PKR ELECTRICAL ENGINEERING

Monday, March 30, 2020

Measuring instruments Questions & Answers

Measuring instruments

1. What are the two main classifications of analog instruments?

The two main classifications of instruments are,
a. Absolute instruments. Example tangent galvanometer.
b. Secondary instruments. Example ammeter, voltmeter. Analog instruments are
classified according to their electrical quantity they measure. Example frequency
meter, voltmeter, etc. Principles they work are moving coil, induction.

2. What are three types of secondary instruments?

The three types of secondary instruments are,
a. Indicating type: It only indicates the electrical quantity measured. Example:
Ammeter, Voltmeter, Frequency meter etc.
b. Integrating type: It integrates (sums up) the quantity being measured. Example:
Energy meter.
c. Recording meter: It records as well as indicates the electrical quantity being
measured. Example: 3 pen graphical recorder.

3. Give three operating forces acting on indicating instruments.

a. Deflecting force.
b. Controlling force.
c. Damping force.

4. What are the advantages of digital instruments over analog instruments?

a. Human errors are avoided (comparative error) because the output is displayed in
form of numbers.
b. Power consumption of digital meters are low as compared to analog meters.

5. What is the range of resistances that can be measured using following.

a. Wheatstone bridge – 1 milli Ω to 11 MΩ.
b. Kelvins double bridge – 0.2 micro Ω to 11 Ω.
c. Megger – Insulation resistances more than 100 kΩ

6. What do you understand by tan delta for a insulating material?

Tan delta measurement is done to find the qualities of insulating material. Tan delta
is angle between current due to surface leakage or current due to capacitance and the
capacitive current. That is Tan δ = Ir / Ic.

7. For what purposes transformer ratio meter can be used?

Transformer ratio meter can be used for,
a. To find the ratio of a transformer.
b. To find the phase angle deviation of primary and secondary voltage of
transformer.
c. To find the magnitude of magnetizing currents.

8. Draw connection diagram of ammeter, voltmeter, energy meter, and wattmeter.

9. Illustrate how can you use a single-phase wattmeter to measure 3 phase reactive
power in a circuit?

We can measure reactive power of 3 phase circuit by dingle phase wattmeter by
connecting the current coil in series with a line or load and connecting the pressure
coil across the other two lines.
Reactive power = 3√ V * I * sinϕ Watts.

10. Draw the basic block diagram of digital meter and explain the function of each
block.

Alternator A/D converter BCD counter decoder & LCD display
Vx
Alternator: It reduces the unknown voltage to a small value. Because the reference
voltage is very less and the unknown voltage is maximum.
A/D converter: It converts the analog signal from alternator to digital signals.
BCD counter: It counts the number of pulses (binary counter).
Decoder and display: It decodes the binary code to decimal form and gives a visual
display of it.

11. Draw a neat sketch and explain the use of CT and PT for measurement of power in a
single-phase circuit?

If wattmeter of proper range is not available or if voltage and current ranges are high
we can usr CT and PT of suitable ratio. Connect the CT and PT as shown in figure.
The reading of wattmeter can be multiplied by the ratio to get the actual power.
CT PT
CC
WATT METER
PC

12. Explain the construction and working of moving coil instrument.

Construction: The moving part is a coil wound on a light aluminium frame,
mounted on a shaft made of stainless steel which is pivoted at either ends on jewel
bearing made of sapphire. The coil is placed between poles of a permanent magnet.
Moving system is made light as far as possible to have high torque and weight ratio.
There are two phosphor bronze springs of very less resistance. This is used as
terminals for the current to pass through the coil and out of coil. It is also serves the
purpose of control force.
Aluminium coil former acts as a damping device by eddy current damping. There is
a knife edge pointer and a counter weight to avoid its sagging.
Working: When ever a current carrying conductor is placed in a magnetic field a
force is experiences by the conductor. Moving coil meters work on this principle.
Force = BINL
As the BNL is constant, force (F) is directly proportional to current (I).
That is FαI. Where B is magnetic flux, N is number of turns and L is length of coil.

13. Explain the procedure for measurement of earth resistance.

14. Explain the working principle of wheat stone’s bridge along with equations under
balanced condition. B

P kG Q
I1 I1
A I2 G
I2 D
X S
C
Wheat stone’s bridge works on kirchoff’s law. It is used to measure medium range
resistances. P and Q are fixed standard resistances. S is standard variable resistances.
X is the unknown resistance. G is galvanometer, kG is galvanometer switch and kB
is battery switch. No current will flow through galvanometer if the potentials across
its terminals are equal. So there will be no deflection of galvanometer. This condition
is called the balanced condition.
This is achieved by varying S and also by varying P/Q ratio.
At balanced condition VAB = VAC And VBD = VCD
I1
*
P = I2*X & I1*Q = I2*S
Dividing both I1
*
P = I2*X
I1*Q = I2*S
Question and answers Electrical Maintenance Unit
- 107 -
= P/Q = X/S
Unknown resistance (X) = P/Q * S Ω

15. Explain the working principle of Kelvins double bridge and procedure for the
measurement of terminal resistance.

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Electrical system Questions & Answers

Electrical system

1. What are the main two divisions of MAPS electrical system and what do you
understand by it?

The two main divisions of MAPS electrical system are

a. Main output system. Output system supplies power to the grid. Generated voltage
is stepped up to 220 kV from 16.5 kV and supplied to grid.
b. Station service system. This system supplies the load inside the power station.
The generated voltage is stepped down to 6.6 kV and 415 V from 16.5 kV and
supplies to auxiliary loads.

