1
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At what principle the alternator operates?
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Principle
of the alternator
- The alternator
works on the principle of electromagnetic induction.
- The stator
conductors cut the flux which is produced by rotating field.
- The field
winding consists of alternate N and S poles therefore the induced emf in the
armature of the alternator is also alternating ( AC ).
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2
|
Describe the advantages of stationary armature and rotating field system
in the alternator.
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The
advantages of stationary armature and rotating field system are
- The output is
directly fed from the stator terminal to the load terminal without any brush
contacts.
- It is very easy to braced armature winding and insulate for high
voltages.
- The use of slip – rings carrying large current at high voltage is
avoided in the stationary armature construction.
- Only two slip rings are
required for DC excitation but it carries small current.
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3
|
Describe the function of the exciter.
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Function
of the exciter
- The exciter is
nothing but small dc generator mounted same shaft as that of an alternator.
- The function of the exciter is to supply direct current to the field winding.
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4
|
Describe the function of stator frame or yoke in the alternator and DC
generator.
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Function
of stator yoke
- The function
of the stator frame in the alternator is to support the armature winding
whereas its function in the DC generator is to carry magnetic flux.
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5
|
Describe the function of radial ventilating holes in the armature of the
alternator.
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Function
of radial ventilating holes
- The function
of the radial ventilating holes is to provide ventilation ( or cooling ) to
the armature winding of the alternator.
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6
|
Describe the advantage of wide open slot as compared to semi closed slot.
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Advantages
of wide open slots
- Easy to form
winding in the slots
- Easy removal
of winding in the case of repair it
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7
|
How many slip – rings are used in the alternator?
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Two slip –
rings
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8
|
Why the diameter of the salient pole alternator is kept large?
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Diameter
of Salient pole alternator
- The salient pole alternator is
designed particularly for low speed.
- Higher number of poles is required for
low speed alternator therefore the diameter of the rotor must be kept large
enough for higher numbers of poles.
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9
|
What happens when the salient pole alternator rotates at high speed?
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Speed of salient pole alternator
- The salient
poles would cause excessive windage loss if it is driven at high speed &
it would produce excessive noise.
- The salient pole structure would not
withstand excessive centrifugal forces at higher speed.
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10
|
How the flux density is distributed in the cylindrical rotor type
alternator?
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Flux
density distribution
- The flux density becomes maximum at
the main pole axis whereas its value gradually falls away on either side.
- The
flux density has minimum value under inter – polar axis.
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11
|
Why the diameter of the cylindrical rotor type alternator is kept small?
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Diameter
of cylindrical rotor
- The cylindrical rotor type alternator
is designed for high speed it means that the rotor consists of low numbers of
poles therefore the diameter of the rotor is kept small.
- If the diameter of
the rotor is kept large, excessive centrifugal force damage the rotor winding
at high speed.
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12
|
Describe the function of damper winding in the alternator and synchronous
motor. Where it is located?
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Function
of the damper winding
- The function
of the damper winding is to reduce hunting effect ( due to fluctuation of the
load ) in the alternator whereas it provides starting torque in the
synchronous motor.
- The damper winding is slotted in the pole-shoes of the
field winding of the alternator.
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13
|
Why turbo alternator is not provided with damper winding?
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The turbo
alternator is not provided with damper winding because the field poles acts
as damper winding.
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14
|
Whether the damper winding carries any current in the running condition
of the alternator? Why?
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Damper
winding
- The damper winding does not carry any
current under running condition of the alternator because alternator always
runs at synchronous speed.
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15
|
What is value of flux density ( a ) At the centre of N pole ( b ) Inter –
polar axis and ( c ) At the center of S pole? Maximum or minimum
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Flux
density distribution
- The value of flux density is maximum
at the N – pole and S – pole and its value becomes minimum at inter – polar axis.
- Therefore the induced emf is maximum at the N – pole and becomes zero at the inter
– polar axis and again induced emf have maximum value but opposite sign at
the S – pole.
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16
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Explain : Pole pitch
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Pole
pitch
- It is defined
as the center to center distance between two adjacent poles.
- If the coil span is
equal to the pole pitch, it is called as full pitch coil otherwise it may be
short pitch coil.
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17
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What is main difference between armature winding of the DC generator and
alternator?
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The armature
winding of the DC generator is closed path whereas in the alternator there is
no closed path in the armature winding.
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18
|
Explain : Full pitch coil and Short pitch coil
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Full
pitch coil
- If the coil having span which is equal to one pole – pitch, it is
called as full pitch coil ( spanning of 180 electrical degrees ).
Short
pitch coil
- If the coil span is less than the pole – pitch, it is called as short
pitch coil.
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19
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Describe the advantages of Short pitch coil over Full pitch coil.
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Advantages
of short pitch coil over full pitch coil
- Save copper
material at the end connections.
- The generated
emf can be made approximate to sinusoidal and distorting harmonics can be
minimized.
- Elimination of
higher order harmonics results in iron losses is reduced.
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20
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Explain : Pitch factor or coil span factor and Distribution factor
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Pitch
factor Kp
- It is defined
as ratio of vector sum of induced emf per coil to the arithmetic sum of induced
emf per coil.
- Kp=
Vector sum of induced emf per coil / arithmetic sum of induced emf per coil
Distribution
factor Kd
- The
distribution factor is defined as ratio of emf induced with distributed
winding to the emf induced with concentrated winding.
- Kd =
Induced emf with distributed winding / Induced emf with concentrated
winding
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