19-09-2017, 11:22 AM
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.
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.