Here are some of the basic questions asked in interviews for EEE students
1. what is the principle of motor?
The electric motor is based on the principle that whenever a current carrying conductor is placed in an magnetic field it experiences a mechanical force i.e turning or twisting movement is called as torque.
2. what is the principle of generator?
The electric generator is based on the principle of faraday's law of electromagnetic induction. Such that whenever a current carrying conductor is placed in an varying magnetic field it induces an emf in it.
3. what are the types of DC motor?
9. Why Single phase induction motor is not self starting?
It consists of two field main flux and rotor flux. If the two rotating magnetic fields are opposite, an equal and opposite torque will be produced and the total torque on the rotor will be zero and the rotor won’t start.
10. Why capacitor is used for starting single phase induction motor?
Based on the double revolving theory, capacitor produces two revolving field which cancels RMF of the main winding and produces the single rotating field which makes the rotor to rotate at desired speed. when the rotor achieves desired speed auxiliary capacitor will be disconnected.
11. What is double revolving theory?
The double-field revolving theory states that, any alternating quantity (here, alternating flux) can be resolved into two components having magnitude half of the maximum magnitude of the alternating quantity, and both these components rotating in opposite direction.
12. What is synchronous speed?
The speed at which the rotor and stator of motor rotates at steady state is called as synchronous speed.
13. What is the speed of synchronous motor?
The 3600 rpm corresponds to 60 revolutions per second. A similar 2-pole permanent magnet synchronous motor will also rotate at 3600 rpm. A lower speed motor may be constructed by adding more pole pairs. A 4-pole motor would rotate at 1800 rpm, a 12-pole motor at 600 rpm.
14. What is slip in an induction motor?
Slip can be defined as the difference between the flux speed (Ns) and the rotor speed (N). Speed of the rotor of an induction motor is always less than its synchronous speed. It is usually expressed as a percentage of synchronous speed (Ns) and represented by the symbol ‘S’.
15. What is the difference between generator and alternator?
Generator and alternator are two devices, which converts mechanical energy into electrical energy. Both have the same principle of electromagnetic induction, the only difference is that their construction. Generator persists stationary magnetic field and rotating conductor which rolls on the armature with slip rings and brushes riding against each other, hence it converts the induced emf into dc current for external load whereas an alternator has a stationary armature and rotating magnetic field for high voltages but for low voltage output rotating armature and stationary magnetic field is used.
16. Why AC motor are preferred over DC motor?
The main advantage of Ac motor over Dc motor is that the speed - torque characteristics. Torque of Ac motor is constant upto certain speed and then decreases, but in case of Dc motor torque linearly decreases as speed increases.
17. Why series DC motor cannot be started without load?
When DC series motor is connected with no load it starts to draw current from the supply which produces the flux. As the flux is directly proportional to torque and inversely proportional to speed, the speed will be higher and the torque will be lower. This produces the back emf which may lead to bursting of motor.
18. What is difference between series and shunt DC motor?
1. what is the principle of motor?
The electric motor is based on the principle that whenever a current carrying conductor is placed in an magnetic field it experiences a mechanical force i.e turning or twisting movement is called as torque.
2. what is the principle of generator?
The electric generator is based on the principle of faraday's law of electromagnetic induction. Such that whenever a current carrying conductor is placed in an varying magnetic field it induces an emf in it.
3. what are the types of DC motor?
- Series motor
- Shunt motor
- Compound motor
- Permanent magnet DC motor
4. What are the types of AC motor?
- Synchronous motor
- Asynchronous motor
5. What is Synchronous motor?
Synchronous motor is a kind of AC motor where the rotation of the shaft is synchronized with the frequency of the supply current at steady state to give the constant speed.
6. What is principle of Synchronous motor?
Synchronous motor is based on the principle of magnetic locking.
7. What is magnetic locking in synchronous motor?
When two unlike poles comes in contact they attract each other, this is called as magnetic locking. In synchronous motor when the
stator winding is excited with 3 phase A.C supply and rotor winding
with D.C supply, two magnetic fields are produced in
the synchronous motor. That two fields attract each other to form magnetic locking. Hence the stator and rotor rotates at same speed.
