Design Of Electrical Machines MCQs – Three Phase Induction Motors MCQs ( Design Of Electrical Machines ) MCQs
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Design Of Electrical Machines MCQs – Three Phase Induction Motors MCQs ( Design Of Electrical Machines ) MCQs
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Main Dimensions
1. What are the parameters which come under the term “Main Dimensions”?
a) diameter
b) length
c) diameter and length
d) area
Answer: c
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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.
Design of Rotor Bars and Slots
1. What is the formula for current in each of rotor bar?
a) current in rotor bar = 2 * slot pitch * window space factor * stator torque * stator current * power factor * rotor slots
b) current in rotor bar = 2 * slot pitch * window space factor / stator torque * stator current * power factor * rotor slots
c) current in rotor bar = 2 * slot pitch * window space factor * stator torque * stator current * power factor / rotor slots
d) current in rotor bar = 2 * slot pitch / window space factor * stator torque * stator current * power factor / rotor slots
Answer: c
Explanation: For the calculation of the current through each rotor bar, firstly we find out slot pitch, window space factor and rotor slots. Then the stator torque and stator current are obtained with respect to stator side.
2. What is the relation between rotor mmf and stator mmf?
a) rotor mmf = 0.85 * stator mmf
b) rotor mmf = 0.80 * stator mmf
c) rotor mmf = 0.75 * stator mmf
d) rotor mmf = 0.70 * stator mmf
Answer: a
Explanation: First the stator mmf is calculated. Then it is multiplied by 0.85 to obtain the rotor mmf.
3. What is the relation of the rotor resistance with respect to the starting torque?
a) rotor resistance is indirectly proportional to the starting torque
b) rotor resistance is directly proportional to the starting torque
c) rotor resistance is indirectly proportional to the square of the starting torque
d) rotor resistance is directly proportional to the square of the starting torque
Answer: b
Explanation: The rotor resistance is directly proportional to the starting torques. High resistance leads to high starting torque.
4. What is the relation of the rotor resistance to efficiency and losses?
a) as rotor resistance, losses increase, efficiency increases
b) as rotor resistance, losses increase, efficiency decreases
c) as rotor resistance, losses decrease, efficiency has no change
d) as rotor resistance, losses decrease, efficiency decreases
Answer: b
Explanation: As the rotor resistance increases, the I2R losses increases and cause heating effects. This increase in losses decreases efficiency.
5. What is the relationship between current density, conductor area and resistance?
a) higher the current density, higher the conductor area, higher the resistance
b) higher the current density, higher the conductor area, lower the resistance
c) higher the current density, lower the conductor area, higher the resistance
d) lower the current density, lower the conductor area, lower the resistance
Answer: c
Explanation: Higher the current density leads to lower conductor area, as current density is the ratio of current per area. As the conductor area decreases, resistance increases.
6. What is the formula for the calculation of rotor resistance?
a) rotor resistance = resistance of the bars + resistance of end rings
b) rotor resistance = resistance of the bars – resistance of end rings
c) rotor resistance = resistance of the bars * resistance of end rings
d) rotor resistance = resistance of the bars / resistance of end rings
Answer: a
Explanation: First the resistance of the bars are obtained. Next, the resistance of the end rings are calculated and the sum gives the rotor resistance.
7. What is the range of current density in rotor bars?
a) 4-9 A per mm2
b) 4-6 A per mm2
c) 4-7 A per mm2
d) 5-6 A per mm2
Answer: b
Explanation: The minimum value of the current density in the rotor bars is 4 A per mm2. The maximum value of the current density in the rotor bars is 6 A per mm2.
8. What is the formula for the area of each bar?
a) area of each bar = current of the rotor bars + current density in rotor bars
b) area of each bar = current of the rotor bars / current density in rotor bars
c) area of each bar = current of the rotor bars * current density in rotor bars
d) area of each bar = current of the rotor bars – current density in rotor bars
Answer: b
Explanation: For calculating the area of each bar, current flowing across the rotor bars should be first calculated. Then the current density in rotor bars should be calculated next and the ratio gives the area of each bar.
9. Closed slots are preferred for small machines.
a) true
b) false
Answer: a
Explanation: Closed slots are preferred for small machines. It is because the reluctance of the air gap is not large owing to absence of slot openings.
