Question { BSL, 35850 }

Which motor has high Starting Torque and Staring current
DC motor, Induction motor or Synchronous motor?

Answer

DC Series motor has high starting torque. We can not start
the Induction motor and Synchronous motors on load, but can
not start the DC series motor without load....

Question { Essar, 16549 }

why semiconductor fuse used in drive ,not ordinary?

Answer

Because semiconductors has negative resistance temperature
co-efficient. When current flows in it, its temperature
increases due to I^2*R losses but resistance decrease
accordingly. The decrease in resistance allows more current
to flow in it which produces more heat. It will melt
ultimately.

Question { 4518 }

Inductor and capacitor ........ power in one phase and
...... it in another phase. if any body found this gaps
answer.please sent me to habib_eu2008@yahoo.com

Answer

Inductor and Capacitor STORE Power in one phase and SUPPLY
or DELIVER it in another phase

Question { 12592 }

how to set time from star to delta starter

Answer

The above answer is correct but at some extent. The
connection is switched from Star to Delta when motor
attained it 70~80 percent speed of its full speed. The time
of attaining this speed is counted and the timer is adjusted
accordingly

Question { Heidelberge Cement, 8218 }

what is the avr

Answer

AVR means Automatic Voltage Regulator. It is usually used in
the excitation of a generator. AVR regulate the excitation
voltage of the generator according to the demand.

Question { AES, 9397 }

Why is copper a better conductor of electricity than
aluminium?

Answer

Copper has less specific resistance than that of Aluminum.......

Question { 8779 }

what is CT saturate?and which effect on CT after its saturate?

Answer

It is possible that relatively low-ratio CTs are applied for
protective relaying of small loads
fed from switchgear and motor controllers of relatively high
short-circuit capacity. Assume
the worst-case scenario of 64kA available fault current from
bus feeding a small motor load
of normal current below 50A. In theory, CTs rated as lows as
50:5 and relay class C10 may
be applied for protection purposes.
Realizing that 64kA of fault current is 1080 times the rated
current of the 50:5 CT, the
magnitude of the problem is evident. Protection class CTs
are designed to work in the linear
range, with minimal errors and minimal waveform distortion,
only up to 20 times the rated
nominal current with the burden as defined by the relay
class (saturation voltage) of the CT
per IEEE Std. C57.13.
Well-established and relatively accurate equations are
available for calculation of the
actual maximum primary current for saturation-free operation
under any specific burden,
any specific X/R ratio, and any specific residual flux in
the CTs. This engineering practice is of
little help here: A CT fed with a primary current hundreds
of times its rated current will
saturate severely - only relatively short duration peaks of
limited current will be observed
from the secondary of the CT. These peaks can be as low as
5-10% of the ratio current, and
will last a small fraction of the half-cycle, down to 1-2ms
in extreme cases. As a result only a
very small portion of the actual ratio current is presented
to protective relays fed from such
severely saturated CTs. In terms of the true RMS value, the
secondary current may be as low
as 1-2% of the expected RMS secondary current.
On the surface it may seem that a severe problem takes place
here – the fault current is
so high that it virtually stops the CT from passing the
signal to the relay. The relay does not
see enough proportional secondary current during severe
faults in order to operate its short
circuit protection. The upstream relay, using CTs of a much
higher ratio, measures the fault
current more accurately and trips. Zone selectivity is lost
because the poor low-ratio CT was
“blinded” to the fault.

Question { Telco, 29469 }

what is demagnetising effect & crossmagnetising effect and how
it is related to electrical system?is there any more effects
are available or only these two??

Answer

With the brushes in the G.N.A. position, there is only
cross-magnetizing effect of armature reaction. However, when
the brushes are shifted from the G.N.A. position, the
armature reaction will have both demagnetizing and cross
magnetizing effects.To neutralized the de-magnetizing
effects we add the more ampere-turns in the main windings
and foe cross magnetising effects we use compensating windings.
The second part is how it is related to electrical system?
These effects disturb our main magnetic field and ultimately
the fundamental behaviour of our DC machine. If the DC
machines are the part of electrical system then the positive
or negative effects influenced on these machine will also be
the part of electrical system, less or more.
At the extent of armature reaction only these two are else
nothing as per my knowledge.
For more details please see the following link.
http://www.circuitmaniac.com/2009/05/10/demagnetizing-and-cross-magnetizing-conductors/

Question { 3411 }

why did lightening arrester rod probe sharp?

Answer

Just to enhance the Mechanical strength

Question { 4284 }

What is the formula that shows relationship between voltage and speed in an induction motor

Answer

V = N * Phi max * f * 4.44
As the induction motor is also an induction transformer

Question { 2997 }

what are the things that can cause a three phase motor to
crawl?

Answer

The phenomena of Crawling in the motor occurs due to the
harmonics. So we can say that all those factors that play
major or minor role to produce the harmonics are also the
producing agencies of the phenomena of Crawling.

Question { NTPC, 4715 }

What will happen if induction motor is supplied with
capacitive load?

Answer

It is quite clear that Poor load current phase angle is
generally the result of an inductive load such as an
induction motor, power transformer, lighting ballasts,
welder etc.
Specifically discussing the case of induction motor we often
use the bank of capacitor to improve the power factor. If
the motor is already being supplied by capacitive load then
it may be fruitful for the motor as well as for the whole
system but up-to some extent. The power factor on capacitive
side has also its own draw backs. The major is to increase
the thermal losses.

Question { 3424 }

why slipring motor , at full speed run as squirrel cage motor?

Answer

The slip ring or wound rotor motor is an induction machine
where the rotor comprises a set of coils that are terminated
in slip rings to which external impedances can be connected.
The stator is the same as is used with a standard squirrel
cage motor.
The slip ring motor is used primarily to start a high
inertia load or a load that requires a very high starting
torque across the full speed range. By correctly selecting
the resistors used in the secondary resistance or slip ring
starter, the motor is able to produce maximum torque at a
relatively low current from zero speed to full speed. A
secondary use of the slip ring motor, is to provide a means
of speed control. Because the torque curve of the motor is
effectivley modified by the resistance connected to the
rotor circuit, the speed of the motor can be altered.
Increasing the value of resistance on the rotor circuit will
move the speed of maximum torque down. If the resistance
connected to the rotor is increased beyond the point where
the maximum torque occurs at zero speed, the torque will be
further reduced.
NOTE:
The all above said characteristic of slip ring motor is due
to slip ring as well as due to external resistance. All
characteristic are related to is starting period or for
speed control. when we disconnected the resistance from the
circuit then slip ring motor and induction motor will be
same. So, hope the reason, "why slipring motor , at full
speed run as squirrel cage motor" will be cleared.