Short questions and answers
EE1251 Electrical Machines II
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.
10. What is the relation between electrical degree and mechanical degree?
(OHFWULFDOGHJUHH HDQGPHFKDQLFDOGHJUHH PDUHUHODWHGWRRQHDQRWKHUE\
the number of poles P, the electrical machine has, as given by the following
equation
H 3 P
11. What is distributed winding?
, When coil-sides belonging to each phase are housed or distributed in more
than one slot under each pole region then the winding is called distributed
winding
A full pitch coil has width of coil otherwise called coil-span as 180º
:KHUH - angle between adjacent slots in electrical degree and x=
1,2,3…
12. Why is short pitch winding preferred over full-pitch winding ?
Advantages
• Waveform of the emf can be approximately made to a sine
wave and distorting harmonics can be reduced or totally
eliminated.
• Conductor material , copper , is saved in the back and front
end connections due to less coil-span.
• Fractional slot winding with fractional number of
slots/phase can be used which in turn reduces the tooth
ripples.
• Mechanical strength of the coil is increased.
13. Write down the formula for distribution factor.
Kd = sin(m PVLQ RU.dn = sin(mn Psin(n
m= number of slots/ pole/ phase
DQJOHEHWZHHQDGMDFHQWVORWVLQHOHFWULFDOGHJUHH
n = order of harmonic
14. Define winding factor.
The winding factor Kd is defined as the ratio of phasor addition of emf
induced in all the coils belonging to each phase winding to their arithmetic
addition.
15. Why are Alternators rated in kVA and not in kW?
The continuous power rating of any machine is generally defined as the power the
machine or apparatus can deliver for a continuous period so that the losses
incurred in the machine gives rise to a steady temperature rise not exceeding the
limit prescribed by the insulation class.
Apart from the constant loss incurred in Alternators is the copper loss, occurring
in the 3 –phase winding which depends on I2 R, the square of the current delivered
by the generator. As the current is directly related to apparent – power delivered
by the generator , the Alternators have only their apparent power in
VA/kVA/MVA as their power rating.
16. What are the causes of changes in voltage in Alternators when loaded?
Variations in terminal voltage in Alternators on load condition are due to the
following three causes:
• Voltage variation due to the resistance of the winding, R
EE1251 Electrical Machines II
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.
10. What is the relation between electrical degree and mechanical degree?
(OHFWULFDOGHJUHH HDQGPHFKDQLFDOGHJUHH PDUHUHODWHGWRRQHDQRWKHUE\
the number of poles P, the electrical machine has, as given by the following
equation
H 3 P
11. What is distributed winding?
, When coil-sides belonging to each phase are housed or distributed in more
than one slot under each pole region then the winding is called distributed
winding
A full pitch coil has width of coil otherwise called coil-span as 180º
:KHUH - angle between adjacent slots in electrical degree and x=
1,2,3…
12. Why is short pitch winding preferred over full-pitch winding ?
Advantages
• Waveform of the emf can be approximately made to a sine
wave and distorting harmonics can be reduced or totally
eliminated.
• Conductor material , copper , is saved in the back and front
end connections due to less coil-span.
• Fractional slot winding with fractional number of
slots/phase can be used which in turn reduces the tooth
ripples.
• Mechanical strength of the coil is increased.
13. Write down the formula for distribution factor.
Kd = sin(m PVLQ RU.dn = sin(mn Psin(n
m= number of slots/ pole/ phase
DQJOHEHWZHHQDGMDFHQWVORWVLQHOHFWULFDOGHJUHH
n = order of harmonic
14. Define winding factor.
The winding factor Kd is defined as the ratio of phasor addition of emf
induced in all the coils belonging to each phase winding to their arithmetic
addition.
15. Why are Alternators rated in kVA and not in kW?
The continuous power rating of any machine is generally defined as the power the
machine or apparatus can deliver for a continuous period so that the losses
incurred in the machine gives rise to a steady temperature rise not exceeding the
limit prescribed by the insulation class.
Apart from the constant loss incurred in Alternators is the copper loss, occurring
in the 3 –phase winding which depends on I2 R, the square of the current delivered
by the generator. As the current is directly related to apparent – power delivered
by the generator , the Alternators have only their apparent power in
VA/kVA/MVA as their power rating.
16. What are the causes of changes in voltage in Alternators when loaded?
Variations in terminal voltage in Alternators on load condition are due to the
following three causes:
• Voltage variation due to the resistance of the winding, R
