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1. Why star delta starter is preferred with induction motor?
Star delta starter is preferred with induction motor due to following reasons:
• Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence it causes
less losses.
• Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times, that
is why current also reduces up to 3 times and hence less motor burning is caused.
• In addition, starting torque is increased and it prevents the damage of motor winding.
2. State 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.
3. Why AC systems are preferred over DC systems?
Due to following reasons, AC systems are preferred over DC systems:
a. It is easy to maintain and change the voltage of AC electricity for transmission and distribution.
b. Plant cost for AC transmission (circuit breakers, transformers etc) is much lower than the equivalent DC
transmission
c. From power stations, AC is produced so it is better to use AC then DC instead of converting it.
d. When a large fault occurs in a network, it is easier to interrupt in an AC system, as the sine wave current
will naturally tend to zero at some point making the current easier to interrupt.
4. How can you relate power engineering with electrical engineering?
Power engineering is a sub division of electrical engineering. It deals with generation, transmission and
distribution of energy in electrical form. Design of all power equipments also comes under power
engineering. Power engineers may work on the design and maintenance of the power grid i.e. called on grid
systems and they might work on off grid systems that are not connected to the system.
5. What are the various kind of cables used for transmission?
Cables, which are used for transmitting power, can be categorized in three forms:
• Low-tension cables, which can transmit voltage upto 1000 volts.
• High-tension cables can transmit voltage upto 23000 volts.
• Super tension cables can transmit voltage 66 kV to 132 kV.
6. Why back emf used for a dc motor? highlight its significance.
The induced emf developed when the rotating conductors of the armature between the poles of magnet, in a
DC motor, cut the magnetic flux, opposes the current flowing through the conductor, when the armature
rotates, is called back emf. Its value depends upon the speed of rotation of the armature conductors. In
starting, the value of back emf is zero.
7. 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’.
8. Explain the application of storage batteries.
Storage batteries are used for various purposes, some of the applications are mentioned below:
• For the operation of protective devices and for emergency lighting at generating stations and
substations.
• For starting, ignition and lighting of automobiles, aircrafts etc.
• For lighting on steam and diesel railways trains.
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• As a supply power source in telephone exchange, laboratories and broad casting stations.
• For emergency lighting at hospitals, banks, rural areas where electricity supplies are not possible
9. Explain advantages of storage batteries
Few advantages of storage batteries are mentioned below:
• Most efficient form of storing energy portably.
• Stored energy is available immediately because there is no lag of time for delivering the stored energy.
• Reliable source for supply of energy.
• The energy can be drawn at a fairly constant rate.
10. What are the different methods for the starting of a synchronous motor.
Starting methods: Synchronous motor can be started by the following two methods:
• By means of an auxiliary motor: The rotor of a synchronous motor is rotated by auxiliary motor. Then rotor
poles are excited due to which the rotor field is locked with the stator-revolving field and continuous rotation
is obtained.
• By providing damper winding: Here, bar conductors are embedded in the outer periphery of the rotor poles
and are short-circuited with the short-circuiting rings at both sides. The machine is started as a squirrel cage
induction motor first. When it picks up speed, excitation is given to the rotor and the rotor starts rotating
continuously as the rotor field is locked with stator revolving field.
11. Name the types of motors used in vacuum cleaners, phonographic appliances, vending machines,
refrigerators, rolling mills, lathes, power factor improvement and cranes.
Following motors are used: -
• Vacuum cleaners- Universal motor.
• Phonographic appliances – Hysteresis motor.
• Vending machines – Shaded pole motor.
• Refrigerators – Capacitor split phase motors.
• Rolling mills – Cumulative motors.
• Lathes – DC shunt motors.
• Power factor improvement – Synchronous motors.
12. State Thevenin’s Theorem:
According to thevenin’s theorem, the current flowing through a load resistance
Connected across any two terminals of a linear active bilateral network is the ratio open circuit voltage (i.e.
the voltage across the two terminals when RL is removed) and sum of load resistance and internal
resistance of the network. It is given by Voc / (Ri + RL).
13. State Norton’s Theorem
The Norton’s theorem explains the fact that there are two terminals and they are as follows:
• One is terminal active network containing voltage sources
• Another is the resistance that is viewed from the output terminals. The output terminals are equivalent to
the constant source of current and it allows giving the parallel resistance.
The Norton’s theorem also explains about the constant current that is equal to the current of the short
circuit placed across the terminals. The parallel resistance of the network can be viewed from the open
circuit terminals when all the voltage and current sources are removed and replaced by the internal
resistance.
14. State Maximum power transfer theorem
The Maximum power transfer theorem explains about the load that a resistance will extract from the
network. This includes the maximum power from the network and in this case the load resistance is being is
equal to the resistance of the network and it also allows the resistance to be equal to the resistance of the
network. This resistance can be viewed by the output terminals and the energy sources can be removed by
leaving the internal resistance behind.
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15. Explain different losses in a transformer.
There are two types of losses occurring in transformer:
• Constant losses or Iron losses: The losses that occur in the core are known as core losses or iron losses.
Two types of iron losses are:
o eddy current loss
o Hysteresis loss.
These losses depend upon the supply voltage, frequency, core material and its construction. As long as
supply voltage and frequency is constant, these losses remain the same whether the transformer is loaded or
not. These are also known as constant losses.