8. What is the principle of single phase induction motors?
When the stator of a single phase motor is fed with single phase supply, it produces alternating flux in the stator winding. This flux induces current in the rotor bars (of the squirrel cage rotor ) according to Faraday's law of electromagnetic induction.These rotating alternating flux is called as Rotating magnetic flux. Now there are two fluxes one is main flux (stator flux), and another is called rotor flux. These two fluxes produce the desired torque which makes the motor to rotate.
9. Why Single phase induction motor is not self starting?
It consists of two field main flux and rotor flux. If the two rotating magnetic fields are opposite, an equal and opposite torque will be produced and the total torque on the rotor will be zero and the rotor won’t start.
10. Why capacitor is used for starting single phase induction motor?
Based on the double revolving theory, capacitor produces two revolving field which cancels RMF of the main winding and produces the single rotating field which makes the rotor to rotate at desired speed. when the rotor achieves desired speed auxiliary capacitor will be disconnected.
11. What is double revolving theory?
The double-field revolving theory states that, any alternating quantity (here, alternating flux) can be resolved into two components having magnitude half of the maximum magnitude of the alternating quantity, and both these components rotating in opposite direction.
12. What is synchronous speed?
The speed at which the rotor and stator of motor rotates at steady state is called as synchronous speed.
13. What is the speed of synchronous motor?
The 3600 rpm corresponds to 60 revolutions per second. A similar 2-pole permanent magnet synchronous motor will also rotate at 3600 rpm. A lower speed motor may be constructed by adding more pole pairs. A 4-pole motor would rotate at 1800 rpm, a 12-pole motor at 600 rpm.
14. What is slip in an induction motor?
Slip can be defined as the difference between the flux speed (Ns) and the rotor speed (N). Speed of the rotor of an induction motor is always less than its synchronous speed. It is usually expressed as a percentage of synchronous speed (Ns) and represented by the symbol ‘S’.
15. What is the difference between generator and alternator?
Generator and alternator are two devices, which converts mechanical energy into electrical energy. Both have the same principle of electromagnetic induction, the only difference is that their construction. Generator persists stationary magnetic field and rotating conductor which rolls on the armature with slip rings and brushes riding against each other, hence it converts the induced emf into dc current for external load whereas an alternator has a stationary armature and rotating magnetic field for high voltages but for low voltage output rotating armature and stationary magnetic field is used.
16. Why AC motor are preferred over DC motor?
The main advantage of Ac motor over Dc motor is that the speed - torque characteristics. Torque of Ac motor is constant upto certain speed and then decreases, but in case of Dc motor torque linearly decreases as speed increases.
17. Why series DC motor cannot be started without load?
When DC series motor is connected with no load it starts to draw current from the supply which produces the flux. As the flux is directly proportional to torque and inversely proportional to speed, the speed will be higher and the torque will be lower. This produces the back emf which may lead to bursting of motor.
18. What is difference between series and shunt DC motor?
- In series dc motor, the field winding is connected in series with the armature where as in shunt dc motor, the field winding is connected in parallel with the armature.
- Series dc motor will have high starting current and torque where as shunt dc motor will have constant starting torque.
19. Application of dc series motor?
Its high starting torque makes it useful for
Its high starting torque makes it useful for
- Electric traction
- hoists
- Cranes
- Trolley cars
- Conveyors
- Elevators
- Vacuum cleaner
20. Application of shunt dc motor?
It is used in
- Lathe machines
- Centrifugal pump
- Fans
- Blowers
- Weaving machines
21. What is squirrel cage induction motor?
A squirrel-cage rotor is the rotating part of the common squirrel-cage induction motor.
It consists of a cylinder of steel laminations, short circuited with aluminum or copper
conductors. The rotor winding has current
induced in it by the stator field, and produces its own magnetic field.
22. What is slip ring induction motor?
Slip ring induction motor also known as wound-rotor motor is a type of induction motor where the rotor wingdings are connected through slip rings to external resistance. External resistance can be adjusted to control the speed/torque characteristic of the motor.
23. What is difference between slip ring and squirrel cage induction motor?
Speed and torque of the slip ring induction motor can be changed by adjusting the external resistance where as speed and torque of squirrel cage induction motor cannot be changed because of short circuited cage.
