A
three-phase balanced winding in the stator of the Induction motor (IM)
is shown in (schematic form). In a three-phase balanced winding, the
number of turns in three windings, is equal, with the angle between the
adjacent phases, say R & Y, is 120° (electrical). Same angle of 120°
(elec.) is also between the phases, Y & B.A three-phase balanced
voltage, with the phase sequence as R-Y-B, is applied to the above
winding. In a balanced voltage, the magnitude of the voltage in each
phase,assumed to be in star in this case, is equal, with the phase angle
of the voltage between the adjacent phases, say R & Y, being 120° .
Four-Pole Stator :
A
4-pole stator with balanced three-phase winding is taken as an
example.The winding of each phase (one part only), say for example, ( R1
− R1′ ) is assumed to be concentrated in one slot each, both for
forward and return conductors, with required no.as needed. Same is the
case for other two phases. The connection of two parts of the winding in
R-phase, is also shown in the same figure. The windings for each of
three phases are in two parts, with the mechanical angle between the
start of adjacent windings being 60° only, whereas the electrical angle
remaining same at 120° . As two pairs of poles are there, electrically
two cycles, i.e. 720° are there for one complete revolution, with each
N-S pair for one cycle of 360° , but the mechanical angle is only 360° .
If we move through one cycle of the waveform, by 360° (electrical), the
axis of the resultant flux in this case moves through a mechanical
angle of 180° , i.e. one pole pair ( 360° -elec.), or half revolution
only. As stated earlier, for the resultant flux axis to make one
complete revolution ( 360° - mech.), two cycles of the waveform ( 720° -
elec.), are required, as No. of poles ( p ) is four (4). So, for the
supply frequency of f = 50 Hz (c/s),the speed of the rotating magnetic
field is given by,
The Reversal of Direction of Rotating Magnetic Field :
The
direction of the rotating magnetic field is reversed by changing the
phase sequence to R-B-Y, i.e. changing only the connection of any two of
the three phases, and keeping the third one same. The schematic of the
balanced
three-phase winding for a 2- pole stator, with the winding of
each phase assumed to be concentrated in one slot, is redrawn in which
is same as shown in . 4(i) (a-d). The space phase between the adjacent
windings of any two phases (say R & Y, or R & B) is 120° , i.e. 2
π / 3 rad (elect.), as a 2-pole stator is assumed. Also, it may be
noted that, while the connection to phase R remains same, but the
phases, Y and B of the winding are now connected to the phases, B and Y
of the supply respectively. The waveforms for the above phase sequence
(R-B-Y) are shown in Please note that, the voltage in phase R leads the
voltage in phase B, and the voltage in phase B leads the voltage in
phase Y. As compared to the three waveforms shown in the two waveforms
of the phases Y & B change, while the reference phase R remains
same, with the phase sequence reversed as given earlier.
The
currents in three phases of the
stator winding are Without going into
the details of the derivation, which has been presented in detail
earlier Note that the second part of the expression within square
bracket is zero. It can be shown that the rotating magnetic field now
moves in the reverse (i.e., anticlockwise) direction , from phase R to
phase B (lagging phase R by 120 °) ,which is the reverse of earlier
(clockwise) direction as shown in , as the phase sequence is reversed.
This is also shown in the final expression of the resultant mmf wave, as
compared to the one derived earlier. Alternatively, the reversal of
direction of the rotating magnetic field can be derived by the procedure
followed in the second method as given earlier. In this lesson the
first one of this module, it has been shown that, if balanced three-
phase voltage is supplied to balanced three-phase windings in the stator
of an
Induction motor, the resultant flux remains constant in
magnitude, but rotates at the synchronous speed, which is related to the
supply frequency and No. of poles, for which the winding (stator) has
been designed. This is termed as rotating magnetic field formed in the
air gap of the motor. The construction of three-phase induction motor
(mainly two types of rotor used) will be described, in brief, in the
next lesson, followed by the principle of operation.
View More Details in :
http://www.sunnerindia.org/
