Product failure mechanisms and reliability testing

Transcription

1 Technology Report Product failure mechanisms and reliability testing Part 1: Motors and transformers Masahiro Yamaura Analysis Center, Calibration Group ESPEC CORP., Reliability Research Headquarters, T 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). 1 Introduction 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 Failure of products used for extended periods of time 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) 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 1

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) No. of product failures Years of use Fig.2 Number of product failures by years of product use (motors) 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) 2

3 No. of product failures n/a Years of use 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 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 3

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 fan 2 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 Product failures caused by transformers Figure 5 illustrates how flyback transformers can cause product failures. 4

5 Lead wire resistance inside 6 ml Cracks formed, causing discharges in the direction of the arrow core 6.1 cm Fig. 5 Product failure caused by flyback transformer 2 3 Product failure mechanisms 3.1 Motors Figure 6 illustrates the mechanism responsible for product failures caused by deterioration of the motor coil s insulation. Temperature Humidity Temperature Humidity Extended use (approx. 30 years) Insulation deterioration (on motor winding) Extended use (approx. 30 years) Lubrication oil leak (from motor bearings) Short-circuiting, sparks Layer shorting Loss of smooth rotation Overheating (in motor coil winding) Fire Fire 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 5

6 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. 6

7 Temperature Humidity Temperature Humidity Extended use (approx. 13 years) Insulation deterioration (on FBT highvoltage coil) Dust Extended use (approx. 19 years) Oil mist Deterioration of insulation (on FBT high-voltage coil) Vaporization of insulation material leads to FBT voltage rise Layer shorting High-voltage leaks Cracks in FBT casing Cracks in FBT casing Product emits smoke 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 Conductor (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 Insulation (h) Water- and humidity-resistant Fig.10 structure of magnet wire 2 7

8 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. Polyamide Polyamideimide Service life (hr) Formal Polyurethane Polyester Polyesterimide Fig.11 Magnet wire service life curves 1 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 8

9 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 9

10 Fig. 13 Summary of mechanisms responsible for product failures linked to transformers 4 Reliability testing designed to reveal potential product failures 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 10

11 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 C Rated value Not set No No - to Oct.85 x1.1 (B) Nov C Rated value 100 C No No Good to Mar.87 85%RH x1.1 10h ON 2h OFF (C) From 70 C Rated value 100 C 121 C No Good Apr.87 85%RH x1.1 10h ON 2h OFF 95%RH 2atm 72h (D) From 70 C Rated value 100 C 121 C Yes Good Mar.94 85%RH x1.1 10h ON 2h OFF 95%RH 2atm 72h Table 1. Evolution of flyback transformer service life test conditions 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. 11

12 1 Examining only components prone to weaknesses is insufficient. Effects on all components need to be evaluated. Example: Abnormal winding temperature rise Deterioration of startup capacitor characteristic 2 Carefully examine indications of deterioration during evaluation. 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.) 3 Extreme testing Evaluation under unanticipated operating conditions Fig. 14 Key features of evaluation testing designed to reveal potential product failures 5 Conclusion 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) 12