• Variable losses or copper losses: when the transformer is loaded, current flows in primary and secondary
windings, there is loss of electrical energy due to the resistance of the primary winding, and secondary
winding and they are called variable losses. These losses depend upon the loading conditions of the
transformers. Therefore, these losses are also called as variable losses.
16. Explain different types of D.C motors? Give their applications
Different type of DC motors and their applications are as follows:-
• Shunt motors: It has a constant speed though its starting torque is not very high. Therefore, it is suitable
for constant speed drive, where high starting torque is not required such as pumps, blowers, fan, lathe
machines, tools, belt or chain conveyor etc.
• Service motors: It has high starting torque & its speed is inversely proportional to the loading conditions
i.e. when lightly loaded, the speed is high and when heavily loaded, it is low. Therefore, motor is used in
lifts, cranes, traction work, coal loader and coal cutter in coalmines etc.
• Compound motors: It also has high starting torque and variable speed. Its advantage is, it can run at NIL
loads without any danger. This motor will therefore find its application in loads having high inertia load or
requiring high intermittent torque such as elevators, conveyor, rolling mill, planes, presses, shears and
punches, coal cutter and winding machines etc.
17. Explain the process of commutation in a dc machine. Explain what are inter-poles and why they are
required in a dc machine.
Commutation: It is phenomenon when an armature coil moves under the influence of one pole- pair; it
carries constant current in one direction. As the coil moves into the influence of the next pole- pair, the
current in it must reverse. This reversal of current in a coil is called commutation. Several coils undergo
commutation simultaneously. The reversal of current is opposed by the static coil emf and therefore must
be aided in some fashion for smooth current reversal, which otherwise would result in sparking at the
brushes. The aiding emf is dynamically induced into the coils undergoing commutation by means of
compoles or interpoles, which are series excited by the armature current. These are located in the interpolar
region of the main poles and therefore influence the armature coils only when these undergo commutation.
18. Comment on the working principle of operation of a single-phase transformer.
Working principle of operation of a single-phase transformer can be explained as
An AC supply passes through the primary winding, a current will start flowing in the primary winding. As
a result, the flux is set. This flux is linked with primary and secondary windings. Hence, voltage is induced
in both the windings. Now, when the load is connected to the secondary side, the current will start flowing
in the load in the secondary winding, resulting in the flow of additional current in the secondary winding.
Hence, according to Faraday’s laws of electromagnetic induction, emf will be induced in both the
windings. The voltage induced in the primary winding is due to its self inductance and known as self
induced emf and according to Lenze’s law it will oppose the cause i.e. supply voltage hence called as back
emf. The voltage induced in secondary coil is known as mutually induced voltage. Hence, transformer
works on the principle of electromagnetic induction.
19. Define the following terms: Reliability, Maximum demand, Reserve-generating capacity, Availability
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(operational).
Reliability: It is the capacity of the power system to serve all power demands without failure over long
periods.
Maximum Demand: It is maximum load demand required in a power station during a given period.
Reserve generating capacity: Extra generation capacity installed to meet the need of scheduled downtimes
for preventive maintenance is called reserve-generating capacity.
Availability: As the percentage of the time a unit is available to produce power whether needed by the
system or not.
20. Mention the disadvantages of low power factor? How can it be improved?
Disadvantages of low power factor:
• Line losses are 1.57 times unity power factor.
• Larger generators and transformers are required.
• Low lagging power factor causes a large voltage drop, hence extra regulation equipment is required to
keep voltage drop within prescribed limits.
• Greater conductor size: To transmit or distribute a fixed amount of power at fixed voltage, the conductors
will have to carry more current at low power factor. This requires a large conductor size.
21. State the methods of improving power factor?
Methods of improving power factor:
• By connecting static capacitors in parallel with the load operating at lagging power factor.
• A synchronous motor takes a leading current when over excited and therefore behaves like a capacitor.
• By using phase advancers to improve the power factor of induction motors. It provides exciting ampere
turns to the rotor circuit of the motor. By providing more ampere-turns than required, the induction motor
can be made to operate on leading power factor like an overexcited synchronous motor.
22. State the factors, for the choice of electrical system for an aero turbine.
The choice of electrical system for an aero turbine is guided by three factors:
• Type of electrical output: dc, variable- frequency ac, and constant- frequency ac.
• Aero turbine rotational speed: constant speed with variable blade pitch, nearly constant speed with
simpler pitch- changing mechanism or variable speed with fixed pitch blades.
• Utilization of electrical energy output: in conjunction with battery or other form of storage, or
interconnection with power grid.
23. What are the advantages of VSCF wind electrical system?
Advantages of VSCF wind electrical system are:
• No complex pitch changing mechanism is needed.
• Aero turbine always operates at maximum efficiency point.
• Extra energy in the high wind speed region of the speed – duration curve can be extracted
• Significant reduction in aerodynamic stresses, which are associated with constant – speed operation.
24. Explain the terms real power, apparent power and reactive power for ac circuits and also the units
used.
• Real Power: It is the product of voltage, current and power factor i.e. P = V I cos j and basic unit
of real power is watt. i.e. Expressed as W or kW.
• Apparent power: It is the product of voltage and current. Apparent power = V I and basic unit of
apparent power is volt- ampere. Expressed as VA or KVA.
