Stepping motor

A stepping motor of the homopolar design with a step angle of 3.6.degree. and having a generally cubic form is disclosed. The motor includes a generally square, single-section laminated stator which has only four stator poles extending radially inwardly from the middle regions of the straight sides, and a concentric rotor secured to a shaft journaled in a pair of bearing bells fixed to the stator at the opposite faces thereof and having the same generally square configuration. The rotor includes a disc-shaped permanent magnet of high remanent energy density (B.times.H).sub.max .gtoreq.10.times.10.sup.6 GOe, and a pair of annular steel rotor elements provided at the working air gap with identical numbers Z.sub.r of rotor teeth of pitch .tau..sub.r =360.degree./Z.sub.r. Each stator pole is provided at the working air gap with four or five stator pole teeth of pitch .tau..sub.s =360.degree./Z.sub.s, where Z.sub.s is the theoretically maximum available number of stator pole teeth and must be evenly divisible by 4, and Z.sub.s =Z.sub.r -1.+-.k where k is an even integer and 0.ltoreq.k.ltoreq.6. A special twin-section insulation member adapted to be axially fitted into the stator from the opposite faces thereof and designed to permit simultaneous machine winding of the field windings onto the respective stator poles is also disclosed. This abstract is not to be taken either as a complete exposition or as a limitation of the present invention, however, the full nature and extent of the invention being discernible only by reference to and from the entire disclosure.

This invention relates to a stepping motor which is adapted to drive, under 
electronic control and in steps in any desired direction of rotation, a 
round plastic disc arranged on the linearly moving carriage of a printing 
machine and having, for example, 100 movable type characters. 
An office machine (typewriter), which works on this principle, is 
described, for example, in the American company brochure "Qume The Printed 
Word" published by Facit-Vertrieb-AG, of Seftigenstr. 57, CH-3000 Berne 
17, Switzerland. 
There are various types of stepping motors on the market. For this reason 
it was tried at first to use the existing stepping motors for the 
above-mentioned special problem. The available stepping motors were, 
however, found to be unsuitable in many respects for use in several 
machines that are currently in the development stage, for a variety of 
reasons. Among these were that such motors had too large a diameter, too 
great a weight relative to their output, either too small or too large a 
step angle, and too high production costs. 
It is the principal object of the present invention, therefore, to provide 
a novel and improved special stepping motor by means of which the 
above-mentioned drawbacks and disadvantages could be eliminated. 
More particularly, the new stepping motor according to the present 
invention works on the known homopolar principle for self-starting 
synchronous motors. This principle is described, for example, in U.S. Pat. 
No. 2,982,872, and to the extent required for an understanding thereof the 
disclosures of that patent are hereby incorporated herein by reference. In 
terms of construction, however, it was found that a motor such as the one 
disclosed in the said patent is not free of the above-mentioned drawbacks 
and that it was necessary, therefore, to abandon the conventional design 
criteria and to make the stator, the rotor, the step angle, the winding 
insulation and the winding technique considerably different from those 
elements as used in the conventional design. 
Basically speaking, the present invention provides a stepping motor of the 
homopolar design which utilizes a layered stator having a plurality of 
radially inwardly directed stator poles which are toothed at the working 
air gap, and an axially magnetized rotor including an axially disposed 
permanent magnet and two magnetically conductive circumferentially toothed 
steel rotor elements sandwiching the magnet therebetween and secured 
jointly with the same on a magnetically non-conductive shaft journaled in 
a pair of bearings mounted in respective magnetically non-conductive 
bearing bells. In its overall configuration, the motor is not cylindrical 
in shape but rather has a generally cubic form, apart from being slightly 
rounded or chamfered along its edges parallel to the shaft axis, by virtue 
of the stator and the bearing bells each being essentially square in 
outline except for having chamfered edges along the corners parallel to 
the shaft axis. The stator is a single-section laminate, however, and thus 
the motor does not have an external yoke housing axially surrounding the 
stator such as is required for a multi-section stator in order to enable 
the magnetic circuit to be completed. The fastening of the two bearing 
bells to the stator in properly centered relation is effected by 
parallelly arranged bolt or rivet-type fastening means extending through 
suitable recesses provided in the stator in the regions of the four 
chamfered corners thereof and corresponding recesses provided in the 
bearing bells at the four corners thereof. 