2. List out the components of station output system.
Main generator, Generator transformer, PT, CT, CVT, lightning arrestor, wave trap,
main 220 kV bus, transfer bus, SF6 circuit breakers and isolators, line protection
scheme, GT and Generator protection scheme, bus bar protection scheme etc.

3. Why earth switches are provided in 220 kV bays?

When bay CB trips, both end (station and grid) CB will trip. The earth switches are
provided because the grid will always be alive so to prevent any shocks to the
operator or maintenance personnel who is working on the line or bay due to
accidental energizing of the bus.

4. What is the purpose of CVT (capacitance voltage transformer)?

Purposes of CVT are
a. To indicate if line is charged or not.
b. To synchronize grid with generator.
c. For power line communication and carrier tripping
.
5. What are the protections provided for 220 kV lines and bus bars?

a. Bus bar differential protection.
b. Distance protection.
c. Over current protection.
d. Earth fault protection.

6. What are the main sources of power supply to 6.6 kV buses?

a. Unit transformer which steps down the generated voltage to 6.6 kV from the
generator.
b. Start up transformer, which steps down the grid voltage to 6.6 kV.

7. List some important loads to 6.6 kV buses.

a. Auxiliary transformers.
b. PHT motors.
c. BFP motors.
Question and answers Electrical Maintenance Unit
- 101 -
d. CEP motors.
e. CCW motors.
f. Chiller motors.
g. Pressuring pump motors.

8. What type breakers are provided in 6.6 kV buses?

ABB. Make, SF6 gas, 1250A and 2000A capacity circuit breakers are provided in
6.6 kV buses.

9. What is the difference in action of lock out pressure signal on 6.6 kV and 220 kV
breakers?

When lock out signal comes to 6.6 kV breakers the breaker will trip. Where as in
case of 220 kV breakers the breaker will not trip. If the breaker is open it will be
open only and can not be closed. If it is in closed condition it will be closed.

10. For how long 220 V DC batteries can supply power UPS?

220 V DC batteries can supply Power UPS for 30 minutes. Within this time class III
power supply should be restored by DG’s.

11. What do you understand by station black out?

When class IV and class III power supply fails and DG’s cannot be started and also
this condition prevails for 5 minutes then it is called station black out condition.

12. What are the sources of power supply to class I bus?

a. Through control UPS 240 V AC.
b. Through control UPS 220 V DC backed by 220 V batteries.
c. Through control UPS 48 V DC backed by 48 V batteries.

13. What are the lighting systems adopted in KGS?

There are two systems.
a. Normal lighting with class IV power supply.
b. Emergency lighting with class II power supplies and in control room with class I
power supplies.

Turbine Generator Basics Question & Answer

Turbine Generator Basics

1. Explain the principle of working of impulse steam turbine.

When steam enters the turbine it suffers a change in direction and momentum, which
gives rise to the rotation of the turbine. There will be no drop in pressure in impulse
steam turbine.

2. Mention the four processes involved in Renkine cycle.

Saturated liquid line.
4 1
Saturated vapour line.
3’
3 2
1 – 2 􀃆 Expansion process.
2 – 3 􀃆 Constant pressure heat rejection.
3 – 3’􀃆 Reverse adiabatic expansion.
3 – 4 and 4 – 1 􀃆 Constant pressure heating.

3. What are the methods of removing moisture from turbine?

a. External method by moisture separator and reheater, which separates the moisture
and reheates the steam.
b. Internally by stainless steel mesh, which reduces moisture (water particles) to
1%.
c. By main steam reheat.

4. Define capacity factor.

Capacity factor can be defined as net power produced by the plant divided by perfect
net power that can be produced in the plant.
Capacity factor = Net power produced / Perfect net power produced.

5. What is the purpose of turbine governing system?

Turbine governing system governs the speed of the turbine with the help of
centrifugal governer. It reduces the steam inlet when turbine over speeds.

6. What are the benefits of feed water heating?

a. It improves the plant efficiency.
b. Feed water is heated nearer to saturation point thus thermal shock to boiler is
avoided.

7. Mention parameters monitored by turbovisory instruments.

a. Speed governing.
b. Eccentricity monitoring.
c. Vibration monitoring.
d. Valve position monitoring.
e. Temperature monitoring.
f. Pressure monitoring.
g. Level monitoring.
h. Gas leak monitoring.
i. Conductivity monitoring.
j. Flow monitoring.

8. What are the purposes of turning gear?

Turning gear is used to run the turbine from rest to low speed and from normal
running speed to low speed with the help of barring motor to eliminate the hogging
and sagging of turbine because of the high temperature.

9. Explain the differences between the two types of feed water heaters.

a. Open type: In which bleed steam and condensed water are mixes directly and there
is also dearation of steam.
b. Closed type (shell type): It has tubes and shell. The water passes through the tubes
and steam passes through shell. The heat exchange takes placcce through the
metal tubes.

10. Why condenser back-pressure must be low? How it is achieved?

Condenser back- pressure must be low, because steam should be dumped into the
condenser so as to recycle it to boiler through the recycle process. It improves
efficiency of the turbine, as the heat rejection is less. It is achieved by the help of
ejectors and also passing cold water in the condenser through the tubes of the
condenser so that maximum vacuum can be obtained.

11. What are the materials used for TG rotor and blades?

TG rotor is made up of alloy steel and blades are made up of stainless steel.

12. Define the term heat rate?

Heat rate is defined as the heat supplied in to the plant in Btu by power generated or
output by the plant in kWh.
Heat rate = Heat supplied in Btu / Power output in kWh.

13. What is the purpose of gland steam system?

Gland steam system is provided to arrest the steam leak from the turbine and to
protect the air ingress into the turbine.