• Reactive Power: It is the product of voltage, current and sine of angle between the voltage and
current i.e. Reactive power = voltage X current X sinj or Reactive power = V I sin j and has no
other unit but expressed in VAR or KVAR.
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25. Define the following: Average demand, Maximum demand, Demand factor, Load factor.
• Average Demand: the average power requirement during some specified period of time of
considerable duration is called the average demand of installation.
• Maximum Demand: The maximum demand of an installation is defined as the greatest of all the
demand, which have occurred during a given period. It is measured accordingly to specifications,
over a prescribed time interval during a certain period.
• Demand Factor: It is defined as the ratio of actual maximum demand made by the load to the
rating of the connected load.
• Load Factor: It is defined as the ratio of the average power to the maximum demand.
26. Explain forward resistance, static resistance and dynamic resistance of a pn junction diode.
• Forward Resistance: Resistance offered in a diode circuit, when it is forward biased, is called
forward-resistance.
• DC or Static Resistance: DC resistance can be explained as the ratio of the dc-voltage across the
diode to the direct current flowing through it.
• AC or Dynamic Resistance: It can be defined as the reciprocal of the slope of the forward
characteristic of the diode. It is the resistance offered by a diode to the changing forward current.
27. How does Zener phenomenon differ from Avalanche breakdown?
The phenomenon when the depletion region expands and the potential barrier increases leading to a very
high electric field across the junction, due to which suddenly the reverse current increases under a very
high reverse voltage is called Zener effect. Zener-breakdown or Avalanche breakdown may occur
independently or both of these may occur simultaneously. Diode junctions that breakdown below 5v are
caused by Zener Effect. Junctions that experience breakdown above 5v are caused by avalanche-effect. The
Zener-breakdown occurs in heavily doped junctions, which produce narrow depletion layers. The
avalanche breakdown occurs in lightly doped junctions, which produce wide depletion layers.
28. Compare JFET’s and MOSFET’s.
Comparison of JFET’s and MOSFET’s:
• JFET’s can only be operated in the depletion mode whereas MOSFET’s can be operated in either
depletion or in enhancement mode. In a JFET, if the gate is forward-biased, excess-carrier injunction
occurs and the gate-current is substantial.
• MOSFET’s have input impedance much higher than that of JFET’s. Thus is due to negligible small
leakage current.
• JFET’s have characteristic curves more flat than that of MOSFET is indicating a higher drain resistance.
• When JFET is operated with a reverse-bias on the junction, the gate-current IG is larger than it would be
in a comparable MOSFET.
30. Explain thin film resistors and wire-wound resistors
a. Thin film resistors- It is constructed as a thin film of resistive material is deposited on an insulating
substrate. Desired results are obtained by either trimming the layer thickness or by cutting helical grooves
of suitable pitch along its length. During this process, the value of the resistance is monitored closely and
cutting of grooves is stopped as soon as the desired value of resistance is obtained.
b. Wire wound resistors – length of wire wound around an insulating cylindrical core are known as wire
wound resistors. These wires are made of materials such as Constantan and Manganin because of their
high resistivity, and low temperature coefficients. The complete wire wound resistor is coated with an
insulating material such as baked enamel
31. What is a differential amplifier? Also, explain CMRR.
Differential Amplifier: The amplifier, which is used to amplify the voltage difference between two input-
lines neither of which is grounded, is called differential amplifier. This reduces the amount of noise
injected into the amplifier, because any noise appearing simultaneously on both the input-terminals as the
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amplifying circuitry rejects it being a common mode signal.
CMRR: It can be defined as the ratio of differential voltage-gain to common made voltage gain. If a
differential amplifier is perfect, CMRR would be infinite because in that case common mode voltage gain
would be zero.
32. Why is transformer flux constant?
When a load is connected to a transformer current (say I2) flows through secondary coil thus an M.M.F
(N2I2) is produced, this produces the secondary flux. This flux reduces the main flux induced in the primary
& also reduces E.M.F E1 in the primary. As a result more current is drawn from the supply. This additional
current drawn is due to the load component I2’ .This I2‘ is anti-phase with I2.This I2‘ sets a flux which
opposes the secondary flux & helps the main flux. The load component flux neutralises the secondary flux
produced by I2.The M.M.F N1I2 balances N2I2.Thus the net flux is always at constant level
33. Why an induction motor is called a rotating transformer?
The rotor of an induction motor receive electric power by induction in exactly the same way as the
secondary of a 2-winding transformer receives its power from the primary. Thus an induction motor can be
treated as a rotating transformer i.e. one in which primary winding is stationary but the secondary is free to
rotate.
34. Why is transformer rating in KVA?
Copper loss of a transformer depends on current and iron loss on voltage. Hence, total transformer loss
depends on volt-ampere (VA) and not on phase angle between voltage and current i.e. independent of load
power factor. That is why rating of transformers is in Kva and not in Kw.
35. How do we specify the rating of a transformer?
Transformer rating is specified in Kilovolt-Ampere (KVA)
36. What are the cooling methods to cool a transformer?
The cooling methods are:
Oil filled self cooled
Oil filled water cooled
Air blast type
37. What is Scott- connected transformer?
A Scott-T transformer (also called a Scott connection) is a type of circuit used to derive two-phase electric
power (2-φ, 90-degree phase rotation) from a three-phase (3-φ, 120-degree phase rotation) source, or vice-
versa. The Scott connection evenly distributes a balanced load between the phases of the source.
