No. 61 Product failure mechanism and reliability testing - Test Navi

Product safety and failure analysis

ESPEC Technology Report No.61

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T

Technology Report

Product failure mechanisms and reliability testing

Part 1: Motors and transformers

Masahiro Yamaura ESPEC CORP., Reliability Research Headquarters,

Analysis Center, Calibration Group

his report is an expanded version of excerpts taken by Espec from

presentations made at a product reliability seminar given by the Reliability

Subcommittee of the Kansai Electronic Industry Development Center (KEC),

and from material published in the journal KEC Jōhō (issue No. 214 of July, 2010).

Motors and transformers comprised mainly of magnet wires are in widespread use, and

are generally highly reliable components. But occasional news stories about fires caused

by electric fans suggest that motor or transformer problems can sometimes lead directly

to serious product failures, and survey indicate that such cases are more common that

might be expected.

This report presents the findings of a survey on the mechanisms responsible for product

failures. It is based on actual cases in which products with motors or transformers

caused serious product failures reported by the National Institute of Technology and

Evaluation (NITE). These findings are followed by a description of a reliability test

method that uses a better understanding of product failure mechanisms to reveal

potential market failures.

2.1 Breakdown by product

This section presents the breakdown of the surveyed product failures by category (for

each material responsible). These failures occurred between 2001 and 2007, and their

causes were classified by NITE as ‘C1’ (failures thought to have been caused by an old

product or a product with degraded performance from extended use).

2.1.1. Product failures caused by motors

Figures 1 and 2 show the breakdown of the 17 motor-linked product failures surveyed.

Electric fans were the most failure-prone product type in this category, accounting for

Introduction 1

Failure of products used for extended periods of time 2



2

about half of these cases. Product failures caused by fan motors were most often due to

deterioration of the insulation on the coil’s magnet wire. The motor-linked product

failures surveyed occurred after 10 to 40 years of product use, with occurrence spread

evenly over this period of time.

Fig. 1 Failure ratio by category (motors)

Fig.2 Number of product failures by years of product use (motors)

2.1.2. Product failures caused by transformers

Figures 3 and 4 show the breakdown of the 35 transformer-linked product failures

surveyed.

Fig.3 Failure ratio by category

(transformers)

No. of product failures

Years of use



3

Fig.4 Number of product failures by years of product use (transformers)

Color TVs were the most failure-prone product type in this category, with insulation

deterioration in color TV flyback transformers responsible for about 95% of these cases.

The remaining two product failures in this category were also caused by transformer

insulation deterioration, one in a microwave oven transformer and the other in a

humidifier. The breakdown of product failures by years of product use shows failures

starting to occur around the 10-year mark, clustering from the 15- to 20-year mark, and

continuing to nearly the 30-year mark.

2.2 Product failure cases

2.2.1 Product failure cases caused by motors

The motor responsible for the surveyed product failures were AC motors driven by a 100

VAC household power supply. Photos 1 and 2 show the most commonly used type of

electric fan motor.

Photo 1. Shading coil motor Photo 2. Single-phase induction motor

The shading coil motor (photo 1) uses an auxiliary coil shunted to a position away from

the main stator coil as its single-phase startup method. It drives the rotor using the

current flowing to the auxiliary coil due to the action of the transformer when the circuit

is live. Shading coil motors have a simple structure and are cheap to manufacture. They

Years of use

n/a

No. of product failures



4

have few components that fail or restrict service life.

The single-phase induction motor (photo 2) uses a capacitor as its startup method and

drives the rotor by staggering the phases of the main coil and auxiliary coil. While this

type of motor is also actually sound, it has been responsible for a greater number of

product failures than the number of electric fan motor failures given in section 2.1, since

product failures also occur for another reason (the capacitor).

Photo 3. Fire damage to electric fan2

Photo 4. Motor coil deterioration (peeling of insulation material)2

This section describes product failures caused directly by motors. Photo 3 shows the

damage done to an electric fan that caught fire. Photo 4 shows the type of motor coil

insulation deterioration from long-term use that has caused product failures. The

survey findings report a product failure in which “Traces of melting were found in the

failed product’s motor winding. Since it was installed for outside use, long-term product

use (over approx. 30 years) caused deterioration of the motor winding insulation,

leading to layer shorting, and resulting in short-circuiting and sparks thought to have

caused fire”.

Some product failures have been caused by mechanical deterioration: “Product use over

33 years is thought to have resulted in the leakage of lubricating oil coating the

bearings’ metal that support the fan motor; this caused a loss in smoothness on the

motor shaft rotation and bearing metal, leading to motor shutdown, then overheating

and catching fire”.

