Synchronous Machine Armature Windings

Armature windings is the most important part to produce a flux in the machine , we can say that it simplified form that every winding doesn’t start and finish under the identical pole. The 2 coil sides of a given coil exist identical magnetic conditions of opposite polarity.

The armature winding start and finish of all the coils in phase A are respectively, as SA and FA. Phase A is shown as a solid line in the figure, phase B as a dashed line, and phase C as a dotted line. When seen from the coil terminals, the EMF produced in the two coil sides add up. If we assume that the poles on the rotor are moving to the left as shown, then the relative motion of the armature conductors is to the right.

The armature winding start and finish of all the coils in phase A are respectively, as SA  and FA. Phase A is shown as a solid line within the figure, phase B as a dashed line, and phase C as a line. When seen from the coil terminals, the EMF produced within the two coil sides add up. If we assume that the poles on the rotor are moving to the left as shown, then the relative motion of the armature conductors is to the correct.

This implies that identical magnetic conditions are going to be seen by conductors of phase A, followed by phase C, followed by phase B. The induced EMFs in phases A, C, and B could also be said to supply a phase sequence of ACBACBA.

The measure between two phases to attain identical magnetic conditions would rely upon the relative speed of motion, and on the spatial separation of the phases. In Fig the phases are so laid out that every phase is separated from another by 120 electrical degrees (360 degrees is defined by the space to attain identical magnetic conditions).

As the space between two adjacent corresponding points on the poles is 180 electrical degrees, we are able to see that the space between the coil side at the beginning of A which at the beginning of C must be 120 electrical degrees. Thus, the leading pole tip of a unit pole moving to the left within the above figure will induce identical voltages in corresponding coil sides A, C, and B, respectively, 120 electrical degrees apart. Note that phase B lags phase A by 240 electrical degrees or leads phase A by 120 electrical degrees.

Half-Coil And Whole-Coil Armature Windings

Half-coil is also called as single-layer windings are sometimes used in small induction motor stators and in the rotors of small wound-rotor induction motors. A cross-section of a half-coil, single-layer winding is shown in Fig (c)(i). Like the dc dynamo armature windings, most commercial armatures for ac synchronous generators are of the full or whole coil two-layer type, shown in cross-section at the right in Fig (c)(ii).

Half-coil is additionally called as single-layer windings are sometimes employed in small induction motor stators and within the rotors of small wound-rotor induction motors. A cross-section of a half-coil, single-layer winding is shown in Fig (c)(i). just like the dc dynamo armature windings, most commercial armatures for ac synchronous generators are of the complete or whole coil two-layer type, shown in cross-section at the proper in Fig (c)(ii).

The whole-coil, two-layer winding gets its name from the fact that there are two coil sides (one coil) per slot. Fig (a) shows a single-layer, half-coil lap windings Fig (b) shows a double-layer, full-coil lap winding. A cross-section of a single layer (half-coil) winding is shown in Fig (c)(i).

The whole-coil, two-layer winding gets its name from the fact that there are two coil sides (one coil) per slot. Fig (a) shows a single-layer, half-coil lap windings Fig (b) shows a double-layer, full-coil lap winding. A cross-section of a single layer (half-coil) winding is shown in Fig (c)(i).

Chorded or Fractional -Pitch Armature Windings

Mostly single-layer windings are full-pitch windings, the two-layer, whole-coil windings are generally designed on an armature as a chorded or fractional-pitch windings. This common practice stems from the very fact that the first advantage of the whole-coil windings is that it permits the utilization of fractional-pitch coils so as to avoid wasting copper. As will be shown later, fractional-pitch windings, when utilized in ac synchronous and asynchronous generator armatures, additionally to saving copper and to

  • Reduce the MMF harmonics produced by the armature winding
  • Reduce the EMF harmonics induced in the windings, without reducing the magnitude of the fundamental EMF wave to a great extent.

For the three reasons cited, fractional-pitch windings are almost universally used in ca synchronous generator armatures.

Distributed Armature Windings and distribution (or Belt) factor

When the slots are distributed around the armature uniformly, the winding that is inserted is called a distributed winding. A distributed lap winding is shown in Fig. Note that two coils in phase belt A are displaced by one slot angle i.e. the angular displacement between two successive slots with respect to each other. The induced voltages of each of these coils will be displaced by the same degree to which the slots have been distributed, and the total voltage induced in any phase will be the phasor sum of the individual coil voltages.

When the slots are distributed round the armature uniformly, the winding that’s inserted is named a distributed winding. A distributed lap winding is shown in Fig. Note that two coils in phase belt A are displaced by one slot angle i.e. the angular displacement between two successive slots with relation to one another. The induced voltages of every of those coils are going to be displaced by the identical degree to which the slots are distributed, and also the total voltage induced in any phase are the phasor sum of the individual coil voltages.

For an armature winding having four coils distributed over say, 2/3 rd of a pole-pitch, in four slots, the four individual coil side voltages are represented by phasors in the above figure as displaced by some angle <span class="katex-eq" data-katex-display="false">\alpha</span> the number of electrical degrees between adjacent slots, known as slot angle. It is 300 for the case of 4 slots per phase belt. Voltages EC1, EC2, etc., are the individual coil voltages, and n is the number of coils in a given phase belt, in general.

For an armature winding having four coils distributed over say, 2/3 rd of a pole-pitch, in four slots, the four individual coil side voltages are represented by phasors in the above figure as displaced by some angle \alpha the number of electrical degrees between adjacent slots, known as slot angle. It is 300 for the case of 4 slots per phase belt. Voltages EC1, EC2, etc., are the individual coil voltages, and n is the number of coils in a given phase belt, in general.

For a machine using n slots for a phase belt, the belt or distribution factor Kd

k_d=\frac { E_\phi }{ nE_c }

For n coils in series per phase, chord AN, is also 2OAsin n\phi/2, and the distribution or belt factor is Kd

k_d=\frac { E_\phi }{ nE_c }=\frac { 2OA\sin { (n\alpha /2) } }{ n.2OA\sin { (\alpha /2) } } =\frac { AN }{ nE_c } =\frac { 2AM }{ n\ast AC }

=\frac { 2AM }{ n\ast 2AB } =\frac { 2AM }{ n\ast 20A\sin { \frac { \alpha }{ 2 } } } =\frac { 2OA\sin { (n\alpha /2) } }{ n.2OA\sin { \alpha /2 } } =\frac { \sin { n\alpha /2 } }{ n\sin { \alpha /2 } }

As the distribution of coils (slots/pole) increases, the distribution factor Kd decreases. It is not affected by the type of winding, lap or wave, or by the number of turns per coil, etc.

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