Development of Magnet Wires Having Excellent Windability
by Ryosuke Obika *, Yoshinori Tatematsu *, Masakazu Mesaki * and Atsushi Higashiura *In recent years there is an increasing demand for enhanced windability of magnet
wires used in household electrical appliances and automobile electrical components. The reasons are that it is becoming more important to make motors and transformers usedin these fields more powerful and reduced in cost than ever before, and to this end, winding
processes have actively been speeded up and automated, and that the slot fill has been
improved to enhance the performance of these products. Under such circumstances, winding
damage given to magnet wires has increased, causing damage to insulation films.
To prevent such damage, magnet wires offering surface lubrication have heretofore beendeveloped by applying wax to the surface of magnet wires and making the surface self-lubricant.
However, these techniques are not enough to meet the requirements for the windability of magnet wires. From a viewpoint different from conventional techniques, we have developed the technique of improving the adhesion between the conductor and the insulation film and succeeded in
producing new magnet wires enhancing the windability greatly. This paper describes the development and properties of the new magnet wires, as well as their applications.

ABSTRACT1.

INTRODUCTION

In recent years labor savings and rationalization are inprocess in the manufacture of electrical equipment, andcoil winding processes for motors and transformers havealso been actively speeded up and automated. However,when an automatic winding machine is used, the possibility of doing damage to the insulation films of magnet wires
is increased because the wires are wound under the conditions of bending, friction together with strong tension,
thereby decreasing the electrical reliability of the coils
themselves. In coil forming, it is desirable to increase theslot fill (percentage of the volume of magnet wires in the
total volume of equipment) as much as possible, which isvery effective in the economy and workability of the whole
equipment. Increasing the slot fill of coils requires applying
increased tension to magnet wires to wind the coils closely. However, when the stator slot in the motor is forcibly
filled with more magnet wires in the winding process, it
may result in damaged insulation films. If such damage
occurs to the insulation films, there is a problem of poor
layer insulation and poor grounding. This requires a highdegree of windability for the magnet wires used in coils. It
is thought that lubrication and abrasion resistance of thesurface of magnet wires greatly influence windability. The
method of imparting surface lubrication has heretofore
been studied by applying wax, oil or the like to the surfaceof magnet wires and curing the surface overcoated withpaint added with polyolefine.

*

Polymer Research Sec., Hiratsuka Laboratory, R&D Div.

However, it cannot be said that such techniques areenough to reach adequate solutions under the present
condition where winding processes have increasingly
been speeded up and automated.
We have recently developed magnet wires offering
excellent windability than ever before to meet theserequirements. This paper describes the properties andeffects of the new magnet wires, in which the adhesion
between the conductor and the insulation film (often called
simply "adhesion" hereafter) is improved more than everbefore.

2.

CONCEPT OF DEVELOPMENT (TECHNIQUES OF IMPROVING WINDABILITY)

Three techniques can be used to enhance the windabilityof magnet wires. Figure 1 shows the outline.
The first is to increase the strength of insulation films,
which makes it hard to get damaged because they resist
better external factors including wear. For example, magnet wires with overcoat of polyamideimide resin containing
thermoplastics belong to this category. 1)
The second is to modify the condition of the surface of
insulation films. For example, when lubrication is imparted
to the surface of insulation films, the lubrication prevents
damage to the insulation films. The techniques of enhancing lubrication include the method of applying wax, oil orthe like and that of self-lubrication in which the surface isovercoated with resin containing polyolefine. The latter, inparticular, offers good slipping ability and abrasion resistant properties, thus coping with high speed winding and155

Table 1

Insulation film structure of magnet wires
AI/EI

Under-coat

Polyesterimide

Over-coat

Polyamideimide

Table 2

Film toughness

Figure 1

High adhesion between
film and conductor

Concept of the technique of improving windability

156

3.

