Electrical Engineering in Japan, Vol. 187, No.

4, 2014
Translated from Denki Gakkai Ronbunshi, Vol. 132-D, No. 10, October 2012, pp. 976–982

Development of Motor Stator with Rectangular-Wire Lap Winding

and an Automatic Process for Its Production

TAKASHI ISHIGAMI, YUICHIRO TANAKA, and HIROSHI HOMMA

Yokohama Research Laboratory, Japan

SUMMARY torque is inferior to that of a stator with distributed wound

coils. A stator structure with distributed wound rectangular
Aimed at providing small high-power motors with wire coils that is small and has high productivity must
excellent efficiency, a motor stator with rectangular-wire therefore be developed.
lap windings and an automatic process for its production A conventional motor stator with round-wire dis-
were developed. The structure of the windings, namely, tributed windings is shown in Fig. 2. This type of winding
two continuous “α-shaped” coils, and the coil-production is called a “concentric winding.” First, round wire coils are
method (involving simultaneous coil forming and coil in- wound outside the stator core. After that, coils are inserted
sertion) enable mass production of this stator. The slot-fill into the stator slots from the inner side of the core. The
rate for a prototype stator was 80.5%, and the total height coils thus overlap in the radial direction. Finally, these over-
of the coil ends was 60 mm to 72 mm. On the basis of the lapped coil ends are formed in the axial and radial directions
ratio of the sectional areas of two types of the motors, its to be housed in the motor case. Rectangular wire is not
efficiency was estimated to be 3.7% higher than the efficien- suitable for forming the overlapped coil ends. Because the
cies of motor stators with round-wire concentric windings wire has edges and directivity, it is impossible to keep the
(at a rotation speed of 1000/min). In addition, the number coil ends insulated after the coils are formed. As a result,
of coil parts and the number of welding points were greatly motor stators with rectangular-wire concentric windings
reduced compared to those of a stator with segmented-coil have not been mass-produced for automotive use.
wave windings. C⃝ 2014 Wiley Periodicals, Inc. Electr Eng The most popular motor stator with rectangular-wire
Jpn, 187(4): 51–59, 2014; Published online in Wiley Online distributed windings that has been mass-produced is shown
Library (wileyonlinelibrary.com). DOI 10.1002/eej.22522 in Fig. 3. First, rectangular-wire bars are bent into a hairpin
shape. They are then inserted into the slots of the stator in
Key words: motor; stator; rectangular wire; dis- the axial direction. After that, the ends of the hairpin coils
tributed winding; lap winding; production process. are bent. Finally, the ends of the hairpin coils are welded
so that the connected coils form a wave shape. This type of
stator has been used for an alternator [5, 6]. Motors with
1. Introduction stators having such “segmented-coil wave windings” are
very small and have excellent performance; however, they
Automotive rotating machinery (motors, generators, have two key disadvantages. First, the segmented coils have
and so on) must be small enough to be mounted on vehicle too many welding points. For example, in the case of 72
bodies and must generate high torque at low rotating speed. slots (with four conductors per slot), the number of segment
To reduce copper loss and improve the heat dissipation of conductors is 147, and the number of terminals is 288.
automotive rotating machinery, therefore, rectangular wire Second, multiturn coils are not suitable for this winding
coils have recently been tested. structure because four conductors per slot is the limit of
For motors utilizing rectangular winding wire in the present mass-production methods.
stator coil, a stator structure with divided cores and concen- Given the above-described background, in the
trated wound coils (Fig. 1) is widely used for hybrid electric present research a motor-stator structure that allows ex-
vehicles. A stator with concentrated wound coils is supe- cellent productivity and design flexibility was developed
rior in terms of downsizing and productivity. However, its and an automatic production process was devised for it.
performance in terms of cogging torque, noise, and output The newly devised basic structure of the stator, namely,
C⃝ 2014 Wiley Periodicals, Inc.

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Fig. 1. Motor stator with rectangular-wire concentrated
winding and segmented cores. Fig. 4. Structure of rectangular-wire lap winding.

Fig. 2. Motor stator with round-wire concentric winding.