14. Explain the main difference between impulse and reaction turbine.

When the inlet pressure of steam to the turbine is equal to outlet pressure of steam
from the turbine the turbine is called the impulse turbine. In this type the heat is
added is very less.
In reaction turbine the outlet pressure of steam is less than the inlet pressure of the
steam. There is reduction in pressure with the increase in kinetic energy.

15. What is meant by hydrodynamic film lubrication?

In high-speed turbines the lubricating oil will be at the sides and there is metal to
metal contact when turbine at rest. When the turbine speeds up there is pressure
pushing the oil through the metal to metal contact. When turbine finally achieves its
speed the oil film breaks the barrier and it takes the load on itself. This is
hydrodynamic lubrication.

16. How does the hydrostatic lubrication differ from hydrodynamic type?

In hydrostatic lubrication which is used in slow speed turbines the lubricating oil is
pressurised externally where as in hydrodynamic system it forms oil film by its speed
which pushes the lubrication oil to form film.

17. What are the functions of dearator?

Dearator removes non-condensable gases (O2), which gets added in the steam and it,
mixes steam with the condensed water for feed water heating. This is a contact type
feed water heater.

18. Why non-return valves are provided in the steam extraction lines?

Non-return valves are provided because when the turbine trips there will be an
instant drop in pressure inside the turbine. But there will be steam in feed water
heaters, which is at high pressure. These will rush in to the turbine and overspeed
will be there in turbine. So non-return valves are provided in steam extraction lines
to prevent over speeding of turbine.

19. What is the function of the steam traps?

During startup the steam traps will bypass turbine drains.

20. What do the term sensible heat and latent heat mean?

Sensible heat: We can measure the rise in temperature. When we add more heat to a
substance. Example – heat that added to water from 0º C to 100º C. This added heat
is measured as sensible heat.
Latent heat: Though there is addition of heat there will be no rise in temperature.
This is latent heat. Example – when water boils at 100º C though we added more
heat the temperature remains at 100º C till all water becomes steam.

21. How are the generator rotor and stator cooled?

Passing highly DM water through the hollow conductor of the generator cools
generator stator and rotor is cooled by hydrogen.

22. What is function of seal oil system?

Seal oil prevents the leakage of hydrogen from the generator casing to the
atmosphere, where it can form explosive mixture. Seal oil is at higher pressure than
hydrogen.

23. What are the base load and peak load power stations?

During certain periods the load demands are very high. Example the morning when
all factories operate. During these time certain power plants like thermal plants gives
this extra power required. These are the Peak load stations, which operates at certain
periods.
But during the rest of period that is when there is no peak power demand there are
some power stations, which cater to the base load always runs giving power to the
grid. These stations are producing power at constant rate. These stations can not be
easily stopped or restated. Nuclear power station comes under Base load power
station category.

24. What are two types of governing system.

Throttle governing system: In this a valve (just like tap water controlling) which
reduces the steam pressure controls the steam flow. This has very less efficiency.
Nozzle governing: In this the steam floe is reduced but the pressure remains the
same. This is achieved by four valves in which when one is closed to 25% of steam
is reduced. This is efficient way of governing.

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Transformers Question & Answer

Transformers

1. What is the colour of silica gel in dry stage and when saturated with moisture in a
breather?

Dry stage – Deep blue.

Saturated stage – Whitish pink.

2. What is the static pressure of diaphragm in the transformer explosion vent?

5 psi.

3. In buchholz relay how the top and bottom mercury switches are connected?

Top mercury switch is connected for alarm and bottom switch for trip.

4. What is the normal value of moisture content allowed in transformer oil?

<10 ppm.

5. What is the vector group of distribution transformer?

Dy11.

6. What are the losses in a power transformer and mention how these losses can be
minimised?

There are two losses in a transformer.
a. Iron losses.
b. Copper losses.

Iron losses – Iron losses constitutes of two losses.
a. Eddy current losses, these are due to the induced emf in the core, which
constitutes a current in the core. These will heats up the core.
Eddy current losses can be minimised by using laminated core immersed in
varnish. This provides a high resistance between the laminations and thus eddy
current in reduced.
b. Hysterisis losses, these are due to the magnetic reversal of current by which there
is friction between molecules of core and heat is generated.
Hysterisis losses can be minimised by selecting proper magnetic material, like
silicon steel.

Copper losses – these losses are due to the resistance of the winding, which is equal
to I2rt (calories). These losses are depends on load. That is the losses are
increased to the square of the load current
I – current through winding.
r – resistance of winding.
t – time duration.

7. Define the percentage voltage regulation and efficiency of a power transformer.

Voltage regulation is the difference between no load voltage and full load voltage by
no load voltage.
Voltage regulation = No load voltage – Full load voltage / No load voltage.
Voltage regulation is mentioned in % (percentage).
% Voltage regulation = No load voltage – Full load voltage * 100 / No load voltage.
Efficiency of a transformer is the ratio of output in watts and input in watts.
Efficiency = Output in watts / Input in watts.
% Efficiency = Output in watts * 100 / Input in watts.
% Efficiency = Output in watts * 100 / Output in watts + losses.

8. Mention the important parts of a power transformer and their purposes.

Conservator:
This allows for shrinkage and swelling of transformer oil. When the oil
is heated up it swells and rises to the conservator. When cools down it goes back to
main tank. Conservator reduces sludge formation of oil because only the oil surface
in conservator is exposed to atmosphere where oil in the main tank is not exposed to
atmosphere.

Breather:
It provides dry sir to conservator when transformer breathes. That is when
there is shrinkage of oil atmospheric air enters conservator through breather. The
moisture is absorbed in breather by silica gel.

