Patent GB-175306-A (GB)
Title: Windings for alternate current dynamo electric machines
IPC: H02K17/14
Filed: 1920-08-05
Abstract: 175,306. Creedy, F. Aug. 5, 1920. Alternating-current induction machines; motors, controlling.-Consists in secondary windings for internally cascaded motors which enable them to run on a large range of pole numbers and which may also be used as primary windings; means to enable such machines to rise from a cascade speed to basal speeds corresponding to the pole numbers; and primary windings for use with such secondaries. If the two numbers of poles have a common factor CF, the machine circumference will present CF indentical zones each having P and Q pole pairs. A method is given for constructing a curve showing the ideal distribution of ampÚrebars or the volts per bar around the circumference for running on P and Q pole pairs and methods are described for designing windings which approximate to such curves. Such windings will act as primary on one and secondary on the other of two numbers of pole pairs the sum or difference of which divided by their H.C.F. is greater than three. The 5-lobed curve shown in Fig. 3 is the ideal curve of ampere-bars for a machine running on 4 and 6 poles. Such curves have, in general, P + Q and P - Q maxima when P and Q pole pairs rotate in opposite and similar directions respectively, the space phase angle between the maxima being 360‹ multiplied by one pole number and divided by the sum or difference of the pole numbers. If n be the number of equally spaced circumferential winding sections then P + Q = n and the winding must be capable of giving n phases for fields rotating in opposite directions. Further additional phases may be introduced to approximate the curve given by an actual winding more closely to the ideal curve. The five triangles in Fig. 3 are the ampere-bar curve given by a 10-pole single phase winding, Fig. 2, occupying 40 out of 60 slots in a rotor. By placing additional conductors of proper phase in the two vacant slots per pole, e.g. 3, 4, 27, 28, the distribution, as shown in dotted lines, comes much closer to the ideal curve. Internal cascade windings adapted for different pole numbers and of which certain bars are eliminated by superposition of two windings may also be designed by reference to the ideal curve. It is shown that if two windings having r n sections and the requisite phases for P and Q pole pairs respectively rotating in opposite directions be superposed, half of the sections will cancel and a single section of intermediate phase will replace those superposed. Generally, if windings for P and Q pole pairs both having r (P + Q) sections be superposed, 1/7 th of the sections may be eliminated P and Q being such that their sum divided by their H.C.F. gives a number greater than three. Adjustment of the component flux densities to obtain closer agreement with the ideal curve may be effected by varying the winding pitches. An example is given of a 24-section winding of the type described in Specification 175,583 for 10 and 14 poles (n = 24, P = 6, Q = 7) in which one half of the sections in the two groups cancel. Internally cascaded machines of the present type may be raised to basal speeds corresponding to the pole members by making extra connections in the winding so as to subdivide it into a number of closed circuits which is not equal to or a submultiple of the sum of the two numbers of pole pairs, this being done by means of slip-rings on the rotor or by means of an additional rotor winding. Fig. 7 shows these methods applied to a winding suitable in a 40-slot armature, as shown in Fig. 5, suitable for 4 and 6 or for 2 and 8-pole fields. The winding is marked in Fig. 5 with phases corresponding to 4 and 6 poles and, as shown in Fig. 11, it comprises a mesh of five coil elements each formed of two coils in series reversed, together with five single elements joined in star between the mesh-points and a common star-point. In Fig. 11 the black and open circles correspond to sections in Fig. 5 consisting of connected upper and lower conductors in two slots, the slot numbers of the upper conductors being used in Fig. 11. In Fig. 7 the phases are shown for an 8-pole field. The dotted lines show an auxiliary star winding placed in the partly filled slots of Fig. 5 and joined to sliprings which may be short-circuited, as shown in the lower part of the Figure, for raising the speed to that of one of the basal pole numbers. Alternatively, the midpoints of the mesh sides may be brought out to other slip-rings, as shown above the Figure, these being