The stator differs further from the known design in that it has only four 
poles each extending inwardly from the middle region of a respective one 
of the four straight sides of the stator, and each stator pole has four or 
five uniformly pitched teeth on its end face disposed along the circular 
locus defining the outer boundary of the working air gap, with the space 
between each two adjacent stator poles being equal to the space that would 
be required to accommodate one or two additional stator pole teeth at the 
same uniform pitch. The rotor on the outer periphery of each of its two 
lateral elements likewise has a set of uniformly pitched teeth. The pitch 
of the stator pole teeth is expressed by the relation .tau..sub.s 
=360.degree./Z.sub.s where Z.sub.s is the theoretically available number 
of stator pole teeth, the pitch of the rotor pole teeth is expressed by 
the relation .tau..sub.r =360.degree./Z.sub.r where Z.sub.r is the number 
of rotor teeth, and the numbers of the two sets of teeth are expressed by 
the relation Z.sub. s =Z.sub.r -1.+-.k where k is an even integer (on the 
basis of the requirement that Z.sub.s .noteq.Z.sub.r) and 
0.ltoreq.k.ltoreq.6, and Z.sub.s must be divisible by four. A step angle 
.phi. of 3.6.degree. is achieved in a stepping motor of this invention 
which has, for example, 25 rotor teeth and 16 or 20 stator pole teeth, 
with the theoretical number of stator pole teeth being 24. 
The rotor magnet is a thin ring disc magnet which is axially magnetized and 
is made of a highly coercive magnetic material, advantageously a cobalt 
and rare earth metal alloy, and has a relatively high remanent energy 
density (B.times.H).sub.max .gtoreq.10.times.10.sup.6 gauss-oersteds (10 
MGOe). The ratio of disc diameter to disc thickness ranges from about 4 to 
about 20. Correspondingly, the two rotor sections or elements on which the 
rotor teeth are provided likewise have a ring disc form and are inwardly 
chamfered along their outer peripheries on the sides facing each other and 
the magnet, the angle of the chamfer being between about 30.degree. and 
60.degree.. 
The insulation of the stator against the four pole windings consists of a 
specially shaped twin-section annular insulating member the sections of 
which can be fitted into the stator from the opposite sides of the latter 
so as to cover all four stator poles but in a fashion which leaves between 
each two adjacent poles a gap not less than 2 mm wide and enables the 
insulating member to serve as wire guides for the simultaneous winding of 
all four stator poles by means of a needle winding machine. The four pole 
or stator field windings may consist of four monofilar or single-wire 
windings, or alternatively of four bifilar or two wire windings with the 
two partial windings of each pole winding being connected in series or in 
parallel, but in either case the windings are so arranged that only one 
pole winding per phase is energized after each step.

Referring now to the drawings in greater detail, a stepping motor 10 
according to the basic principles of the present invention includes (FIG. 
1) a laminated steel stator 11 having a plurality of inwardly directed 
poles 12 provided with teeth 13 on their circularly curved inwardmost 
faces (FIG. 2), and a rotor 14 mounted on a rotatable shaft 15 and 
consisting of an axially magnetized annular permanent magnet 16 and a pair 
of annular rotor elements 17 and 18 sandwiching the magnet therebetween. 
