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Short questions and answers EE1251 Electrical Machines II
Post: #1

Short questions and answers EE1251 Electrical Machines II


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1. Why almost all large size Synchronous machines are constructed with rotating
field system type?


The following are the principal advantages of the rotating field system type
construction of Synchronous machines:
· The relatively small amount of power, about 2%, required
for field system via slip-rings and brushes.
· For the same air gap dimensions, which is normally decided
by the kVA rating, more space is available in the stator part
of the machine for providing more insulation to the system
of conductors, especially for machines rated for 11kV or
above.
· Insulation to stationary system of conductors is not
subjected to mechanical stresses due to centrifugal action.
· Stationary system of conductors can easily be braced to
prevent deformation.
· It is easy to provide cooling arrangement for a stationary
system of conductors.
· Firm stationary connection between external circuit and
system of conductors enable he machine to handle large
amount of volt-ampere as high as 500MVA.

2. Write down the equation for frequency of emf induced in an Altenator.

Frequency of emf induced in an Alternator,f ,expressed in cycles per second or
Hz, is given by the following equation
F = (PN)/120 Hz,
Where P- Number of poles
N-Speed in rpm

3. How are alternators classified?

According to type of field system
· Stationary field system type
· Rotating field system type
According to shape of field system
· Salient pole type
· Smooth cylindrical type

4. Name the types of Alternator based on their rotor construction.

Alternators can be classified into the following two types according to its rotor
construction
· Smooth cylindrical type alternator
· Salient pole alternator

5. Why do cylindrical Alternators operate with steam turbines?

Steam turbines are found to operate at fairly good efficiency only at high
speeds. The high speed operation of rotors tends to increase mechanical losses
and so the rotors should have a smooth external surface. Hence, smooth
cylindrical type rotors with less diameter and large axial length are used for
Synchronous generators driven by steam turbines with either 2 or 4 poles.

6. Which type of Synchronous generators are used in Hydro-electric plants and why?

As the speed of operation is low for hydro turbines use din Hydro-electric
plants, salient pole type Synchronous generators are used. These allow better
ventilation and also have other advantages over smooth cylindrical type rotor.

7. What are the advantages of salient pole type construction used for Synchronous
machines?


Advantages of salient-pole type construction are :
· They allow better ventilation
· The pole faces are so shaped that the radial air gap length
increases from the pole center to the pole tips so that the
flux distribution in the air-gap is sinusoidal in shape which
will help the machine to generate sinusoidal emf
· Due to the variable reluctance the machine develops
additional reluctance power which is independent of
excitation

8. Why is the stator core of Alternator laminated?

The stator core of Alternator is laminated to reduce eddy current loss.

9. How does electrical degree differ from mechanical degree?

Mechanical degree is the unit for accounting the angle between two points
based on their mechanical or physical placement.
Electrical degree is used to account the angle between two points in rotating
electrical machines. Since all electrical machines operate with the help of
magnetic fields, the electrical degree is accounted with reference to the
magnetic field. 180 electrical degree is accounted as the angle between
adjacent North and South poles.
Post: #2
1. Why almost all large synchronous machines are built with
type of rotating field system?
The following are the main advantages of the rotary field system
type construction of synchronous machines:
• The relatively small amount of energy, about 2%
necessary for the field system through sliding rings and brushes.
• For the same dimensions of the air gap, which normally
decided by the qualification kVA, more space is available in the
stator part of the machine to provide more insulation
the system of conductors, especially for classified machines
to 11kV or higher.
• The isolation of the stationary system of conductors does not
subjected to mechanical stresses due to the centrifugal action.
• The stationary driver system can be easily supported
to avoid deformation.
• It is easy to provide refrigeration arrangements for
stationary system of conductors.
• Firm fixed connection between the external circuit
and system of conductors allow the machine to handle
large amount of volt-ampere as high as 500MVA.

2. Record the equation for the induced emf frequency in an Altenator.
Frequency of emf induced in an alternator, f, expressed in cycles per second
or Hz, is given by the following equation
F = (PN) / 120 Hz,
Where P- Number of poles
N-Speed in rpm

3. How are alternators classified?
According to type of field system
• Type of stationary field system
• Type of rotating field system
Depending on the shape of the field system
• Outgoing pole type
• Flat cylindrical type

4. Name Alternator types based on your rotor construction.
The alternators can be classified into the following two types according to their
rotor construction
• Smooth cylindrical type alternator
• Outboard pole alternator

5. Why do cylindrical alternators work with steam turbines?
Steam turbines are known to operate with fairly good efficiency only at high speeds.
The high-speed operation of the rotors tends to increase the losses
the rotors must have a smooth outer surface. Therefore, smooth cylindrical type
rotors with smaller diameter and large axial length are used to
generators powered by steam turbines with 2 or 4 poles.

6. What type of synchronous generators are used in hydroelectric plants and why?
Since the speed of operation is low for the turbines hid
Synchronous generators are used. These allow for better
ventilation and also have other advantages over the cylindrical smooth rotor type.

7. What are the advantages of pole pole construction
Synchronous machines?
The advantages of the pole-type construction are:
• Allows better ventilation
• The faces of the poles are shaped such that the length of the radial air gap
increases from the center of the pole to the ends of the poles,
the flow distribution in the air gap is sinusoidal
which will help the machine to generate sinusoidal fem
• Due to the variable reluctance of the machine
develops an additional reluctance power that is
independent of excitation

8. Why is the alternator stator core laminated?
The alternator stator core is laminated to reduce loss of eddy currents.

9. How does the electrical grade differ from the mechanical grade?
The mechanical grade is the unit to account for the angle between two
points based on their mechanical or physical placement.
The electric rating is used to take into account the angle between two points in rotary electric machines. As all electric machines work
magnetic fields, the electrical rating is counted with reference to the
to the magnetic field. The electric rating 180 is considered as the angle
between adjacent North and South poles.
 

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