N-AI/HPE(conventional)(developed)

Polyamideimide

Polyamideimide

General Properties of magnet wires

Conductor diameter (mm)Surface lubrication
and/or hardness

automatic winding.
The third is a technique we have recently approached,and it is intended to improve the adhesion between insulation film and conductor. The technique is aimed at preventing the insulation films from being peeled off readily
from the conductor thereby enhancing windability.
To be more specific regarding the third technique in particular, the techniques of improving the adhesion include
the technique of treating conductor surface and that ofmodifying resin.
For the conductor surface treatment, for example, it isalready known that the treatment is feasible by applying
imidazole or the like to the conductor surface. The technique has an advantage of eliminating the need for resin
modification, while it poses problems of increasing manufacturing processes and of doing precise coating longitudinally, which is difficult to be controlled accurately.
For the resin modification, it should be noted that there
is a difference in the level of adhesion depending on thematerial of insulation films, i.e., decreasing in the order ofpolyester resin, polyesterimide resin, and polyamideimide.
The difference probably results from the abundance of terminal groups with a high polarity like -OH group containedin the resin. Consequently, improving the adhesion
requires introducing polar groups having high interaction
with the conductor into resin. Though adhesion may
improve with this method, concerns are raised about thechanges in resin properties.
We introduced polyamideimide (hereinafter referred toas "(AI/) HPE") magnet wires using THEIC (Tris-hydroxyethyl Isocyanurate) modified class H polyester (hereinafter referred to as "HPE") in the previous report. 2)
These magnet wires are improved more than polyesterimide resin in the adhesion between conductor and insulation film. We succeeded further in improving the adhesion
by resin modification.
In the present study, by combination of these techniques of improving windability, we achieved a highdegree of windability exceeding the magnet wires thatwere previously studied, that is, the wires with enhanced
surface lubrication only.

AI/HPE

H class polyester H class polyester

AI/EI

AI/HPE

N-AI/HPE

0.85

0.85

0.85

Insulation thickness (µm)

30

30

30

Breakdown voltage (kV)

10.2

10.1

10.5

Cut-through (°C)

420

420

420

NEMA heat shock 220°C x 30 min

1D OK

1D OK

1D OK

Flexibility 220°C x 6 hr

1D OK

1D OK

1D OK

PROPERTIES OF DEVELOPED MAGNET WIRES

We have evaluated three types of magnet wires this time.
Table 1 gives the structure of the insulation layers of themagnet wires including N-AI/HPE, the wire newly developed, and Table 2 their general properties. It is seen that
all magnet wires are nearly equivalent to one another interms of general properties.
The following are the evaluation and examination
results of the enhancement in the windability of magnet
wires, with adhesion in focus in particular.
3.1 Adhesion
There are several methods of evaluating adhesion.
A peel twist test is a simple evaluation test among themethods that have been used. With this method, one endof a magnet wire having a longitudinal slit made with acutter is fixed, and the wire is horizontally stretched tohave a gauge length of 150 mm and a load of 500 grams.
And the other end is twisted 360 degree to count the number of twists until the insulation film peels off to bare theconductor. This method of evaluation serves somewhat togive a measure of the strength of adhesion, but the number of peel twists may vary depending on the flexibility of
the insulation film itself.
Therefore, we tried a new method to directly evaluate
the adhesion force between the conductor and the insulation film. First, a magnet wire was crushed at a certain
compression ratio, and two parallel slits were longitudinally made at an interval of about 0.6 mm. And one end of
the slit insulation film was peeled off from the conductor tomeasure the tension when the film was peeled off at a
speed of 2 mm/min in the direction of 90 degrees withrespect to the wire axis.
The peel twist tests in Figure 2 reveal that the AI/HPE
magnet wire, following the N-AI/HPE wire, has more numbers of twists before the insulation film peels off to barethe conductor than AI/EI.
Next, Figure 3 shows the results achieved by the new

Furukawa Review, No. 19. 2000

Table 3

Number of twist (times)

90

80

70

60

50

AI/EI

AI/HPE

Figure 2

N-AI/HPE

Peel-twist test results

60

Adhesion (gf/mm)

50
N-HPE/AI
40
AI/HPE

30

AI/EI

20
10
0
0

20

40

60

Compression ratio (%)

Figure 3

Measurement results of adhesion force between conductor and film

method of evaluating adhesion described above. The
adhesion force given on the vertical axis refers to thepeeling force as a magnet wire is stamped in a rectangular cross-section with a certain compression ratio. The
compression ratio given on the horizontal axis indicatesthe ratio at which magnet wires are compressed, that is,the displacement during the stamping. This method was
adopted to simulate actual wire winding, during which
wires are often deformed considerably. A compression
ratio of 60% was used as a critical value. It was observedthat the adhesion force tended to decrease with increasein compression ratio. N-AI/HPE shows the same tendency
as for the peel twist test, and it is higher than AI/EI and
AI/HPE magnet wires in terms of adhesion force. Special
attention should be paid to the fact that the adhesion forceof N-AI/HPE magnet wires is as high as 6 gf/mm evenafter working at a compression ratio of 60%, higher thanthat of other wires.
The above two adhesion evaluations demonstrate that
N-AI/HPE magnet wires are remarkably improved in theadhesion as compared with conventional AI/HPE magnet
wires.
3.2 Windability
Next, we carried out unidirectional abrasion tests, repeated scrape tests and NEMA windability tests to verify theeffects of the level of adhesion on windability. Table 3
gives the results.