Buchholz relay (gas operated):

If there is an initial fault, heating up of core, high
resistance joints heating up by conduction through insulation and supports. There is
heating up of oil, which breaks down and gases are released. This gas actuates the
mechanics in the relay, There by closing contacts of mercury switches for alarm.
Also if there is a short circuit, the buchholz relay will trip the transformer. Also if
there is any leakage of oil through bushing etc and oil level comes down the relay
will give alarm and also will trip the transformer if transformer oil level comes down
the point. Gases can be taken from the relay to identify nature of fault.
Explosion vent: It provided on transformer main tank, provided with two Bakelite
diaphragm which break when the pressure exceeds 5 psi in the transformer tank and
relieve the pressure.

Core:
To provide low reluctance path for the magnetic lines of force. It carries both
the HV and LV windings.

HV Winding:
High voltage is given to HV winding and low voltage is taken from
the LV winding.

LV Winding:
Low voltage is given to LV winding and high voltage is taken from the
HV winding.

Cooling tubes:
These are provided to cool the transformer oil so that the heat of oil
will be given to the atmosphere.

HT bushing:
Carries the HV terminals.

LT bushing:
Carries the LV terminals.

Tap changer: this is provided so that we can get the required voltage out put. There
are two types of tap changer. Online tap changer and off line tap changer.

9. Mention the properties of transformer oil.

Properties of transformer oil are,
a) Colour – pale yellow.
b) Moisture content - <10 ppm.
c) Acidity (KOH/gramme of oil) – <0.1.
d) Dielectric strength – limit value is 45 kV and preferable value is 60 kV.
e) Flash point - 141º C.
f) Inter surface tension – 30 to 40 dynes / cm or 0.3 to 0.4 Newton.
g) Resistivity –

10. How explosion vent works?

Explosion vent is provided on the transformer tank to relieve pressure if the pressure
in the transformer exceeds 5 psi. It is swan neck shaped having two Bakelite
diaphragms. One at top and another at bottom. These break if the static pressure
increases to 5 psi. Wire meshes are provided below the bottom diaphragm and above
the top diaphragm. When there is any breakage due to excess pressure the bottom
wire mesh prevents broken pieces from entering transformer tank and the wire mesh
provided above the top diaphragm protects the diaphragm from any external damage.
There is an oil level indicator provided above the bottom diaphragm. It indicates the
level of oil in the vent if the bottom diaphragm ruptures.
A ruptured diaphragm must be immediately replaced. Also we should check the top
diaphragm for any external damage.

11. Explain the operation of silica gel breather.

Silica gel breather is used in a transformer to provide dry atmospheric air to the
conservator when transformer breathes. The breather consists of an inner container
and outer container. The inner container contains silica gel, which absorbs moisture.
An oil bath in provided at the bottom of breather so that the silica gel will not be in
direct contact with the atmosphere. Also it will trap dust and dirt entering the
breather. Dry silica gel will be deep blue in colour. After it gets saturated with
moisture it will turn into white pink. The change of colour silica gel can be viewed
externally through transparent viewer provided on the breather. When the silica gel is
saturated with moisture it must be replaced or regenerated or recharged. Silica gel is
recharged by heating it to a temperature of 250º F to 300º F till the deep blue colour
of silica gel is got back.

12. Explain with diagram the operation of on load tap changer.

Tap changer (ON LOAD type) can be used to increase or decrease transformer
output voltage without break in the voltage to the load.
The tap changer consists of a diverter switch. The odd taps are taken on one side and
even on another side as shown in figure.
The diverter switch is provided so that there will be no break in the supply to the
load and also no cut of transition resistance when the tap changing is achieved.
Transition resistors are provided to limit the current when the windings are shortcircuited
by the diverter switch.
Operation: In the above figure, the voltage at tap 2 is 406 V. the position of diverter
switch is shown. It short-circuited with transition resistance.
We want to increase voltage to 420 V at tap 5. When we begin to change the tap the
diverter switch connects 2 transition resistance and when the tap changeover is
achieved that is when the tap changer reaches tap 5, the diverter switch short circuits
transition resistance and thus the resistance is eliminated. The diverter switch
switching time is very high. This is to reduce arcing, which can decompose the oil.
The whole assembly is immersed in oil. OLTC is connected to HV side, because in
LV side current handled will be more. But in HT side current to be handled is lesser
than LV side.

13. Explain the procedure for finding out the vector group of a transformer

Procedure: Take the nameplate details. See from nameplate what group the
transformer belongs. Suppose nameplate says that transformer belongs to Dy11.
Take the IR value between
a) HV and LV with body grounded. That is between A2 – a2, B2 – b2, C2 – c2.
b) HV and body with LV grounded and
c) LV and body with HV grounded. A2 c2 a2
C2 B2 b2
Connect A2 to a2 and give low voltage (415 V) to HV side. Measure voltage between
a. C2 – b2 􀃆 410 V (example).
b. C2 – c2 􀃆 395 V (example).
c. B2 – b2 􀃆 395 V (example).
d. B2 – c2 􀃆 395 V (example).
Draw the diagram of Dy11 and check that the readings got are correct.
A2
a2
30º
b2
c2
According to the fig. C2 B2
C2 c2 = B2 b2 = B2 c2.
And C2 b2 will be greater than C2 c2 , B2 b2, B2 c2.
That is C2 b2 >> C c2.
If these conditions are satisfied then that transformer belongs to vector group Dy11.
A2
/ a2
For Dy1 transformer
B2 c2 will be greater than B2 b2, C2 c2, C2 b2.
That is B2 b2 >> B b2. c2
C2
b2 B2
For Yy0 transformer A2/a2
B2 c2 = C2 b2
B b2 = C c2
B b2 and C c2 will be lesser than B2 b2 and C2 b2.
C2/c2 B2/b2

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Conventional system Question & Answer

Conventional system

1. How turbine oil purification is achieved?

Oil purification is achieved by centrifugal operation.

