is concentrated into a series of standard frames. The standard frames are used to cover a wide range of power ratings.
8. What is the frame used in the standard frames?
a) set of standard values
b) mechanical structure required to house a rotor of given outside diameter
c) mechanical structure required to house a stator of given outside diameter
d) mechanical structure required to house a stator of given outside length
Answer
Answer: c [Reason:] The frame is a mechanical structure required to house a stator of given outside diameter. The frame also includes housing the bearings, end covers, terminal box and maximum length.
9. How can the variation in ratings be obtained?
a) alternative windings
b) alternative core lengths
c) alternative pole pitch
d) alternative pole length
Answer
Answer: b [Reason:] The frame is a mechanical structure required to house a stator of given outside diameter, housing the bearings, end covers, terminal box and maximum length. The variation in the ratings can be obtained using alternative core lengths.
10. What is the value of the alternative core lengths below which variation in rating can be obtained?
a) 0.7 * length
b) 0.4 * length
c) 0.3 * length
d) 0.7 * length or 0.5 * length
Answer
Answer: d [Reason:] The variation in the ratings can be obtained using alternative core lengths. The value of the alternative core lengths below 0.5 * length or 0.7 * length provides the variation in the ratings.
11. What is the range of the central heights used in the standard frames?
a) 50-1000 mm
b) 90-1000 mm
c) 100-1000 mm
d) 56-1000 mm
Answer
Answer: d [Reason:] The modern motors of small and medium sizes are built with standard frame sizes as specified in IEC 72. The IEC 72 lists a coherent range of main dimensions with the central heights ranging between 56-1000 mm.
12. The frame size is designated by a number which is its centre height expressed in mm?
a) true
b) false
Answer
Answer: a [Reason:] The IEC 72 lists a coherent range of main dimensions with the central heights ranging between 56-1000 mm. The frame size is designated by a number which is its centre height expressed in mm. The frame designated 180 has a centre height of 180 mm.
Set 5
1. What is the definition of current transformer?
a) it is used for measuring high voltage
b) it is used for measuring low voltage
c) it is used for measuring high current
d) it is used for measuring low current
Answer
Answer: c [Reason:] For the measuring of high currents, the current transformer is made use of. The measured current is scaled down to lower values equivalently.
2. How many classifications are present for the current transformers?
a) 1
b) 2
c) 3
d) 4
Answer
Answer: b [Reason:] The current transformers is divided into 2 types. They are I) measuring current transformer and II) protective current transformer.
3. What is the definition of the ideal current transformer?
a) the primary and secondary windings are in exact ratio and same phase relationship
b) the primary winding and secondary winding ratio is greater than 1 and same phase relationship
c) the primary and secondary winding ratio is lesser than 1 are in exact ratio and different phase relationship
d) the primary and secondary windings ratio is greater than 1 and different phase relationship
Answer
Answer: a [Reason:] The primary and secondary winding ratio are same. The phase relationship of the windings are also same.
4.How many types of errors are present in the current transformers?
a) 1
b) 2
c) 3
d) 4
Answer
Answer: b [Reason:] There are 2 types of errors present in the current transformers. They are ratio error and phase angle error.
5. What is the formula of the angle between secondary induced voltage and secondary current?
a) phase angle = tan-1 *[(reactance of the secondary winding – reactance of the external burden) / (resistance of the secondary winding + resistance of the external burden)].
b) phase angle = tan-1 *[(reactance of the secondary winding – reactance of the external burden) / (resistance of the secondary winding – resistance of the external burden)].
c) phase angle = tan-1 *[(reactance of the secondary winding * reactance of the external burden) / (resistance of the secondary winding + resistance of the external burden)].
d) phase angle = tan-1 *[(reactance of the secondary winding + reactance of the external burden) / (resistance of the secondary winding + resistance of the external burden)].
Answer
Answer: d [Reason:] The reactance of the secondary windings and the external burden is first calculated. Next, the resistance of the secondary windings and external burden is calculated and on substitution gives the value of the phase angle.
6. What is the formula of the phase angle of the secondary load circuit?
a) phase angle of secondary load circuit = tan-1 * (reactance of the external burden/resistance of the external burden)
b) phase angle of secondary load circuit = tan-1 * (reactance of the external burden + resistance of the external burden)
c) phase angle of secondary load circuit = tan-1 * (reactance of the external burden – resistance of the external burden)
d) phase angle of secondary load circuit = tan-1 * (reactance of the external burden * resistance of the external burden)
Answer
Answer: a [Reason:] The reactance and resistance of the external burden is first calculated. Next the value is taken tan inverse to obtain the phase angle of secondary load circuit.
7. What is the formula of the ratio error in the current transformers?
a) ratio error = turns ratio – regulation / regulation
b) ratio error = turns ratio + regulation / regulation
c) ratio error = turns ratio * regulation / regulation
d) ratio error = 1 / turns ratio * regulation
Answer
Answer: a [Reason:] First the turns ratio is calculated. Next the regulation of the current transformer is obtained and on substitution gives the ratio error.
8. The ratio of active conductor section to total conductor section is called space factor?
a) true
b) false
Answer
Answer: a [Reason:] The space factor is a term which is used in the design of the magnet coils. It is the ratio of the active conductor section to the total conductor section.