2.2.2. Product failures caused by transformers

Figure 5 illustrates how flyback transformers can cause product failures.



5

Extended use (approx. 30

years)

Insulation deterioration (on motor winding)

Layer shorting Short-circuiting,

sparks

Fire


years)

Lubrication oil leak (from

motor bearings)

Overheating (in motor coil winding)

Loss of smooth rotation

Fire

Fig. 5 Product failure caused by flyback transformer2

3.1 Motors

Figure 6 illustrates the mechanism responsible for product failures caused by

deterioration of the motor coil’s insulation.

Product failure mechanisms 3

6.1 cm

6 ml

core

Lead wire resistance inside

Cracks formed, causing

discharges in the direction

of the arrow

Temperature Humidity Humidity Temperature

Fig.6 Mechanism for product failures

caused by deterioration of motor coil’s

insulation

Fig. 7 Mechanism for product failures caused

by mechanical deterioration in motor



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Fig.7 illustrates the mechanism responsible for product failures caused by mechanical

deterioration.

The mechanism responsible for product failures caused by deterioration of the motor

coil’s insulation is as follows: The insulation material on the magnet wire used in the

motor coil deteriorates due to stress from factors such as temperature and humidity

over many years of product use. Peeling of the insulation material lowers insulation

performance (see photo 4) then leads to layer shorting that causes short-circuits and

sparks, resulting in fires.

In product failures caused by mechanical deterioration, temperature and humidity

stress in drive components (particularly motor bearings) over many years causes the

lubricating oil (grease) to leak, creating in abnormal rotary operation loads. These loads

cause the motor winding to overheat, leading to fires.

3.2 Transformers

Figure 8 illustrates the mechanism responsible for smoking-generating product failures

caused by flyback transformers (sometimes abbreviated ‘FBTs’). Figure 9 shows the

mechanism for fire-generating product failures caused by flyback transformers.

As with motors, deterioration of the coil insulation is a key feature in product failures

caused by flyback transformers.

When flyback transformers are subjected to temperature and humidity stress over

many years of use, their coil insulation deteriorates (particularly in high-voltage

regions) resulting in layer shorting and high-voltage leaks. The shorting and leaks

sometimes vaporize the insulation material, causing cracks in the casing that cause the

product to emit smoke. In other cases, high-voltage leaks cause fires. Since flyback

transformers are components of indoor products (TVs), product failures can sometimes

be caused by dust and oil mist accumulating on the casing and preventing the proper

release of heat.



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years)

Insulation deterioration (on

FBT high-voltage coil)

Layer shorting

Vaporization of insulation

material leads to FBT voltage rise

Cracks in FBT casing

Product emits smoke

Extended use (approx. 19 years)

Deterioration of insulation (on FBT high-voltage coil)

Cracks in FBT casing

High-voltage leaks

Product catches fire

Fig.8 Mechanism for smoke-generating prod- Fig.9 Mechanism for fire-generating

-uct failures caused by flyback transformers product failures caused by flyback

transformers

3.3 Magnet wires

This section describes the magnet wire that is a key cause of product failures linked to

motors and transformers.

Magnet wire is the general term for the electrical wire used in electrical appliance

windings. It converts electrical energy to magnetic energy or vice versa. Figure 10

shows its structure and main requirements.

Main requirements for magnet wire:

(a) Thin insulation of uniform thickness

(b) Good electrical characteristics (such as insulation

destructive voltage and insulation resistance)

(c) Strong coating with ability to withstand external forces

such as bending, stretching and rubbing

(d) Heat-resistant

(e) No application of solvents, chemicals or varnishes

(f) Non-hydrolytic

(g) Stable when combined with other insulation materials

(h) Water- and humidity-resistant

Temperature Humidity Temperature Humidity

Oil mist Dust

Conductor

Insulation

Fig.10 structure of magnet wire2



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Magnet wire uses extremely thin insulation material to maintain insulation

performance and protect the wire from various environmental factors, and has a major

effect on component service life. Since the life of insulation the material used

determines the life of the magnet wire, plastic are usually used.

Fig.11 Magnet wire service life curves1

Magnet wire is classified according to the heat resistance characteristic of its insulation

material (into classes Y, A, E, B, F, H and C). In conformance with the Arrhenius law,

the service life of each insulation class is normally 40,000 hours (or 20,000 hours under

the UL standard) at the maximum tolerance temperature for the class. The class of

motor insulation used is determined by the temperature rise caused by the heat of the

motor and by the operating environment. In addition to the insulation service life

characteristic, insulation performance is greatly affected by any contamination, by

impurities or damage to the insulation during manufacture. Similar insulation

Service

life (hr)

Polyester

Polyesterimide

Polyamide

Polyamideimide

Formal

Polyurethane



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problems can also be caused when the insulation is applied to the coil.