38. Does the transformer draw any current when its secondary is open?
Yes, no-load primary current
39. Is Cu loss affected by power factor? Why?
Yes, Cu loss varies inversely with power factor
Cu loss depends on current in the primary and secondary windings. It is well-known that current required is
higher when power factor is lower.
40. What are Instrument Transformers? Current Transformer and Potential Transformer are Instrument
transformers. Current Transformer (CT) is a step-up transformer which steps down the current to a
known ratio. The primary of this transformer consists of one or more turns of thick wire connected in series
with the line. The secondary consists of a large number of turns of fine wire and provides for the measuring
instruments and relays a current which is a constant fraction of current in the line
Potential Transformer (PT) is a step down transformer and steps down the voltage to a known ratio. The
primary of this transformer consists of a large number of fine wire connected across the line. The secondary
winding consists of a few turns and provides for measuring instruments and relays a voltage which is a
known fraction of the line voltage.
41. What is the difference between earth and neutral?
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Neutral is a return current path for equipments and earthing is protection for humans. Earth is a support for
neutral. If neutral is not present, the machine we are using will get in full phase and we will get a shock. So
earthing is done to avoid such shocks.
42. What is the difference between an Insulator and a Dielectric?
Dielectrics are used to store the electric charges, while insulators are used to block the flow of electric
charges. All dielectrics are insulators (they don’t allow the flow of electric charges through them) but all
insulators aren’t dielectric because they can’t store charges unlike dielectrics.
43. What are the disadvantages of low power factor?
The disadvantages are:
Large KVA rating of equipment
Greater conductor size
Large copper losses
Poor voltage regulation
Reduced handling capacity of system
44. Why is earth pin longest and thickest?
More the thickness of the wire/conductor less is the resistance. A thick earth pin provides a less resistant
path for the current to be grounded. This is for safety so that the leakage current if any will take the less
resistant path from earth pin to ground rather than a person operating an appliance
45. What is Hysteresis loss?
Hysteresis loss is a heat loss caused by the magnetic properties of the armature. When an armature core is
in a magnetic field, the magnetic particles of the core tend to line up with the magnetic field. When the
armature core is rotating, its magnetic field keeps changing direction. The continuous movement of the
magnetic particles, as they try to align themselves with the magnetic field, produces molecular friction.
This, in turn, produces heat. This heat is transmitted to the armature windings. The heat causes armature
resistances to increase To compensate for hysteresis losses, heat-treated silicon steel laminations are used
46. Which is the motor used in fan?
Single-phase induction motor is used in fan
47. Why do we use starter for a DC motor?
Large DC motors with field windings instead of permanent magnets have a very heavy load when starting.
To prevent fuses blowing, a series of small resistors are sequentially switched in series with the motor. As
the motor picks up speed, a back EMF in opposition to the applied voltage limits the maximum current.
When the motor reaches its running speed, the ‘starter’ is out of circuit
48. What is Form Factor?
The form factor of an alternating current waveform (signal) is the ratio of the RMS (Root Mean Square)
value to the average value (mathematical mean of absolute values of all points on the waveform)
49. Which is more dangerous-shock due to AC or DC?
Shock due to DC is more dangerous than AC
50. What are the practical applications of variable resistance?
Its applications are:
Volume control
Fan speed control
Light dimmer
51. What is slip?
The difference between the synchronous speed Ns and the actual speed N of the rotor is known as slip
52. What is the purpose of compensating windings?
The function of compensating winding is to neutralize the cross magnetizing effect of armature reaction. In
the absence of compensating windings, the flux will be suddenly shifting backward and forward with every
change in load
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53. What is Cogging and Crawling?
Cogging: The rotor of a squirrel cage motor sometimes refuses to start at all, particularly when the voltage
is low. This happens when the number of stator teeth is equal to the number of rotor teeth and is due to the
magnetic locking (cogging) between the stator and rotor teeth
Crawling: Induction motors, especially the squirrel cage type, sometimes exhibit a tendency to run stably at
speeds as low as one-seventh of their synchronous speed Ns. This phenomenon is known as crawling of an
induction motor
54. What are Dummy coils?
These are used with wave winding and are resorted to when the requirements of the windings are not met
by the standard armature punching. They are simply to provide the mechanical balance for the armature
because an armature having some slots without windings would be out of balance mechanically
55. What are CT and PT?
Current Transformer (CT): It is a step-up transformer which steps down the current to a known ratio. The
primary of
this transformer consists of one or more turns of thick wire connected in series with the line. The secondary
consists of a large number of turns of fine wire and provides for the measuring instruments and relays a
current which is a constant fraction of current in the line.
Potential Transformer (PT): It is a step down transformer and steps down the voltage to a known ratio. The
primary of this transformer consists of a large number of fine wires connected across the line. The
secondary winding consists of a few turns and provides for measuring instruments and relays a voltage
which is a known fraction of the line voltage
56. What is a Universal Motor?
A universal motor is defined as a motor which may be operated either on direct or single phase ac supply at
approximately the same speed and output.