2. Why morpholine is used?

Morpholine is used for pH control.

3. By which material condenser tubes are made?

Condenser tubes are made up of aluminium brass.

4. How boiler level is controlled?

Feed water control valves controls boiler level.

5. Where magnetic filter is used?

Magnetic filter is used in stator water lines to remove magnetic particles from the
DM water.

6. What is the purpose of accelerator governer?

The purpose of accelerator governer is to cut of steam momentarily when large
electrical loads are taken to prevent turbine speeding up.

7. What is the use of jacking oil pump?

Jacking oil pump is used to initial lifting of turbine rotor by hydrostatic lubrication.

8. What is the use of supplementary oil tank?

Supplementary oil tank is used to collect the oil drains from the CIES valves.

9. How dearator pressure is maintained?

Dearator pressure is normally maintained by extraction steam.

10. How dearator pressure is maintained after turbine trip?

Pegging steam is used to maintain dearator pressure after turbine trip.

11. Where trust bearing is provided?

Trust bearing is provided between HP turbine and LP turbine rotor.

12. How dissolved oxygen control is achieved in feed water system?

Hydrazine is added to feed water system to control dissolved oxygen.

13. How seal oil pressure is maintained?

Differential pressure regulator maintains the seal oil pressure at 0.7 kg / cm2 higher
than H2 pressure.

14. What is used to purge hydrogen from the generator casing?

During generator purging CO2 is used to purge out H2 from the casing.

15. Why class B trip is provided?

Class B trip is provided to prevent damage and over speeding of the turbine.

16. How lub oil supply is maintained?

Lubrication oil is supply is maintained by outlet oil from turbine oil pumps during
normal operation and jacking oil pump during startup.

17. What is the function of speeder gear?

Speed raising beyond governor takeover speed upto full speed is achieved by raising
and lowering the speeder gear.

18. How gland-sealing steam is supplied?

Gland sealing speed is supplied from main steam line.

19. Why exhaust sprays are provided?

Over heating of last stage LP blades is avoided by exhaust sprays by CEP.

20. What is the use of vacuum breaker?

In case of loss of seal oil to generator seals vacuum breaker is used to bring TG to
rest very quickly.

21. How relay oil is supplied?

Relay oil is supplied from the main oil pump for the operation of governing system.

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Motorised valve actuator Question & answer

Motorised valve actuator
1. What are the advantages of motorised valve actuators?

Advantages
a. Can be used to operate in remote areas, high heat areas etc.
b. Suitable logics can be wired up easily.
c. Hammer blow mechanism (which will release valve in stuck open or close). The
motor has high torque.

2. What are the four basic parts of any valve actuators?

a. Valve motor.
b. Gear mechanism.
c. Limit switch and torque switch assembly.
d. Terminal box.

3. Explain how the motor motion is transmitted to the valve stem?

The motor shaft is connected to a spar gear. It engages on a worm wheel. The worm
wheel has dog teeth. This dog teeth engages or hits the dig teeth on moving or sliding
clutch. The sliding clutch has splines and these are on the splines of valve stem. So
when the sliding clutch rotates the valve also rotates simultaneously.

4. Do you require separate limit switch for closing and separate limit switch for
opening? Ans. YES.

5. What does the limit switch mechanism senses to operate?

Limit switch mechanism senses whether the open and close travel of the motor has
exceeded the limit setting of the motor to operate.

6. What does the torque switch mechanism senses to operate?

Torque switch senses whether the torque of motor has exceeded the set point
irrespective of position of valve.

7. Explain the operational aspects of limit switch and torque switch in rotork valve
actuator.

a. When limit function is selected?
b. When torque function is selected?
Limit function – when limit function is selected in rotork valve, the limit switches
will operate when the limit set points are reached. Suppose limit switches fails to
operate the torque switches will act and cuts off the supply to the motor (both torque
switch and limit switches can act when selected to limit function).
Torque function – when torque function is selected the torque switches will act when
set point is reached. The limit switches will not act (only torque switch will act when
selected to torque function).

8. Indicate how you will select the limit switch contacts for indication and for cutting
the supply to the motor?

For cutting off the supply to the motor normally closed (NC) contacts of limit switch
(LS) should be wired in series with interposing relay coil of respective direction of
the valve so that when valve reaches their respective direction contact will open and
cut the control supply. For indication normally open contacts (NO) of opposite
direction of valve should be used so that when valve fully closes open indication
contact should remain NC only and vice-versa.
So for close direction
NC contact of close direction

9. What indication will you get in control room when

a) valve is open – green
b) valve is closed – amber
c) valve is intermediate – both
d) valve is closing and torque switch operate – both

10. What is the function of hammer blow mechanism?

Hammer blow mechanism allows motor to rotate freely for ½ or ¼ turn and the dog
teeth on worm gear comes against the dog teeth on sliding clutch with a blow. This is
use full when valve is stuck in fully open or closed condition.

11. How will you proceed to operate the valve manually after an electrical operation?

After electrical operation to operate manually we must tilt the lever provided on the
actuator to hand (manual) position by which the sliding clutch gets engaged with
hand drive.

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Insulation classification and testing Question & Answer

Insulation classification and testing

1. What is good dielectric break down value for insulating oil?

60 kV.

