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WorksheetsCFE AC/DC Machines
Total questions: 75
Worksheet time: 40mins
What is the work of the frame of dc machines?
to reduce the voltage
to reduce the flux
to carry the flux
to carry the current
Why is the length of the yoke made larger?
to protect the armature windings
to cover the armature windings
to cover the field windings
to cover and protect the field windings
What is the formula for the depth of the yoke?
depth of yoke = thickness/2
depth of yoke = thickness
depth of yoke = 2*thickness
depth of yoke = 3*thickness
In large machines, the thickness is relatively larger to the diameter.
true
false
What is the formula in order to check the rigidity?
moment of inertia ≥ (weight of magnetic frame * radius2 * 10-6) / 225
moment of inertia ≤ (weight of magnetic frame * radius2 * 10-6) / 225
moment of inertia = (weight of magnetic frame * radius2 * 10-6) / 225
moment of inertia < (weight of magnetic frame * radius2 * 10-6) / 225
What is the formula for the thickness of the ac machines?
thickness = 40 * inner diameter of frame/12
thickness = 40 + inner diameter of frame/12
thickness = 40 - inner diameter of frame/12
thickness = 40 * inner diameter of frame*12
What is the formula for the breadth of the ac machine?
breadth = 6 + 0.01 * inner diameter of frame
breadth = 6 - 0.01 * inner diameter of frame
breadth = 6 * 0.01 * inner diameter of frame
breadth = 6 / 0.01 * inner diameter of frame
What is the formula for the checking of rigidity of induction machines?
moment of inertia ≥ radius / length of stator core * 90
moment of inertia ≥ radius * length of stator core * 90
moment of inertia ≥ radius * length of stator core / 90
moment of inertia ≤ radius / length of stator core * 90
What is the formula for the radius at the centre of gravity?
radius at the centre of gravity = inner diameter1.5/6.3
radius at the centre of gravity = inner diameter2/6.3
radius at the centre of gravity = outer diameter1.5/6.3
radius at the centre of gravity = outer diameter2/6.3
What is the formula of the centrifugal force?
centrifugal force = weight of revolving body * 39.43 * speed2 * radius of circular path
centrifugal force = weight of revolving body / 39.43 * speed2 * radius of circular path
centrifugal force = weight of revolving body * 39.43 / speed2 * radius of circular path
centrifugal force = weight of revolving body * 39.43 * speed2 / radius of circular path
What is the use of the wire bands of rotor?
used for bracing the rotor windings
used for circulating the current in the rotor windings
used for the encircling of the rotor windings
used for the protecting the rotor windings
Where are the wire bands placed?
active portions of rotor conductors
inactive portions of rotor conductors
active or inactive portions of rotor conductors
active and inactive portions of the rotor conductors
What are the factors on which the sizes of bands placed on depend?
length of air gap
method of cooling of armatures
length of air gap and method of cooling of armatures
method of cooling of armatures or length of air gap
In what machines are the wire bands along the active length of windings placed?
dc or ac machines
dc and ac machines
dc machines
ac machines
What is the range of the width of each band that should not be exceeded?
10-15 mm
15-20 mm
20-25 mm
18-23 mm
What is the maximum value above which the total width of the bands should not exceed?
25-35% of the axial length of armature core
30-35% of the axial length of armature core
25-30% of the axial length of armature core
35-40% of the axial length of armature core
What is the formula for the breadth of the ring slot?
breadth of the ring slot = (number of turns in a band + 1)*diameter of band wire - 2*constant
breadth of the ring slot = (number of turns in a band + 1)*diameter of band wire + 2*constant
breadth of the ring slot = (number of turns in a band + 1)*diameter of band wire * 2*constant
breadth of the ring slot = (number of turns in a band + 1)*diameter of band wire / 2*constant
What is the value of the constant used in the calculation of the breadth of the ring slot for the diameter of band wire < 1.5 mm?
1 mm
1.5 mm
2 mm
3 mm
What is the maximum width of the bands placed on the end windings of induction machines and high speed dc machines?
30 mm
35 mm
40 mm
45 mm
What is the diameter of the wire bands made of tin, steel or bronze wire?