Windability properties of magnet wires
AI/EI

AI/HPE

N-AI/HPE

Unidirectional abrasion (N)

15.2

16.2

18.6

Repeated scrape (times)

200

200

205

NEMA windability test (strokes)

23

26

30

In the unidirectional tests, a piano wire was moved in
the direction of a magnet wire while a load was applied on
the piano wire intersecting perpendicularly the magnet
wire, and the load was varied at a given rate to measure
the load when the insulation film of the magnet wire broke
to bare the conductor.
In the repeated scrape tests, repeated scrapes were
implemented with a given load applied to count the number of times until the conductor was bared.
In the NEMA windability tests, one turn of magnet wire
was wound around a mandrel, and the mandrel was reciprocated in the longitudinal direction of the magnet wirewith a tension applied to the wire. A voltage of 1,500 volts
was applied between the magnet wire and the mandrel tocount the number of strokes before three shorted points ormore were detected.
The unidirectional abrasion tests show that N-AI/HPE
magnet wires offer a higher value of abrasion load than
AI/EI and AI/HPE magnet wires, indicating that the NAI/HPE magnet wire is considered the best.
The repeated scrape tests revealed that N-AI/HPE magnet wires were almost equivalent to AI/EI and AI/HPE
magnet wires and that there was no significant differencein the number of abrasion times.
For the NEMA windability tests, N-AI/HPE magnet wires
offer more numbers of strokes than AI/EI and AI/HPE
magnet wires, indicating that the N-AI/HPE magnet wire isconsidered the best.
We could thus verify that the N-AI/HPE magnet wirewith improved adhesion between the conductor and theinsulation film showed an improvement effect in the unidirectional abrasion tests and windability tests and that thewindability was enhanced.
3.3 Discussion (Adhesion and Windability)
Since an enhancement in windability was observed by
improving the adhesion as mentioned above, we gave
consideration to knowing its contribution or analyzing its
factor.
First, in order to make the factor clear, we decided to
study how adhesion and lubrication interact with eachother. We used three types of wires with different levels ofadhesion, i.e., increasing in the order of AI/EI, AI/HPE,
and N-AI/HPE; and four levels of lubrication expressed bystatic friction coefficient: non-lubricated (non-treated) =0.13, wax-treated = 0.08, self-lubricated upperpolyamideimide resin = 0.03, and the same = 0.05.
Figure 4 shows the relation with unidirectional abrasion.
Studies reveal that an effect resulting from the enhancement in surface lubrication is not shown, but that an effectresulting from the enhancement in adhesion is obvious. It
is apparent that the N-AI/HPE magnet wire is improved in

157

35
N-AI/HPE
AI/HPE
AI/EI

22

NEMA windability test (strokes)

Unidirectional abrasion (N)

24

20
18
16
14

Self-lubrication treated
No.1
No.2

12

Wax treated

Non-treated

10

30

25

20

0.05

0.1

0.15

0

Effect of lubrication and adhesion on unidirectionalabrasion properties

0.05

Repeated scrape (times)

