Both stator and rotor have poly phase windings so dc

Both stator and rotor have poly-phase windings
So
DC motor/machines:
The stator winding is connected to dc supply and rotor winding flows ac current.
Stator is made of salient pole and Commutator is connected to the rotor so rotor
winding is supply ac power.
Induction machines:
In induction motor the ac supply is connected to stator winding and rotor and
stator are made of poly-phase windings.
Synchronous machines:
In this type machines the stator is connected to ac supply but rotor winding is
excited by dc supply. The rotor is made of both salient pole and slip rings and
stator is made of cylindrical.
Q. 46
When stator and rotor windings of a 2-pole rotating electrical machine are
excited, each would produce a sinusoidal mmf distribution in the airgap with
peal values F
s
and F
r
respectively. The rotor mmf lags stator mmf by a space
angle d at any instant as shown in figure. Thus, half of stator and rotor surfaces
will form one pole with the other half forming the second pole. Further, the
direction of torque acting on the rotor can be clockwise or counter-clockwise.
The following table gives four set of statement as regards poles and torque. Select
the correct set corresponding to the mmf axes as shown in figure.
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GATE SOLVED PAPER - EE 2003
Stator
Surface
ABC forms
Stator
Surface
CDA forms
Rotor
Surface
abc forms
Rotor
surface
cda forms
Torque
is
(A) North Pole
(B) South Pole
(C) North Pole
(D) South Pole
South Pole
North Pole
South Pole
North Pole
North Pole
North Pole
South Pole
South Pole
South Pole
South Pole
North Pole
North Pole
Clockwise
Counter
Clockwise
Counter
Clockwise
Clockwise
Sol. 46
Given that
F
s
is the peak value of stator mmf axis. F
r
is the peak value of
rotor mmf axis. The rotor mmf lags stator mmf by space angle
d . The direction of torque acting on the rotor is clockwise or counter clockwise.
When the opposite pole is produced in same half portion of stator and rotor
then the rotor moves. So portion of stator is north-pole in ABC and rotor abc is
produced south pole as well as portion surface CDA is produced south pole and
the rotor cda is produced North pole.
The torque direction of the rotor is clock wise and torque at surface is in counter
clockwise direction.
Hence (C) is correct option.
Q. 47
A 4-pole, 3-phase, double-layer winding is housed in a 36-slot stator for an ac
machine with 60 c phase spread. Coil span is 7 short pitches. Number of slots in
which top and bottom layers belong to different phases is
Sol. 47
Given that:
A 4-pole, 3- f , double layer winding has 36 slots stator with 60 c phase spread, coil
span is 7 short pitched
so,
Pole pitch
pole
slot
=
4
36
9 = =
Slot/pole/phase 3 =
so,
3-slots in one phase, if it is chorded by 2 slots then
Out of 3 2 " have different phase
Out of 36 24 " have different phase.