2 mm
1 mm
4 mm
3 mm
What is the function of the bands when it is placed on overhang?
used to reduce the centrifugal forces
used to increase the centrifugal forces
used to balance the centrifugal forces
used to withstand the centrifugal forces
What is the function of the bands when they are distributed along the axial length of armature?
used to reduce the centrifugal forces
used to increase the centrifugal forces
used to decrease the centrifugal forces
used to withstand the centrifugal forces
What is the position of centre of gravity?
mean diameter at the position of centre of gravity = Inner diameter + diameter of stator wires
mean diameter at the position of centre of gravity = Inner diameter * diameter of stator wires
mean diameter at the position of centre of gravity = Inner diameter / diameter of stator wires
mean diameter at the position of centre of gravity = Inner diameter - diameter of stator wires
What is the value of permissible stress for bronze wire for the diameter of branding wire of 1 mm?
350 NM per m2
250 NM per m2
300 NM per m2
450 NM per m2
What is the value of permissible stress for steel wire for the diameter of branding wire of 0.5-1.2 mm?
570 NM per m2
600 NM per m2
650 NM per m2
700 NM per m2
What happens in the rotating electrical machine?
armatures are slotted
armatures are slotted and results in the movement of rotor
the rotor remains stationary
the rotor slots are rotating
What is pulsation losses?
flux pulsations are caused due to the slotted armature
flux pulsations are caused due to the rotation of machine
flux pulsation occurs due to rotor slots are rotating
flux pulsations occur due to the change in reluctance
In which machine part/parts does the pulsation loss occurs?
teeth
pole face
conductors
teeth and pole faces
How are the pulsation losses aggravated?
if the air gap is small compared with slot openings
if the air gap is reduced
if the air gap is increased
if the air gap is made larger than the slot openings
The slotting produces harmonic fields which cause high frequency losses near the gap surface.
true
false
In which machine is the pulsation losses considerable?
synchronous motors
induction motors
dc shunt motors
dc series motors
What are the factors the permeance depends upon in the zigzag leakage?
relative position of stator
relative position of rotor
relative position of stator and rotor
stored energy at any position
What is the formula for the stored energy at any position?
stored energy at any position = mmf per slot2 * permeance in a particular position
stored energy at any position = 2 * mmf per slot2 * permeance in a particular position
stored energy at any position = 1/2 * mmf per slot2 * permeance in a particular position
stored energy at any position = 1/3 * mmf per slot2 * permeance in a particular position
What is the formula for the zigzag permeance?
zigzag permeance = average width of the rotor tooth / (1/2 * mmf per slot2)
zigzag permeance = average width of the rotor tooth * (1/2 * mmf per slot2)
zigzag permeance = 1/average width of the rotor tooth *(1/2 * mmf per slot2)
zigzag permeance = average width of the rotor tooth *(1/2 / mmf per slot2)
What is the formula of the zigzag specific permeance?
zigzag specific permeance = average width of the rotor tooth * length / (1/2 * mmf per slot2)
zigzag specific permeance = average width of the rotor tooth / length * (1/2 * mmf per slot2)
zigzag specific permeance = average width of the rotor tooth * length * (1/2 * mmf per slot2)
zigzag specific permeance =1/ average width of the rotor tooth * length * (1/2 * mmf per slot2)
What are the factors the overhang leakage reactance is obtained?
length of the overhang
diameter of the overhang
shape of the overhang
length of the overhang along with the shape of the overhang
The overhang leakage reactance depends on the degree of saturation in the ferromagnetic parts.
true
false
What is the relation between the overhang specific permeance and the slot pitch?
overhang specific permeance is directly proportional to the slot pitch
overhang specific permeance is indirectly proportional to the slot pitch
overhang specific permeance is directly proportional to the square of the slot pitch
overhang specific permeance is indirectly proportional to the square of the slot pitch
How many factors are present in the relation between rating and dimension of rotating machines?
6
7
8
9
What is the formula of the total magnetic loading?
total magnetic loading = total flux around the armature periphery at the air gap
total magnetic loading = total number of ampere conductors around the armature for periphery
total magnetic loading = number of poles/armature total flux
total magnetic loading = total flux/number of poles
What is the formula of the specific electric loading?
specific electric loading = total armature ampere conductors * armature periphery at air gap
specific electric loading = total flux around the air gap / area of flux path at the air gap
specific electric loading = total armature ampere conductors / armature periphery at air gap
specific electric loading = total flux around the air gap * area of flux path at the air gap
How many terms can be used to express the output of a machine?
3
4
5
6
How many factors affecting the size of rotating machines?
3
4
5
2
How many factors are used to determine the specific magnetic loading?
2
3
4
5
How many factors are used to determine the specific electric loading?
2
3
4
5
What is the relation between the output and the flux density?