The rotor elements are provided with respective sets of external teeth 19 
and 20 the circular locus of the outermost surfaces of which is spaced 
from the circular locus of the inwardmost surfaces of the stator pole 
teeth 13 by a working air gap 21. The stator poles are covered, except at 
the teeth, by a twin-section insulation member 22 over which the various 
field windings 23 for the individual poles are wound. The windings are 
covered by a pair of housing elements or end bells 24 and 25 which are 
secured to the stator at opposite sides thereof and which at their central 
regions support a pair of bearings 26 and 27 for the shaft 15. The motor 
10, it should be pointed out, is defined as being a stepping motor of the 
homopolar design, because of the fact that the magnet 16 is axially 
magnetized, which means that all the poles that are formed at one of the 
toothed rotor elements 17 and 18 are "N" poles while all the poles that 
are formed at the other one of the said rotor elements are "S" poles. 
As will be seen, to a certain extent the motor structure outlined above 
generally parallels that of the motor disclosed in U.S. Pat. No. 
2,982,872. Nevertheless there are a number of major differences between 
the patented motor and that of the present invention, which will now be 
described. Inasmuch as a great many considerations, supported by extensive 
calculations and tests, entered into the determination of the various 
individual features of the stepping motor of the present invention, 
however, it is deemed advisable to discuss the changes and modifications 
made in the different motor components individually. 
(A) Modifications in the Stator 
These are represented in FIGS. 1 and 2. The stator 11 is a single-section 
sheet steel lamination formed with the aid of four rivets 28 and, as shown 
in FIG. 2, has a generally square form with four sides 11a which are 
straight except for being chamfered or slightly rounded at the corners, as 
indicated at 11b. Since the stator is a single-section structure (FIG. 1), 
devoid of any medial transaxial air gap, there is no need for a yoke to 
complete the magnetic circuit. The external magnetic stator shell which is 
required in the patented motor for the magnetic field return thus has been 
eliminated. 
The stator 11 is provided with only four inwardly directed stator poles 12 
onto which the windings 23 are wound. Each such pole at its inwardmost 
surface facing the working air gap 21, is circularly curved and concentric 
with the axis of the shaft 15. The stator pole teeth 13 advantageously are 
five in number as shown in FIG. 2 although they may be only four in number 
(not shown). The maximum possible number of stator pole teeth, which is 
the sum of the teeth 13 actually provided plus those teeth which are 
missing due to the provision of the gaps or spaces between the adjacent 
stator poles 12 (at the rate of one or two per gap, as the case may be), 
is denoted Z.sub.s, and the pitch of the stator pole teeth based on the 
maximum number Z.sub.s is denoted .tau..sub.s. It should be noted that the 
number of stator pole teeth, i.e. one or two, which are omitted between 
adjacent stator poles is determined by production considerations. Thus, it 
is an aspect of the present invention to provide a capability for the 
field windings to be wound be machine simultaneously onto the four stator 
poles, and the interpolar gaps are required to accommodate the machine 
needles. As a practical matter, therefore, the interpolar gap size cannot 
be less than about 2 mm, and it will be clear that although in a motor of 
a given size it may be possible to attain this goal through the omission 
of only four stator pole teeth overall, in a somewhat smaller motor the 
omission of eight stator pole teeth may be required. 
In order to ensure best utilization of the available space and to enable 
minimization of the size of the motor relative to its output or 
performance, the four stator poles 12 are arranged in the middle regions 
of the straight sides 11a of the stator, and the winding spaces 29 are 
disposed in the corner regions. In these corner regions, the stator is 
further provided with U-shaped recesses 30 adapted to accommodate 
respective throughbolts or like fastening means (not shown) to enable the 
two end housing members or bearing bells 24 and 25 to be secured in 
properly centered relation to the stator and each other (centering may 
also be aided by suitable internal chamfers on the bearing bells 
corresponding to the chamfers 11b on the stator if the bearing bells are 
designed to accommodate at least in part the circumference of the stator). 
The external configuration of the bearing bells is the same as that of the 
stator. The overall motor configuration thus is essentially cubic, which 
makes the motor of the present invention adaptable to utilization in some 
environments and applications where a motor with the conventional round or 
cylindrical configuration cannot be as advantageously used. 
