Electrical Motors Interview Question and Answers
1. What are causes of hot bearings?
The causes are
• lack of oil
• belt too tight
• armature not centred with respect of pole pieces
• bearings too tight or not in line
2. What causes heating of armature?
The causes are
• eddy currents
• moisture which almost short-circuits the armature
• unequal strength of magnetic poles
• Operation above rated voltage and below normal speed.
3. What is commutator pitch of a 4-pole DC armature having 49 commutator bars?
YC = (49+–1)/2 = 24 or 25.
4. How may the direction of rotation of a DC motor be reversed?
This can be done by reversing either the field current or current through the armature. Usually, reversal of current through the armature is adopted
5. What happens if the direction of current at the terminals of a series motor is reversed?
It does not reverse the direction of rotation of motor because current flows through the armature in the same direction as through the field
6. What will happen if a shunt motor is directly connected to the supply line?
Small motors up to 1 KW rating may be line-started without any adverse results being produced
High rating motors must be started through a suitable starter in order to avoid the huge starting current which will
– Damage the motor itself
– Badly affect the voltage regulation of the supply line
7. A DC motor fails to start when switched on. What could be the possible reasons and remedies?
Any one of the following reasons could be responsible:
• Open-circuit in controller-should be checked for open starting resistance or open switch or open fuse
• Low terminal voltage-should be adjusted to name-plate value.
• Overload-should be reduced if possible otherwise larger motor should be installed.
• Excessive friction-bearing lubrication should be checked.
8. Why does a DC motor sometime run too fast when under load? Give different possible causes and their remedies.
Different possible causes are as under:
• Weak field-remove any extra resistance in shunt field circuit
• Line voltage too high-reduce it to name-plate value
• Brushes back of neutral-set them on neutral
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9. What causes are responsible for over-heating of Commutator in a DC motor?
It could be due either to the brushes being off neutral or being under excessive spring pressure. Accordingly, brushes should be adjusted properly and the spring pressure should be reduced but not to the point where sparkling is introduced
10. How is magnetic leakage reduced to a minimum in commercial transformers?
This is done by interleaving the primary and secondary windings
11. Mention the factors on which Hysteresis loss depends?
• Quality and amount of iron in the core
• Flux density
• Frequency
12. How do changes in supply voltage and frequency affect the performance of an induction motor?
High voltage decreases both power factor and slip, but increases torque. Low voltage does just the opposite. Increase in frequency increases power factor but decreases the torque. However, per cent slip remains unchanged. Decrease in frequency decreases power factor but increases torque leaving per cent slip unaffected as before
13. What is, in brief, the basis of operation of a 3-phase induction motor?
The revolving magnetic field which is produced when a 3-phase stator winding is fed from a 3-phase supply
14. Why induction motors are called asynchronous?
Because their rotors can never run with the synchronous speed
15. How does the slip vary with load?
The greater the load, greater is the slip or slower is the rotor speed
16. Enumerate the possible reasons if a 3-phase motor fails to start?
Any one of the following reasons could be responsible:
• One or more fuses may be blown
• Voltage may be too low
• The starting load may be too heavy
• Worn bearings due to which the armature may be touching field laminae, thus introducing excessive friction
17. A motor stops after starting i.e., it fails to carry load. What could be the reasons?
Following can be the reasons
• Hot bearings, which increase the load by excessive friction
• Excessive tension on belt, which causes the bearings to heat
• Failure of short cut-out switch
• Single-phasing on the running position of the starter
18. Which is the usual cause of blow-outs in Induction motors?
The most common cause is single-phasing
19. Can a 3-phase motor be run on a Single-Phase Line?
Yes, it can be. But a Phase-Splitter is essential
20. What is a meant by a Phase-Splitter?
It is a device consisting of a number of capacitors so connected in the motor circuit that it produces, from a single input wave, three output waves which differ in phase from each other
21. What is the standard direction of rotation of an Induction motor?
Counter clock wise, when looking from the front end i.e. non-driving end of the motor
22. How would you reverse the direction of rotation of a capacitor start-induction-run motor?
By reversing either the running or starting-winding leads where they are connected to the lines. Both must not be reversed
23. What could be the reasons if a split-phase motor fails to start and hums loudly?
It could be due to the starting winding being open or grounded or burnt out.
