Question { 15758 }

In a synchronous motor armature current flows due to
revolving flux which in turn rotates the load,but in no
load condition armature current is zero hence thr is no
revolving flux then how motor rotates?

Answer

This is funny , how on earth could u assume that the no
load condition of a motor means open circuit condition ,
now listen the 3 phase supply given to stator produces a
rotating magnetic flux which reacts wit the initially at
rest field conductors to set up an induced emf lagging the
flux by 90* , but the field mmf set up leads the stator mmf
and rotates in the same direction as the rotating field (
note that i m already considering that the motor has
starting mechanism i.e. damper winding and later on dc
excitation ) thus the two rotating mmfs are stationary wrt
to each other but rotating at synchronous speed wrt
stator , since the rotor mmf i.e field mmf has the tendency
to follow the resultant air gap mmf so electromagnetic
torque acts that sets the rotor rotating . it must be noted
that the field mmf is proportional to field current while
the armature mmf is prprtnl to armtr crrnt , hence at no
load the armature mmf is maxm, because the arm current is
maxm, while with increase in load the current decreases and
hence the arm mmf , however the reasultant air gap mmf
always remain constant till voltage supply and the
frequemncy are maintained constant, it is only that the
field mmfs and the armature mmfs change in orientation and
magnitude.

Question { 21279 }

Why does the power consumed is always constant even if a fan
speed is altered?

Answer

it must be clearly mentioned here that a fan always uses a
single phase capacitor run induction motor , where there
are two windings to provide the two phae supply to generate
the revolving magnetic field , the torque developed is
proportional to the prdct of starting current , main
current and the sine of the angle btwn them , nw whe we
increase the resistance betwn the main wndng and voltage
supply , the voltage remainds the same while the current
decreases , the electrical equivalent of the mech power
developed i.e eicos# decreases , thus power dusnt remain
constant

Question { 5013 }

what happens when parallel resonance occurs?

Answer

in parallel resonance the power factor of the circuit is
unity as the reactive current drawn by the circuit is 0,
but the impedance of the circuit is maximum in parallel
resonance as the line current is minimum at this case . the
overall impedance at parallel resonance is l/rc, where lis
the inductance and c the capacitance having the dimensions
of ohms .

Question { 38704 }

what is armature reaction?explain in simple words?and what
is its infleunce on dc motor

Answer

well armauture reaction is the effect of armature flux on
the main field flux , the effect of armature reaction in a
dc machine is cross magnetising that is to distort the main
field flux and if the effect of saturation is taken into
account then it leads to demagnetisation as well. in case
of a dc motor the armature flux the flux at the leading
pole tips are weakened while it is strenghtened at the
trailing pole tips .

Question { 10008 }

In IDMT RELAY the term minimum (M) indicates what?

Answer

well all the people who answered this please note that an
idmt relay is different from an inverse realy in the manner
that it shows the inverse operating behaviour only uptill
the pick up value is attained and after that it attains a
constant operating behaviour that is its operating time
becomes independent of the magnitude of the fault current .

Question { 6028 }

wat is the difference between rotor 1st stage fault and 2nd
stage fault?

Answer

Hi friend i m glad dat u came up with a question like dat .
i appreciate ur indepth study of power system .
well basically the rotor system of altenators are unearthed
due to which the single gound fault at the rotor doesnt
actually disturbs the rotor as there is no fault current ,
however if a second gorund to earth faukt develops then the
short circuit between the windings results as the fault
current gets a path to flow . the former is called the 1st
stage fault while the latter is called the 2nd stage fault.

Question { ABC, 10096 }

To transmit the Power which must be present…?

a) Reactance b) Resistance c) Capacitance
d) none of above

Answer

my dear freinds this question is answered in almost every
power system book , and i am amazed that after so many days
of posting this question is stil awaiting a correct
response. my freinds if there is no reactance in the lines
then there can be no transmission of electrical power .

Question { L&T, 37260 }

Why Generator rotate only clock wise ?

Answer

Now it must be noted that in a generator the prime mover
torque is applied against the direction of electromagnetic
torque . the field system prodeuces the field flux and 90
lagging to it is produced the armature flux . the resultant
air gap flux somewhere between the two , all the three
fluxes revolving in the same counterclockwise direction and
hence the required electromagnetic torque is set up in the
counterclockwise direction and hence the primemover torque
is applied in the clockwise direction .

Question { 12075 }

working principle of induction motor is similar to
transformer. but what makes it to rotate? then why the
transformer doesnt?

Answer

a transformer doesnt have a concentic winding so a 3 phase
supply in a transformer doesnt produce a rotating magnetic
field , however the 3 phase windings in a induction motor
develop a rotating magnetic field and hnce a rotational
torque is produced in an induction motor .

Question { GATE, 11566 }

If the Brsh of DC motor is shifted by 90 deg, then MNA will
shift by --degree.

Answer

mna refers to the axis along which the resultant flux is
0 , which coincides with the geometric neutral axis when no
armature current flows but the flow of armaue current
shifts the 0 flux position or the mna in the direction of
rotation in a generator and opposite to it in a motor ,
ideally the brushes should be along the mna as then it
would be unde the influence of no rotational emf induced
due to earlier pole once the the coil crosses the brush or
undergoes commutation , the shift of brushes has got
nothing to do with the shift of mna that is the mna should
remain at the same position , however for the sparkless
commutation mna shoild be along the brush and hence it
would be good if the mna also gets shifted by 90 .

Question { 4274 }

can u guys pls suggest a good material for learning abt
switch yard and its components

Answer

believe me though extensive but u cant get a better book
for any topic of power system than jb gupta

Question { 8637 }

what happens when we give ac supply to the dc motor?and why?

Answer

It all depends upon whether the motor is a series motor or
shunt , if the motor is series then it would be running but
quite unsatisfactorily with increased sparking during
commutation and heating. however a shunt motor wont run
because of the high inductance of its field windings ( due
to greater no. of turns ) it will oppose the change in the
magnitude and direction of current which is th case with
ac .

Question { Birla, 14143 }

what is IDMT relay.

Answer

An inverse definite time minimum relay as the name suggests
shows a an inverse behaviour that is the operating time is
inversely proportional to the magnitude of fault current,
however the operating characteristics become constant once
the pick up value of the relay is attained , this is
achieved by using a core material that saturates at the
pick up value of the relay

Question { 30480 }

what will happen when excitation of a synchronous machine
is suddenly reduce to zero when it is running at full
load..??

Answer

in case of a synchronous motor once the excitation is cut
off , then the motor starts to run as an induction motor,
this is because a synchronous motor by itself is not self
starting , and is first started as a squirrel cage
induction motor , once the motor attains about 95% of
synchronous speed then the excitation is applied due to
which the the copper bars are effectively removed from the
circuit as they revolve with the sane speed as the rotating
flux , once the excitation is cut off , the relative motion
between flux and the copper bars come into action and the
motor starts operating as a induction motor.

Question { 28584 }

Why a transformer is known as a static device?

Answer

a transformer has no moving parts , there is no rotating
magnetic field in a transformer like the induction motor ,
and hence no torque is is generated to keep one of the
windings in motion.