(B) Modifications in the Rotor 
As a concomitant to the reduction of the stator dimensions, the diameter of 
the rotor 14 has likewise been reduced. It was necessary, however, that 
this be done as far as possible without loss of magnetic output. 
Accordingly, the permanent magnet 16 of the rotor 14 is a highly coercive 
disc magnet having a very high remanent energy density in excess of 
10.times.10.sup.6 gauss-oersteds, i.e. (B.times.H).sub.max .gtoreq.10 
MGOe. Suitable materials for this purpose are certain alloys of cobalt and 
rare earth metals which are commercially available, such as a 
samarium/cobalt alloy (Sm/Co.sub.5) marketed by Brown, Boveri & Cie, A.G. 
of Mannheim, Germany, and a cobalt/rare earth metal alloy (exact 
composition not known at this time) marketed by Fried. Krupp GmbH of 
Essen, Germany under the registered trademark KOERMAX. This choice was 
based on a number of considerations. Thus, a calculation of the magnetic 
circuits of various motor models showed that through the use of rotor 
magnets of this type, even where the same are relatively thin magnet discs 
and are accompanied by an increase in the air gap induction, it might be 
possible for the rotor volume to be considerably reduced. These 
calculations were subsequently fully confirmed by actual test models. In 
particular, it was found that the use of a high coercitivity magnet made 
of the aforesaid type of cobalt/rare earth magnetic material makes it 
possible to attain, in a motor of small dimensions such as here 
contemplated, a relatively high torque over the entire step frequency 
range. The use of this material also is significant from a manufacturing 
standpoint, in that it makes it possible to provide the motor with a 
relatively large working air gap through the use of a smaller rotor and 
thereby to minimize the production costs of such a motor. Still further, 
the use of the said material prevents a possible demagnetization of the 
rotor in the event of a high-amplitude pulsating field excitation of the 
stator poles, and it enables the usually required exterior steel housing 
or yoke for completing the magnetic circuit axially of the stator to be 
dispensed with. 
The optimum ratio of magnetic disc diameter to magnetic disc thickness 
depends on the axial length of the stepping motor, i.e. of the rotor. For 
the purposes of the present invention, this ratio is about 20 in axially 
short stepping motors and about 4 in axially long stepping motors. 
As previously stated, the rotor magnet 16 is axially magnetized and is 
arranged between two magnetically conductive plane-parallel annular discs 
or rotor elements 17 and 18. Thus, by virtue of the axial magnetization of 
the disc magnet 16, only "N" poles are formed at one and only "S" poles 
are formed at the other of the toothed rotor elements 17 and 18. In order 
to minimize the magnetic stray field losses, however, necessitated by the 
small thickness of the disc magnet 16, the discs 17 and 18 are provided, 
on the sides thereof facing the disc magnet 16, with respective 
peripherally beveled extensions 17a and 18a (FIG. 1), the bevels extending 
down to the outside diameter of the magnet at an angle of between about 
30.degree. and 60.degree. and preferably at a mean angle of about 
45.degree.. 
The two sets of teeth 19 and 20 provided on the rotor discs or elements 17 
and 18 are, of course, identical, the number of rotor teeth on each disc 
being denoted Z.sub.r and their pitch being denoted .tau..sub.r (FIG. 2). 
For reasons well known to those skilled in the art, however, the two sets 
of teeth are circumferentially offset or staggered relative to each other 
in known manner by a half rotor tooth pitch (FIG. 1). Both the rotor 
magnet 16 and the two rotor discs 17-18 are fixedly secured to the motor 
shaft 15, for example by being cemented thereto, and the shaft, in order 
to avoid a magnetic shunt, is made of nonmagnetizable steel or the like. 