57. What is Voltage Regulation?
It is the change in voltage when the load is reduced from rated value to zero, expressed as percentage of the
rated load voltage
58. What are Interpoles used for?
As the polarity of Interpoles is the same as that of the main pole ahead, they induce an emf in the coil
(under commutation) which helps the reversal of current. Another function is to neutralize the cross
magnetizing effect of armature reaction
59. What is the principle of DC motor?
The action of DC motor is based on the principle that when a current carrying conductor is placed in a
magnetic field, it experiences a mechanical force whose direction is given by Fleming’s Left Hand Rule
and whose magnitude is given by F=BIL Newton
60. Explain commutation in DC machines
The process by which current in the short circuited coil is reversed due to reversal of current while it
crosses the M.N.A. is called commutation. The brief period during which coil remains short circuited is
known as commutation period TC
61. What are Damper Windings?
It is a winding consisting of several conducting bars on the field poles of a synchronous machine, short-
circuited by conducting rings or plates at their ends, and used to prevent pulsating variations of the position
or magnitude of the magnetic field linking the poles. It is also known as Amortisseur winding
62. How do you start an Induction motor?
The stator windings when supplied with 3-phase currents produce a magnetic flux, which is of constant
magnitude but which revolves at synchronous speed. This revolving flux induces an AMF in the rotor by
mutual induction
63. What are the uses of Servomotors?
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These are used in computers, machine tools, process controllers, and robots, radars, tracking and guiding
units
64. What is Regenerative Braking?
Regenerative braking is a system in which the electric motor that normally drives a hybrid or pure electric
vehicle is essentially operated in reverse (electrically) during braking or coasting. Instead of consuming
energy to propel a vehicle, the motor acts as a generator that charges the onboard batteries with electrical
energy that would normally be lost as heat through traditional mechanical friction brakes. As the motor
“acts in reverse,” it generates electricity. The accompanying friction (electrical resistance) assists the
normal brake pads in overcoming inertia and helps slow the vehicle
65. What will happen if we give 220 Volt DC supply to a bulb or tube light?
Bulbs (devices) for AC are designed to operate such that it offers high impedance to AC supply. Normally
they have low resistance. When DC supply is applied, due to low resistance, the current through lamp
would be so high that it may damage the bulb element
66. Which motor has high Starting Torque and Staring current – DC motor, Induction motor or
Synchronous motor?
DC Series motor has high starting torque. We cannot start the Induction motor and Synchronous motors on
load, but can start the DC series motor without load
67. What is 2 Phase motor?
A two phase motor is a motor with the starting winding and the running winding having a phase split. E.g.
AC servo motor, where the auxiliary winding and the control winding have a phase split of 90 degree
68. Give two basic speed control schemes of DC shunt motor?
By using flux control method-in this method a rheostat is connected across the field winding to control the
field current. So by changing the current the flux produced by the field winding can be changed, and since
speed is inversely proportional to flux speed can be controlled
Armature control method-in this method a rheostat is connected across armature winding by varying the
resistance the value of resistive drop(IaRa)can be varied and since speed is directly proportional to (Eb-
IaRa) the speed can be controlled
69. What is the difference between Synchronous Generator & Asynchronous Generator?
In simple, Synchronous generator supply’s both active and reactive power but Asynchronous generator
(Induction generator) supplies only active power and absorbs reactive power for magnetizing. These types
of generators are used in windmills
70. Why synchronous generators are used for the production of electricity?
Synchronous machines have capability to work on different power factor(or say different imaginary power
varying the field EMF. Hence used for the production of electricity
71. Enlist types of DC Generators?
DC Generators are classified into two types
Separately excited DC generator
Self excited DC Generator which is further classified into Series, Shunt and Compound (which is
further classified into Cumulative and Differential)
72. What is Automatic Voltage Regulator (AVR)?
AVR is an abbreviation for Automatic Voltage Regulator. It is an important part in Synchronous
Generators; it controls the output voltage of the generator by controlling its excitation current. Thus it can
control the output Reactive Power of the Generator
73. What is Eddy Current Loss?
It is a phenomenon caused by the rate of change in an induced magnetic field. The relative motion causes a
circulating flow of electrons or current within the conductor, leading to efficiency loss
74. What is plugging of an Induction motor?
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Plugging is a method of braking used in induction motors. Plugging involves interchanging the supply to
two of the stator phase windings. This method is used in applications which require immediate stop
applications. When the phase supply is reversed, torque is produced in the opposite direction. This leads to
braking of the electric motor
75. What is the principle of operation of tube light?
A fluorescent lamp is a glass tube containing some mercury vapour, with a phosphorescent coating on the
interior surface of the tube. The mercury vapour is made to glow by using a high voltage across its
electrodes that sets off an electric arc discharge in the tube. The discharge can also be described as
conducting plasma. The resulting flow of current through the mercury vapour is stable and well-controlled
by external ballast, or loading device, which consists of a high-inductance choke-coil that is connected in
series with the tube. The continuous discharge glow is invisible to the human eye but it causes the
phosphorescent coating on the interior surface of the tube to emit visible light
76. What are Brushless Alternators?
A brushless alternator is composed of two alternators built end-to-end on one shaft. Smaller brushless
alternators may look like one unit but the two parts are readily identifiable on the large versions. The larger
of the two sections is the main alternator and the smaller one is the exciter. The exciter has stationary field
coilsand a rotating armature (power coils). The main alternator uses the opposite configuration with a
rotating field and stationary armature
77. What is equalizer ring used for?
An equalizer, or equalizer connection, connects two or more generators operating in parallel at a point
where the armature and series-field leads join, thus connecting the armatures in multiple and the series coils
in multiple so that the load will divide between the generators in proportion to their capacities