Unidirectional abrasion

Self-lubrication treated
No.1

Wax treatedNot effective

Effective

Repeated scrape

Effective

Not effective

NEMA windability

Not effective

Effective

No.2

0.05

0.1

0.15

Static friction coefficient

Figure 5

Adhesion

Non-treated

50

0

0.15

Effect of various factors on windability properties
Surface lubrication

150

100

0.1

Effect of lubrication and adhesion on NEMA windability test

N-AI/HPE
AI/HPE
AI/EI

0

Effect of lubrication and adhesion on repeatedscrape properties

particular. This is probably because the adhesion between
conductor and insulation film has a great effect when, as
in the case of the unidirectional test, such a great external
force as to peel off the insulation film is used.
Next, Figure 5 shows the relation with repeatedscrapes. Studies reveal that an effect resulting from theenhancement in surface lubrication is noticeably shown incontrast to unidirectional abrasion, but that there is noeffect resulting from the improvement in adhesion and that
there is no difference between improved magnet wires
and conventional magnet wires. This is probably because,in view of the fact that the repeated scrape test is basedon repeated scrapes under a relatively light load, only thesurface treatment of magnet wires had an effect.
Figure 6 shows the relation with NEMA windability tests.
Studies reveal that an effect resulting from the enhancement in surface lubrication is not shown, but that an effectresulting from the enhancement in adhesion is obvious,and the same results as for unidirectional abrasion wereobtained.
During the windability test, the adhesion force between
the conductor and the insulation film seems to decrease
little by little, and thus the insulation film becomes abnormally nonuniform. This causes defects such as cracks inthe portion where the insulation film is extraordinarily elongated. The N-AI/HPE magnet wire offers a smaller degreeof decreased adhesion than the AI/HPE magnet wire,
which may result in the increased number of strokes.

158

Figure 6

Table 4250

200

Non-treated
N-AI/HPE
AI/HPE
AI/EI

Static friction coefficient

Static friction coefficient

Figure 4

Wax treated

15

10
0

Self-lubrication treated
No.1
No.2

Table 4 presents the relation between windability andthe factors thus obtained. In the past, surface lubrication
has been aggressively studied as a technique of improving windability. The enhancement in lubrication permits
reducing damage to the insulation film caused by contact
between magnet wires or with the coil slot. As a result, it isassumed, damage to the insulation film can probably be
prevented. However, when a great external force that cannot be reduced by surface lubrication only --such as is thecase with unidirectional abrasion test or windability test-is applied, it is presumed that the enhancement in theadhesion between the conductor and the film becomes
effective in preventing damage to the insulation film.
Windability exceeding conventional magnet wires willprobably be achieved by a combination of the enhancement in the adhesion and the enhancement in the lubrication of the surface of a magnet wire.
3.4 Refrigerant Properties
The AI/HPE magnet wire we have recently evaluated has
many applications including refrigerators and air conditioners as a refrigerant-resistant wire. In these applications,the magnet wire is used as is immersed in refrigerant. We
also evaluated the reliability of the magnet wire used inrefrigerant.
A magnet wire was immersed in refrigerant R22 usedmostly in air conditioners and refrigerant R410A beginningto be used as an alternative refrigerant at 150°C for 1,000
hours to evaluate its blister resistance.
When a magnet wire is immersed in refrigerant, takenout and then rapidly heated up to a given temperature,
blister occurs on the surface of the wire. Blister resistancemeans the degree of resistance to blister occurrence.
Table 5 gives the evaluation results.
Comparison of AI/HPE and N-AI/HPE magnet wires
reveals that both magnet wires are provided with high

Furukawa Review, No. 19. 2000

Table 5

Refrigerant resistance properties

R22+oil (150°C x 1000 hr)
Blister

R410A+oil (150°C x 1000 hr)
Blister

AI/HPE

N-AI/HPE

120°C

Good

Good

140°C

Good

Good

160°C

Good

Good

120°C

Good

Good

140°C

Good

Good

160°C Slight foaming

Slight foaming

resistance to blistering in the refrigerant R22 and that
there is no difference recognized in the blister resistance
between the two.
The same results were obtained also with the newrefrigerant R410A.

4.

CONCLUSION

1) The newly developed magnet wire (N-AI/HPE) isenhanced in adhesion between the conductor and theinsulation films as compared with the conventional

magnet wire (AI/HPE). This results in the enhanced
properties of unidirectional abrasion and NEMA windability, thereby providing high windability.
2) It was also found that the requirements for windability
becoming more and more strict could be met by a
combination of the developed magnet wire and thesurface treatment offering high lubrication.
3) Magnet wires using the technique of improving adhesion are equivalent to conventional magnet wires inother basic properties including refrigerant resistance,
and thus application to the same areas as before will
be possible.

REFERENCES

1) James J Xu et al., "Dynamic mechanical properties of toughmagnet wire", EMCW Proceedings 1998, p163-168
2) Y.Tatematsu et al., "Development of self-lubricating magnet
wires for alternative refrigerant", EEIC/EMCW 1995
Proceedings, p427-430
Manuscript received on December 24, 1999.

159