2. What is the temperature coefficient of insulating materials?

Insulators are negative temperature coefficient materials.

3. What is gap between the electrodes in transformer oil testing kit?

0.1 Inch.

4. What is the life insulation if temperature increased by 10°C?

The life of the machine insulation decreases by half if the temperature of the
insulation increases by 10°C.

5. What is the value of vacuum maintained by vacuum pump in oil filteration machine?
27 Hg.

6. What is the DC HV test voltage range?

1.7* 1.5* rated voltage.

7. What do you mean by term insulating resistance? How it is measured?

Insulating resistance: insulating resistance is the opposition offered by an insulating
material to the flow of current (electrons) through it when an high potential is
applied across it.
Insulating resistance are measured by megger.
First the equipment whose resistance is to be measured is disconnected from supply.
If the machine is a large one, there may be accumulated static charge on the machine.
So we have to discharge it by connecting a wire between the terminals and ground
for 15 minutes. Otherwise megger will give wrong reading.
After this we should remove the wire and we have to connect megger terminals (live
& earth) to the motor terminal and earth. The rating of the megger should be selected
properly. Then rotate the megger at rated speed of 160 rpm and take the readings.

8. What you mean by dielectric absorption test?

Whenever we apply a potential from the megger to test the IR value, initially the
needle of the megger will go to low value of the resistance. This is due the
capacitance effect of the insulation material and after some seconds the needle will
start moving towards the higher value. Because in the insulating material there is
strain on the molecules when the potential is applied. Polarization of the molecules
occurs and they form a Di – pole. The negative charges are attracted to positive
terminal and positive charges are attracted to negative terminal. So there is a strain
on the insulation molecules and they align themselves parallel. This aligning may
take more time. This test is done to know the condition of insulating material.
I
(μ Amps)
(A)
(B)
Time
If the insulation is good the graph is as shown as B and if there is dirt, moisture the
graph will flatten early as shown in A.
After the test terminals to be discharged so that molecules may return to their
unstressed state.

9. Draw the transformer drying out curve and explain each stage.

At this point the heaters are
IR and Switched off.
Temp. Temp.
IR
1st 2nd 3rd
Time in hours
When we start the filtering process initially the temperature will be low, as the
insulation value is high. But on temperature increases the IR value starts to decrease
because the moisture entrapped in the coils are released due to rise in temperature
and this causes the IR value to go down. This is the first stage.
Then comes the point where all the moisture is released and then will be no decrease
in IR value or rise in the temperature. This is the second stage.
At this point the heaters are switched off. Now the moisture is removed by the oil
filters and the IR value goes up and as the heaters are off the temperature decreases.
This is the third stage.

10. The insulation resistance of a DC motor is observed to be 15 MΩ at a temp. of 70°C.
what is its value corrected to 40°C. the correction factor for 70°C is 8.0.
Observed resistance at 70°C – 15 MΩ.
Temperature correction factor – 8.
Rm = kt * Rt kt – correction factor.
Rm = 8 * 15 Rt – resistance measured at +°C.
Rm = 120 MΩ. Rm – corrected value to 40°C
The IR of DC motor corrected to 40°C is 120 MΩ.

11. The armature of a 600 kW, 0.24 k, 1000 rpm DC generator has an indicated IR to
ground of 2 MΩ at a temp. of 30°C. what is the recommended value of insulation? Is
it advisable to put the machine in service? Give reason. Correction factor for 30°C is
0.5.
Data given are
kV – 0.24
Indicated IR – 2MΩ
Temp. - 30°C
Correction factor – 0.5
Recommended value (Rm) = kV + 1 MΩ
= 0.24 + 1
= 1.24 MΩ
Indicated IR at 30°C = 2MΩ
Correction factor – 0.5
So value corrected to 40°C = Rm = kt * Rt
= 0.5 * 2
= 1MΩ
The generator cannot be put in service because the corrected value is lesser than
recommended value. It should be sent for IR re-conditioning .
Q

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Motor, Generator and Exciter Question & Answer

Motor, Generator and Exciter

1. What are the main classifications of alternator?

a. Salient pole.
b. Non – salient pole.

2. What is the emf equation of alternator?

Flux cut per second by each conductor = 2 φm f
Average emf generated in each conductor = 2 φm f Z
Average emf generated per phase = kd kc 2 φm f Z
r.m.s emf generated per phase = kf kd kc 2 φm f Z
For sinusoidal waveform when kf is 1.11 then emf generated
= 1.11 *2 kd kc φm f Z
= 2.22 kd kc φm f Z
kf 􀃆form factor
f 􀃆 Frequency
φm 􀃆 Flux maximum
Z 􀃆 Turns per phase
kd 􀃆 Breadth factor or distribution factor
kc 􀃆 Coil span

3. Why conductors in alternator are transposed?

To reduce eddy current losses.

4. What is the effect of frequency and high voltage at the start of motor.

For a constant load if frequency decreases motor current will increase and at the start
if voltage is more motor current also increases.

5. What is the minimum voltage required for starting of 6.6 kV motors?

Minimum 80% of rated voltage.

6. What are the limits of vibration measurement for motors?

50 microns for displacement and 5 mm/second for velocity.

7. What you mean by SPDP?

Screen protected drip proof.

8. What is the current in single phasing?

2 times of rated current.

9. What is the impedance per phase of delta connected motor?

1.5 times the total impedance.

-
10. What is the slip of an induction motor during normal running?
More than zero.

11. What is the effect of increased load on power factor of induction motor?

Power factor of an induction motor increases with loading.

