COMPUTER AIDED ELECTRICAL DRAWING (CAED) 10EE65 part2
Short chorded winding or Fractional slot winding:
• Coil pitch in poly phase machines is usually less than pole-pitch and such a winding arrangement is called short pitch or chorded or fractional slot winding.
• Usually the coil pitch varies from 2/3 pole pitch to full pole pitch.
• A coil span less than 2/3 pole pitch is not used in practice. Because a chording more than 1/3 pole pitch would noticeably reduce the phase emf.
• Advantages of short pitched,( chorded, fractional slot) windings are:-
– The amount of copper used in the overhang (end winding) reduced and hence a saving on copper,
– The magnitude of certain harmonics in the emf and also mmf is suppressed.
Note :-
• In integral full pitch winding, a slot contains coil sides of the same phase.
• In integral chorded pitch winding, some slots contain coil sides pertaining to different phases.
• Interconnection between the phase belts of chorded three phase winding is done in a similar manner to that explained earlier for full pitch winding.
For example :
Consider a motor stator with 36 slots wound for six poles.
. If the same stator is rewound for the lower speed say, 750 rpm, i.e., for 8 poles, the number of slots per pole per phase will then be:-
. In induction motors such cases usually arise when stators with the same number of slots are wound for more than one speed or number of poles
. For fractional slot windings, however, from the view point of symmetry, the number of slots must be divisible by the number of phases. i.e 3
. Limitations of fractional slot windings are
- It can be used only with double-layer windings
- The number of parallel circuits is limited
. The fractional-slot winding differs from the integral-slot winding in that it must be composed of coil groups with different numbers of coils and each phase must occupy the same number of slots, otherwise the winding would be unbalanced.
. Usually, the fractional-slot winding is a combination of two types of coil groups:
. One in which the number of coils in the group is equal to the integer part of the number of slots per pole per phase.
. The other in which the number of coils is one greater than in the first type.
• If for example, the number of slots per pole per phase is 2 ½, the winding will be built up of alternating coil groups containing two and three coils each, every two-coil group being followed by a three-coil group.
2-3-2-3-2-3…….
• Because of the alternation, the number of slots per pole per phase is:-
• Sometimes the fractional number of slots per pole per phase is expressed as an improper fraction, i.e.
Arranging fractional slot windings with the aid of tables:
. The coil groups in a fractional-slot winding are easily arranged with the aid of a table.
. Taking a sheet of millimeter lined paper, the table is drawn with as many horizontal lines as there are poles, and each line is divided into 3C boxes, where C is the numerator of the improper fraction representing the slots per pole per phase and 3 is no. of phases.
. The table is next divided by vertical lines forming three equal columns for the three phases with C boxes per phase.
. Following this, in ordinal succession, the boxes are filled in with the numbers of the slots at intervals of d boxes, where d is the denominator of the fraction expressing the number of slots per pole per phase.
Example -
Given:- S = 27, p = 6, m = 3, q = 1½ = 3/2
Solution
The largest common factor t for S = 27 and p = 6 is:-
S = 27 = 3 x 3 x 3
p = 6 = 2 x 3
then, t = 3 and S/(t x m) = 27 / (3x3) = 3 is a whole number.
1. draw a table where no. rows = no. of poles and each column of three phases with C no. of sub columns, where, C is the numerator of the improper fraction.
2. Fill the boxes starting from the extreme left top box with cross or consecutive numbers (representing adjacent slots) as shown in table below. Proceed to the right marking crosses/numbers separated from each other by denominator of the improper fraction of no. of slots per phase per pole.
Winding table Interpretation:
. Reading the table horizontally line by line, write down the letter of the respective phase each time a cross/number appears in its column.
. This reveals the following sequence of the coils of each phase under consecutive poles.
RRBYY, RBBY, RRBYY, RBBY, RRBYY, RBBY
. Each letter indicates the coils of each phase, and like letters succeeding one another indicate how many coils of the same phase the group will contain.
. Thus, in our example, the sequence shows that it is necessary to prepare nine groups of two coils each and nine single coils.
. They will occupy (9 x 2) + 9 = 27 slots with the following arrangement.
Summary on Fractional-slot Winding:
. When the integer before the fraction is greater than unity, the numbers in the sequence table must be that integer and a number increased by one.
. Thus, for example, when q = 1 ½ , the sequences will contain repeating single and two-coil groups (1-2), while in the case where q = 2 ½ the repeating sequences will contain two-coil and three coil groups (2-3).
. The number of integers in a period is equal to the denominator d of the improper fraction expressing the slots per pole per phase; the sum of the integers is equal to c, the numerator of the improper fraction.
. Thus, when the period consists of five integers, (1-2-1-2-2), the sum of the integers is 8, i.e., it is equal to the numerator of the fraction.
Ex. 6. Design and draw the developed winding diagram of an AC motor with following details: No of poles
= 6 no. of phases = 3, No. of slots = 27, double layer lap winding, star connected.
Soln:
MUSH WINDING:
. This winding is very commonly used for small induction motors having circular conductors.
. This is a single layer winding where all the coils have same span (unlike the
concentric winding where coils have different spans).
. Each coil is wound on a former, making one coil side shorter than the other.
. The winding is put on the core by dropping the conductors, one by one into previously insulated slots.
. The short coil sides are placed first and then the long coil sides. The long and short coil sides occupy alternate slots.
. It will be also observed that the ends of coil situated in adjacent slots cross each other i.e. proceed to left and right alternatively.
. That is why sometimes it is known as a basket winding.
Points to be remembered:
The following should be kept in mind while designing a mush winding, that is
.The coils have a constant span.
. There is only one coil side per slot and therefore the number of coil sides are equal to number of slots.
. There is only one coil group per phase per pole pair and therefore, the maximum number of parallel paths per phase is equal to pole pair.
. The coil span should be odd. Thus for a 4 pole 36 slot machine, coil span should be 36/4=9 while for a 4 pole 24 slot machine, the coil span should not be 24/4=6; it should be either 5 or 7 slots. This is
because a coil consists of a long and a short coil side. The long and short coil sides are placed in alternate slots and hence one coil will be in an even numbered slot and the other in an odd numbered slot giving a coil span which is an odd integer.
Ex. Design and draw the developed winding diagram of an AC motor with following details: No of poles = 4 no. of phases = 3, No. of slots = 24, single layer mush winding. Soln:
No. of poles = 4; No. of conductors = 24;
Pole pitch = 24/4 = 5 (should be odd)
no of slots/pole /phase = 24 / (4x3) = 2
Slot angle = 180 x 2 /pole pitch = 180 x 2 / 24 = 15
Connections: Rs = 2, Bs = 2 + 120/15 = 10; Ys = 2 + 240/15 = 18
References:
1. A Course in Electrical Machine Design – A. K. Sawhney
2. Design of Electrical Machines – V. N. Mittle
3. Performance and Design of A C Machines – M. G. Say
4. Electrical Engineering Drawing – S. F. Devalkar
5. Electrical Engineering Drawing – K. L. Narang
6. www.google.com and related websites
8. Krishna Vasudevan et. al. Electrical Machines II, Indian Institute of Technology, Madras
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