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.

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.


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 x^{3} : 1 but the output in the ratio of
x^{4} : 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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