24. What could be the reasons if a split-phase motor runs too slow?
Any one of the following factors could be responsible
• Wrong supply voltage and frequency
• Overload
• Grounded starting and running windings
• Short-circuited or open winding in field circuit
25. What are the two types of turbo-alternators?
Vertical and horizontal
26. What is direct-connected alternator?
One in which the alternator and engine are directly connected. In other words, there is no intermediate gearing such as belt, chain etc. between the driving engine and alternator
27. What is the difference between direct-connected and direct-coupled units?
In the former, alternator and driving engine are directly and permanently connected. In the latter case, engine and alternator are each complete in itself and are connected by some device such as friction clutch, jaw clutch or shaft coupling
28. Can a generator be converted into an alternator?
Yes, by providing two collector rings on one end of the armature and connecting these two rings to two points in the armature winding 1180 apart
29. For what service are synchronous motors especially suited?
They are especially suited for high voltage service
30. Which has more efficiency; synchronous or induction motor?
Synchronous motor
31. Mention some specific applications of synchronous motors
Its applications are
• constant speed load service
• reciprocating compressor drives
• power factor correction
• voltage regulation of transmission lines
32. What is a synchronous capacitor?
An over excited synchronous motor is called synchronous capacitor, because, like a capacitor, it takes a leading current
33. What could be the reasons if a synchronous motor fails to start?
It is usually due to following reasons
• Voltage may be too low
• Some faulty connection in auxiliary apparatus
• Too much starting load
• Open circuit in one phase or short circuit
• Field connection may be excessive.
34. Under which conditions a synchronous motor will fail to pull into step?
The conditions are
• No field excitation
• Excessive load
• Excessive load inertia
35. Do stepper motors have internal or external fans?
No, because the heat generated in the stator winding is conducted through the stator iron to the case which is cooled by natural conduction, convection and radiation
36. Any disadvantage of having more phases?
Advantages are
• More leads have to be brought out from the motor
• More interconnections are required to the drive circuit
• More switching devices are needed.
37. Will there be any harm if the rotor of a hybrid stepper motor is pulled out of its stator?
Yes. The rotor will probably become partially demagnetized and, on reassembling, will give less holding torque
38. What are the advantages and disadvantages of Compact Fluorescent Lamps?
The advantages are
• CFLs are up to four times more efficient than incandescent bulbs
• While initially they may cost more, CFLs are less expensive in the long run as they last much longer than incandescent bulbs
• CFLs are highly versatile and can be used in any setting that you would normally use incandescent bulbs
The disadvantages are
• CFLs when used outdoors need to be covered and protected from the elements. They are also sensitive to temperature
• While CFLs are supposed to last about 10,000 hours, turning them on and off too frequently can reduce that lifetime substantially
• CFLs are not suitable for focused or spot lights or where narrow beams of light are required. They are meant only for ambient light.
39. Give an example of integrating instruments
Simple example for integrating instrument is energy meter that is used in houses which indicates the summation of readings (units) consumed
40. Can a series motor be started without any load?
No, because if a DC series motor is started without load, the armature winding (winding that doesn’t spin) current will be at a minimum, resulting in the motor going to maximum speed / runaway, which can cause things to fly apart. As Armature current increases, the speed of the motor will decrease. The armature current will be forced to increase / speed of the motor will be forced to decrease as more and more load is applied
41. What is specified in synchronous watts?
It is the unit of torque an is defined as
Tsy = Rotor input in watts/2*pi*Ns
42. What is Phase Swinging?
When a synchronous motor is used for driving a varying load, then a condition known as phase swinging or hunting may occur. Hunting may also be caused if supply frequency is pulsating (as in the case of generators driven by reciprocating internal combustion engines)
43. Are single phase induction motors self-starting?
In single phase induction motor, there is no self starting as initially torque is zero.
but we can make it self-stating by adding an extra winding known as starting or auxiliary winding and space the two by 90 degrees. Due to this, the two currents will produce revolving flux what will make the induction motor self starting
How to conserve electricity at homes?