78. What is a Ground line?
The purpose of the ground line is to provide a continuous, unbroken path to allow a circuit fault current to
return to the distribution panel to trip the circuit’s over current device
79. How may the number of parallel paths in an armature be increased?
By increasing the number of magnetic poles
80. How are brushes connected in DC generator?
Usually, all positive brushes are connected together and all negative brushes together
81. How can field distortion be remedied?
By using compensating winding which are embedded in the slots in the pole-shoe and are connected in
series with the armature
82. What is meant by Normal Neutral Plane?
It is a plane which passes through the axis of armature perpendicular to the magnetic field of the generator
when there is no flow of current through the armature
83. What is the best way of minimizing eddy currents in an armature?
Lamination is the best way
84. Should the brushes of a loaded generator be placed in the neutral plane?
No, because the brushes must be advanced by a certain angle (called brush lead) beyond the neutral plane
to prevent sparking.
85. What causes sparking at the brushes?
It is due to the self-induction of the coil undergoing commutation.
86. What is the standard direction of rotation of the DC generators?
Clockwise when viewed from the end opposite to the driven end
87. What is meant by voltage build up of a generator?
It means the gradual increase in the generator voltage to its maximum value after the generator is started
from rest
88. How should a generator be started?
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It is usually brought up to speed with the help of the driving engine called prime mover
89. What is the procedure for shunting down the generator?
First, the load should be gradually reduced, if possible, by easing down the driving engine, and then when
the generator is supplying little or no current, the main switch should be opened. When the voltmeter reads
almost zero, then brushes should be raised from the commutator
90. Can a generator be reversed by reversing the connections between armature and field coils?
No, because if these connections are reversed, the generator will not build up at all
91. What is the permissible rise of temperature in a well-designed generator?
270C above the surrounding air
92. 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
93. 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.
94. 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
95. 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
96. 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
1. Damage the motor itself
2. Badly affect the voltage regulation of the supply line
97. 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.
98. 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
99. What causes are responsible for over-heating of Commutator in a DC motor?
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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
100. How is magnetic leakage reduced to a minimum in commercial transformers?
This is done by interleaving the primary and secondary windings
101. Mention the factors on which Hysteresis loss depends?
Quality and amount of iron in the core
Flux density
Frequency
102. 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
103. 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
104. Why induction motors are called asynchronous?
Because their rotors can never run with the synchronous speed
105. How does the slip vary with load?
The greater the load, greater is the slip or slower is the rotor speed
106. 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
107. 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
108. Which is the usual cause of blow-outs in Induction motors?
The most common cause is single-phasing
109. Can a 3-phase motor be run on a Single-Phase Line?
Yes, it can be. But a Phase-Splitter is essential
110. 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
111. 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
112. 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
113. 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.
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114. 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
115. What are the two types of turbo-alternators?
Vertical and horizontal
116. 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
117. 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
118. 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
119. For what service are synchronous motors especially suited?
They are especially suited for high voltage service
120. Which has more efficiency; synchronous or induction motor?
Synchronous motor
121. 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
122. What is a synchronous capacitor?
An over excited synchronous motor is called synchronous capacitor, because, like a capacitor, it takes a
leading current
123. 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.
124. Under which conditions a synchronous motor will fail to pull into step?
The conditions are
No field excitation
Excessive load
Excessive load inertia
125. 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
126. Any disadvantage of having more phases?
Advantages are
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More leads have to be brought out from the motor
More interconnections are required to the drive circuit
More switching devices are needed.
127. 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
128. 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.
129. 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
130. 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
131. 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)
132. 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
133. What is an oscillator?
An oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or
a square wave. They are widely used in many electronic devices. Common examples of signals generated
by oscillators include signals broadcast by radio and television transmitters, clock signals that regulate
computers and quartz clocks, and the sounds produced by electronic beepers and video games
134. Why positive feedback is given to the oscillator?
Positive feedback is used in oscillators to keep them running or oscillating. An oscillator is basically an
amplifier which has a feedback path from the output back to the input. This permits a portion of the output
signal to get back to the input to “keep things going” in stage – to keep things oscillating. The feedback
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must be positive so that it will “contribute” to the stage being able to keep oscillating. If the feedback was
negative, it would serve to damp the oscillation and to “kill” the oscillator
135. Does oscillator has an input?
No. In an oscillator, positive feedback is used to supply its own input signal for its operation.
136. What are freewheeling diodes? A flyback diode (sometimes called a snubber diode, freewheeling diode,
suppressor diode, or catch diode) is a diode used to eliminate flyback, the sudden voltage spikes seen across
an inductive load when its supply voltage is suddenly reduced or removed
137. What is Phase Margin?
Phase margin (PM) is the difference between the phase, measured in degrees, of an amplifier’s output
signal (relative to its input) and 180°, as a function of frequency
138. What is modulation?
Modulation is the process of varying one or more properties of a high-frequency periodic waveform, called
the carrier signal, with a modulating signal which typically contains information to be transmitted
139. What are the uses of PLL circuits?
PLL circuits are now frequently being used to demodulate FM signals, AM demodulators, FSK decoders,
two-tone decoders and motor speed controls
140. What is a Low Pass filter?
A low-pass filter is an electronic filter that passes low-frequency signals but attenuates (reduces
theamplitudeof) signals with frequencies higher than the cut off frequency. The actual amount of
attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble
cut filter when used in audio applications
141. What are the uses of Schmitt trigger circuits?
Schmitt triggers are typically used in open loop configurations for noise immunity and closed
loopconfigurations to implement function generators
142. What is the one main difference between UPS & inverter ?
Uninterrupted power supply is mainly used for short time, means according to UPS VA it gives backup.
