Question { 8008 }

A D.C. series motor should

(1) always be run at constant speed

(2) always be started on load

(3) not be used where starting torque is high

(4) None of these

Answer

(2) always be started on load
otherwise it will very high dangerous speed.

Question { NTPC, 6969 }

If current flows through conductor it produce magnetic
field only.but if current flows through vaccum both
magnetic & electrical field produce why ?

Answer

i think he is talking about spark in vaccum

Question { MPSEB, 10010 }

how will protect transformer agint overcurrent protection

Answer

by buchholz relay for large tranformer

Question { 51686 }

why transformer does not obey ohm law?

Answer

simple in transformer two winding are not connected electrically(they are linked magneticlly) so we
cannot apply apply ohm law between primary and
secondary. but you can apply it to individually
in primary and secondary side.

Question { NTPC, 22996 }

how the inductor will work on dc supply?whether it will
work or not?

Answer

when dc circuit will switch on and off inductor will play same like ac (i.e it oppose the change in current as during switching on or off as time high frequency transients will appear) After that it will act same as resistor (as inductor have no of turns hence its resistance is high)

Question { NTPC, 7416 }

what is the difference b/w neutral and earth?what is the
need of earthing and what are the different methods for
earthing?

Answer

neutral is reference from where phase voltage is
measured also it serve the purpose of return path
in three phase.it is possible in Y connection .in
delta it is virtual.while earth is always at O
potential.

it is used reduce the shock hazards in equipment.

earthing is done by connecting the equipment with earth using conductor.
there is no as such method.

there is 3 methods of neutral grounding(not earthing)
1.solidly ground
2.RNS
3.INS

Question { NTPC, 26824 }

1)Why electric shock is feel ?? n in a Electric Train why
we didnt feel that ?

2)What is the reason for transformer Noise ??

3)What is relation b/w Transmission Lines & Communication
Lines ?

4)Which material is used for making Transformer oil & How u
measure its dielectric strength & Name the test ?

Answer

1)when electrical current passes through our body
then it interacts with electrical impulse in our
body due to which our muscles contracts and expand (depending on type of current i.e ac or dc ,in dc
it contracts while ac in ac it contracts and expands
but very high speed depends on frequency),its called
shock.
in case of electrical train, it is grounded through wheel
so current follows that path(train body to wheel)
rather through our body. so didn't feel shock.
2)transformer noise is due magnetostriction (expansion
and contraction of core in high frequency as in ac
polarity changes rapidly ,i.e it has frequency).
3) transmission line carries high power, have no
insulation ,low frequency operation
communication line carries low power signal,have
insulation
,high frequency operation.
4) transformer oil is usually made from petroleum as it having high Di-electrical strength and high temperature rating and it is less reactive then normal oil.
sorry friend i don't know exactly name of test.
its probably dielectrical strength test.

Question { NTPC, 20821 }

Why no-load primary current is very small
as compared to full load current in transformer (as 1% of
full load current?

Answer

when secondary is open circuit the current in primary
is low i.e only magnetizing is present to create flux
in core.but when load is connected in secondary side it
produces its own ampere turn N2I2 which reduces the flux
in core . in order to maintain the flux constant, primary current increases to balance N2I2 i.e
N1I1=N2I2(neglecting magnetizing current)
therefore primary current increases when load
is connected.

Question { 6486 }

in single phase circuit if i connect a heater between phase
and neutral, then what will be the current path? phase to
neutral or neutral to phase.

Answer

As it is having ac nature therefore in both direction current will flow (as no diode is connected).

Question { NHPC, 21714 }

what is the basic difference between synchronous
motor,synchronous generator and induction motor

Answer

synchronous motor and generator are same i.e
constant speed type machine but only difference
is that motor consume power and generator produce
power .it is doubly excited.
where induction motor is asynchronous machine.
its singly excited.

Question { 5070 }

What will happen when a DC series motor suppllied by an AC
source ?

Answer

in ac both polarity changes which changes the flux and current
direction in field and armature simultaneously. as torque is product of flux and current(in armature)is always positive hence it will rotate in one direction only.for this series motor is also called universal motor.

Question { NTPC, 8294 }

what is the power factor?please tell me uses.

Answer

its cosine of angle between voltage and current of the given system.
its present in both ac and dc circuit but in case of dc its always 1 as angle between voltage and current in this case is always 0 degree.so generally we don't talk about pf in dc.
but in case of ac it varies from 0 to 1.

Question { NTPC, 8847 }

WHAT IS THE DIFFERENCE BETWEEN A DC MACHINE OUTPUT AND
RECTIFIER OUTPUT...

Answer

if rectifier is half-wave type then there is difference . in case of dc machine the output will same like full-wave rectifier.as we know that inside dc machine emf induced as sinusoidal but due to commutator the output becomes like full-wave rectifier type.

Question { NTPC, 14200 }

400 kvtransmission lines in india are connected in star or delta

Answer

delta
as for long transmission it is economical to transfer power with 3 wire rather with 4 wire.

Question { Eskom, 7662 }

what is a motor

Answer

A machine which converts electrical energy into mechanical rotational energy (not only mechanical energy as in case of buzzer,loud speaker electrical energy is converted into mechanical vibration but it is not a motor) is called motor.