Fig. 5. Method of producing a diamond coil for

mid-sized rotating machinery.

2. Stator with Rectangular-Wire Lap Winding

2.1 Stator for small-lot production

In small-lot production for medium- and large-sized

Fig. 3. Motor stator with segmented-coil wave winding. rotating machinery, rectangular-wire lap winding coils, a
kind of distributed winding, are typically used as stator
coils [7]. The stator with rectangular-wire lap windings
for medium- and large-sized rotating machinery is shown
a rectangular-wire lap winding, is explained first. Then, schematically in Fig. 4. The rectangular wire coils, called
problems concerning the stator structure and the production “diamond coils,” are shaped like hexagons (Fig. 4(a)). One
technology for its mass production are described. After that, of the two straight parts of a coil inserted into the stator
the structure with two continuous α-shaped coils, which slots is inserted into the inner side of a slot (Fig. 4(b)),
makes it possible to reduce the number of welding points and the other is inserted into the outer side of another slot
and miniaturize the coil ends, is explained. The new pro- (Fig. 4(c)). The two oblique parts (Figs. 4(e) and (f)) that
duction method, which allows simultaneous coil forming sandwich the top of the coil (Fig. 4(d)) are positioned in
and automatic coil insertion, is described. Next, the results radially different paths. In this configuration, interference
of tests using experimental devices to verify the effects of between the adjoining oblique parts of coil ends is avoided
the newly designed coil shape and of the production method and the coil ends are small.
are presented. Finally, the measured efficiency of the trial Next, the production process for making the diamond
motor (with rectangular-wire lap windings) and that of a coil for medium- and large-sized rotating machinery is ex-
conventional motor (with round-wire concentric windings) plained as follows. First, rectangular magnetic wire, around
calculated from the fraction of the cross-sectional area of which a tape is spirally wound, is wound onto a truck-
the coil that can be arranged in a stator slot are compared. shaped coil. Then, adhesive sheets are inserted between
The productivity of the manufacturing process of the stator the wires of the coil, and the coil is heated and hardened
with rectangular-wire lap windings and that of the manu- (Fig. 5(1)). Next, the two straight parts and two heads of
facturing process of the stator with segmented-coil wave the coil are mechanically clamped, and the coil is formed
windings are also compared. into a diamond shape by moving these clamps (Fig. 5(2)).

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Lastly, glass tape is wound spirally around the diamond coil
for strengthening of insulation (Fig. 5(3)).

2.2 Application to mass production

of small motors

It was planned to apply the stator with rectangular-

wire lap windings to mass production of small motors, es-
pecially those for automobiles. This structure makes small
coil ends of the stator with rectangular-wire distributed
windings possible. Small rotating machinery does not need
high dielectric strength, so that mica tape or glass tape is not
needed for the insulation of its coils. Therefore, insulation
paper was selected as the insulator for the coils, in the Fig. 6. Problem concerning processing of terminal lines.
same manner as in popular small-size rotating machinery.
Magnetic wire with an adhesive layer was selected as the
material for the coils. Therefore, the basic coils are heated
and hardened by a flowing electric current, and the paper-
insulated parts of the coils are inserted into the stator slots.

3. Problems Concerning Stator Structure and Its

Mass Production

The two main problems concerning the structure of a

stator with rectangular-wire lap windings and its mass pro-
duction are described below. The model stator considered
in this research has 48 slots, and its NSPP (number of slots
per pole per phase) is 2.

Fig. 7. Assembly of diamond coils to stator core by hand.

