CA1139345A - Multi speed polyphase motor arrangement - Google Patents

Abstract

MULTI-SPEED POLYPHASE MOTOR ARRANGEMENT
ABSTRACT OF THE DISCLOSURE
A polyphase multiple speed motor arrangement in the form of a three-phase two speed motor along with a generalized technique for designing, winding and connecting such a motor employing concentric winding techniques to provide a ratio of the torque in one speed mode to the torque in another speed mode of a desired value within a wide range of possible values while also obtaining desirable performance at the specified torque ratio in both speed configurations is disclosed.

Description

1~3~ '45 BACKGROU~D OF THE INVENTION
.
The present invention relates generally to polyphase multi-speed alternating current winding arrangements and more particularly to such winding arrangements as might be used in 5 an induction motor. Even more specifically, the present invention relates to polyphase induction motors of the speed changing variety and techniques for fabricating such motors to meet certain specified design requirements.
The classical polyphase alternating current dynamo-electric machine ha~ a slotted magnetic core in which threeseparate phase windings are lap-wound with the individual phase windings being wye or delta connected to a three-phase source of alternating current. Such a lap-wound stator is not, however, easily adapted to automated production techniques since each coil of that lap-winding has one side turn portion disposed in the bottom ~radially outermost) part of the stator core slot while another side turn portion is disposed in the top area of another stator core slot. ThUS, each coil has one of its side turn portions covered by a side turn portion of another coil and with this requirement the several coils are generally hand-placed in their respective slots.
In recent years, the use o~ concentric winding arrange-ments has become quite popular since for any qiven lap-winding arrangement there is an equivalent concentric winding arrange-ment and the concentric winding arrangement is rather ~asilyadapted to mass production techniques by forming the concentric winding~ external of the stator core and then simultaneously or sequentially machine inserting those concentric windings into the appropriate stator core slots. It should be noted that there are concentric winding arrangements having no equiv-alent la~-winding arranaement.
It is frequently desirable to provide an induction motor 113~345 which may have its windin~ appropriately interconnected to operate at a selected one of, for example, two different operating speeds. Such multi-speed or pole changing motors, while not unknown in the polypllase induction motor art, S are often found in single phase motor designs. These arrangements for changing the operating speed of an induction motor may include windings operable in either speed mode and additional windings which are not operable in all of the speed modes, thereby giving an effective number of poles which differs from mode to mode. Also known is the provision of a specified number of wound poles which are interconnected in one operating mode to be of alternatin~ magnetic polarity while in another operating mode these wound poles have their interconnections such that consecutive wound poles are of the same magnetic polarity, thereby inducing between each pair of such wound poles, a consequent pole of an opposite magnetic polarity, thereby effectively doubling the number of poles when the stator connections are such as to induce the corresponding consequent poles.
For many multi-speed motor installations it is important to be able to design the motor so as to give a torque ratio tailored to the particular environment. This is especially important in hermetic motor~, such as employed to drive com-pressors of the type employed in refrigerating and air condition-ing systems. In a sinqle-phase two-speed motor of the consequent-pole variety, this tailoring of the motor torque ratio to suit the needs of the particular installation is frequently accomplished by providing a so-called extended main winding which is used only in one of the pole configurations to red~e the torque in that pole configuration while not detrimentally affecting motor opera-tion in the other pole configuration, in which that extended main winding is idle. Such an approach is not, however, easily adapted 1~3~ 5 to polyphase motor winding arrangments.
The typical consequent pole polyphase winding arrangement has the wound poles of either lap or concentric configuration of full pitch for the lower speed mode of operation and therefore of fifty percent pitch for the higher speed mode of operation.
Such a winding arrangement of a type made and sold by the assignee of the present invention is illustrated in concentric form in Fig. 4. The pitch factors for this arrangement become apparent when the equivalent lap-winding is sketched.
To modify the torque of a motor such as depicted in Fig.
4, in one of its pole number operating modes, without deleteriously affecting the motor performance in the other of its pole number operating modes, is not easily accomplished. Merely decreasing number of turns gives an increased flux density in the stator and increased I2R losses deteriorating both efficiency and power factor in the other of the pole number operating modes. Known winding arrangements including the one illustrated in Fig. 4, due in part to the large number of teeth spanned by the outermost of the concentric coils, experience substantial end turn in-sulating problems which while not insurmountable do increase bothinsulating material requirements and fabrication time thereby leading to an increased overall cost for the motor, Also,with a Fig. 4 winding configuration, the latitude of achievable tor-que ratios within allowable current density and flux density 2S limitations is quite limited.
SUMMARY OF THE INVENTION