UPS is of two types: Online and Offline. Online UPS has high Volt and Amp for long time backup with
high DC voltage. But UPS start with 12v DC with 7 amps. But inverter starts with 12v, 24v DC to 36v DC
and 120 Amp to 180 Amp batteries with long time backup
143. What is an UPS?
An uninterruptible power supply, also uninterruptible power source, UPS or battery/flywheel backup, is an
electrical apparatus that provides emergency power to a load when the input power source, typicallymains
power, fails
144. What are the equipments used in sub- station?
The elements of a sub- station are
Primary power lines
Ground wire
Overhead lines
Transformer for measurement of electric voltage
Disconnect switch
Circuit breaker
Current transformer
Lightning arrester
Main transformer
Control building
Security fence
Secondary power lines
145. What is the difference between Relay and Fuse?
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A relay is a switch; it is not usually meant to open under fault conditions (high current).
A fuse is a fault protection device. If a short circuit develops, large currents will flow, and the fuse link will
melt, causing the circuit to open
146. What is PLCC?
Power line communication or power line carrier (PLC), also known as power line digital subscriber line
(PDSL), mains communication, power line telecom (PLT), power line networking (PLN), or broadband
over power lines (BPL) are systems for carrying data on a conductor and are also used for electric power
transmission. A wide range of power line communication technologies are needed for different
applications, ranging from home automation to Internet access
147. Why is alternating current used for transmission?
AC is used for transmission because transformers can step it up to high voltage for long distance, and
down to low voltage for local distribution and use. The output of power stations comes from a rotary
turbine, which by its nature is AC and therefore requires no power electronics to convert to DC
Also, it is much easier to change the voltage of AC electricity for transmission and distribution and the cost
of plant associated with AC transmission (circuit breakers, transformers etc) is much lower than the
equivalent of DC transmission
AC transmission provides a number of technical advantages. When a fault in the network occurs, large fault
current flows. In an AC system, this becomes much easier to interrupt, as the sine wave current will
naturally tend to zero at some point making the current easier to interrupt
148. What is corona?
The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission
line is known as Corona
149. What is skin effect?
The tendency of alternating current to concentrate near the surface of a conductor is known as Skin Effect
150. Why is voltage stepped up before transmission?
Voltage is stepped up before transmission due to the following reasons:
Reduces volume of conductor material
Increases transmission efficiency
Decreases percentage line drop
151. What are standard Transmission and Distribution voltages?
Primary Transmission: 66 kV, 132 KV, 220 kV and 400 kV
Secondary Transmission: 33 kV
Primary Distribution: 11 kV, 3-phase, 3-wire
Secondary Distribution: 400 V between two phases (3-phase, 4-wire) and 230 V between any one phase
and neutral
152. Explain Transmission and Distribution systems
Electric-power transmission system is the system used for bulk transfer of electrical energy, from
generatingpower plants to electrical substations located near demand centers.
The different systems of transmission are:
DC system
Single phase AC system
Two phase AC system
Three phase AC system
The part of power system which distributes electric power for local use is known as Distribution System.
The different distribution systems are:
AC Distribution system
Primary distribution system
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Secondary distribution system
DC Distribution system
2 wire DC system
3 wire DC system
153. What are Lightning Arrestors?
A Lightning Arrestor is a device used in electrical power systems to protect the insulation on the system
from the damaging effect of lightning
154. Explain the principle of operation of a Relay and a Circuit breaker
RELAY: An electromagnetic relay works on two principles
Electromagnetic attraction
Electromagnetic induction
The force of attraction produced has two components-one is constant and the other is fluctuating at a
frequency, twice the natural frequency. This force in terms of current is represented as-
F= K1 – K2 * I2
K1=K2 * I2
Since K1 and K2 both are constants, so the current I will be constant. This is the condition when the relay is
at the verge of operation and F=0.
CIRCUIT BREAKER: Once a fault is detected, contacts within the circuit breaker must open to interrupt
the circuit; some mechanically-stored energy (using springs or compressed air) contained within the
breaker is used to separate the contacts, although some of the energy required may be obtained from the
fault current itself. When a current is interrupted, an arc is generated. This arc must be contained, cooled,
and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage
in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium in which the
arc forms
155. What is a Bus bar?
A Bus bar is a strip of copper or aluminium that conducts electricity within a switchboard, distribution
board,substationor other electrical apparatus. They form a link between the incoming and outgoing circuits.
The size of the bus bar determines the maximum amount of current that can be safely carried
156. What is Arc formation?
When a short circuit occurs, a heavy current flows through the contacts of the circuit breaker and they are
opened by the protective system. At the instant when the contacts begin to separate, the contact area
decreases rapidly and large fault current causes increased current density and hence rise in temperature.
The heat produced in the medium between the contacts is sufficient to ionise the medium. The ionised air
or vapour acts as a conductor and an arc is struck between the contacts.
