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
• 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.
10. What is the relation between electrical degree and mechanical degree?
Electrical degree e and mechanical degree m are related to one another by the number of poles P, the electrical machine has, as given by the following equation
e (P/2) m
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.
10. What is the relation between electrical degree and mechanical degree?
Electrical degree e and mechanical degree m are related to one another by the number of poles P, the electrical machine has, as given by the following equation
e (P/2) m
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