Among the several objects of the present invention may be noted the provision of an integrated technique for the .. . . .
~ design and fabrication of a wide variety of multi-speed poly-phase motors to meet certain design specifications; the pro-vision of a design and fabrication technique as suggested by the previous object wherein predetermined torque ratios may 113~ 45 be achieved by changing the winding pitch; the provision of a stator assembly for a polyphase multi-speed motor having short pitch windings and characterized by its compact end turn arrangement; the provision of a two speed three-phase S motor design which meets predetermined design requirements such as a desired torque ratio for the respective speed modes;
and the provision of a design and fabricating technique for multi-speed polyphase motors which obviate the somewhat haphazard prior art approach to such motor design. These as well as other objects and advantageous features of the present invention will be in part apparent and in part pointed out hereinafter.
In general, a stator assembly for use in a pole chang-ing motor which provides a predetermined torque ratio for spec-ified pole number operating modes is fabricated by selecting a stator core having a rotor accepting bore with a plurality of winding accepting core slots communciating with the bore, deter-mining the number of turns of a selected wire size which may be placed in selected core slots, determining the number of effec-tive turns per pole in each of the specified pole number opera-ting modes, determining by, for example, bench tests, a torqueratio for the motor in the specified pole number operating modes and changing the winding distribution in the core slots to change the number of effective turns per pole in each of the specified pole number operating modes to bring the torque ratio into closer conformity with the predetermined torque ratio. The winding distribution may be changed by changing the pole pitch or by changing the number of turns in certain ones of the concentric coils forming the coil groups. The winding distribution may, for example, be changed by changing the number of core slots spanned by each of the concentric coils in each of the plurality of pole groups by a li~e amount. The foregoing sequence of s~eps subsequent to the selection of a stator core may in some 113~ 5 cases be repeated until the difference between the predetermined torque ratio and the determined or measured torque ratio is less in magnitude than some predetermined value.
Also in qeneral and in one form of the invention, a polyphase induction motor has a stator including a primary core member with end faces, a yoke section, and a number of teeth sections forming coil accommodating slots, and a bore with first, second and third winding phases displaced in phase from one another carried by the core and each of the winding phases including at least two pole groups each formed by at least two concentric coils with an outermost coil in each pole group spanning the most teeth sections for that pole group.
The three winding phases may be delta connected to a polyphase source with the two pole groups of each phase connected in series to be of the same magnetic polarity for inducing a like number of consequent poles for each phase when operating the motor in a lower speed mode and the three winding phases wye connected to a polyphase source with the two pole groups of each phase connected in parallel to be of opposite magnetic polarity for operating the motor in a higher speed mode. The three winding phases may, alternatively, be wye connected to a polyphase source with the two pole groups of each phase connected in series to be of the same magnetic polarity for inducing a like number of consequ2nt poles for each phase for operating the motor in a lower speed mode and the three winding phases delta connected to a polYphase source with the two pole groups of each phase connected in series to be of opposite magnetic polarity for operating the motor in a higher speed mode. In either of these alternative arrangements, each winding phase may consist of two pole groups each formed by~four concentric coils with adjacent coil side turn portions separated by but a single stator tooth, and in each arrangement the pole pitch 113~345 in the higher speed mode is less than one while the pole pitch in the lower speed mode is also less than one in the first alternative arrangement and is less than one in the second alternative arrangement.
S BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 is an end view of a stator according to the present invention illustrating schematically the positioning of the several wlndings;
Figs. 2-3 are schematic diagrams illustrating the inter-connection of the windings of Fig. 1 for low speed and high speed modes of operation respectively;
Fig. 4 is an end view of a stator similar to Fig. 1 but illustrating schematically a known arrangement of the several windings;
Fig. 5 is a schematic diagram illustrating the inter-connection of the windings of Fig. 4 for a low speed mode of opera-tion; ~
Fig. 6 is a schematic diagram illustrating the interconnec-tion of the windings of Fig. 4 for a high speed mode of operation;
Fig. 7 is an end view of a stator similar to Figs. 1 and 4 but illustrating schematically another variation on the position-ing of the several windings according to the present invention;
Fig. 8 is a schematic diagram illustrating the interconnec-tion of the windings of Fig. 7 for a low speed mode of operation;
Fig. 9 is a schematic diagram illustrating the interconnec-tion of the windings of Fig. 7 for a high speed mode of operation; and Figs. 10 and 11 illustrate magnetomotive force diagrams for the motor of Figs. 1-3 in the higher and lower speed modes respectively.
Corresponding reference characters indi-cate corresponding parts throughout the several views of the drawing.
The exemplifications set out herein illustrate a preferred embodiment of the invention in one form thereof and such exem-113~;~4S