3.1 Reduction of number of terminal points and
provision for ease of wiring

The number of coil terminals is 96 (48 slots × 2) in the schematically in Fig. 7. Diamond coils can be interferingly
case of a single coil winding like that in mid- and large-size inserted without interference into the stator slots, and the
rotating machinery. The number of coil terminals is one- insertion points are shifted by one slot (as shown in Fig. 7
third that of a stator with segmented-coil wave windings (state 1) to (state 3)). However, after the state shown in
(72 slots, four conductors per slot; number of coil terminals Fig. 7 (state 3), no more coils can be inserted because some
288). However, 96 terminals is never a small number: that parts of the coils have already been inserted in the inner of
is to say, too many welding points degrade productivity and the stator slots. Therefore, the already inserted parts of coils
increase the height of the coil end. In addition, the start of must be pulled out from the stator slots, as shown in Fig. 7
the winding is located at the bottom of the coil layer, so that (state 4). The straight parts of uninserted coils that are to be
it is difficult to pull out the start line from between the coil located on the outer of the stator slots are passed below the
ends automatically. For these reasons, a coil structure that pulled-out coil parts and are inserted into the slots. Finally,
has few terminal lines and whose wiring is easy is needed. the insertion of all the coils is completed (Fig. 7 (state 5)).
As mentioned above, the coil insertion of mid- and
large-size rotating machinery has depended on manual
3.2 Automation of coil forming and coil work up until now, so that its automation has been impos-
insertion into stator core sible. To make it possible to mass produce small rotating
machinery with rectangular-wire lap windings, it is neces-
The manual process for fitting coils in a stator core in sary to automate the forming and insertion of the diamond
the case of mid- and large-size rotating machinery is shown coils.

53
 Fig. 8. Two continuous α-shaped coils.

4. Two Continuous 𝛂-Shaped Coils and Simultaneous

Coil Forming and Coil Insertion

To solve the problems mentioned in Section 3, a

motor structure and production process were devised as
explained in the following sections.

4.1 Two continuous 𝛂-shaped coils

To reduce the number of terminal lines and make their

wiring easy, a design with two continuous α-shaped coils
was devised as shown in Fig. 8. The basic shape of an α-
shaped coil is a standard design. The two parts of a coil are
wound in opposite directions. The coil’s terminal lines can
be pulled out from the outside, so that wiring and welding
them is easy.
The NSPP of the stator considered in this research is Fig. 9. Simultaneous coil forming and coil insertion.
2, and two coils in neighboring stator slots have the same
phase. An α-shaped coil is wound as the base of the two
continuous coils. The two parts of the coil that are wound of the diamond coils into the stator. First, all outer straight
in different directions and are connected by an extending parts (48 pieces) of the basic coils are positioned in stator
line (Fig. 8(1)) are treated as two independent coils. Two slots. Next, metal supports (Fig. 9(2)) with ground surfaces
continuous diamond coils can be formed by twisting the and corners cut into a round shape are positioned on both
α-shaped coil. By making two continuous diamond coils, end faces of all teeth to prevent damage to the teeth at the
the number of terminal lines (Fig. 8(2)) is reduced from 96 bent points of the coils. The width of the basic coil is set
to 48. Furthermore, both terminal lines are located on the so that the inner straight parts of the coils are located inside
outermost layers of the coils (Fig. 8(3)), so that there is no the stator. Next, a gear is inserted into the inner of the stator
need to pull out the terminal lines from between adjoining axially, and all inner parts of the basic coils are inserted into
coil ends. In addition the extending line (Fig. 8(4)) that the slots of the gear (Fig. 9(3)). After that, the gear and the
connects two diamond coils is located below their coil ends, stator core are rotated relatively; as a result, all the coils are
and therefore all coil ends assembled in a stator are small. simultaneously twisted (Fig. 9(4)). The gear is rotated by an
angle equivalent to the angle between the two straight parts
of the diamond coil. After rotation, the slots of the gear and
4.2 Simultaneous coil forming and coil insertion the slots of the stator are located face-to-face again. At this
time, the outer parts of the coils are located in the slots of
A simultaneous coil-forming and coil-insertion the stator, and the inner parts of the coils are located in the
method (shown schematically in Fig. 9) was developed to slots of the gear. Next, the blades are projected out of the
automate the forming of the basic coils and the insertion gear in the radial direction and the inner parts of the coils

54
 Table 1. Specifications of prototype stator

Item Specification

Outer diameter ϕ 180.5 mm

Inner diameter ϕ 130 mm
Core stack height 110 mm
No. of slots 48
Coil pitch 37.5◦
No. of conductors in a slot 6
Conductor size 1.4 mm × 3.5 mm
NSPP 2
Coil-connection type Y

Fig. 11. Insulation-paper assembly machine.