157. What is 3-Phase supply?
It is a type of poly phase system and is the most common method used by grids worldwide to transfer
power. In a three-phase system, three circuit conductors carry three alternating currents (of the same
frequency) which reach their instantaneous peak values at different times
158. Why do we require protection against lightning?
The lightning surges may cause serious damage to the expensive equipments in the power system (e.g.
generators, transformers etc.) either by direct strokes on the equipment or by the strokes on the
transmission lines that reach the equipment as travelling waves
159. What is a Sub-station?
The assembly of apparatus used to change some characteristics (e.g. voltage, AC to DC, frequency, power
factor) of the electric supply is called a sub-station
160. What is grading of cables?
The process of achieving uniform electrostatic stress in the dielectric of cables is called grading of cables
161. What is transmission cable made up of?
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The principal insulating materials used in cables are rubber, vulcanised India rubber, impregnated paper,
varnished cambric and polyvinyl chloride.
162. Explain the purpose of fuse
When a short circuit or overload occurs, the current through the fuse increases beyond its rated value which
raises the temperature and fuse element melts (or blows out), disconnecting the circuit protected by it
163. What is ACSR cable and where we use it?
ACSR means Aluminium conductor steel reinforced. This conductor is used in transmission & distribution
164. What will happen when power factor is leading in distribution of power?
If there is high power factor, i.e. if the power factor is close to one
Losses in the form of heat will be reduced,
Cable becomes less bulky and easy to carry, and very cheap to afford, &
It also reduces over heating of transformers.
165. What are the faults in a three phase line?
The types of faults in a three phase line are
Phase-To-Earth Fault
Phase-To-Phase Fault
Phase-To-Phase-To-Earth Fault
Three-Phase Fault
Three-Phase-To-Earth Fault
Phase-To-Pilot Fault
Pilot-To-Earth Fault
166. What is the approximate power generation capacity in India?
Sector MW
State Sector 83,563.65
Central Sector 56,572.63
Private Sector 42,553.34
Total 1,82,689.62
167. Is corona more in AC or DC?
Corona Loss p = .00241[(f+25)/$]*[root of (r/d)]*square of[Vp-Vo]
f= supply frequency
$= air density correction factor
Vp = operating voltage
Vo= critical disprutive voltage
r= radius of conductor
d= distance of separation for fine weather
Now for DC f=0, there is a certain power loss by corona but less than AC corona loss.
168. What is current chopping?
Current chopping in CB is the incident of arc current interruption before normal current zero is reached. If
this current is enough to deliver a voltage which is greater than restriking voltage then the arc will continue
again. But gradually the current falls and certainly the voltage also reduces and finally falls down the
restriking voltage and ultimately arc is extinguished. It results in voltage oscillation in the line which
affects the electrical equipments
169. Which circuit breaker uses current chopping circuit?
Vacuum circuit breaker uses current chopping circuit
170. What is SCADA?
SCADA (supervisory control and data acquisition) generally refers to Industrial Control Systems (ICS):
computer systems that monitor and control industrial, infrastructure, or facility-based processes
171. Why do we use a wave trap?
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Line trap also is known as Wave trap. It helps in trapping the high frequency communication signals sent
on the line from the remote substation and diverting them to the telecom/ tele-protection panel in the
substation control room (through coupling capacitor and LMU). The Line trap offers high impedance to the
high frequency communication signals thus obstructs the flow of these signals in to the substation Bus bars
172. Which are preferred – Underground or Overhead cables?
Although many low voltage, local electric distribution lines are buried underground, almost all high-voltage
electric transmission lines are tried to be placed as overhead cables due to the cost difference
173. What is an IDMT relay?
It is an Inverse Definite Minimum Time relay. In IDMT relay, operating is inversely proportional to fault
current and also a characteristic of minimum time after which this relay definitely operates
174. What is a diversity factor?
Diversity Factor = Sum of Individual Maximum Demand/ Maximum Demand on Power Station
175. What is the basic structure of a rotating electrical machine?
It mainly consists of the following circuits
Magnetic circuit
Electric circuit
Dielectric circuit
Thermal circuit
Mechanical parts
176. What is Specific Electric Loading?
The number of Armature (or Stator) Ampere Conductors per Metre of Armature (or Stator) periphery at the
air gap is known as Specific Electric Loading
177. What is Specific Magnetic Loading?
The average flux density over the air gap of a machine is known as Specific Magnetic Loading
178. What are the various types of ratings of a machine?
The different types of ratings are
Continuous duty
Short time duty
Intermittent periodic duty
Intermittent periodic duty with starting
Intermittent periodic duty with starting and braking
Continuous duty with intermittent periodic duty
Continuous duty with starting and braking
Continuous duty with periodic speed changes
179. List some ventilating systems
The different ventilating systems are
Radial ventilating system
Axial ventilating system
Combined radial and axial
180. What is a Runaway speed?
It is defined as the speed which the prime mover would have, if it is suddenly unloaded when working at its
rated load
181. What are the modes of Heat Dissipation?
The heat can be dissipated by
Conduction
Convection
Radiation
182. What is a Field Form Factor?
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It is the ratio of the average flux density over the pole pitch to the maximum flux density. It is also known
as flux distribution factor
183. What is a Window Space Factor?
It is defined as the ratio of copper area in the window of the total window area
184. What are the different types of cores?
Different Types of cores are
Rectangular core
Square core
Stepped core