plifications are not to be construed as limiting the scope of the disclosure or the scope of the invention in any manner.
DESCR~PTION OF T B PREFERRED EMBODIMENT
Referring now to the drawing generally, a stator assembly 11 or 15, for use in a pole changing motor, is fabricated to have a predetermined torque ratio for specified pole number operating modes by first selecting a stator core 17 or 21 having a rotor acce~ting bore 23 or 27, and a plurality of winding accepting core slots, such as 29 or 33, with each slot communicating with the bore. As illustrated, the stator cores 17 and 21 happen to be identical so-called forty frame cores with twenty-four stator core slots and these cores are illustrated at about 85 percent of their actual size.
The core slots, such as slot 29 in Fig. 1, are peripher-ally determined by the stator core yoke section 35 and the ad-jacent stator teeth 37 and 39 and by the bore 23 periphery with some minor clearance being allowed between the tooth tip ends near the bore to accommodate bore insulating wedges, if desired.
For a selected wire size, the number of turns which may be placed in a selected core slot may be determined either ex-perimentally or graphically or by known empirical relationships determined by the technique of placing the coils in the slots.
The number of effective turns per pole in each of the specified pole number operating modes is next determined by summing for each coil within a pole group the product of the number of turns in that coil and the sine of 90 times the ratio of the number of teeth spanned by that coil to the number of teeth per pole for the given stator configuration. Thus, for example, in the twenty-four slot stator illustrated in Fig. 1, in its two pole configuration, there are twelve slots per pole, or equivalently a maximum of twelve teeth which could be spanned by the outermost coil in a pole group. Thus, if coil 41 has 113~45 Nl turns and spans eight stator teeth, while 43 has N2 turns spanning six coil teeth, coil 45 has N3 turn-q spanning four coil teeth, and coil 47 spanning a pair of coil teeth has N4 turns, the number of effective turns per pole in the two pole mode would be given by:
N - Nl sin(t8/12) 90)+ N2 sin((6/12) 90)+ N3 sin (4/12) 90)+ N4 sin((2/12) 90~
This result may of course be appropriately scaled depend-ing upon the specific number of phases to get a representative value for the entire motor winding.
A torque ratio for the motor in the specified pole number operating modes may now be determined by actual bench testing of the motor employing known textbook techniques, by creating a mathematical model for the motor, or more simply, but only approximately, by the known approximation that the tor~ue is inversely proportional to the square of the number of effective turns. If this determined torque ratio does not meet the desired design parameters for the motor, the winding distribution in the core slots may be changed to change the number of effective turns per pole in each of the specified pole number operating modes to bring the torque ratio into closer conformity with a predetermined ratio. This change in the winding distribution may be accomplished either by changing the specific number of turns in given coils or by changing the numbers of teeth spanned by various coils or by some combination of these two techniques. In some cases it may be desirable to repeat the foregoing sequence of determina-tions to better fit the resulting design to the initially est-ablished design parameters.
Referrin~ now specifically to Fig. 1, the stator 11 has one end face thereof visible and is seen to includ~ a primary core member 17 having a number of teeth sections, such as 37 and 3~, extending radially inwardly from the stator core yoke 1139;~45 section 35 with adjacent teeth sections formin~ coil accommoda-ting slots~ such as 29 and 53, as well as defining the rotor accepting bore 23. A first windin~ phase including the con-centric coils 41, 43, 45 and 47, as one pole group thereof, and the pole group interconnecting leads 61 and 63, is located radially intermediate to other winding phases, with the phases being displaced in phase one from the other. The outermost coil in each pole group, such as coil 41, is seen to span the greatest number of teeth sections for that pole group while having less than full pole pitch. This may perhaps be most easily seen by sketchin~ the equivalent lap-winding which has a throw from slots 1 to 6, thereby encompassing five of the twelve teeth per pole in the illustrated two-pole configuration.