the two sides of the winding form [(a)] rotate so that the
wire [(d)] is wound on the winding form by the rollers [(c)]
on the disks. Only one disk-roller set is shown in Fig. 10;
another disk-roller set is located against this one. The two
rollers rotate in opposite directions. As a result, two contin-
uous α-shaped coils are wound. The rotation speed of the
rollers is 75/min and the wire pressure force exerted by the
roller is 251 N. The two continuous α-shaped coils can be
wound in 2.4 s.
Fig. 10. Winding machine. (2) Adhesion of coils and insulation paper by elec-
trical heating
An electric current (300 A, 300 W, 108 s) was passed
are inserted into the slots of the stator (Fig. 9(5)). Finally, through the wire of the trial coils. As a result, the tem-
the gear and the supports are withdrawn from the stator perature of the adhesive layer of the coils rose to about
(Fig. 9(6)). 180◦ C, so that the wires adhered. In this process, the coil
All the coils are inserted at the same time, so that ends were cooled by blowing air, and only the straight
automatic assembly of the coils without interference is parts of the coils to be inserted into the stator slots were
possible. Furthermore, the productivity of forming of the heated and adhered. The reason for this selective adhe-
diamond coils is improved by forming the coils simultane- sion is that if the coil ends adhere tightly, the insulation
ously. layer will separate from the wire during the forming of
the coil into a diamond shape. In addition, in the actual
mass production of the proposed stator, it is planned to
5. Verification of Coil Structure and connect multiple sets of two continuous coils in series to
Manufacturing Process raise the total resistance and shorten the current flowing
time.
To verify the devised coil structure and its manufac- A side view of the experimental insulation-paper as-
turing process, a trial stator with that coil structure was sembly machine used for the trial stator is shown in Fig. 11.
fabricated and experimentally tested. The specifications of This machine fixes U-shaped pieces of insulation paper
the trial stator are listed in Table 1. to the straight parts of the coils that are inserted into the
slots.
(3) Effect of two continuous α-shaped coils
5.1 Verification of the two continuous 𝛂-shaped The trial coils are shown in Fig. 12(a), and the on
coil approach by trial production which all the coils were assembled is shown in Fig. 12(b).
The number of terminal lines could be reduced from 96 to
(1) Coil winding 48 and all the terminal lines were positioned on the exterior
The winding machine used to wind the trial stator is of the coils. The line extending between the two continuous
shown in Fig. 10. Two disks [(b) in the figure] placed on coils could be positioned under the coil ends.

55
 Fig. 12. Prototype coils.

Fig. 14. Motion of machine for forming and inserting

coils.

was 3300 Nm, and this value was set as a design parameter
of the machine. The inner parts of the coils in the gear slots
are then pushed out radially into the stator slots (Fig. 14
Fig. 13. Machine for forming and inserting coils. (step 5)). Finally, the supports are removed from both ends
of the stator core and the stator is taken out of the experi-
mental machine (Fig. 14 (step 5). After that, paper wedges
5.2 Verification of simultaneous coil forming are inserted into the stator slots to prevent the coils from
and coil insertion dropping from the slots, and the terminal lines are welded,
thereby completing the stator.
A top view of the experimental machine for simulta- It was verified that all 48 coils were simultaneously
neously forming and inserting coils is shown in Fig. 13. A formed and inserted into the stator slots. It is thus concluded
stator [(b) in the figure] is positioned on the axle of the gear that the problem of automating the production of a stator
[(a)]. The blade in each slot of the gear pushes coils into the with lap winding coils was solved.
stator slots in the radial direction by moving a cone that was
built into the experimental machine. The stator core is on a
6. Verification of Motor Performance
rotatable table [(c)]. A disc cam positions supports on both
end faces of all teeth to prevent damage at the bent points
The performance of the motor with rectangular-wire
of the coils.
lap windings was compared with that of a motor with
The six steps of the method for simultaneous coil
round-wire concentric windings and that of a motor with
forming and coil insertion are shown in Fig. 14. First, a sta-
segmented segmented-coil wave windings.
tor core is inserted into the experimental machine (Fig. 14
(step 1)). Next, the supports are positioned on both end
faces of all teeth from the outer side of the core (Fig. 14 6.1 Comparison of a motor with
(step 2)). After that, the basic coils are inserted into the rectangular-wire lap windings and a motor
large slots that are formed by the gear slots and the stator with round-wire concentric windings
slots (Fig. 14 (step 3)). Next, the gear and the stator core
are rotated relatively so that all the coils are twisted at the (1) Heights of coil ends
same time (Fig. 14 (step 4)). At this point, the estimated The trial stator with rectangular-wire lap windings is
torque for forming all the basic coils into diamond shapes shown in Fig. 15. The stator has a wire connection board