10. What is the relation of closed slots with leakage reactance?
a) closed slots give no leakage reactance
b) closed slots give high leakage reactance
c) closed slots give low leakage reactance
d) closed slots give negative leakage reactance
Answer: b
Explanation: It is an advantage that closed slots give large leakage reactance. If the leakage reactance is large, the current at the starting can be limited.
11. What is the relation of closed slots with leakage reactance and overload capacity?
a) closed slots give high leakage reactance, and increases the overload capacity
b) closed slots give high leakage reactance, and decreases the overload capacity
c) closed slots give low leakage reactance, and decreases the overload capacity
d) closed slots give low leakage reactance, and increases the overload capacity
Answer: b
Explanation: The closed slots have the main advantage of giving high leakage reactance. A high leakage reactance gives the advantage that the current at the starting can be limited.
12. What is the relation between surface of rotor and the operation?
a) smooth surface leads to the quiet operation
b) rough surface leads to the quiet operation
c) smooth surface leads to the noisy operation
d) rough surface leads to the noisy operation
Answer: a
Explanation: For the design of the rotor bars of the three phase induction machine, smooth surface is preferred. Smooth surface helps in the silent operation.
13. Rectangular shaped bars and slots are preferred to circular bars and slots.
a) true
b) false
Answer: a
Explanation: Rectangular shaped bars and slots are preferred to circular bars and slots. This is because while using the rectangular shaped bars, rotor resistance increases and this leads to the improvement of starting torque.
14. What is the relation between clearances and slots?
a) high clearances are provided for salient slots
b) low clearances are provided for skewed slots
c) low clearances are provided for salient slots
d) high clearances are provided for skewed slots
Answer: d
Explanation: When the skewed slots are being used, higher clearances are provided. High clearances can lead to the smooth and efficient operation of the machine for skewed slots.
15. What is the range of clearance that can be left between rotor bars and the core?
a) 0.1-0.4 mm
b) 0.2-0.4 mm
c) 0.15-0.4 mm
d) 0.4-0.6 mm
Answer: c
Explanation: The range of clearance is chosen based on whether the slots are skewed or not. The range is usually chosen between 0.15-0.4 mm.
Design of End Rings
1. How does the revolving field produce emf in the bars?
a) revolving field produces emf of fundamental frequency in the bars
b) revolving field produces emf of third frequency in the bars
c) revolving field produces emf of no frequency in the bars
d) revolving field produces emf of sinusoidal frequency in the bars
Answer: a
Explanation: The stator winding is 3 phase distributed winding and thus produces a revolving field. This revolving field produces emfs of fundamental frequency in the bars.
2. What happens if the resistance of the end rings is negligible?
a) resistance coming in each current path is resistance of three bars
b) resistance coming in each current path is resistance of four bars
c) resistance coming in each current path is resistance of two bars
d) resistance coming in each current path is resistance of five bars
Answer: c
Explanation: If the resistance of end rings is negligible then the resistance of combined bars are taken into account. Generally the resistance of two bars are taken into account.
3. What factors does the current in the bars depend on?
a) emfs, position of bars in magnetic field
b) instantaneous emfs, position of bars in magnetic field
c) emf
d) instantaneous emf
Answer: b
Explanation: The current that the bars carry are proportional to their instantaneous emfs. The instantaneous emfs are proportional to the position of the bars in the magnetic field.
4. The end resistance, if not negligible, will tend to distort the bar current distribution from being sinusoidal.
a) true
b) false
Answer: a
Explanation: The end resistance is proportional to the current distribution. If it is not negligible then the resistance will distort the bar current distribution.
5. What is the formula for the maximum current in end ring, if the current in all bars are maximum at the same time?
a) maximum current in the end ring= bars per pole * 2 * current per bar
b) maximum current in the end ring= (bars per pole / 2) * current per bar
c) maximum current in the end ring= bars per pole / 2 / current per bar
d) maximum current in the end ring= bars per pole * 2 / current per bar
Answer: b
Explanation: First the bars per pole are obtained. Then the current per bar is calculated. Then substituting in the above formula, the maximum current in the end rings.
6. Given the bars per pole is 6 and the current per bar is 20 A, what is the value of the maximum current in the end rings?
a) 60 A
b) 80 A
c) 90 A
d) 70 A
Answer: a
Explanation: maximum current in the end ring= bars per pole / 2 * current per bar
Maximum current in the end ring = (6/2)*20 = 3 * 20 = 60 A.