When this same winding is employed in a consequent-pole mode, the equivalent lap-winding would encompass five of the six teeth associated with each pole, so it is clear that in either pole configuration the winding arrangement illustrated in Fig.
1 is a short pitch winding.
Fig. 3 illustrates the manner of interconnecting the winding leads of the Fig. 1 stator assembly to operate as a two-speed three-phase motor in a two-pole mode where every motor pole is a wound pole. Identifying pole groups by their end lead numbers, it will be seen that one phase winding includes two ~ole groups connected to be of opposite magnetic polarity.
Thus, pole group 57-59 and pole group 75-77 form the two poles for one phase, and are connected as at 79 to one phase of a three-phase source. The other two phases of this three-phase source are connected to leads 81 and 83 which are in turn con-nected to a parallel arrangement of each pair of pole groups for the corresponding phase. Thus, the Fig. 3 interconnection provides a high-speed wye-connected winding arrangement.
Fig. 2 illustrates the three winding phases in a delta 113~4S 03-l~M-5246 connection to the same polyphase source 79, 81, 83 but here each phase of two pole groups i9 series connected to create like magnetic poles and to therefore induce a similar number of consequent poles for operating the motor in a lower speed mode. Thus, comparing Figs. 3 and 2, it will be noted that the current flow at some instant of time will be for one pole group from lead 57 to lead 59, while the current flow for the other pole group of that same phase will be from line lead 75 to lead 77. As noted earlier, this arrangement creates wound poles of opposite ~agnetic polarity. However, when the current flow is from lead 57 to lead 59, in the winding connection of Fig. 2, the current flow through the other coil group for this phase is from lead 77 to lead 75, thereby effectively creating two like magnetic poles for this phase. The other pole groups are similarly interconnected as is easily followed by comparing the lead identification numbers 72, 65, 67, 69, 71, 73, 63, 61, 77, 75, 57, 59 in Figs. 1 through 3. The particular scheme in which the several pole group leads are interconnected for higher and lower speed operating modes has been used as illus-trated by the corresponding sequence of lead interconnectionsin Fig. 5, where beginning at the three phase source terminal 79, leads 87, 93, 99, 105 join one phase winding in a delta configuration to three phase source terminal 81, while leads 95, 101, 107 and 89 in that order connect a second phase across three phase terminals 81 and 83, with the delta inter-connection being completed by the third phase in order 103, 85, 91, 97. The corresponding wye configuration for a higher speed mode of operation is illustrated in Fig. 6.
Fig. 4 illustrates a known winding arrangement on the same stator core configuration as depicted in Fig. 1, and tests have been run comparina these two winding arrangements when applied to substantially the same stator core. It will be noted ~139345 03-1~M-5246 that the span of each coil in a pole group, as illustrated in Fig. 4, is one stator tooth greater than the corresponding span of coils in the pole groups as depicted in Fig. 1. The Fig. 4 span of the several coils corresponds to a pitch factor of one in the four pole mode of operation, and a pitch factor of 0.5 in the two pole running mode. Again, this can be most easily seen by sketching the equivalent lap-winding and noting that each coil thereof spans six stator teeth or equivalently has a throw of 1-7. A ta~ulation of experimental verification of the torque ratios achievable with the stator arrangement as exemplified by Figs. 1 and 7, as compared to the known winding arrangement of Fig. 4, is tabulated as follows:

Fia. 1 Fig. 4 Fig. 6 coil throw for equiv-alent lap winding 1 - 6 1 - 7 1 - 8 pitch

2 pole 41.7 50 5B.3 4 pole 83.3 100 83.3 break-down torque in oz. ft.
2 pole 455 391 821 4 pole 440 476 436 4 pole/2 pole .97 1.22 .53 maximum running torque in oz. ft.
2 pole (3000 rpm; 60 Hz) 390 330 607 4 pole (1500 rpm; 60 Hz) 355 373 362 4 pole/2 pole .91 1.13 .60 locked rotor current in amps.
2 pole 102 86.3 177 4 pole 53 S7.4 51.2 efficiency 2 pole rated load 86.3 85.3 90.0 2 pole peak value 89.5 90.7 90.9 4 pole rated load 84.3 84.9 83.8 4 pole peak value 85.5 85.6 84.7 power factor 2 pole rated load 92.5 92.6 88.6 2 pole peak value 93.0 92.6 90.7 4 pole rated load 70.5 69.9 69.0 4 pole peak value 71.0 75.0 75.0 113~45 03-~M-5246 Fi~. 1 Fig. 4 Fig. 6 wire diameter .0453 .0453 .0605 in inches (bare diameter) number of turns Phase 1 29-30-30-30 29-28-29-28 16-17-17-17 Phase 2 29-30-30-30 29-28-29-28 16-17-17-17 Phase 3 2~-30-30-31 2~-28-29-29 16-17-17-18 line to line resistance in ohms 2 pole .938 .985 .463 4 pole 1.251 1.307 1.403 effective turns 2 pole 34.85 38.51 51.15 4 pole 48.23 47.56 27.04 relative harmonic voltage levels 2 pole 3rd .604 .462 .250 5th .027 .145 .204 7th .156 .111 .020 9th .10~ .191 .250 4 pole 3rd .500 .707 .500 5th .067 .259 .067 7th .067 .259 .067 9th .500 .707 .500 Numerous salient features of the present invention~are apparent from the foregoing tabulation. Thus, for example, a designer faced with the problem of increasing, for eY.ample, the two pole torque in his motor design would, with the Fig. 4 arrangement, merely decrease the number of turns in the several windings, thereby increasing substantially the flux density in the core, giving a new motor design with increased two pole torque and increased four pole torque with the ratio of the four pole to two pole maximum run-ning torques remaining about 1.13 as tabulated. This design change would, however, substantially increase the losses in the motor and deteriorate the power factor. If, however, the designer approached the problem according to the teachings of the present invention, he would, at least in concept, increase the two pole torque while decreasing the four 1~39~45 pole torque, which while providing him with the needed two pole torque, would not deleteriously effect the efficiency or power factor to a significant degree. Thus, a four pole to two pole torque ratio of .91 would be achieved in the Fig. 1 con-figuration with the listed design parameters, while a four poleto two pole torque ratio of .6 would be achieved in the Fig. 7 configuration, all while maintaining the other operating para-meters within acceptable limits. Similarly, note that the efficiencies in the two pole configuration for the winding arrangement of Fig. 7 have been improved while not substantially diminishing the four pole efficiencies as compared to the known Fig. 4 winding arrangement, while the efficiency figures between the Fig. 1 and Fig. 4 configurations are substa~tially identical.
Another significant advantage in the configuration of Fig. 1 as compared to that illustrated in Fig. 4 does not appear from the tabulated values. Either winding arrangement could be and is,as shown in the afore-mentioned table, wound as a graded winding arrangement. This simply means that the two pole groups of the outermost phase have longer end turn portions than those of the intermediate phase, while the innermost phase has the end turn portions of its coil groups shorter than either of the other phases. This is done both to save material and to obviate any buckling in the end turn portions of the inner positioned phase windings due to excessive material therein.
To balance the reactance between the several phases, an additional turn or two is sometimes added in the outermost coil of that radially innermost pole group and this additional turn shows up in each of the three configurations compared in the tabulation in Phase 3. Ilowever, since the winding arrangement of Fig. 1 is short pitched, grading of the windings is particularly effective to separate the several phases, one from another, without the need for additional insulation between the several phase winding 1~9~45 03-t~M-5246 end turn portions. This advantage which saves both material and assembly time is peculiar to the configuration of Fig. 1.
The winding arrangement illustrated in Fig. 7 and its two depicted operating modes, as illustrated in Figs. 8 and 9, are unique, not only in providing the unusually low four pole to two pole torque ratio illustrated in the table, but also that both winding interconnection schematics ha~e the pole groups for each phase connected in series.
In the low speed four pole configuration illustrated in Fig. 8, one source phase terminal 79 connects to the series combination of the fir-qt phase winding defined by leads 119, 125, 131 and 113. The central or neutral connections of the wye configuration is then coupled to another source phase ter-minal by leads 121, 115, 109 and 127, while the third source lS phase is connected to the third phase winding by leads 111, 117, 123 and 129, which again terminates at the neutral center connection. This arrangement provides two consequent poles for each pair of wound poles, so current flow fromlead 119 to lead 125 provides the same magnetic polarity of a wound pole a~s does current flow from lead 131 to lead 113. It will be noted, however, that current flow from lead 119 to lead 125 necessitates that same current flow from lead 113 to lead 131 in the connection illustrated in Fig. 9, and hence the two magnetic poles associated with that phase are unlike and the Fig. 9 interconnection provi~es a high speed or two pole mode of operation.
Figs. 10 and 11 are magnetomotive force diagrams for the winding arrangement illustrated in Fig. 1, with Fig. 10 corres-ponding to the two pole high speed winding interconnection of Fig. 3, while Fig. 11 illustrates the four pole consequent pole lower speed interconnection of these windings, as depicted in Fig. 2. Before proceed~ng with a discussion of these magneto-1139- ~4~