12. Explain the behavior of generator when operating alone and operating parallel with
grid.

Generator operating alone.
a) The power factor of generator depends on load power factor.
b) The terminal voltage decreases when generator is loaded.
c) Governor decides the frequency of generator.
d) Increase in excitation increases the terminal voltage of the generator.

Generator parallel with grid.

a) If we increase the steam input to the generator the frequency of the generator will
not change. It will remain practically constant as same as grid frequency. That is
grid decides the frequency of the generator.
b) Increase in the excitation will not increase the terminal voltage. Instead the
reactive power out put of the generator increases. This reactive power supplies for
the magnetizing current of motors, transformers and etc.
c) Increase in the steam input increases the active power of the generator.
d) If generator is under excited it will draw leading reactive current from the grid.

13. Draw and explain following.

a) Load current Vs terminal voltage at different power factors.

1. At leading power factor as the load current increases the terminal voltage also
increases.
2. At lagging power factor as the load current increases the terminal voltage
drops.
3. At unity power factor as the load current increases there is slight drop in
terminal voltage.
Leading power factor
Unity power factor
Lagging power factor
Ter. Vol.
Load current

b) Torque Vs slip characteristics of induction motor.
Torque (T) = φ I2 cosφ2
Where φ - main flux.
I2 – rotor current Pull out torque (R = XL) I
Cosφ2 – rotor power factor.
Starting current
Starting torque (1.5 times)
Torque Full load current
1 Slip 0
When motor is started from rest the slip is 1 (one) at time of starting and starting
torque is 1.5 times of rated torque. As the motor accelerates slip reduces and torque
increases. Because the power factor of rotor is improving due to the decrease in rotor
frequency and at certain slip the rotor reactance is equal to rotor resistance. At that
time torque is maximum (pull out torque). When the motor accelerates to the rated
speed the torque comes to the rated value, which is less than the starting torque.
Torque is zero when slip is zero, because there will be no relative motion between
stator magnetic field and rotor.
c) Generator capability curve
This curve gives the operating limits of the turbine generator at different power
factor what should the power output of the generator to avoid the heating of
generator stator winding, rotor parts and end parts.

14. What is the cooling medium for different parts of the turbo generator?

Stator – DM water.
Rotor – Hydrogen.
Bearing – Oil.

15. What are the effects of unbalanced voltage on induction motor?

There will be negative sequence current, which will heat up the stator winding and
weakens the insulation. These currents will induce emf in rotor and heat up the rotor
bars and cause breakage in them. Also due to high leakage fluxes due to negative
phase sequence current the end parts heating will be more.

16. What are the types of bearings are adopted for small motors and large motors?

Small motors (LT motors)
Horizontal mounted – deep groove ball bearing at both ends.
Medium motors
Roller bearing at DE and deep groove bearing at NDE.
Large motors (HT motors above 750 kW)
Horizontal mounted – sleeve bearing (pedestal) cooled by lub oil.
Vertical mounted – face to face angular contact ball bearing at NDE and roller or ball
bearing at DE.

Sunday, March 29, 2020

BASIC ELECTRICAL ENGINEERING QUESTIONS & ANSWERS PART-4 201-225 QUESTIONS

201. Why single-phase motors are not self-starting?

When a 1φ supply is given to the single winding of the
single phase motor, the field produced by it changes in
magnitude and direction sinusoidally (pulsating flux).

Such and alternating field is equivalent to two fields
of equal magnitude and speed rotating in opposite
direction. Such rotating magnetic fields produces two
torque’s on the rotor. So the rotor can not rotate in
any direction. Because the net torque developed by the
motor is equal to zero. So a single-phase motor is not
self-starting.

202. What are the methods to self-starting of single-phase
motor?

a. Splitting one phase into two phases.
b. By using capacitor.
c. By using repulsion method.
d. By shading the poles.
e. By connecting the field in series with the rotor
having winding with commutator (AC series motor or
universal motor).

203. What are the methods to control the speed of singlephase
motor?

In AC single-phase motors speed control can not be
achieve as smooth as in DC motor. There are following
few methods of speed control.
a. By changing the number of poles of stator.
b. By changing the applied voltage to the stator.
c. Frequency control method.
d. Rotor rheostat control.
e. By operating two motors in concatenation or cascade
or tandem method.
f. By injecting an emf in the rotor circuit.
g. By changing slip.

204. What are the classifications of electrical measuring
instruments?

a. Absolute instruments. These instruments give the
value of the quantity to be measure in terms of the
constant of the instrument and their deflection only.
There is no any calibrated scale.
b. Secondary instruments. Secondary instruments are
those, which are calibrated in comparison with some
absolute instrument so as to indicate the electrical
quantity to be measured with the deflection of needle
or pointer of that meter over a calibrated scale.

205. What are the operating principles of electrical
measuring instruments?

a. Magnetic effect.
b. Electro dynamic effect.
c. Electro magnetic effect.
d. Thermal effect.
e. Chemical effect.
f. Electro static effect.
206. What are the classifications of secondary instruments?
a. Indicating instruments.
b. Recording instruments.
c. Integrating instruments.

207. What are the essentials of indicating instrument?

a. Deflecting torque or force (effect of electricity).
b. Controlling torque or force (spring control and
gravity control).
c. Damping torque or force (air friction, eddy current
and fluid friction).

