Development of a Magnet Wire
with Superior Inverter Surge Resistanceby Ryousuke Obika *, Tadashi Ishii *, Yoshinori Tatematsu *2 and Katsumi Osada *2

In recent years there is a trend for electrical equipment and industrial motors to
be inverter controlled, as energy saving is promoted together with the downsizing
and performance improvement of these equipment. While it has been suggested that excessive
surges accompanied by inverter control adversely affect motor insulation systems, this does notmake an exception for magnet wire. Thus magnet wires are required to have better resistanceagainst inverter surges than before, because, when the wires are exposed to such voltages,
degradation of the insulation layer is caused by corona discharge, unless sufficient insulationand creepage distance are provided between the wires. With respect to driving motors forelectric vehicle (EV) and electric hybrid vehicle (EHV) also, similar phenomena are observable
depending on the design of motors, thereby leading to a burgeoning demand for a magnet wirethat is well adapted for inverter-controlled equipment.
To satisfy these demands, the authors have recently developed a new magnet wire withsuperior inverter surge resistance. Characteristics of this wire will be described in this report,
whereby the life under high-frequency voltage has been improved significantly.

ABSTRACT1.

INTRODUCTION

In recent years there is a trend for electrical equipment
and industrial motors to be inverter controlled, as energy
saving is promoted together with the downsizing andperformance improvement of these equipment. While ithas been suggested that excessive surges generated byinverter-controlled equipment can enter into the motors
to adversely affect their insulation systems, this does notmake an exception for magnet wire. Thus, when the wires
are exposed to such surges, degradation of the insulation
layer is caused by corona discharge, unless sufficientinsulation and creepage distance are provided between
the wires. To prevent the corona discharge degradation
due to surges, approaches to reduce such voltages
based on motor design are oftentimes taken includingchanges of peripheral circuits. On the other hand, inorder to reduce the size and cost of these motors, there
is a demand for improving manufacturing efficiency of
motors, including elimination of the inter-phase insulation
paper between the wires. Therefore, magnet wires are
required to have better resistance against inverter surge
than before. With respect to driving motors for electricvehicle (EV) and electric hybrid vehicle (EHV) also, similar
phenomena are observable depending on the design of**2

Ecology & Energy Lab.
Engineering Dept., Magnet Wire Div.

motors, thereby leading to a burgeoning demand for amagnet wire that is well adapted for inverter-controlled
equipment.
To satisfy these demands, the authors have recently
developed a new enameled wire with superior inverter
surge resistance. Characteristics of this wire will be
described in this report, whereby the life under highfrequency voltage has been improved significantly.

2.

DEVELOPMENT CONCEPTREGARDING INVERTER SURGERESISTANCE

Surges of inverter-controlled motors can reach 1.5~2.0
times the input voltage, depending on the cable length
between the inverter and the motor. Because such surges
exceed the corona inception voltage of the magnet wire
within the motors, corona discharge occurs in thesewires, in case sufficient insulation reinforcements are notprovided, producing adverse effects such as a substantial
depletion of the insulation layer. Thus it is essential
for inverter-controlled motors to improve the insulation
reliability against surges.
Improvement techniques for inverter surge resistanceof magnet wires come in two types. The first technique isto make the corona inception voltage higher. When thecorona inception voltage of the wire insulation is improved
to be higher than the inverter surge voltage (i.e., high-

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2003

65

Development of a Magnet Wire with Superior Inverter Surge ResistanceV = 163 (t /ε ) 0.46
(V)(1)
where, t is the thickness of insulation layer in µm,
and ε is permittivity.
It can be seen from the equation that the corona
inception voltage can be maximized by reducing thepermittivity of the insulation layer as low as possible,
and by increasing the insulation layer thickness as large
as possible. However, reduction of the permittivity islimited by the availability of appropriate resins and by thepractical limit of engineering applicability; and increasing
the insulation layer thickness may result in changing the
motor design in view of slot fill factor.
The second technique aims at extending the life evenin case of occurrence of corona discharge due to surges,
and the technique is based on introducing insulation
layers having corona discharge resistance. There aremany examples reported, in which the resistance against
inverter surges are provided by adding inorganic fillers
to the organic enamel resin. Such techniques, however,result in a decrease in the flexibility of insulation layers,presenting a problem of insulation degradation under the
influence of excessive stresses due to wire winding work.
In this development, we have studied the second
technique mentioned above, i.e., insulation layerswith resistance against inverter surges, carryingout development of insulation layer materials andinvestigation of application to enameled wires.
Improvement of resistance against inverter surges can
suppress insulation degradation due to the surges,
thus improving insulation reliability even when exposed
to excessive surges caused by inverter-controlled
equipment. We have been successful, by dispersing
inorganic particles into the enamel resin using a special
process, in improving the inverter surge resistance,
thereby also succeeding in solving the existing problem of
degradation of this resistance after excessive mechanicalstresses.