7. What is the formula for the maximum value of current through end ring, when the current is not maximum in all the bars under one pole at the same time?
a) maximum current in end ring= (2*3.14) / (bars per pole/2*no of poles) * current per bar
b) maximum current in end ring= (2/3.14) * (bars per pole/2*no of poles) / current per bar
c) maximum current in end ring= (2*3.14) * (bars per pole/2*no of poles) * current per bar
d) maximum current in end ring= (2*3.14) / (bars per pole/2*no of poles) / current per bar
Answer: c
Explanation: The bars per pole is first obtained. Then the no of poles is calculated along with the current per bar. On substituting in the formula the maximum current in the end ring with the current through all the bars under one pole is not maximum.
8. What is the formula for the maximum current through each bar?
a) maximum value of the current through each bar = 2 * current through each bar
b) maximum value of the current through each bar = (2 * current through each bar)1/2
c) maximum value of the current through each bar = (2 * current through each bar)2
d) maximum value of the current through each bar = (2 * current through each bar)1/3
Answer: b
Explanation: Firstly, the current through each bar is calculated. Then it is multiplied by 2 and its square root provides the maximum value of current through each bar is obtained.
9. What is the formula for the rms value of the end ring current?
a) rms value of end ring current = (bars per pole * current per bar) / (3.14*no of poles)
b) rms value of end ring current = (bars per pole * current per bar) * (3.14*no of poles)
c) rms value of end ring current = (bars per pole * current per bar) / (no of poles)
d) rms value of end ring current = (bars per pole * current per bar) / (3.14+no of poles)
Answer: a
Explanation: First the bars per pole is obtained. Then the current per bar is calculated and the no of poles is calculated. Substituting in the above formula, the rms value of the end ring current is obtained.
10. The value of the current density is chosen for the end rings such that the desired value of rotor resistance is obtained.
a) true
b) false
Answer: a
Explanation: The value of current density is chosen for the end rings should be chosen such that the desired value of the rotor resistance is obtained. If not it can lead to the starting problems in the machine.
11. How is the current density of the rotor bars chosen with respect to the end rings?
a) current density of rotor bars < current density of end rings
b) current density of rotor bars > current density of end rings
c) current density of rotor bars = current density of end rings
d) current density of rotor bars <= current density of end rings
Answer: a
Explanation: The ventilation is generally better for the end rings. Thus, the current density of the end rings should be greater than that of the rotor bars.
12. What is the formula for the area of the ring?
a) area of the ring = depth of the end ring + thickness of the end ring
b) area of the ring = depth of the end ring – thickness of the end ring
c) area of the ring = depth of the end ring / thickness of the end ring
d) area of the ring = depth of the end ring * thickness of the end ring
Answer: d
Explanation: For calculation of the area of end rings, first the depth of end rings is calculated. Next, the thickness of the end ring is calculated and substituting in the above formula gives the area of the ring.
Design of Wound Rotor
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: c
Explanation: 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: c
Explanation: 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: c
Explanation: 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: b
Explanation: 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: a
Explanation: 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: c
Explanation: 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: d
Explanation: 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: a
Explanation: 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: c
Explanation: 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.
Design Of Synchronous Machines MCQs
10. The value of the current density of rotor is chosen almost equal to that in the stator.
a) true
b) false
Answer: a
Explanation: 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: a
Explanation: 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: a
Explanation: 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: b
Explanation: For the small motors mush windings are made use of. For the large motors, a double layer bar type winding is made use of.
No Load Current
1. How many methods are present to obtain all the machine performance characteristics?
a) 3
b) 2
c) 1
d) 4
Answer: b
Explanation: 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
Explanation: 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 90° leading the voltage
c) the magnetizing component is 90° lagging the voltage
d) the magnetizing component is 90° out of phase with the voltage
Answer: d
Explanation: The magnetizing current component is 90° 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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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) 30°
b) 45°
c) 60°
d) 90°
Answer: c
Explanation: The flux tube crossing the air gap at 60° 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 60° 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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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%
Answer: b
Explanation: For output of 0.75 kW, the no load current is 50% of full load current. For output of 75 kW and above, the no load current is 27% of the full load current.
