16
|
How the core loss of rotating machine is related
to flux density?
|
|
Core
Losses in the rotating machines
- The specific
core loss ( Core loss per unit volume ) of rotating machine is directly
proportional to square of flux density.
- The core loss
of rotating machine is directly proportional to flux density.
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17
|
Describe the effect of specific magnetic loading
on iron loss.
|
|
Effect
of Specific magnetic loading on iron loss
- The iron loss
in the rotating machine is directly proportional to specific magnetic
loading.
- Therefore the
higher value of specific magnetic loading means higher iron loss and vice
versa.
|
18
|
Describe the effect of linear dimensions on the
iron loss and output of machine.
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|
Iron
loss and Output of Machine
- The two
machines having linear dimension in the ratio of x : 1, the iron loss of the
two machines in the ratio of x3 : 1 but the output in the ratio of
x4 : 1.
|
19
|
Which factors greatly influence due to specific
electrical loading?
|
|
Factor
affecting specific electrical loading
- Temperature
rise
- Voltage
- Size of
machine
- Current
density
|
20
|
Describe the relation between cooling coefficient
and specific electrical loading.
|
|
Cooling
coefficient and specific electrical loading
- The specific
electrical loading of the machine is inversely proportional to cooling
coefficient.
- The cooling
coefficient depends upon cooling condition of the machine. Lower value of
cooling coefficient for better ventilation and vice versa.
- Therefore the
higher value of specific electrical loading allows for better cooling
condition of any machine.
|
21
|
Define : Space factor
|
|
Space
factor
- It is defined
as the ratio of bare conductor area to the total slot area.
|
22
|
On which factors the cooling coefficient of
machine depends?
|
|
Ventilation
condition
|
23
|
Give reason : The specific electrical loading for
high voltage machine is less than the low voltage machine.
|
|
Specific
Electrical Loading
- The specific electrical
loading of a given machine is directly proportional to space factor.
- The value of
space factor is low for high voltage machine due to higher insulation
thickness.
- Therefore the
specific electrical loading for high voltage machine is lower than that of
low voltage machine.
|
24
|
Why the round conductors are used in place of
rectangular conductors in the rotating machines?
|
|
Space
factor for round conductors
- The value of
space factor depends upon shape of the conductor.
- The space
factor for round conductor is lower than that of rectangular conductor
therefore lower value of specific electrical loading is used for round
conductors.
|
25
|
Describe the relation between efficiency and
linear dimension of rotating machines.
|
|
Efficiency
– Linear dimensions
- The efficiency
of rotating machine increases with its linear dimension. Therefore the
efficiency of large machine is much higher than that of fractional HP motor.
|
26
|
Describe the relation between heat dissipating
surface and linear dimensions of rotating machine.
|
|
The heat
dissipating surface of rotating machine is directly proportional to square of
linear dimension of machine.
|
27
|
Describe the effect of temperature rise on linear
dimension of rotating machines.
|
|
Effect
of temperature rise on Linear dimension of Machines
- The
temperature rise of given machine is directly proportional to its linear
dimensions but this is not strictly true.
- As the size of
the machine increases, it requires better cooling condition in order to keep
temperature rise within limits.
- Therefore the
fractional HP motors are self cooled than the large dimension machines.
|
28
|
Why the Pole section of the DC machine should be
kept circular?
|
|
Pole
section of the DC Machine
- The pole
section of the DC machine should be kept circular in order to kept mean
length of field winding smallest.
- However the
circular pole requires solid pole structure.
|
29
|
What should be ratio of L / τ in the rotating
machine?
|
|
The ratio of
stator core length to pole pitch is in the range of 0.7 to 0.9.
|
30
|
Whether the peripheral speed of DC machine is
limited?
|
|
Peripheral
speed
- The peripheral
speed of the rotating machine is limited by considering diameter of rotor.
- It should not
exceed 30 m/s.
- If the
peripheral speed increases, the rotor may distorted due to excessive
centrifugal forces.
|
31
|
On which factors the rotor diameter of the
machine depends?
|
|
Peripheral
speed of machine
|
32
|
Define : Frame and Frame size
|
|
Frame
- It is
mechanical structure provided to cover stator for diameter D, core length L,
bearings, end covers and terminal box.
Frame
size
- The frame size
number indicates its centre height from shaft center which is expressed in
mm. The first number indicates shaft center and second letter indicates frame
length.
- The letter S
indicates small core length; M stands for medium core length and L for large
core length.
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