motive force diagrams, it should be recalled that a three phase voltage soùrce may be depicted as three superimposed sinusoidal wave forms each displaced from its predecessor by 120 electrical degrees. If time is "frozen" when one of these sinusoidal wave forms is at a maximum, the other two phase wave forms will be at one-half this maximum magnitude and in an opposite sense.
Therefore, in the circuit diagram of Fig. 3, if one ampere were exiting terminal 81, one-half ampere would be entering each of terminals 79 and 83 at this instant. It is the magnetomotive wave form which occurs at this time which is depicted in Figs.
10 and 11.
The stator core of Fig. 1 has been cut along a slot adjacent to slot 53, uncurved and laid out in a straight line to aid understanding of these magnetomotive force wave forms.
Thu~, a single conductor representing coil 41 and a single conductor representing coil 55 are depicted in a first slot, coil 43 depicted in a second slot and the remaining coils of one pole group from Fi~. 1 identified by their corresponding reference numerals 45 and 47. Further, the conductor exem-plifying a coil of a particular phase, say phase 1, is illus-trated as being circular, while phase 2 is depicted as a triangular conductor and phase 3 as a square conductor, all merely for ease of understanding. At the aforementioned frozen instant of time, current flowing in the direction toward the observer from the drawing is illustrated by a dot within each conductor, while current flow in the direction away from the observer is depicted b~ a cross within each conductor.
The cumulative effect of these current flows gives rise to the north and south poles of the illustrated somewhat stepped increasing and decreasing air gap mmf at this instant of time.
When the several phase pole groups are interconnected as illustrated in Fig. 2, the slot positioning of the individual 113~;~4S
03-aM-5246 coil indicating conductors remains unchanged, however, the direction of current flow in certain ones of those conductors changes and the contribution of each is again summed, this time giving the magnetomotive force diagram illlustrated in Fig. 11.
Of course, the designation of phases as being Phase 1, Phase 2, and Phase 3 is completely arbitrary and as discussed may not correspond to the mechanical order of placing the coils in the core slots. However, interchanging two phases, as is well known, merely results in a reversal of the direction of motor operation and is not significant to the present invention.
In summary then, the series-delta two-pole, series-wye four-pole arrangement of Figs. 7 through 9 pro~ides good two-pole and four-pole performance since typically the higher torque requirement is in the two pole configuration and this basic design may be "tuned" by slight modification of the numbers of turns to provide torque ratios of the four-pole to the two-pole configuration in the range of .5 to 1. The parallel-wye two-pole and series-delta four-pole arrangement of Figs. 1 through 3 has a short (1-6) pitch with a very desirable end turn configuration and requires the interconnection of only six leads to accomplish switching between its two pole number operating modes. This Fig. 1 through 3 configuration provides reasonably good two pole and four pole performance without excessive losses and with acceptably low harmonic content in each of the pole configurations, providing an overall desirable motor design.
From the foregoing it is now apparent that a novel method of fabricating a stator assembly for use in a pole changing motor as well as novel polyphase induction motor stator arrangements 1139~4~

