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All questions of Synchronous Machines for Electrical Engineering (EE) Exam
Which of the following method would give a lower than actual value of regulation of an alternator?- a)zpf method
- b)mmf method
- c)Emf method
- d)ASA method
Correct answer is option 'B'. Can you explain this answer?
Which of the following method would give a lower than actual value of regulation of an alternator?
a)
zpf method
b)
mmf method
c)
Emf method
d)
ASA method
 | Sanvi Kapoor answered |
Mmf method is called optimistic method because it gives voltage regulation lower than the actual value.
The flux per pole in a synchronous motor with the field circuit ON and the stator disconnected from the supply is found to be 25 mWb. When the stator is connected to the rated supply with the field excitation unchanged, the flux per pole in the machine is found to be 20 mWb while the motor is running on no-load. Assuming no-load losses to be zero, the no-load current drawn by the motor from the supply- a)is zero
- b)lags the supply voltage
- c)is in phase with the supply voltage
- d)leads the supply voltage
Correct answer is option 'D'. Can you explain this answer?
The flux per pole in a synchronous motor with the field circuit ON and the stator disconnected from the supply is found to be 25 mWb. When the stator is connected to the rated supply with the field excitation unchanged, the flux per pole in the machine is found to be 20 mWb while the motor is running on no-load. Assuming no-load losses to be zero, the no-load current drawn by the motor from the supply
a)
is zero
b)
lags the supply voltage
c)
is in phase with the supply voltage
d)
leads the supply voltage
 | Rajat Kumar answered |
Since after connecting the stator to supply, flux per pole is reduced to 20 mWb from initial value of 25 mWb, therefore effect of armature reaction is demagnetizing i.e. Ff > Fr Due to which motor is over-excited and runs at leading power factor.
A synchronous motor is operating on no-load at unity power factor. If the field current is increased, the pf will become- a)leading and the current will increase.
- b)lagging and the current will decrease.
- c)leading and the current will decrease.
- d)lagging and the current will increase.
Correct answer is option 'A'. Can you explain this answer?
A synchronous motor is operating on no-load at unity power factor. If the field current is increased, the pf will become
a)
leading and the current will increase.
b)
lagging and the current will decrease.
c)
leading and the current will decrease.
d)
lagging and the current will increase.
 | Ankita Das answered |
Motor becomes over-excited and starts operating at leading pf with the increase in field current.
Here, Ia will increase.
Here, Ia will increase.
The four methods of calculating voltage regulation of a 3-phase alternator are:
1. Emf method
2. Saturated synchronous reactance method
3. New ASA method
4. Mmf method
The correct sequence in the ascending order of the values of regulation obtained by these methods is- a)4, 3, 1,2
- b)4, 3, 2, 1
- c)3, 4, 2,1
- d)3, 4, 1,2
Correct answer is option 'B'. Can you explain this answer?
The four methods of calculating voltage regulation of a 3-phase alternator are:
1. Emf method
2. Saturated synchronous reactance method
3. New ASA method
4. Mmf method
The correct sequence in the ascending order of the values of regulation obtained by these methods is
1. Emf method
2. Saturated synchronous reactance method
3. New ASA method
4. Mmf method
The correct sequence in the ascending order of the values of regulation obtained by these methods is
a)
4, 3, 1,2
b)
4, 3, 2, 1
c)
3, 4, 2,1
d)
3, 4, 1,2
 | Kajal Yadav answered |
Explanation:
The voltage regulation of a 3-phase alternator refers to the deviation in the output voltage from the rated voltage when the alternator is subjected to a load. The four methods mentioned are different techniques used to calculate the voltage regulation. The correct sequence in the ascending order of the values of regulation obtained by these methods is option 'B' - 4, 3, 2, 1.
1. MMF Method:
- In the MMF (magnetomotive force) method, the synchronous reactance and armature resistance drop are considered.
- The voltage regulation is calculated as the percentage deviation of the terminal voltage from the rated voltage.
- This method gives the highest value of regulation among the four methods.
2. New ASA Method:
- The New ASA (American Standards Association) method is an improved version of the ASA method.
- It takes into account the synchronous reactance and armature resistance drop, as well as the effect of armature reaction.
- The voltage regulation obtained using this method is lower than that obtained by the MMF method.