44. Electricity can be conserved by following methods
• Switch off lights, fans and other gadgets when not required
• Do not leave your computer and TV on standby
• Set the AC in economy mode so that it switches off when the room cools to the required temperature
• Buy energy efficient home appliances
After removing plagiarism
Question and Answer Electrical Motors
1. What are the causes of hot bears?
The causes are as follows
• lack of oil
• The belt is too tight
• Non-centered armature in relation to pieces of pole
• The bearings are too tight or out of line
2. What causes the heat of the weapon?
The causes are as follows
• eddy currents
• a relatively short humidity around the gun
• unequal strength of magnetic poles
• Operation above average power and below normal speed.
3. What is the rotating voice of a 4-pole DC weapon with 49 commutator bars?
YC = (49 + -1) / 2 = 24 or 25.
4. How can the rotation direction of the DC motor be changed?
This can be done by reverting the current or current field to a weapon. Normally, current conversion with a weapon is acceptable
5. What happens if the current power direction in the car series terminals is reversed?
It does not reverse the vehicle’s rotation indicator because the current flows within the same direction as the field
6. What will happen if the car shunt is connected directly to the supply line?
Small motors up to 1 KW can be started on the line without the production of side effects
High-level motors should be started with a suitable start to avoid the first large stream to start
– Damage to the car itself
– Adversely affect the power control of the supply line
7. DC vehicle failed to start when turned on. What could be the reasons and possible solutions?
Any one of the following reasons may be liable:
• Open circuit in controller – should be tested for first open resistance or open button or open fuse
• Low storage voltage – should be adjusted to nominal-plate value.
• Overcrowding – should be reduced if possible otherwise a large vehicle should be installed.
• Excessive lubrication should be checked.
8. Why does a DC car sometimes run so fast when loaded? Give different possible causes and solutions.
The various causes are as follows:
• Weak field – remove any additional resistance in the shunt field region
• The electric power of the line is too high – lower it to the name-plate value
• Brushes after neutrality – apply to neutral
•
9. What are the causes of Commutator overheating in a DC car?
It can be caused by the brushes being neutral or under intense spring pressure. Likewise, the brushes should be well adjusted and the spring pressure should be reduced but not where the glitter is applied.
10. How is the leakage of magnetic field reduced to a minimum in commercial transformers?
This is done by incorporating the first and second spirits
11. Name the factors that depend on the loss of Hysteresis?
• Quality and value of steel in the center
• Frequency Flux
• Frequency
12. How do changes in supply power and frequency affect the performance of the import vehicle?
High power reduces electric power and smoothness, but increases torque. Low power does the opposite. The increase in frequency increases the power factor but reduces the torque. However, the percentage smoothness remains unchanged. Decreasing frequency reduces the power factor but increases torque leaving the percentage slip as untouched as before
13. What, in short, is the basis for a three-phase import vehicle?
A rotating magnetic field produced by the insertion of a stator 3 zigzag from a 3-phase feed
14. Why are motor induction motors called asynchronous?
Because their rotors cannot run at sync speeds
15. How does a slip differ from a load?
Heavy load, sliding or accelerating rotor speed
16. What are the possible reasons for a third-party vehicle failing to start?
Any one of the following reasons may be liable:
• One or more flies may be used
• The voltage may be too low
• The initial load can be very heavy
• Bears are worn out because the arm can affect field laminae, thus causing excessive friction
17. The car stops after starting ie, unable to carry the load. What could be the reasons?
Following may be reasons
• Hot bearings, which increase the load by excessive collision
• Excessive tension in the belt, causing the bear’s warmth
• Failure for a short cutting switch
• One drop in star star launch
18. What is the most common cause of Induction motor explosions?
The most common reason is a single category
19. Can a 3-segment car be driven on the same single-segment line?
Yes, it can. But Phase-Splitter is important
20. What does Category-Splitter mean?
It is a tool that contains many capacitors that are so closely connected to the automotive cycle that it produces, from a single input wavelength, three different output waves in phase
21. What is the typical indicator of Induction car rotation?
The calculation clock is smart, when you look from the previous end i.e. the end of the car’s non-driving
22. How can you change the rotation direction of a capacitor start-induction-run motor?
By changing the running track or first-twist when connected to the line. Both should not be changed
23. What would be the reasons if a segmented car fails to start and is highly praised?
It may be because the wind is open or concentrated or hot.