208. What are the possible errors in induction (energy
meter) measuring instruments?

a. Phase error: Field flux in induction meter does not
lag 90° behind the supply voltage due to its
resistance. This can be adjusted by copper shading
rings, which are placed at the central limb of the
shunt magnet.

b. Speed error: An error in speed, which is tested on
the non-inductive load, can be eliminated by
correctly adjusting the position of the brake magnet.

c. Friction error: It can be reduced very much by
providing two copper shading st the both outer limbs.

d. Creeping error: Some time slow, continuous rotation
of the disc (rotor) when only the pressure coil is
excited, but no current flowing in the circuit (no
current in current coil) may happen. It may be caused
due to incorrect friction compensator, stray magnetic
field, and excess voltage. This can be rectified by
drilling two holes in the disc on the opposite sides
of the spindle. This causes sufficient distortion of
the field to prevent rotation, when one of the holes
comes under one of the pole of the shunt magnet.

209. What is illumination?

The quantity of a light emitted by a lighting source is
known as illumination. Heating effect of electric
current is used in producing illumination. When a solid
or vapour is heated it begins to radiate energy in the
surrounding media.
Lux is the unit for illumination. Lux is the
illumination produced by a uniform source of candle
power on the inner surface of a sphere of radius one
(1) meter.

210. What are the laws of illumination?

a. Illumination ‘E’ is directly proportional to the
luminous intensity ‘I’ of the source. ie E ∝ I.

b. Inverse square law: The illumination of the surface
is inversely proportional to the square of the
distance of the surface from the source. ie E ∝ 1/d2.

c. Illumination ‘E’ is directly proportional to the
cosine of angle made by the normal to the
illumination surface and the direction of the
incident light and is known as lam pod’s cosine
angle.

211. What are the factors to be considered for correct
illumination?

a. Nature of work.
b. Determine the foot-candle illumination required after
studying the nature of work. Example for precision
work – 100 foot candle, for fine engraving – 50 foot
candle, for reading, typing, drawing, fine machine
works 25 foot candle etc.
c. Design of apartment using for the proper projection
of illumination for better work or purpose.

212. What are the types of lighting?

a. Direct lighting: light directly comes from the source
to the surface.
b. Indirect lighting: light reflects from the wall,
reflector or ceilings etc.
c. Semi direct lighting: light comes through the shade.

213. What are the properties of good illumination?

a. It should have sufficient light.
b. It should not strike the eyes.
c. It should not produce glares.
d. Light should be uniform.
e. It should be of harmonious.
f. It should be of correct type as needed.
g. It should have suitable shade and reflector.
h. Economically productive.

214. What are the sources of light?

a. Incandescent lamps.
b. Carbon arc lamps.
c. Gas discharge lamps.

215. What are the materials used in Neon sign tube lamps
for different colors?

Following are the materials used in neon sign tubes for
different colors.
For, Red – Neon gas.
Reddish orange – Neon gas + Argon gas.
Blue – Vapour of mercury.
Golden – Neon gas + Helium gas.
Green – mixture of Neon gas and mercury in yellow
glass tube.
By depositing fluorescent powder on the inner surface
of the tube varying colors in intensity can be made.

216. What is the material used in florescent tube?

The fluorescent tube is filled with argon gas at law
pressure and some mercury after evacuating the tube.
This argon gas gives initial starting at quick manner.
Initially the mercury is in the form of globules on the
inside of the tube surface. As the temperature
increases the liquid takes globules mercury changes
into vapour form and takes over the conduction of the
current.

217. What are the importances of conversion of AC into DC?

a. For traction purpose a DC series motor is most
important. Examples in railways, in electrical lifts
etc.
b. For electrolytic and electro chemical processes such
as electro plating, electrolysis, electro refining
only DC is essential.
c. DC is essential for battery charging, running arc
lamp torch, cinema projector and for arc welding.
d. It is required for operating relays, timer,
telephone, circuit breakers etc.

218. What is rectifier? Write few types of rectifiers.

Rectifier is a device which converts AC supply into DC.
Following are the types of rectifiers generally used.
a. Copper oxide rectifier.
b. Selenium rectifier.
c. Electrolytic rectifier.
d. Mercury arc rectifier.
e. Tungar rectifier.

219. What are the parts of paper insulated lead covered
cable?

a. Core.
b. Insulation of cable.
c. Metallic sheath.
d. Bedding.
e. Armouring.
f. Serving.

220. What are the factors considered for selecting a cable?

Following factors considered for the selection of the
cable.
a. System voltage.
b. Condition of installation.
c. Continuous current to be carried.
d. Maximum operating conductor temperature (70°).
e. Ambient air temperature (40°).
f. Thermal resistivity of soil.
g. Depth of laying.
h. Short circuit current. Ish = Ka / (t/2). Where ‘K’ is
constant (K = 109 for copper cables), ‘a’ is area and
‘t’ is time duration of short circuit in seconds.

221. What are the advantages of high voltage transmission?

a. Saving in conductor materials.
b. Low power loss (I2R) of transmission lines due to
decrease in current.
c. Better efficiency of line due to fewer losses.
d. Better voltage regulation due to less voltage drop in
line due to less transmission current.
e. Due to the less cross section of conductor distance
between the poles increases and the cost decreases
and the labour cost also decreases.

222. What are the types of distribution system?
a. Radial distribution system.
b. Ring distribution system.
c. Grid distribution system.

223. What are types of distribution of supply?

a. Over head distribution system.
b. Under ground distribution system.
Question and answers Electrical Maintenance Unit

224. What are the main items used in over head distribution
system?

a. Conductor material.
b. Pole.
c. Cross arm.
d. Insulators.
e. Strain insulator.
f. Post insulator.
g. Stay wire.
h. Support with insulator and stay lightner.

225. Write types of lightning arrestor.

a. Horn gap lightning arrestor.
b. Oxide film lightning arrestor.
c. Pellet lightning arrestor.
d. Thyrite lightning arrestor.

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#Static_gk
The Highest Peaks

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