(C) Modifications in the Step Angle 
A printing machine 31 of the type for which the stepping motor of the 
present invention is particularly well suited is illustrated schematically 
in FIG. 9. Only the salient elements of the machine are shown, namely the 
supply/takeup rolls 32 for the paper tape 33, the plastic printing disc 
34, the typeactuating striker device 35, and the stepping motor 36. As 
previously indicated, in this machine the plastic disc 34 to be driven by 
the stepping motor has generally one hundred type characters 37 (FIG. 9A) 
which are provided on radially arranged, elastically movable tongues 38. 
In order to meet the requirements of small disc diameter and low moment of 
inertia in such a system, it is necessary, at a given type spacing, to 
utilize the full disc circumference. Accordingly, it becomes necessary to 
use a stepping motor with a step angle .phi. of 3.6.degree. 
(360.degree./100). 
Motors of the known homopolar type operating at a small step angle, e.g. 
motors such as the one shown in U.S. Pat. No. 2,982,872, require for the 
customary two-phase control thereof either stator poles with an integral 
number, i.e. forty-eight, of stator pole teeth conforming to the relation 
Z.sub.s '=Z.sub.s -v, where Z.sub.s ' denotes the total number of existing 
stator pole teeth, v denotes the number of stator pole teeth omitted 
between the adjacent poles for enabling the field windings to be applied 
to the stator poles, and Z.sub.s denotes the maximum possible number of 
stator pole teeth. At the same time, the rotor consists of a permanent 
magnet with a relatively low energy density, i.e. (B.times.H).sub.max &lt;6 
MGOe, and includes two magnetically well conductive rotor pole caps each 
of which has a different integral number Z.sub.r, i.e. fifty, of rotor 
teeth on its circumference. The step angle of this stepping motor, as is 
known, thus is 1.8.degree.. 
It will be appreciated, therefore, that the abovementioned requirements for 
a small stepping motor diameter and a step angle of 3.6.degree. cannot be 
met with such a motor as is disclosed in U.S. Pat. No. 2,982,872, for the 
following reasons: 
1. For the above described known stepping motor with eight stator poles, 
the stator pole tooth pitch .tau..sub.s and the rotor tooth pitch 
.tau..sub.r are expressed by the relations 
.tau..sub.r =360.degree./Z.sub.r and 
.tau..sub.s =360.degree./Z.sub.s =360.degree./(Z.sub.r -2). 
It follows, therefore, that 
EQU Z.sub.s =Z.sub.r -2. 
In addition, for a symmetrical stepping motor with faultless step angle, 
the number of stator pole teeth must be divisible by the number of stator 
main poles without a remainder, which, it is easy to see, is a requirement 
that cannot be met for a 3.6.degree. step angle motor where the number of 
rotor teeth Z.sub.r must be 25. 
2. Another reason which argues against this known stepping motor design is 
the fact that at the required small motor diameter, the introduction of 
eight stator main pole windings and their wiring would only be possible at 
extremely high production costs. 
By way of contrast, in the new stepping motor according to the present 
invention, as already mentioned in section A above, only four stator 
poles, and correspondingly only half the number of stator pole windings 
specified for the known motor, are required. 
A symmetrical design of the stator with four poles for a faultless step 
angle can be achieved if two conditions are satisfied; first, 
.tau..sub.r =360.degree./Z.sub.r and 
.tau..sub.s =360.degree./Z.sub.s =360.degree./(Z.sub.r -1.+-.k) 
where k denotes an even integer (to ensure that Z.sub.s .noteq.Z.sub.r) and 
0.ltoreq.k.ltoreq.6, and second, the number of stator pole teeth Z.sub.s 
must be divisible by the number of stator poles without a remainder. 
With Z.sub.r =25, it follows that Z.sub.s =24 at k=0. Both requirements are 
thus met. 
