Question { 2896 }

Why is ignitor used in sodium vapour lamp

Answer

The ignitor is used to provide the high voltage required to
break down the gas (a mixture of neon and argon). This will
ionise the gas and allow it to conduct current.

Question { 2688 }

If 110 volts ac is mixed up ith 12 volts dc what will be the
resultant voltage and what is the wave form?

Answer

The wave form will still be the same.
It will be offset from the zero line by 12 volts. The
positive half cycle would then be to 122 volts and the
negative half cycle would be 98 volts at peak.

Question { 3718 }

explain why reduced voltage starters are required for large
tree phase inudction motor

Answer

The main reason for using reduced voltage starters is to
limit the starting current drawn by the motor. This current
can be in excess of 7 times the full load current.

If the current is not limited during starting excessively
large cables must be used and larger transformers etc. to
provide the extra current.

Question { 3393 }

Why primary batteries not rechargeable and secondary batteries
are rechargeble???
If any one know please send me.

Answer

Both primary and secondary batteries provide voltage
through a chemical reaction. When the chemicals are used up
the battery is flat.

The difference is that the chemical reaction in the
secondary batteries is reversable but it is not in primary
batteries.

Applying a voltage slightly above the normal fully charged
voltage of the secondary batteries will reverse the
chemical reaction.

Question { 21564 }

why in a transformer frequency remains constant? if frequency
is changing what will happen?

Answer

As the secondary voltage is induced by the primary current
the frequency will follow each other. Normally the
frequency of the supply is constant so the output frequency
will remain the same.

If the supply frequency changes the output frequency will
follow this change and change also.

Question { Bhel, 7009 }

why do we prefer alternating current in india where as in
USA prefers DC?

Answer

Most of the world including India and the USA use AC power.
the main reason for this is the ease with which the
voltages can be changed using transformers. The 2
differences are that USA uses power of 120 volts and 60 Hz
but it is still AC.

Question { AFMC, 2952 }

what is a,b,c,d checks diesel generator?

Answer

Most generator manufacturers have a requirement of the sets
being regularly serviced. The are also different codes
(e.g. fire codes) which require the regular servicing. The
type of service is usually designated as A to D.
An A service is usually a check of the generator to make
sure all systems are functioning correctly.
This steps up through oil and filter change to replacement
of consumable parts at D.
Consumable parts are the parts such and belts, hoses,
coolant pump, injectors, etc.
Each of these services are set by a time period or hours of
operation similar to the servicing of a car.

Question { 4196 }

What is the function of the auto reclosure relay?

Answer

The auto reclosure relay is often used in a distribution
system. If a breaker opens on a fault the fault may be
cleared by the fault current of the fault may clear it
self. An example of this is where something hits the power
lines during a storm. It may cause a short circuit open the
breaker. The could be a tree limb shorting across the lines
or pushing the lines together and then falling clear.
In circumstances like this the breaker would normally stay
open until someone access it and reclosed it.
What the reclose relay does is that after a set time delay
it will send a close signal to the breaker. This will then
restore the supply. If all is well the electricity company
can check for the problem later.
If there is still a fault the breaker will open and the
reclose relay will close one more time. If the fault still
exists the reclose relay will lock out until reset.

Question { Transco, 5311 }

why insulators are colured in brown only?

Answer

Insulators are generally white inside. A glase is put on
them during firing. The colour of the glase depends on the
manufacturer. Many have used brown but many others have
used white or blue glase as well.
As far as the insulating properties of the insulator it
generally does not matter what the colour is as long as it
does not conduct.

Question { MR Cables, 4894 }

Why do we twist copper to make cable?

Answer

This is the part of the manufacturing process. When the
cable is passing through the dies to put insulation on it
the twisting holds the copper in the centre so the
insulation is even all around.

For uninsulated cables the strands would separate making
handling impossible.

Question { 3411 }

In 3 phase induction motor even with low Voltage it reaches
nearly it's rated speed why? (for example instead of 400V it
reaches with 250V)

Answer

With an induction motor the speed is dependant on the
frequency and is the same as the frequency minus slip. For
a 4 pole motor this is about 1440 to 1460 RPM.
It does not depend on the voltage. The torque is dependant
on the voltage.
A 400 volt motor supplied at 250 volts would not have
anywhere the power output.

Question { 2874 }

CAN WE GENERATE AC WITHOUT DC EXCITATION?

Answer

Yes
By replacing the field winding with permanent magnets. The
problem with doing this is that the output voltage is
controlled only by the speed of the alternator. This means
that at constant speed (frequency)the voltage will fall as
the load is increased. If the voltage must be constant the
speed must be increased with load. So the frequency will
not be constant.

Question { 4623 }

what is difference b/w bus duct & bus bar.

Answer

Busbar is a conductor of electrical power. It is usually
made of copper but sometimes also of aluminium. It is
usually rectangular in cross section. Busbars are used
inside switchboards where it is normally not insulated but
protected from access by panels. The busbars are mounted on
insulating blocks.

A bus duct is an enclosure which contains a number of
busbars (phases, neutral and earth). It is used for
connection between switchboards. Here the enclosure us used
to protect the busbars from being touched. the busbars are
also held in insulating blocks.

Question { ABB, 5153 }

what is the rating of Insulation tester for measure 15 KW, 3
phase 415V 50 HZ induction motor?

Answer

500 volts

For LV it is usually 2 times the phase to earth voltage

Question { 3134 }

what is FET

Answer

field-effect transistor