Question { 5105 }

what is the difference between synchronous motor and
induction motor?

Answer

Synchronous motor runs at synchronous speed only while
induction motor runs at speed lower than the synchronous
speed. 3 phase Synchronous motor needs 3 phase AC voltage at
its stator terminals i.e the armature wdg and its poles form
Ns and Ss, the armature flux rotated in one direction. and
DC supply at its field winding.this poles are not
synchronized with the stator flux thts y its not self
starting hence its requires some starting methods. So that
rotor field gets locked with stator flux and rotor rotates
at the synchronous speed i.e. at the speed of the rotating
magnetic field.

while 3 phase induction motor only needs 3 phase rated AC
voltage to operate and it will run the less than synchronous
speed and its speed is almost constant speed so that it is
called a asynchronous motor. In synchronous motor,
its working operation its almost like a transformer

Question { Coast Guard, 3367 }

how much air gap in induction motor as per the Indian
standards?

Answer

According the IS the air gap between the stator and rotor
air gap of induction motor is 0.5mm Maximum

Question { 5419 }

why the field winding of synchronous machine is excited by dc source

Answer

The field wdg is placed on rotor of an synchronous machine
is exicited with dc source.when the exicitation is given to
rotor of the field wdg in its current will flow. so this
current setup an flux which expernice the torque will be
setup on rotor.acc to the farady law of electro magnetic
induction. at the same time revoluoing flux will be setup
on the aramature wdg.so that the stator aramature flux of
the mmf and stationary fileld flux will not be zero.after
certain speed the magntic locking tendcy will come to
picturei.e stator and rotor poles will be magentic inter
locking so that it will rotate with synchronous speed.

If rotor field wdg is is exicited with Ac source, in wdg
alternating flux will be setup on rotor.that the stator
aramature flux of the mmf and stationary fileld flux will
zero so that the machine does not provide any output

Question { 7800 }

WHY WE NEED NEUTRAL FOR 3PH 400 OR 415 VOLTS POWER SUPPLY (4
WIRE SOURCE).

Answer

As to the domestic loads i.e lightning loads required the
neutral bcz of This High voltage supply its being connected
with the distribution substations here this voltage is being
step down low voltage by using transformer. Normally the
power transformer primary is formed in delta connection to
avoid the leakage currents and its secondary is formed in
star connection. as required above with the same.
Neutral & Earthing are tied together. neutral its carrying
charging current also acts as return path

Question { 7467 }

which of the folowing has negative temperature coefficient
of resistance?

a. mercury
b. copper
c. lead
d. carbon

Answer

The mercury has negative temperature coefficient
of resistance