As a practical matter, in a motor according to the present invention the 
value of k, absent special considerations related to the magnitude of the 
step angle required (depending on whether the number of type characters is 
somewhat above or below 100, the step angle may have to be somewhat 
smaller or greater than 3.6.degree.), will always be chosen to be 0. It 
should be understood, however, that by choosing k&gt;0 (but always an even 
number), it also becomes possible, for a given stator (or rotor) 
construction, to vary the number of rotor (or stator) teeth, and thus the 
step angle, within certain limits in both directions, as the situation 
demands. The significance of the number k will be more clearly understood 
from the following examples, which are presented merely by way of 
illustration. 
EXAMPLE 1 
Assuming Z.sub.s =24 and k=0, then Z.sub.r =25 and 
.phi.=360.degree./4.times.25=3.6.degree.. 
EXAMPLE 2 
Assuming Z.sub.s =24 and k=2, then Z.sub.r =27 and 
.phi.=360.degree./4.times.27=3.33.degree.. 
EXAMPLE 3 
Assuming Z.sub.s =20 and k=4, then Z.sub.r =25 and .phi.=3.6.degree.. 
EXAMPLE 4 
Assuming Z.sub.s =20 and k=6, then Z.sub.r =27 and .phi.=3.33.degree.. 
From these examples it can be seen that by suitably using the equation 
Z.sub.s =Z.sub.r -1.+-.k or its equivalent Z.sub.r =Z.sub.s +1.+-.k, one 
can achieve different step angles by providing different numbers of rotor 
teeth for a given number of stator pole teeth (Examples 1 and 2), or one 
can achieve identical step angles by providing different number of stator 
pole teeth for a given number of rotor teeth (Examples 1 and 3), etc. 
Other combinations are, of course, also possible, and which of the various 
combinations (whether specified above or not) is selected as best suited 
for any given application will be decided on a case by case basis. The 
stepping motor of the present invention thus can be adapted to any 
required number or spacing of the type characters at a given plastic disc 
diameter. 
(D) Measures for Simplifying and Reducing the Costs of Stator Windings 
These are represented in FIGS. 3-3A-3B-3C, 4-4A-4B, and 5 to 8. 
In actual practice, stepping motors of the known types having eight stator 
poles are made with the individual pole or field windings consisting of 
two bifilar partial windings each, which are so wired that a symmetrical 
winding with center tap is formed. All stator pole windings wich center 
tap thus formed are connected in parallel, phase by phase. This, however, 
entails relatively high wiring costs. 
In accordance with one aspect of the present invention, therefore, it is 
contemplated to utilize, as a means for minimizing the winding and wiring 
costs incident to the manufacture of stepping motors with only four stator 
poles, four monofilar or single-wire windings W-1, W-2, W-3 and W-4, one 
for each pole 12 of the stator 11 (FIG. 3). Concomitantly it is proposed 
to connect the windings into a control circuit 39 (FIG. 3A), which has a 
power source 40 and a pair of reversing control switches 41 and 42 (these 
are illustrated in conventional form but in actual practice are electronic 
switches such as transistors, flip-flops, etc.) shiftable between 
respective positions or conductive states I/II and III/IV. In the circuit 
39, the two windings of each of the diametrally opposed pairs of windings 
W-1/W-3 and W-2/W-4 have a respective common connection between those 
windings, designated 43 and 44 in FIG. 3A, connected to the same terminal 
45 of the power source 40. The arrangement of the control circuitry is 
such that after any given step only one winding per phase is energized, 
i.e. either W-1 or W-3 and either W-2 or W-4. 
To this end it will be understood that the various electronic switches 41 
and 42, composed of suitable circuit elements and configurations, e.g. 
transistors, gates, flip-flops, or the like, well known to those skilled 
in the art and not necessary to illustrate or describe in detail, are 
themselves controlled and shifted from one current carrying state to 
another by means of suitably timed electronic pulses or input signals. The 
pulse sequence (and hence the winding energization sequence) for the 
circuit arrangement represented purely schematically by FIG. 3A and the 
resultant sequence of stator pole polarities are diagrammatically 
illustrated in FIGS. 3B and 3C, respectively. In FIG. 3B, T.sub.f denotes 
the time duration of the energized state of each winding (it is the same 
for all windings), I.sub.n denotes the current amplitude in each energized 
winding (it is the same in all windings and is also denoted I.sub.1 to 
I.sub.4 for the windings W-1 to W-4, respectively), and T.sub.u denotes 
the time interval overlap between energization states of the windings 
which is required to effect the polarity changes of the windings in each 
phase. 