have been disclosed meeting the objects and advantaseous features se~ out hereinbefore as well as others and that modifications as to the precise configurations, shapes and details may be made by those having ordinary skill in the art without departing from the S spirit of the invention or the scope thereof as set out by the claims which follow.

Claims (10)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. In a method of fabricating a stator assembly for a two-speed pole-changing three-phase motor having a predetermined torque ratio for the two operating speeds comprising the steps of: selecting a stator core having a rotor accepting bore and a plurality of winding accepting core slots communicating with the bore; determining the number of turns of a selected wire size and a corresponding concentric winding configuration which may be placed in selected core slots to form a plurality of pole group windings;
determining the number of effective turns per pole in each of the specified pole number operating modes; and determining a ratio of the torque of the motor at the different operating speeds; the improvement comprising: changing the torque ratio so as to bring it into closer conformity with the predetermined torque ratio by changing the pole pitch and thereby changing the winding distribution in the core slots so as to change the number of effective turns per pole in each of the operating modes of the motor.

2. The method of claim 1 wherein the windings comprise concentric coils, and changing the pole pitch is accomplished by changing the number of core slots spanned by concentric coils in each of the plurality of pole groups by a like amount.

3. In a polyphase induction electric motor, a stator comprising a primary core member having end faces, a yoke section, and a number of teeth sections forming coil accommodating slots and a bore; and first, second and third winding phases displaced in phase one from the other carried by said core;
each of said winding phases including at least two pole groups each formed by at least two concentric coils with an outermost coil in each pole group spanning the most teeth sections for that pole group while having less than full pole pitch; the three winding phases being delta connected to a polyphase source with the two pole groups of each phase connected in series to be of the same magnetic polarity for inducing a like number of consequent poles for each phase for operating the motor in a lower speed mode and the three winding phases being wye connected to a polyphase source with the two pole groups of each phase connected in parallel to be of opposite magnetic polarity for operating the motor in a higher speed mode so that the pitch factor for each winding in either speed mode connection is less than one.

4. The stator of claim 3 wherein each winding phase consists of two pole groups each formed by four concentric coils.

5. The stator of claim 4 wherein adjacent coil side turn portions are separated by one stator tooth.

6. The stator of claim 5 wherein the core member has twenty-four teeth and the number of teeth spanned by the respective concentric coils of each pole group is two, four, six and eight respectively.

7. In a polyphase induction electric motor; a stator comprising a primary core member having end faces, a yoke section, and a number of teeth sections forming coil accommodating slots and a bore; and first, second and third winding phases displaced in phase one from the other carried by said core;
each of said winding phases including at least two pole groups each formed by at least two concentric coils with an outermost coil in each pole group spanning the most teeth sections for that pole group; the three winding phases being wye connected to a polyphase source with two pole groups of each phase connected in series to be of the same magnetic polarity for inducing a like number of consequent poles for each phase for operating the motor in a lower speed mode and the three winding phases being delta connected to a polyphase source with the two pole groups of each phase connected in series to be of opposite magnetic polarity for operating the motor in a higher speed mode.

8. The stator of claim 7 wherein each winding phase consists of two pole groups each formed by four concentric coils.

9. The stator of claim 8 wherein adjacent coil side turn portions are separated by one stator tooth, the pitch factor for each winding in the higher speed mode being greater than one-half.

10. The stator of claim 9 wherein the core member has twenty-four teeth and the number of teeth spanned by the respective concentric coils of each pole group is four, six, eight and ten respectively.