3.

CHARACTERISTICS OF DEVELOPEDWIRE

Table 1 shows the insulation layer structure of wire
samples evaluated here.
3.1 Corona Inception Voltage
The corona inception voltage was measured using
twisted samples under conditions of 50 Hz in frequency
and 10 pC in electric charge detection limit. The results
are shown in Table 2. The corona inception voltage ofthe developed wire is seen to be equivalent to that of theconventional wire, indicating no degradation due to theuse of the developed resin.

Table 1

Insulation layer structure of wire samples evaluated.
Developed wire

Conventional wire (AIHPW)

Wire size: 0.9 mm

Wire size: 0.9 mm

Insulation layer: 30 µ m thick

Insulation layer: 30 µ m thickUpper layer: Polyamideimide resin
Lower layer: H-class resin including
developed resin

Table 2Upper layer: Polyamideimide resin
Lower layer: H-class polyester resin

Corona inception voltages.
Developed wire Conventional wire (AIHPW)Corona inception voltage (Vp),805 V

at 50 Hz and electric charge

798 V

detection of 10 pC

43

Voltage (kV)frequency impressed voltage), corona discharge does notoccur extending the service life of the wire. The corona
inception voltage V of an enameled wire is given by theexperimental equation of Dakin 1) as shown below.

Developed wire2
Conventional wire

1

0
0.001

0.01

0.1

1

10

100

1000

Life (hr)Figure 1

Comparison of V-t characteristics between
the developed and conventional wires.

3.2 Evaluation of High-frequency Characteristics
To simulate the mechanical stresses due to elongationand deformation at the time of assembly into motors
and insertion into slots, the wires are evaluated under
an elongation of 5 %, 10 %, or 20 % depending on thecircumstances. This time, the wires were twisted after
being extended by 20 % --corresponding to about a half
the breakage elongation-- to prepare test samples.
3.2.1 Sinusoidal Wave Life
The high-frequency characteristics of the conventionaland developed wires were evaluated by measuring thetime until breakdown (i.e., life) of twisted wire samples
after 20 % elongation, on which a sinusoidal voltage of
10 kHz and in excess of the corona inception voltage
was impressed. The results are shown in Figure 1. It has
been shown that the developed wire has a life about 100
times the conventional wire under an impressed voltageof 1.5 kV. The main cause for the significant improvementof life, regardless of about the same insulation layer
thickness and corona inception voltage in the case of
the wire developed here, is thought to be that the speedof depletion due to corona discharge is suppressed by
the newly developed resin layer, thereby significantly
extending the time until breakdown.

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Development of a Magnet Wire with Superior Inverter Surge Resistance

180

1000
Inverter surge

160
140

2.12 kVp

Life (hr)120Life (hr)1.41 kVp

100

100
80
60

10
1
0.1

40

0

Figure 22.84 kVp

0.01

20Developed wire

0.001

Conventional wire

Comparison of life under inverter surge between
the developed and conventional wires.

15

25

35

45

55

Layer thickness ( µ m)

Figure 4

Layer thickness dependence of the sinusoidal
wave life under selected voltages and the life under
inverter surge.

4

Voltage (kV)

Table 3

Total insulation layer thickness:

3

50 µ m

1

0.1

1

10

100

φ 0.9 mm

2D

1D

OK

OK

Unidirectional abrasion (N)

20.0

15.5

Reciprocated abrasion (times)

210

205

NEMA heat shock test,
220°C × 30 min

23 µ m0.01

AIHPW, Single,

Flexibility after 20 % elongation

30 µ m00.001

Developed wire,
Single, φ 0.9 mm

Item

44 µ m2

Windability

1000

Life (hr)Figure 3

Comparison of V-t characteristics of the developed
wire with different insulation layer thicknesses.