3. Saturated Synchronous Reactance Method:
- The saturated synchronous reactance method considers the effect of armature reaction and saturation of the synchronous reactance.
- It gives a lower value of regulation compared to the New ASA method.
4. EMF Method:
- The EMF (electromotive force) method is the simplest method used to calculate voltage regulation.
- It only considers the synchronous reactance drop and neglects the effect of armature reaction and saturation.
- This method gives the lowest value of regulation among the four methods.
Therefore, the correct sequence in the ascending order of the values of regulation obtained by these methods is option 'B' - 4, 3, 2, 1. The MMF method gives the highest value of regulation, followed by the New ASA method, the saturated synchronous reactance method, and finally, the EMF method gives the lowest value of regulation.
The voltage regulation of a 3-phase alternator refers to the deviation in the output voltage from the rated voltage when the alternator is subjected to a load. The four methods mentioned are different techniques used to calculate the voltage regulation. The correct sequence in the ascending order of the values of regulation obtained by these methods is option 'B' - 4, 3, 2, 1.
1. MMF Method:
- In the MMF (magnetomotive force) method, the synchronous reactance and armature resistance drop are considered.
- The voltage regulation is calculated as the percentage deviation of the terminal voltage from the rated voltage.
- This method gives the highest value of regulation among the four methods.
2. New ASA Method:
- The New ASA (American Standards Association) method is an improved version of the ASA method.
- It takes into account the synchronous reactance and armature resistance drop, as well as the effect of armature reaction.
- The voltage regulation obtained using this method is lower than that obtained by the MMF method.
3. Saturated Synchronous Reactance Method:
- The saturated synchronous reactance method considers the effect of armature reaction and saturation of the synchronous reactance.
- It gives a lower value of regulation compared to the New ASA method.
4. EMF Method:
- The EMF (electromotive force) method is the simplest method used to calculate voltage regulation.
- It only considers the synchronous reactance drop and neglects the effect of armature reaction and saturation.
- This method gives the lowest value of regulation among the four methods.
Therefore, the correct sequence in the ascending order of the values of regulation obtained by these methods is option 'B' - 4, 3, 2, 1. The MMF method gives the highest value of regulation, followed by the New ASA method, the saturated synchronous reactance method, and finally, the EMF method gives the lowest value of regulation.
Which of the following graphs represents the speed-torque characteristic of a synchronous motor?- a)
- b)
- c)
- d)
Correct answer is option 'A'. Can you explain this answer?
Which of the following graphs represents the speed-torque characteristic of a synchronous motor?
a)
b)
c)
d)
 | Yashvi Shah answered |
For a synchronous motor, N = Ns for all value of torque.
In figure the characteristic that corresponds to the variation of synchronous reactances of synchronous machine with the field current is
- a)Curve A
- b)Curve B
- c)Curve C
- d)Curve D
Correct answer is option 'B'. Can you explain this answer?
In figure the characteristic that corresponds to the variation of synchronous reactances of synchronous machine with the field current is
a)
Curve A
b)
Curve B
c)
Curve C
d)
Curve D
 | Akash Singh answered |
B.
THERE IS NO RELATION BETWEEN SYNCHRONOUS REACTANCE AND FIELD CURRENT.. PRACTICALLY IT SHOULD REMAIN ALMOST SAME....
THERE IS NO RELATION BETWEEN SYNCHRONOUS REACTANCE AND FIELD CURRENT.. PRACTICALLY IT SHOULD REMAIN ALMOST SAME....
A winding is distributed in the slots along the air-gap periphery
1. to add mechanical strength to the winding.
2. to reduce the amount of conductor material required.
3. to reduce the harmonics in the generated emf.
4. to reduce the size of the machine.
5. for full utilization of iron and conductor materials.
From these, the correct answer is - a)1, 3, 4, 5
- b)1,2, 4, 5
- c) 1,2, 3, 5
- d)1,3,5
Correct answer is option 'D'. Can you explain this answer?
A winding is distributed in the slots along the air-gap periphery
1. to add mechanical strength to the winding.
2. to reduce the amount of conductor material required.
3. to reduce the harmonics in the generated emf.
4. to reduce the size of the machine.
5. for full utilization of iron and conductor materials.
From these, the correct answer is
1. to add mechanical strength to the winding.
