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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