short-circuited for raising the speed. The two methods may be combined by short-circuiting each phase of the cascade winding through a phase of the auxiliary winding. Examples an given of the adaptation of windings as in Figs. 5 and 11 for other pole and phase members by alterations in the numbers of slots and conductors without alteration in the slot pitch and coil connections. Motors having secondary windings as described may be given additional speeds by using the windings as primaries. Thus, the winding of Fig. 5 may be converted into a 4-phase primary by connecting the conductors not used in the cascade winding, e.g. in slots 3 and 6, into pairs of coils and joining the lower conductor of each coil to the mid-point of the adjacent mesh side, thus giving a 20-side mesh with a 20-armed star joined to it. The 4-phase supply terminals are obtained by joining the first, fifth, ninth ... arms, the second, sixth, tenth ... arms together, and so on. Other examples of such conversions are given. A stator winding for use with such a secondary is shown in Fig. 20 and represents a type having any pair of pole numbers which divided by their G.C.M. give an odd and an even quotient. This winding is adapted for primary use on 16 or 12 poles and as a secondary on 4 or 8 poles alternatively or vice versa, and has 24 sections arranged in groups of four, the members of each group, e.g. 1, 7, 13, 19, being evenly spaced around the stator. Threephase energy is supplied at L1, L2, L3. With the switch thrown to the left or right hand, a 16-pole mesh or 8-pole star-winding respectively is given. The winding, by rearrangement as in Fig. 21, gives 12 poles, the supply in this case being at L1 and at L2, L3 the extremities of a transformer W from which three of the groups in series take off 0.866 of the line voltage. Other examples of such stator windings to give 2 and 6 poles are shown with and without the use of variable resistances for starting. If the sections forming the groups are not evenly spaced on the iron, overlapping sections may be used to compensate the out-of-balance voltages. Stator windings of this nature may be used for fields rotating similarly or oppositely and when the secondary is designed for 1 and 3 pole pairs and a 12 or 18-phase supply is required for producing the requisite 2-pole primary field, this supply may be obtained as described in Specification 123,395 or by means of an auxiliary winding on the primary member. A 48-section star-mesh winding for use on several pole numbers which acts as a cascade winding if alternate star terminals are short-circuited is shown in Fig. 24. This may be brought up to a basal speed if the open-circuit terminals are connected through resistances or to cascade speed with half of the terminals open-cirenited by connecting resistances across tappings of the machine. Alternatively, the secondary terminals which are short-circuited on cascade speed may be connected through variable resistances for starting. For bringing the machine up to a basal speed, a set of sliprings s<1> and resistances r<1> are employed, together with a switch SW<1> on the rotor. This switch is partly shown as four-groups of a 2-way 24-pole switch, the contacts of which bear the same numerals as the sections to which they are connected. For running up to cascade speed those terminals which have to be short-circuited for cascade running are connected to the rings. With the switch in the position shown in dotted lines on the left-hand, these terminals are short-circuited and instead of them alternate star terminals for cascade running are connected to the slip-rings in readiness for running up to a basal speed. Fig. 25 shows a motor having a rotor F with windings I - - VI in parallel groups adapted to run as a secondary on more than one pole number, say 4, 8 or 12. D is a star-connected 12-pole stator winding and E another stator winding giving 4 or 8 poles, the terminals being connected to correspondingly numbered contacts on switches G, H. K. With switch G alone closed, the winding E is inoperative and the motor runs on 12 poles. With H closed, the rotor supplies current to the stator winding E causing it to produce 4 poles to which the rotor acts as a secondary while also so acting in the 12-pole field D, giving a cascade speed corresponding to 16-poles. With K closed, the winding F runs as a secondary in the 8-pole field due to coils E at a 20-pole-speed. The number of speeds obtainable with the star-mesh windings described may be increased by means of a switch which reverses the star sections with respect to the mesh sections.