Abstract
The temperature-rise, which electrical machines may safely withstand is determined by the limiting temperature of the insulating materials used in them. It is therefore, a vital requirement to qualify electrical insulating materials thermally by determining their insulation class. In this study, 25 sample varieties of Nigerian cloth fabrics were experimented with to determine their insulation classification. The samples were cut into definite dimensions and weighed. Each type of cloth fabric was made into two samples; one sample left in its ordinary state, while the other sample was impregnated with insulating varnish. Both samples were subjected to a heat-run in a sealed industrial oven, while measuring the insulation resistance of the given sample at regular temperature intervals, until the sample burns out. The measured values of weight, insulation resistance and temperature are shown in tables. Curves were plotted to show the variation of insulation resistance with temperature. From the experiments, only two of the unimpregnated samples can be used for class Y insulation whose limiting temperature is 90°C, while 24 impregnated fabrics can be used for class Y insulation. Only one cloth fabric is unsuitable for class Y insulation even with impregnation.
INTRODUCTION
The thermal stability or limiting temperature of machine insulating materials determines the temperature rise, which the machines can withstand safely. By using materials of higher limiting temperature, machines of the same physical size may be rated for greater power output. Great efforts have been made to qualify electrical insulating materials thermally and to improve their thermal capabilities. The IEEE and IEC have developed practices and procedures for the thermal evaluation of insulation systems for electrical machines.
Modern machines using synthetic or inorganic materials such as glass and mica in a matrix of polyester, epoxy or silicon resins exhibit long life at elevated temperatures (Raja et al., 2007; Ali and Hackam, 2008; Lamarre and David, 2008; Takala et al., 2008). Insulating materials are classified thermally as Y, A, E, B, F, H and C. The allowable temperature for each of these classes is defined in NEMA, BSI and IEC standards are 90, 105, 120, 130, 155, 180 and >180°C, respectively. The tables are based on a 20 years working life under average conditions. Reddy and Ramu (2007) examined the intrinsic thermal stability in HVDC cables, while Bassapa et al. (2007), Berlijn et al. (2007), Chong et al. (2007),
Kannus and Lahti (2007), Reddy andn Ramu (2007) and Wieck et al. (2007), investigated the dielectric performance of insulators under iced and hot temperature conditions while, the influence of water absorption on the dielectric quality of insulators was studied by Kyritsis et al. (2000) and Hong (2009a, b). High quality insulating materials are expensive and for many developing countries, they are imported. In their research, Paraskevas et al. (2006), Fu (2007), Kikuchi et al. (2008), Rui-Jin (2008) and Ishikawa (2009) investigated the influence of ambient and operating temperature on the dielectric properties and aging of insulating materials. This experimental study is an effort to thermally qualify cloth fabrics available in Nigeria.
By measuring, their maximum operating temperature, their insulation class can be determined, thus, helping to determine their level of use as insulating materials for electrical machines and if they can serve as viable alternative insulating materials to imported ones. Conducting, the experimentation with both impregnated and unimpregnated samples will help to evaluate the improvement in insulation resistance resulting from impregnation. Twenty-five sample varieties of Nigerian cloth fabrics were used in the experimental research.
MATERIALS AND METHODS
The twenty-five cloth fabrics used in the experi-mentation were:
Preparation of the sample of cloth fabrics: Each sample of the 25 types of cloth fabric measured 10x5 cm. The thickness of each fabric was maintained as manufactured in order not to alter the integrity of the fabric. Each type of fabric was made into two samples. One of the samples was impregnated by immersing in hot insulating varnish for ten hours and then dried slowly for 2 days while, the other sample was left unimpregnated.
The weights of the samples before impregnation, immediately after impregnation and after drying, as well as the initial insulation resistance (at room temperature) of both samples of each fabric are shown in Table 1.
Heat run: The two samples of each of the twenty-five cloth fabrics were subjected to heat run in a well-lagged industrial oven shown in Fig. 1. 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 cloth samples during the heat-run.
| Table 1: |
Initial parameters of samples of cloth fabrics |
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| Table 2: |
Heat and insulation resistence measurement of sample of cloth fabrics(v=varnish,nv=non varnish) |
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| Fig. 1: |
The inner chambers of the oven |
RESULTS AND DISCUSSION
The curves of the variation of insulation resistance with temperature for the impregnated samples of the twenty-five cloth fabrics are shown in Table 2. From Table 2, it can be seen that even the impregnated samples of cloth fabrics could not withstand high temperatures. Many of the impregnated samples could barely withstand 110°C, while, at 130°C, all the samples were burnt. Only two samples of unimpregnated fabrics (Cotton and Cord Lace) had insulation resistance up to 8 MΩ at 90°C. The rest twenty-three unimpregnated samples had insulation resistances ranging from 1.8 MΩ to 7 MΩ and so are unsuitable for use at temperatures up to 90°C. For the impregnated samples, only one fabric (Ribbon) had insulation resistance below 8 MΩ at 90°C. Thus, it is unsuitable as insulating material for temperatures up to 90°C. The remaining twenty-four impregnated fabrics had insulation resistances up to and exceeding 8 MΩ at 90°C. Since 90°C is the limiting temperature for class Y insulation, these twenty-four fabrics can be used as insulating materials for class Y operation.
CONCLUSION
Out of the twenty-five types of cloth fabrics used in this experimental research, only two samples of unimpregnated materials (Cotton and Cord Lace) had insulation resistance up to 8 MΩ at 90°C. Thus, only these two cloth fabrics can be used for class Y insulation without impregnation. For the twenty-five impregnated samples, only Ribbon had insulation resistance below 8 MΩ at 90°C and so is unsuitable for class Y insulation even when impregnated. The remaining twenty-four impregnated cloth fabrics had insulation resistances up to and exceeding 8 MΩ at 90°C. Thus, apart from Cotton and Cord lace, the other twenty-two cloth fabrics would require impregnation to make them suitable for class Y insulation.
