Abstract

Ambient temperatures in tropical countries across the equatorial belt are adversely high. Thus, the operating temperatures of electrical machines in these countries are uncomfortably high even when, the machines operate at rated load. Nigeria as a tropical and developing country has had to contend with this unfavorable thermal condition for electrical machines. Most of the drive motors in Nigeria are imported and many of them come with class B insulation. In this study, eight sample varieties of Nigerian wood species-Mahogany, Obeche, Iroko, Afara, Danta, Opepe, Abeza and Aper and the two most commonly imported class B insulating materials-fiber glass and leatheroid are experimented with to compare their ability to maintain insulation integrity at higher temperatures. Impregnated samples of the eight wood species and samples of leatheroid and fiber glass were subjected to a heat-run in a sealed industrial oven. The insulation resistance of each sample was measured at regular temperature intervals, until the sample burns out. Tables show the values of weight, insulation resistance and temperature. The results show that at the class B limiting temperature of 130°C, leatheroid had an insulation resistance of 6 MΩ, while that of fiber glass was 8 MΩ. The eight wood samples had insulation resistance above 30 MΩ. At the higher temperature of 150°C, the insulation resistances of leatheroid and fiber glass collapsed to 1 and 1.4 MΩ, respectively while that of the wood species was a minimum of 15 MΩ. At a much elevated temperature of 170°C, both leatheroid and fiber glass were burnt but, the insulation resistances of the eight wood species were up to 8 MΩ. Thus, the wood species exhibited higher insulation integrity at higher temperatures than leatheroid and fiber glass. These wood species through some careful chemical synthesis could be made more malleable and consequently, serve as alternative and cheaper machine insulating materials to the imported leatheroid and fiber glass.

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INTRODUCTION

The effect of temperature on insulation of a machine is of great importance and is a limiting factor in design. The behavior of insulating materials under different thermal and environmental conditions is an important subject of investigation. For example, Peng et al. (2007) Wieck et al. (2007), Kannus and Lahti (2007) and Berlijn et al. (2007) examined the flashover performance of insulators and breakers under iced conditions while, Abderrazzaq (2008), Du (2005), Lan and Gorur (2008) Yousfi et al. (2005) investigated the influence of dryness, wetness, atmospheric pressure and heat on the dielectric characteristics of insulators. Mayoux (2000), Oraee (2000), Nelson et al. (2000), Hudon et al. (2000) and Crine (2005) examined the degradation of insulating materials under electrical stress and the problem of aging and life expectancy of motor insulation. In their research, Paraskevas et al. (2006), Fu et al. (2007), Kikuchi et al. (2008), Rui-Jin et al. (2008) and Ishikawa et al. (2009) investigated the influence of ambient and operating temperatures on the dielectric properties and aging of insulating materials while, the influence of water absorption on the dielectric quality of insulators was studied by Kyritsis et al. (2000) and Hong et al. (2009a, b). In their own contributions, Ohki and Hirai (2007) examined the electrical conduction and breakdown properties of several biodegradable polymers, while D.C ramp test is used to determine the dielectric response of a stator winding insulation (David et al., 2007; Ohki and Hirai, 2007). Chakradhar and Ramu (2008), Murakami et al. (2008) and Maity et al. (2008) investigated the thermal breakdown of polymer nanocomposites as insulation for HVDC cables; the electrical breakdown properties of magnesium oxide/low-density, polyethylene (MgO/LDPE) nanocomposites, as well as the degradation of polymer dielectrics with nanometric metal oxide fillers due to surface discharges.

The operating temperatures of electrical machines in tropical countries are uncomfortably high. Nigeria as a tropical and developing country has had to contend with this unfavorable thermal condition for electrical machines. Most of the drive motors in Nigeria are imported and many of them come with class B insulation. The permissible maximum temperature for class B insulation is 130°C. Leatheroid and fiber glass are very common imported class B insulating materials used in electrical machine winding and slot insulation in Nigeria. The wood species of Abeza, Afara, Aper, Danta, Iroko, Mahogany, Obeche and Opepe are abundantly available in Nigeria. The research experimented with these wood species as well as leatheroid and fiber glass to compare their ability to maintain insulation integrity at higher temperatures. The research also evaluated if these wood species can serve as viable alternative and cheaper machine insulating materials to the imported leatheroid and fiber glass.

MATERIALS AND METHODS

The eight wood species used in the experimental research were:

•

Abeza

•

Afara

•

Aper

•

Danta

•

Iroko

•

Mahogany

•

Obeche

•

Opepe

The two imported class B insulating materials were;

•	Fiber glass

•	Leatheroid

Preparation of the samples: Each of the eight wood species was cut into a sample measuring 10x5x0.5 cm. They were impregnated by immersing in a hot insulating varnish for 20 h and then slowly dried for 3 days. Both leatheroid and fiber glass were cut into samples measuring 10x5 cm. But, the thickness of each sample was maintained as manufactured in order not to alter the material integrity of the sample. The weight of the samples before impregnation, immediately after impregnation and after drying as well as the initial insulation resistances (at room temperature) of each sample material are shown in Table 1.

 

Table 1:	Initial parameters of samples

 

 

Table 2:	Heat run and insulation resistance measurement of sample materials (MΩ)

 

Heat-run: Each sample of the eight wood species as well as samples pf leatheroid and fiber glass were subjected to heat run in a well-lagged industrial oven. The insulation resistances of the samples were measured at regular temperature intervals of 20°C, until the given sample burns out. Table 2 shows, the insulation resistance measurement of the samples during the heat-run.

RESULTS AND DISCUSSION

Table 2 presents, some fascinating results. Fibreglass and leatheroid are the two most commonly imported class B insulating materials for electrical machine winding and slot insulation in Nigeria. At the class B limiting temperature of 130°C, leatheroid had an insulation resistance of 6 MΩ, while that of fiber glass is 8 MΩ. The insulation resistance of impregnated samples of the eight wood species at this temperature was a minimum of 30 MΩ. Aper and Iroko each had an insulation resistance of 50 MΩ at this temperature. When, the oven temperature was raised to 150°C, the insulation resistances of leatheroid and fiber glass collapsed to 1 and 1.4 MΩ, respectively. But, the insulation resistance of the wood samples was a minimum of 15 MΩ. In fact, Aper and Iroko each had an interesting figure of 20 MΩ. At an elevated oven temperature of 170°C, both leatheroid and fiber glass were burnt. Of the eight wood species, four (Abeza, Afara, Mahogany, Obeche) had an insulation resistance of 8 MΩ while, Aper, Danta, Iroko and Opepe had an insulation resistance of 9 MΩ. Thus, the wood samples exhibited higher insulation resistances at higher temperatures than leatheroid and fiber glass, the imported class B insulating materials.

CONCLUSION

The experiments showed that the impregnated samples of the eight wood species-Abeza, Afara, Aper, Danta, Iroko, Mahogany, Obeche and Opepe exhibited higher insulation integrity at higher temperature than leatheroid and fiber glass, the imported class B insulating materials. These wood species through some careful chemical synthesis could be made malleable and consequently, serve as alternative and cheaper machine insulating materials to the imported leatheroid and fiber glass.

How to cite this article:

A.M.O. Obiazi. Comparative Analysis of the Insulation Resistance of Some Nigerian Wood Species with Imported Class B Insulating Materials.
DOI: https://doi.org/10.36478/ijepe.2009.238.241
URL: https://www.makhillpublications.co/view-article/1990-7958/ijepe.2009.238.241