In operation: At step 0 (time 0), while the electronic switch 42 is still 
in the state thereof denoted III in FIG. 3A, so that winding W-2 is and 
remains "on," the switch 41 is shifted to its state I. As a result, 
current I.sub.2 continues to flow in winding W-2, and winding W-1 turns 
"on" as current I.sub.1 begins to flow in winding W-1. Both of these 
windings thus provide "N" polarities at their respective stator poles 
(FIG. 3C), while windings W-3 and W-4 are "off." At step 1 (time 1/2 
T.sub.f) following a time overlap interval T.sub.u of concurrent 
energization of the windings W-1 and W-2, the switch 42 is shifted to its 
state IV while the switch 41 remains in its state I; the winding W-2 thus 
is deenergized as its current I.sub.2 is turned "off" while the winding 
W-4 is energized as the flow of current I.sub.4 commences (FIG. 3B). The 
"N" polarity of winding W-1 thus remains but the winding W-4 now provides 
a "S" polarity for its respective stator pole (FIG. 3C). The manner of 
continuation of the winding energization sequence and the polarity 
shifting sequence thereafter can be readily understood from FIGS. 3B and 
3C and will thus not be further described, except to point out that at 
step 4 the overall cycle resumes again, with the various conditions at 
that time once more being the same as at the previous step 0. 
The sequence of polarity changes effected by the current flow sequence in 
the windings W-1 to W-4 is, of course, the means by which, in known 
fashion not necessary to describe in detail, the permanent magnet rotor 14 
is moved stepwise in a given direction of rotation as a consequence of the 
magnetic interactions between the stator and the rotor resulting from the 
different numbers of rotor and stator pole teeth and their relative 
instantaneous dispositions. It will also be apparent to those skilled in 
the art that by appropriately changing the pulse or energization sequence 
in the pairs of windings W-1/W-3 and W-2/W-4, it is possible to reverse 
the motor, i.e. to rotate the rotor in a direction opposite to the 
direction resulting from the sequence described above. 
The principles of the present invention as so far described can also be 
applied to a stepping motor utilizing bifilar or two-wire windings 
W-1.1/W-1.2, W-2.1/W-2.2, W-3.1/W-3.2 and W-4.1/W-4.2 on the stator poles 
12 (FIG. 4). In this type of system, however, there exists the further 
possibility of varying the internal resistance of the motor windings in 
order to adapt the motor, without any change in the basic control 
principle, to operation with different control circuits. Thus, the 
resistance of the windings can be varied as required by the gate voltage 
or current flow characteristics of the transistors, flip-flops, etc. used 
in the control circuit, by suitably selecting certain connections between 
the various coils or winding sections internally of the motor. Merely by 
way of example, a resistance-decreasing parallel connection of the stator 
partial windings W-1.1/W-1.2 etc., utilizing respective paired leads 46, 
47, 48 and 49, is shown in FIG. 4A, and a resistance-increasing series 
connection of the stator partial windings W-1.1/W-1.2 etc., utilizing 
respective single leads 50, 51, 52 and 53, is shown in FIG. 4B. 
In accordance with another aspect of the present invention, the insulation 
member 22 (FIG. 1) is a twin-section specially shaped structure 54 made of 
any suitable electrically non-conductive synthetic plastic material and 
best shown in FIGS. 5 to 8. The insulation member is constructed to enable 
all four stator poles to be covered on all sides of the same, leaving only 
an opening or gap about 2 mm wide between adjacent stator poles for the 
introduction of the wire guides of a needle winding machine, to permit all 
four stator pole windings to be inserted simultaneously. Since the 
insulation member 54 consists of two identical sections 54a and 54b, a 
brief description of one of these will suffice for both. 