15. The no load power factor is the ratio of full load current to no load current.
a) true
b) false
Answer: a
Explanation: For obtaining the no load power factor first the no load current value is obtained. Then the full load current value is obtained and the ratio gives the no load power factor.
Dispersion Coefficient
1. How many factors influence the power factor of an induction motor?
a) 3
b) 2
c) 1
d) 4
Answer: b
Explanation: 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: a
Explanation: 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: b
Explanation: 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: a
Explanation: 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: d
Explanation: 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: a
Explanation: 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: c
Explanation: 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: d
Explanation: 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: b
Explanation: 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: d
Explanation: 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: b
Explanation: Overload capacity is indirectly proportional to the dispersion coefficient. The overload capacity of induction motors decreases with an increase in the 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: a
Explanation: 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 the 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: c
Explanation: 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: c
Explanation: 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: a
Explanation: 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.
Losses and Efficiency
1. How many losses are present in induction motors?
a) 4
b) 3
c) 5
d) 2
Answer: c
Explanation: 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
Explanation: 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
Explanation: The additional losses are divided into 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
Explanation: 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
Explanation: 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
Explanation: 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 effects 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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: 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
Explanation: The additional iron losses are a small amount when compared with the supplied power. They are 0.5% of the supplied power.
Stator Winding
1. What type of winding is generally used for the stators?
a) double layer wave winding
b) double layer lap winding
c) single layer wave winding
d) single layer lap winding
Answer: b
Explanation: The double layer wave winding is generally used for stators. The wave winding is with diamond coils is used for stators.
2. What type of winding is made use of small motors?
a) single layer mush winding
b) single layer lap winding
c) single layer wave winding
d) double layer wave winding
Answer: a
Explanation: Small motors consisting of small number of slots have a large number of turns per phase. These small motors use single layer mush windings.
3. What class does the slot and phase insulation belong to?
a) B
b) Y
c) H
d) E
Answer: d
Explanation: The modern insulating materials for diamond coils belong to class E, B and F. The slot and phase insulation is Polyester foil coated with compressed fibre for Class E.
4. What class does the plastic foil baked with polyamide fibres belong to?
a) Y
b) B
c) F
d) H
Answer: c
Explanation: The modern insulating materials for diamond coils belong to classes E, B, and F. The plastic foil baked with polyamide fibres belong to class F.
5. What is the formula for flux per pole?
a) flux per pole = average magnetic flux * pole pitch * length
b) flux per pole = average magnetic flux / pole pitch * length
c) flux per pole = average magnetic flux * pole pitch / length
d) flux per pole = average magnetic flux * pole pitch + length
Answer: a
Explanation: First the average magnetic flux is calculated. Then the pole pitch is calculated and then the length of the pole is calculated.
6. What is the initial assumption for the value of winding factor?
a) 0.9
b) 0.95
c) 0.93
d) 0.92
Answer: b
Explanation: The winding factor may be initially assumed as 0.955. It is the value of winding factor for infinitely distributed winding with full pitch coils.
7. What is the formula for stator turns per phase?
a) stator turns per phase = Stator voltage per phase / 4.44 * f * maximum flux / stator winding factor
b) stator turns per phase = Stator voltage per phase * 4.44 * f * maximum flux * stator winding factor
c) stator turns per phase = Stator voltage per phase / 4.44 * f * maximum flux * stator winding factor
d) stator turns per phase = Stator voltage per phase * 4.44 * f * maximum flux / stator winding factor
Answer: c
Explanation: For the finding out of stator turns per phase, first the stator voltage per phase is obtained. Next, the maximum flux is calculated, then the stator winding factor is calculated.
8. What should be the range of current density in the stator windings?
a) 2-5 A per mm2
b) 4-5 A per mm2
c) 3-5 A per mm2
d) 2-3 A per mm2
Answer: c
Explanation: The minimum value for the current density in stator winding is 3 A per mm2. The maximum value of the current density in the stator windings should not exceed 5 A per mm2.
9. For the lower values of current, round conductors would be convenient to use.
a) true
b) false
Answer: a
Explanation: For lower values of current, round conductors would be the most convenient to use while for higher current bars. It should be less than 2 or 3 mm in diameter or else it is difficult to wind.
Design Of Electrical Machines MCQs – Three Phase Induction Motors MCQs ( Design Of Electrical Machines ) MCQs