2. to reduce the amount of conductor material required.
3. to reduce the harmonics in the generated emf.
4. to reduce the size of the machine.
5. for full utilization of iron and conductor materials.
From these, the correct answer is
a)
1, 3, 4, 5
b)
1,2, 4, 5
c)
1,2, 3, 5
d)
1,3,5
 | Prisha Sen answered |
Distributed Winding in Slots of a Machine
Distributed winding is a type of winding that is evenly distributed in the slots along the air-gap periphery of a machine. This type of winding has several advantages, which are discussed below.
Mechanical Strength
The distributed winding adds mechanical strength to the machine by holding the coils firmly in place, reducing the risk of the coils coming loose during operation. This is because the distributed winding is wound around the entire circumference of the machine, which provides a more secure hold on the coils.
Reduced Conductor Material
The distributed winding requires less conductor material because the winding is distributed over the entire circumference of the machine, reducing the amount of conductor material required to make the coils. The reduction in conductor material also reduces the cost of the machine.
Reduced Harmonics
The distributed winding reduces the harmonics in the generated emf because the winding is evenly distributed in the slots along the air-gap periphery. This results in a more sinusoidal waveform, which reduces the harmonic distortion.
Reduced Size of the Machine
The distributed winding reduces the size of the machine because the winding is evenly distributed in the slots along the air-gap periphery. This results in a more compact design, which is beneficial for applications where space is limited.
Full Utilization of Iron and Conductor Materials
The distributed winding allows for the full utilization of iron and conductor materials because the winding is evenly distributed in the slots along the air-gap periphery. This results in a more efficient use of materials, which reduces the cost of the machine.
Conclusion
In conclusion, the correct answer is option D, which states that the distributed winding is used to add mechanical strength to the winding, reduce harmonics in the generated emf, reduce the size of the machine, and for full utilization of iron and conductor materials. Additionally, distributed winding also reduces the amount of conductor material required, which reduces the cost of the machine.
Distributed winding is a type of winding that is evenly distributed in the slots along the air-gap periphery of a machine. This type of winding has several advantages, which are discussed below.
Mechanical Strength
The distributed winding adds mechanical strength to the machine by holding the coils firmly in place, reducing the risk of the coils coming loose during operation. This is because the distributed winding is wound around the entire circumference of the machine, which provides a more secure hold on the coils.
Reduced Conductor Material
The distributed winding requires less conductor material because the winding is distributed over the entire circumference of the machine, reducing the amount of conductor material required to make the coils. The reduction in conductor material also reduces the cost of the machine.
Reduced Harmonics
The distributed winding reduces the harmonics in the generated emf because the winding is evenly distributed in the slots along the air-gap periphery. This results in a more sinusoidal waveform, which reduces the harmonic distortion.
Reduced Size of the Machine
The distributed winding reduces the size of the machine because the winding is evenly distributed in the slots along the air-gap periphery. This results in a more compact design, which is beneficial for applications where space is limited.
Full Utilization of Iron and Conductor Materials
The distributed winding allows for the full utilization of iron and conductor materials because the winding is evenly distributed in the slots along the air-gap periphery. This results in a more efficient use of materials, which reduces the cost of the machine.
Conclusion
In conclusion, the correct answer is option D, which states that the distributed winding is used to add mechanical strength to the winding, reduce harmonics in the generated emf, reduce the size of the machine, and for full utilization of iron and conductor materials. Additionally, distributed winding also reduces the amount of conductor material required, which reduces the cost of the machine.
The phasor diagram of a synchronous machine connected to an infinite bus is shown below. The machine is acting as a
- a)generator and operating at a lagging pf.
- b)generator and operating at a leading pf.
- c)motor and operating at a leading pf.
- d)motor and operating at a lagging pf.
Correct answer is option 'A'. Can you explain this answer?
The phasor diagram of a synchronous machine connected to an infinite bus is shown below. The machine is acting as a
a)
generator and operating at a lagging pf.
b)
generator and operating at a leading pf.
c)
motor and operating at a leading pf.
d)
motor and operating at a lagging pf.
 | Hiral Kulkarni answered |
As 
therefore machine is acting as a generator and operating at lagging Pf.
therefore machine is acting as a generator and operating at lagging Pf.
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