Die Grinding Machine :
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.
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