The aging of these problems caused during manufacture can cause problems that were

not initially apparent to exacerbate product failure factors.

3.4 Summary of product failure mechanisms

Figure 12 summarizes the mechanisms responsible for product failures linked to motors,

based on the findings of surveys on actual product failures and the characteristics of

magnet wire. Figure 13 summarizes the mechanisms responsible for product failures

linked to transformers.

Fig. 12 Summary of mechanisms responsible for product failures linked to motors



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Fig. 13 Summary of mechanisms responsible for product failures linked to transformers

4.1 Flyback transformer service life evaluations

This section describes the gradual improvements made on evaluation methods through

careful examination of actual product operating environments. These improvements

have enabled both better end-of-life reproducibility and accelerated life testing.

Table 1 summarizes how flyback transformer service life test conditions have evolved.

Since magnet wire service life conforms to Arrhenius law, service life testing initially

only examined ambient temperature, and couldn’t capture the correlation to problems

in the market [(A) in table 1]. When analysis of market failures showed that humidity

was a cause, this factor was added to the test conditions, enabling reproduction of the

destruction mode in the market [(B) table 1]. However, since it took six months to

evaluate reliability under these test conditions, researchers looked for a way to shorten

this time by working with the water absorbency of the insulation plastic. They found

that using a pressure cooker test (PCT) to preprocess the plastic reduced evaluation

time to about one month [(C) in table 1]. The final improvement came when researchers

modeled the deterioration of flyback transformer surface resistance caused by the dust

accumulated and humidity absorbed under actual operating conditions. Researchers

found that coating materials with silver paint is an effective way to detect structurally

Reliability testing designed to reveal potential product failures 4



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weak points [(D) in table 1].

Test

method

Period

used

Atmosp

here

Output

voltage

Core

temp.

PCT Silver

paint

coating

End-of-life

mode

reproduci

bility

(A) Jul.80

to

Oct.85

80°C Rated value

x1.1

Not set No No -

(B) Nov.85

to

Mar.87

70°C

85%RH

Rated value

x1.1

10h ON

2h OFF

100°C No No Good

(C) From

Apr.87

70°C

85%RH

Rated value

x1.1

10h ON

2h OFF

100°C 121°C

95%RH

2atm

72h

No

Good

(D) From

Mar.94

70°C

85%RH

Rated value

x1.1

10h ON

2h OFF

100°C 121°C

95%RH

2atm

72h

Yes Good

Table 1. Evolution of flyback transformer service life test conditions1

4.2 Key features of evaluation testing designed to reveal potential product failures

Figure 14 lists the key features of evaluation testing designed to reveal potential

product failures.



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Examining only components prone to weaknesses is insufficient. 1

•→ Effects on all components need to be evaluated.

Example: Abnormal winding temperature rise → Deterioration of startup capacitor characteristic

Carefully examine indications of deterioration during evaluation. 2

•→ End-state conditions in test results need to be carefully examined.

Example: Insulation deterioration

•→ Evaluation result: At least XX hours through XX MΩ or less

•→ How would the product perform after that? (Were safety devices triggered? Was there abnormal overheating?, etc.)

Extreme testing 3 •→ Evaluation under unanticipated operating conditions

Fig. 14 Key features of evaluation testing designed to reveal potential product failures

As with my failure analysis, the opportunity to survey product failures led to finding a

more detailed survey of fire causes than anticipated. Product failures thought to be

linked to device service life factors account for many of the product failures in the

market. I found that some products are used for a much greater length of time than

likely anticipated by their designers. Products such as electric fans are used until they

stop working, which can be longer than their design service life. The designers of these

products must therefore now consider not only the product’s design service life but also

its end-of-life failure mode. This sort of reliability tailored to product characteristics is

something we will likely be seeing greater demand for in the future, along with

corresponding reliability evaluation methods.

Bibliography

1. Journal of the Television Society [in Japanese], Volume 49, No. 11 (1995)

2. “Fire Cause” [in Japanese], website of Fire Investigation Research Team for

Firefighters (www7a.biglobe.ne.jp/~fireschool2/m-cause2.html#A1)

Conclusion 5

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No. 61 Product failure mechanism and reliability testing - Test Navi