Squirrel cage induction motor has lower starting torque where as slip ring induction motor has higher starting torque.
Slip ring induction motor will have rotor bars and slip rings which leads to losses where squirrel cage induction motor doesn't have any rotor bars.
24. What is cogging effect?
When the slots of the rotor are equal in number with slots in the stator, they align themselves in such way that both face to each other and at this stage the reluctance of the magnetic path is minimum and motor refuse to start.This characteristic of the induction motor is called cogging.
25. What is Cogging torque?
Cogging torque of electrical motors is the torque due to the interaction between the permanent magnets of the rotor and the stator slots of a Permanent Magnet (PM) machine. It is also known as detent or 'no-current' torque.
26. How to reduce cogging effect?
Cogging effect can be reduced by using unequal rotor and stator poles.
27. What is Crawling?
Mostly squirrel cage type induction motor has a tendency to run at very low speed compared to its synchronous speed, this phenomenon is known as crawling. The resultant speed is nearly 1/7th of its synchronous speed. This is due to the fact that harmonics fluxes produced in the gap of the stator winding of odd harmonics like 3rd, 5th, 7th etc. These harmonics create additional torque fields in addition to the synchronous torque. At this stage motor does not reach up to its normal speed and continue to rotate at a speed which is much lower than its normal speed.
28. How to reduce crawling?
Crawling can be reduced by eliminating the harmonics.
29. What is universal motor?
The universal motor is a type of electric motor that can operate on either AC or DC power. It is a commutated series-wound motor where the stator's field coils are connected in series with the rotor windings through a commutator. It is often referred to as an AC series motor.
30. Which motor is used in refrigerators?
Single-phase induction motor having one or more auxiliary short-circuited windings acting on only a portion of the magnetic circuit.
31. Which motor is used in fans?
Capacitor start capacitor run induction motor.
Slip ring induction motor also known as wound-rotor motor is a type of induction motor where the rotor wingdings are connected through slip rings to external resistance. External resistance can be adjusted to control the speed/torque characteristic of the motor.
23. What is difference between slip ring and squirrel cage induction motor?
Speed and torque of the slip ring induction motor can be changed by adjusting the external resistance where as speed and torque of squirrel cage induction motor cannot be changed because of short circuited cage.
Squirrel cage induction motor has lower starting torque where as slip ring induction motor has higher starting torque.
Slip ring induction motor will have rotor bars and slip rings which leads to losses where squirrel cage induction motor doesn't have any rotor bars.
24. What is cogging effect?
When the slots of the rotor are equal in number with slots in the stator, they align themselves in such way that both face to each other and at this stage the reluctance of the magnetic path is minimum and motor refuse to start.This characteristic of the induction motor is called cogging.
25. What is Cogging torque?
Cogging torque of electrical motors is the torque due to the interaction between the permanent magnets of the rotor and the stator slots of a Permanent Magnet (PM) machine. It is also known as detent or 'no-current' torque.
26. How to reduce cogging effect?
Cogging effect can be reduced by using unequal rotor and stator poles.
27. What is Crawling?
Mostly squirrel cage type induction motor has a tendency to run at very low speed compared to its synchronous speed, this phenomenon is known as crawling. The resultant speed is nearly 1/7th of its synchronous speed. This is due to the fact that harmonics fluxes produced in the gap of the stator winding of odd harmonics like 3rd, 5th, 7th etc. These harmonics create additional torque fields in addition to the synchronous torque. At this stage motor does not reach up to its normal speed and continue to rotate at a speed which is much lower than its normal speed.
28. How to reduce crawling?
Crawling can be reduced by eliminating the harmonics.
29. What is universal motor?
The universal motor is a type of electric motor that can operate on either AC or DC power. It is a commutated series-wound motor where the stator's field coils are connected in series with the rotor windings through a commutator. It is often referred to as an AC series motor.
30. Which motor is used in refrigerators?
Single-phase induction motor having one or more auxiliary short-circuited windings acting on only a portion of the magnetic circuit.
31. Which motor is used in fans?
Capacitor start capacitor run induction motor.
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