What is the relation between the output and the flux density?
flux density is directly proportional to the output
flux density is indirectly proportional to the output
flux density is directly proportional to the square of the output
flux density is indirectly proportional to the square of the output
What is the relation between output and the specific electric loading?
specific electric loading is directly proportional to the output
specific electric loading is indirectly proportional to the output
specific electric loading is directly proportional to the square of the output
specific electric loading is indirectly proportional to the square of the output
What is the relation of the specific electric loading and the diameter?
specific electric loading is directly proportional to the diameter
specific electric loading is indirectly proportional to the diameter
specific electric loading is directly proportional to the square of the diameter
specific electric loading is indirectly proportional to the square of the diameter
What is the formula of the I2R loss?
I2R loss = number of conductors / copper loss in each conductor
I2R loss = number of conductors + copper loss in each conductor
I2R loss = number of conductors - copper loss in each conductor
I2R loss = number of conductors * copper loss in each conductor
What is the formula of the efficiency of the machine?
efficiency = output / output + losses
efficiency = output * output + losses
efficiency = output - output + losses
efficiency = output + output + losses
The fractional horsepower motors have efficiency of order of 98%.
true
false
Why are the DC motors preferred for traction applications?
Torque and speed are inversely proportional to armature current
Torque is proportional to armature current
Torque is proportional to square root of armature current
The speed is inversely proportional to the torque and the torque is proportional to square of armature current
Which of the following load application normally needs starting torque more than the rated torque?
Blowers
Conveyors
Air compressors
Centrifugal pumps
Which motor has almost replaced DC shunt motor from its applications?
Wound-rotor induction motor
Differential compound motor
Air motor
Squirrel caged induction motor
Which of the following motors can be used to drive the rotary compressor?
DC shunt motor
DC series motor
Universal motor
Synchronous motor
In world today, around 25% of the motors are manufactured are DC motors.
True
False
Maximum torque in a DC machine is limited by ______________
Commutation
Heating
Losses other than heating
Stability
Commutator performs rectification so that output of the machine is bi-directional.
True
False
How total number of brushes in a commutator are determined in a given DC machine?
Speed of armature
Type of winding
Voltage
Amount of current to be collected
Reason behind the rapid wear of brushes is __________
Abrasion from dust
Excessive spring pressure
Rough commutator bars
Abrasion from dust, excessive spring pressure and rough commutator bars
In a _________ circuit, the total resistance is greater than the largest resistance in the circuit.
Series
Parallel
Either series or parallel
Neither series nor parallel
In a ____________ circuit, the total resistance is smaller than the smallest resistance in the circuit.
Series
Parallel
Either series or parallel
Neither series nor parallel
Which is the most cost efficient connection?
Series
Parallel
Either series or parallel
Neither series nor parallel
Which of the following is not the operating characteristics of Dc generator?
No-load characteristics
Load characteristics
External characteristics
Internal characteristics
Characteristics drawn at Ia = 0 is also called as ____________
Magnetization characteristics
Non-magnetization characteristics
Anti-magnetization characteristics
Cannot be determined
Open circuit characteristics (OCC) is generally drawn across __________
Ea vs If, Ia=constant (not equal to rated)
Ea vs If, Ia=0
Ea vs If, Ia=constant
Ea vs If, Ia=constant (rated)
Characteristics of a DC generator drawn across Vt vs If at rated armature current and constant speed, is called as ____________
Load characteristics
No-load characteristics
External characteristics
Armature characteristics
Armature characteristic is _____________
No-load characteristic
Load characteristic
OCC Current
Cannot be determined
External characteristics is drawn with constant term/s ____________
Field current
Speed
Both Field current and Speed
Load
Armature characteristic is drawn across ________
Ia vs If
Ia vs Vt
If vs Vt
Ea vs Ia
What is the first step in drawing the armature characteristic?
If is made 0
Field current is adjusted to give Voc equal to rated value
Switch connecting armature circuit with external circuit is made open
Cannot be determined
What is Self-excitation in DC shunt generator?
Field winding is connected in series of armature
Field winding is connected in parallel of armature
Field winding is not connected to the armature
Field Winding is not excited
For a self-excited DC shunt generator Ia = _____________
IL - If
IL + IL
- IL - IL
- IL + IL
75. Characteristics of separately excited DC generator are drawn by keeping _____
a) Field current and speed both constant
b) Field current and speed both variable
c) Field current constant and speed variable
d) Field current variable and speed constant
a
b
c
d
