Question { Tata Elxsi, 43625 }

What does negative slip of an induction motor signify?

Answer

For an induction motor we have
s= (Ns-Nr)/Ns
so slip can be negative in the region where the value of
rotor speed exceeds that of the synchronous speed. usually
while working as an induction motor the speed of the rotor
cant touch the synchronous speed. because as it does so
there will be no cutting of the flux lines by the rotor
conductors. so there will be no emf induced so no current ,
so no force and no torque.
but when the induction motor is run by an external
primemover over the synchronous speed now rotor speed
increases that of the Ns. so rotor emf E2> stator applied emf.
the induction motor now functions as an induction generator.

Question { NTPC, 18439 }

why synchronous motor has zero starting ?

Answer

a synchronous motor is not self starting.
when three phase current displaced in time by 120 degree is
applied to the three phase stator windings displaced in
space by 120 degree then a rotaing magnetic field is
produced in the air gap. the rotor tends to allign with this
rotating field. the rotor windings are supplied by dc . so
consatnt poles are produced on the rotor.
now suppose the instant when the rotating stator field has
its north pole in line with the south pole of the rotor. the
rotor will be attracted and tend to move with the stator.
but due to its inertia before it starts to move with the
stator the south pole of the stator comes in line with the
rotor south pole. so now the rotor gets repelling force and
it tends to move in the opposite direction as that of the
stator. but again before it starts to move the stator north
pole comes in line with the rotor south pole. thus overall
the efect is that the syn. motor is not self starting. once
it is brought to the syn speed by an external primemover it
rotates at syn. speed irrespective of the load.

Question { Siemens, 21749 }

how kva rating of transformer is calculated.

Answer

the kva rating can be determined using the formula
kva=power supplied(in KW)/power factor.
so if the transformer is feeding a load of 100MW industrial
load. then the KVA rating of transformer to be used has to be
100/.85=117.647mva
we haver assumed the power factor for industrial load to be .85