56
 Fig. 15. Prototype of stator with rectangular-wire lap
windings.

Fig. 18. Comparison of motor efficiencies.

(containing bus bars made of magnetic wire) on one end,

and the terminal lines are connected to this board by TIG
welding. In the case of this stator, the coil-end height on the
wire-connection side is 43 mm, and the coil-end height on
the opposite side is 29 mm, so that the total coil-end height
is 72 mm. On the other hand, when the terminal lines are
Fig. 16. Spiral terminal arrangement. arranged in a spiral shape and solderless terminals are used,
the coil-end height on the wire-connection side is 31 mm,
so that the total coil-end height is 60 mm.
Table 2. Comparison of rectangular-wire lap winding The total coil-end heights and slot-fill rates for the
and round-wire concentric winding stator with round-wire concentric windings and the stator
with rectangular-wire lap windings are listed in Table 2.
Rectangular-wire Round-wire In accordance with the specifications of conventional prod-
lap winding concentric winding ucts, the slot-fill rate for a stator with round-wire concentric
Winding type (Measured value) (Estimated value) windings is assumed to be 64%. The total coil-end height of
Total coil-end 60–72 mm About 70 mm such a stator is assumed to be 70 mm. That for the trial sta-
size tor with rectangular-wire lap windings is greater, namely,
Slot-fill rate 80.5% 64% 80.5%, and the total coil-end height of the trial stator is
Resistance of 0.0172 mΩ 0.0216 mΩ lower than that of the round-wire concentric-winding stator.
one phase
(20◦ C) (2) Motor efficiency
The efficiency of the trial motor with rectangular-
wire lap windings was investigated by using load-test
equipment. At the same time, the input electric power Pi
to the motor was measured by a power meter and the motor
output Po (= T ω) was calculated from the output torque
T and the measured motor rotation speed ω. The efficiency
El of the motor at each rated rotation speed (“■” marks in
Fig. 18) was then calculated by the following formula:

El = Po∕Pi. (1)

The cross-sectional area of the round-wire concentric

winding coils can be assumed to be 0.79 times that of the
rectangular-wire lap winding coils based on the slot-fill
rates for each type of coil. The coil lengths for each type of
coil were assumed to be nearly the same. The copper loss
Fig. 17. Torque-characteristic curve. of the motor with round-wire concentric windings (W c)
was thus calculated by using the measured copper loss of