As shown, each insulator section is an annular structure and includes a 
flat ring portion 55 the outer diameter of which is substantially the same 
as the diameter of the circular locus of the surface portions 29a (see 
also FIG. 2) of the stator winding spaces 29. At one side of the ring 55 
there are provided four radially inwardly depending planar webs 56 
alternating with four laterally extending, circumferentially elongated, 
hollow protuberances 57, of which the latter are shaped and dimensioned to 
fit precisely into the spaces 29 in the stator and are provided at their 
radially inward sides 57a with respective axially extending gaps 57b not 
less than 2 mm wide, while the former are adapted precisely to overlie the 
lateral outside faces of the stator poles 12 at one side of the stator. 
Extending laterally rom each of the four planar webs 56 but in a direction 
opposite to that of the protuberances 57 are four circularly curved 
arcuate webs 58 which are circumferentially spaced from one another by 
respective axially extending gaps 59 not less than 2 mm wide and precisely 
aligned with the gaps 57b. The portions 57a of the protuberances 57 in 
essence are lateral continuations of the arcuate webs 58 but are somewhat 
thinner. In this manner, therefore, there are provided a set of arcuate 
shoulders 60 which extend to opposite sides of the inward faces of planar 
webs 56 and engage the stator poles 12 generally coextensively with the 
teeth-bearing portions of the latter. 
In addition, two diametrically opposed ones of the protuberances 57 are 
provided at their free ends with a circumferential edge region of 
externally slightly reduced thickness, as shown at 57c for the insulation 
section 54a in FIGS. 6 and 8, and correspondingly the other two 
protuberances are provided at their free ends with a circumferential edge 
region of internally slightly reduced thickness, as shown in FIGS. 6 and 8 
at 57d for the insulation section 54b. The arrangement is such that in 
use, the two insulator sections 54a and 54b are fitted into the stator 11 
but displaced 90.degree. out of mirror image relation to each other. Thus, 
the two sets of protuberances are aligned with one another, but they are 
in an alternating sequence telescoped slihgtly into each other, i.e. two 
opposed ones of the protuberances on one insulator section fit slightly 
into the protuberances of the other section aligned therewith, and vice 
versa for the other two. The aligned gaps 59/57b, of course, constitute 
the previously mentioned winding gaps while the interiors of the aligned 
protuberances 57 define the spaces for accommodating the field windings of 
the motor. 
The implementation of the individual but interrelated innovations described 
hereinabove in sections A, B, C and D, has been found to lead to the 
production of a novel stepping motor which fully meets all expectations in 
terms of minimization of size without sacrifice of power, increased 
efficiency, the choice of step angle, and the lowering of production 
costs. Thus, over and above the advantages already adverted to 
hereinbefore, the cubic form of the motor not only means lesser space 
requirements for it, but also that the square 4-pole construction of the 
stator which makes the cubic form attainable can itself be manufactured 
with an economical use of materials and a minimum amount of waste. Still 
further, in a motor of the present invention the amount of available 
winding space for its small size is maximized. 
A motor according to the present invention, when made and used as a 
stepping motor, is particularly suitable for its special task as herein 
disclosed. It is to be noted, however, that a stronger motor, built with 
only four stator poles according to the homopolar principle and operated 
as a self-starting synchronous motor, tends to run very noisily due to 
unbalanced radial forces occurring in the bearings. The use of such a 
stronger motor as a self-starting synchronous motor would, therefore, be 
not highly recommended. 
It will be understood that the foregoing description of various aspects of 
the present invention is for purposes of illustration only, and that the 
structural and operational features and relationships herein disclosed are 
susceptible to a number of modifications and changes none of which entails 
any departure from the spirit and scope of the present invention as 
defined in the hereto appended claims.