24. What could be the reasons if a classification car is moving too slowly?
Any of the following factors can be addressed
• Electrical transmission power and frequency
• Overcrowding
• Active and running base
• Short or open rotation in the field circuit
25. What are the two types of turbo-alternators?
Straight and horizontal
26. What is a direct connected alternator?
One where the alternator and engine are directly connected. In other words, there is no interlocking link like belt, chain etc between the steering engine and another alternator
27. What is the difference between directly and indirectly connected units?
Previously, another connecting and driving engine was connected directly and permanently. In the latter case, each engine and alternator is fully self-contained and connected to a specific device such as a friction clutch, jaw clutch or shaft coupling
28. Can the generator be converted into an alternator?
Yes, by donating two collector rings to one side of the weapon and connecting the two rings to two points in the armature rotating in 1180
29. Which service synchronous motor is particularly suitable?
They are especially suitable for high power service
30. More efficient; synchronous or induction motor?
Synced car
31. Name some specific applications of synchronous motors
Its applications
• Regular loading service
• reset compressor drivers
• power factor adjustment
• power control in transmission lines
32. What is a synchronizing capacitor?
The over-the-counter synchronous motor is called a synchronous capacitor, because, like a capacitor, it takes the current lead
33. What could be the reasons if a compatible vehicle fails to start?
Often for the following reasons
• The voltage may be too low
• Some incorrect connection to auxiliary items
• Too much of a load
• Open circuit in one section or short circuit
• Field connection may be excessive.
34. Under what circumstances does a compliant vehicle fail to pass the step?
Circumstances exist
• No field excitement
• Overload
• Excessive load inertia
35. Do stepper motors have internal or external fans?
No, because the heat generated by the stator rotation is made of stator metal in the cooled case for natural conduction, convection transmission and radiation
36. Is there a lack of pluralism?
The benefits are there
• More tracks should be removed from the vehicle
• More connections are required in the drive region
• Multiple switching devices required.
37. Will there be any damage if the rotor of the hybrid stepper motor is removed from its stator?
Yes. The rotor will probably be partially electrically grounded and, when reassembled, will provide less torque to hold
38. What are the advantages and disadvantages of Compact Fluorescent Lamps?
The benefits are there
• CFLs are four times more efficient than incandescent bulbs
• Although initially expensive, CFLs are less expensive over time because they last much longer than incandescent bulbs
• CFLs are versatile and can be used in any setting you can use incandescent bulbs
What is worse
• CFLs when used outdoors need to be covered and protected in buildings. They are also sensitive to temperature
• While CFLs should last about 10,000 hours, opening and closing them more often can shorten the life expectancy
• CFLs are not suitable for focused or spotted lights or where light beams are required. They are designed only for available light.
39. Give an example of integration tools
A simple example of assembling a household energy meter to show a summary of the readings (units) used
40. Can a consecutive car be started without a load?
No, because if the DC motor series is started without a load, the current armature (winding) will be minimal, which has led to the car going too fast / running, which could cause things to split. As the current Armature increases, the speed of the car will decrease. The current armature will be forced to increase / speed the car will be forced to decrease as additional load is applied
41. What is specified in the corresponding watts?
The torque unit i is defined as
Tsy = Rotor Installation in watts / 2 * pi * Ns
42. What is the Swinging Category?
When a parallel motor is used to drive a different load, then a condition known as phase swinging or hunting is possible. Hunting can also be triggered when the supply frequency is active (as in the case of generators driven to replace internal combustion engines)
43. Do single-phase import engines start?
In a single-phase induction car, there is no starting automatically as the initial torque is zero.
but we can do so by adding an additional component known as a helpful start or stop and setting the space by 90 degrees. As a result, the two currents will produce a dynamic flow that will allow the import vehicle to start itself
How can you save energy in your home?
44. Electricity can be stored by following methods
• Turn off lights, fans and other gadgets when needed
• Do not leave your computer and TV in standby
• Set the AC to economic mode to turn off when the room cools to the required temperature
• Buy energy-efficient household appliances
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