57
 Table 3. Comparison of rectangular-wire lap winding total number of conductor bars is 147. The trial stator with
and segmented-coil wave winding rectangular-wire lap winding has only one kind of coil, only
24 coils in all, and only 48 terminal lines. Therefore, its
Rectangular-wire Segmented-coil associated productivity is superior to that with 10 coil types,
Winding type lap winding wave winding
a total of 147 coils, and a total of 288 terminals.
Number of slots 48 72 In the case of the stator with segmented-coil wave
Types of coils 1 10 windings, the terminal lines are clamped on the outer side
Total number of coils 24 147 of the coil end and the inner side of the coil end radially, and
Number of terminals 48 288 are welded. For that reason, when the stator that has three
Possible number of 2n (≥4) 2 or 4 hairpin coils (six conductors per slot), the two middle termi-
conductors in a slot
nal lines of the stator cannot be clamped and welded. This
means that it is impossible to design a stator with segmented
coil-wave windings that has more than six conductors per
the trial motor with rectangular-wire lap windings (W l) slot. In the case of the stator with rectangular-wire lap
according to the following formula: windings, more than four conductors (even numbers only)
can be arranged in a stator slot. The numbers of conductors
W c = W l∕0.79. (2) per slot of this trial stator is six. Other trial stators with
four and eight conductors per stator slot have already been
The efficiency E C of the motor with round-wire
built as trial products. It is thus concluded that the newly
concentric windings at each rotating speed (“∙” marks in
developed stator with rectangular-wire lap windings has
Fig. 18) was calculated by the following formula:
better design flexibility than the stator with segmented-coil
Ec = (Po − (W c − W l))∕Pi. (3) wave windings, whose maximum number of conductors in
a stator slot is 2 or 4.
The copper loss of the stator with rectangular-wire
lap windings was estimated to be about 20% smaller
than that of the stator with round-wire concentric wind-
7. Conclusions
ings. It follows that the efficiency of the rectangular-
wire lap-winding motor was 3.7% higher at low rotation
A stator with rectangular-wire lap windings was de-
speed (1000/min) and 1.1% higher at high rotation speed
signed to achieve a small, high-efficiency motor allowing
(9000/min) than that of the round-wire concentric-winding
high productivity and design flexibility. An automatic pro-
motor.
cess for mass production of the stator was also developed.
In particular,

6.2 Comparison of a motor with 1. The “two continuous α-shaped coil” design was
rectangular-wire lap winding and a motor devised to reduce the numbers of terminal lines of the motor
with segmented-coil wave winding with rectangular-wire lap windings and to make wiring
easy.
Under the assumption that the sizes of the mo- 2. A “simultaneous coil forming and insertion
tors are the same, the total coil-end height of the motor method” for forming all the coils into a diamond shape and
with segmented-coil wave windings is assumed to be 55 inserting them into the stator slots at the same time was
mm, which is slightly shorter than that of the motor with developed.
rectangular-wire lap windings. In addition, the efficiency of 3. Two continuous α-shaped coils were made as a
the motor with segmented-coil wave windings is assumed production trial, and the effect of reducing the number of
to be roughly the same as that of the motor with rectangular- terminal lines (i.e., total number of terminal lines = 96 in
wire lap windings. single coils and 48 in two continuous coils) and their easy
The productivities and design flexibilities in the cases wiring was confirmed.
of the two motors (one with rectangular-wire lap windings 4. It was confirmed that all 48 coils could be si-
and one with segmented-coil wave windings) are compared multaneously formed and inserted into stator slots by an
in Table 3. When the stator has 72 slots and four conduc- experimental machine using the newly devised method for
tors per slot, the stator with segmented-coil wave windings simultaneously forming and inserting the coils. In this way,
needs jumper coils to connect the inner and outer hairpin the problem of automating the production of a stator with
coils as well as conductor bars for the input lines and the rectangular-wire lap windings was solved.
neutral point. For this reason, a stator with segmented-coil 5. The efficiency of the trial motor with rectangular-
wave windings has 10 kinds of conductor bars, so that the wire lap windings was measured and the efficiency of

58
the round-wire concentric windings was calculated from REFERENCES
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AUTHORS (from left to right)

Takashi Ishigami (member) graduated from Yokohama National University and joined Hitachi, Ltd. in 1992. He has
been engaged in the development of motor manufacturing and coil-winding technologies. He is now a senior researcher at the
Yokohama Research Laboratory of Hitachi, Ltd. He received a Ph.D. degree in engineering from Yokohama National University
in 2011. He is a member of IEEJ and of the Japan Society for Precision Engineering.

Yuichiro Tanaka (nonmember) received a master’s degree from Kyushu University and joined Hitachi, Ltd. in 1995. He
has been engaged in R&D related to semiconductors, liquid-crystal displays, and electric motors. He is now a senior researcher
at the Yokohama Laboratory of Hitachi, Ltd. and a member of the Japan Society for Precision Engineering.

Hiroshi Homma (nonmember) joined Hitachi, Ltd. in 1991. He has been engaged in R&D related to semiconductors and
electrical motors. He is now a researcher at the Yokohama Laboratory of Hitachi, Ltd. and is a member of the Japan Society for
Precision Engineering.

59