ELECTRIC MOTORS LAB VIVA Questions and Answers :-
1. What are causes of hot bearings?
The causes are:
1. lack of oil
2. belt too tight
3. armature not centred with respect of pole pieces
4. bearings too tight or not in line
1.1 What causes heating of armature?
The causes are:
1. eddy currents
2. moisture which almost short-circuits the armature
3. unequal strength of magnetic poles
4. Operation above rated voltage and below normal speed.
1.2 What is commutator pitch of a 4-pole DC armature having 49 commutator bars?
YC = (49+–1)/2 = 24 or 25.
2. 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
3. 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
4. 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
5. 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.
6. 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
7. 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
8. How is magnetic leakage reduced to a minimum in commercial transformers?
This is done by interleaving the primary and secondary windings
9. Mention the factors on which Hysteresis loss depends?
- Quality and amount of iron in the core
- Flux density
- Frequency
10. 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
11. 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
12. Why induction motors are called asynchronous?
Because their rotors can never run with the synchronous speed
13. How does the slip vary with load?
The greater the load, greater is the slip or slower is the rotor speed
14. 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
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
15. Which is the usual cause of blow-outs in Induction motors?
The most common cause is single-phasing
16. Can a 3-phase motor be run on a Single-Phase Line?
Yes, it can be. But a Phase-Splitter is essential
17. 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
18. 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
19. 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
20. 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.
21. 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
22. What are the two types of turbo-alternators?
Vertical and horizontal
23. 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
24. 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
25. 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
26. For what service are synchronous motors especially suited?
They are especially suited for high voltage service
27. Which has more efficiency; synchronous or induction motor?
Synchronous motor
28. 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
29. What is a synchronous capacitor?
An over excited synchronous motor is called synchronous capacitor, because, like a capacitor, it takes a leading current
30. 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.
31. Under which conditions a synchronous motor will fail to pull into step?
- The conditions are
- No field excitation
- Excessive load
- Excessive load inertia
32. 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
33. 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.
34. 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
35. 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.
36. 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
37. 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
38. What is specified in synchronous watts?
It is the unit of torque an is defined as
Tsy = Rotor input in watts/2*pi*Ns
39. 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)
40. 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
41. How to conserve electricity at homes?
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