3.2.2 Life under Inverter Surge
The life under inverter surge was evaluated, using an
industrial inverter commercially available, by measuring
the time until breakdown of twisted wire samples,
whereby the inverter output terminal and the sample
were connected using a cable about 100 m in length,under conditions of an inverter input voltage of 480 V
and a carrier frequency of 15 kHz, which resulted in a
maximum voltage of 1.5 kV impressed on the sample.
The results are shown in Figure 2. It has been shownthat the developed wire has achieved a life improvementof 30 times over the conventional wire --a significant
improvement. It should be noted that this improvement
has been achieved using wire samples with 20 %elongation. It is therefore believed that the developed
wire maintains this inverter surge resistance even in caseof undergoing mechanical stresses during a winding
process, thus overcoming conventional concerns.
3.2.3 Effects of Layer Thickness on High-frequency Life
Figure 3 shows the relationship, between the impressedvoltage and the life under 10-kHz sinusoidal voltage, ofthe developed wires having insulation layers of different
total thicknesses constituted by the newly developed
resin. It can be seen that the lower the impressed voltage
is the longer the life becomes, and there is a tendency inparticular that as the layer thickness increases, the life
extends significantly.

In practical applications in inverter equipment, thecarrier frequency, pulse frequency, surge voltage, etc.
considerably change depending on the type of theinverter, so that the insulation reliability of the wire
also changes depending on the environment of use.
Figure 4 shows the layer thickness dependence of thesinusoidal wave life under selected voltages and the life
under inverter surge. It can be seen that the logarithm ofsinusoidal wave life is in a linear relationship with the layer
thickness in the case of the same impressed voltage, anda similar tendency can be seen with the life under inverter
surge. This fact suggests that a life under inverter surge
can be predicted with a certain extent of accuracy basedon the relationship of the sinusoidal wave life.
3.3 Windability
Table 3 shows the windability. When an automatic
winding machine is used in the winding process of motor
manufacturing, it is known that the wire undergoes
elongation, bending, friction, etc. under a strong tension,
so that the insulation layer is likely to be damaged.
The wire developed here has, in comparison with theconventional wire, better abrasion characteristics, asmaller degree of flexibility degradation after 20 %elongation, and a satisfactory NEMA heat shock property.
Thus the developed wire presents no problem of
degradation in insulation properties at the time of winding
processes, while this problem constituted a major concern
for conventional wires that included inorganic fillers
added.

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Development of a Magnet Wire with Superior Inverter Surge Resistance

Table 4General characteristics.

Item

4.

Developed wire, AIHPW, Single,
φ 0.9 mm
Single, φ 0.9 mm

Finished size (mm)

0.956

0.956

Conductor size (mm)

0.896

0.896

Insulation layer thickness ( µ m)

0.030

0.030

Flexibility

1D, Good

1D, Good

Adhesion

Good

Good

Abrasion resistance (N)

20.0

15.5

Insulation breakdown voltage (kV)

13.5

13.3

Cut-through temperature (°C)

410

408

1D, Good

1D, Good

Heat shock, 220°C × 1 hr

3.4 General Characteristics
General characteristics of the developed wire are shownin Table 4. The developed wire has general characteristics
equivalent to those of H-class conventional wires in termsof insulation breakdown voltage and adhesion, so thatthe wire is applicable to various industrial and automobile
motors. Thus when used in such applications whereconcerns are raised about degradation of insulation
reliability due to inverter driving, the wire is expected tooffer high reliability.

IN CONCLUSION

1) The wire developed here has achieved an
improvement in high-frequency life, thus offeringsuperior resistance against inverter surges, incomparison with conventional wires.
2) Also, even after 20 % elongation, the wire improvesin the life under inverter surges.
3) Basic characteristics of the developed wire are
equivalent to those of conventional wires, so that thewire is applicable to various industrial and automobile
motors.

REFERENCE
1) Dakin: IEEE, 5 (1959), 155

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