Effect of Winding Coil Diameter on AC Insulation Breakdown Voltage of Polyamideimide/Nanosilica Wire

Abstract

The AC insulation breakdown voltage was investigated for seven types of winding coils made of polyamideimide (PAI), flexural PAI (nanosilica 5 wt%) and anti-corona PAI (nanosilica 15 wt%) wires with various winding coil diameters of φ5, φ15 and φ25 mm. The winding coil was made of enameled wire with an enamel thickness of 30~50 μm, and the rectangular copper wire had a thickness of 0.77~ to 0.83 mm and width of 1.17~ to 1.23 mm, respectively. The insulation breakdown voltages of the original PAI coils with diameters of φ5, φ15 and φ25 mm were 7.30, 6.58, and 5.95 kV, respectively, and those values decreased as the winding coil diameter increased, regardless of the wire types.

1. INTRODUCTION

In recent years, environmental issues have emerged as a global concern, and the energy efficiency in industrial motors has become more important, with many modern industrial motors now equipped with inverters or adjustable-speed drives to achieve a high efficiency. Also, nverter power supplies implement the pulse modulation (PWM) technique, for which insulated gate bipolar transistors (IGBTs) are used as high speed switching devices, to improve the controllability of the motor torque and to reduce the switching loss [1-3].

Polyamideimide (PAI) enamelled wires exhibit good thermal and electrical insulation properties. Polyamide is a type of polymer with a high insulation breakdown strength and good heat resistance, mechanical strength and chemical resistance. Therefore polyamide-insulated wires have been used in harsh environmental conditions [4-6]. Polyimides are used in inverter-fed motor windings and in high-pressure coils for electric translators, and in recent years, elements allowing for high-speed switching, such as insulated-gate bipolar transistors (IGBTs), have been developed for use as inverter power devices.

The electrical strength of the enamelled wire, is evaluated by measuring the insulation breakdown voltage using various methods [7], including (1) close contact with the cylinder, (2) twisted pairs, (3) U-bend specimen in shot bath and (4) Coil-wound specimen in a shot bath. All test voltages ars applied with an AC voltage with a nominal frequency of 50 Hz or 60 Hz. The test voltage is applied from zero and increases at a uniform rate until breakdown.

In this study, the insulation breakdown voltage was investigated using coil-wound specimen in shot bath prepared using PAI enameled rectangular wires.

 

2. EXPERIMENTS

The PAI wires were prepared starting wihh rectangular copper wires with thickness of 0.77 to 0.83 mm and width of 1.17 to 1.23 mm , resprctively, and coating them with seven types of PAI/nanosilica (15 wt%) enamels (Sam Dong Co., Ltd., Korea) with enamel thickness of 30~to 50 μm. The first wire was coated with original PAI, the three types for the second group were coated with double layers of flexural PAI and anti-corona PAI (nanosilica 15 wt%), and the three types for the third group were coated with double layers of flexural PAI (nanosilica 5 wt%) and anti-corona PAI (nanosilica 15 wt%). Type_1 and Type_4 were dried at 220℃, Type_2 and Type_5 were dried at 240℃, and Type_3 and Type_6 were dried at 260℃.

The AC insulation breakdown voltage was tested in a shot bath, as seen in Fig. 1, using coil-wound specimens with winding coil diameters of φ5, φ15 and φ25 mm, as shown in Figure 2. A sinusoidal waveform voltage from 0 ~ to 12 kV with a frequency of 50~ to 1,000 Hz was applied to the winding coil specimens using a withstanding voltage tester (APCP-12 kV-100, Sky Innotek Co., Ltd.) for the frequency acceleration. In order to apply A high voltage was applied by removing the , enamel insulation layer from one side of the wire and arranging the specimen in the shot bath, as shown in Figure 1, The insulation breakdown voltage was measured at a speed of 0.5 kV/s until the electrical insulation breakdown took place.

Fig. 1.Coil-wound specimen for the breakdown voltage test in a shot bath.

Fig. 2.Coil-wound specimen for the insulation breakdown voltage test and photo of the insulation breakdown trace in an enamelled wire.

Figure 3 shows photo of the insulation breakdown trace in the enamelled wire.

Fig. 3.Weibull statistical analysis for the insulation breakdown voltage of the original PAI wires with various winding coil diameters at 60 Hz.

 

3. RESULTS AND DISCUSSION

Figure 3 shows the Weibull statistical analysis (Weibull++ 7.0) for an insulation breakdown voltage of the original PAI wires with various winding coil diameters at 60 Hz, and the Weibull parameters such as shape, scale, and B10 value were obtained from each Weibull plot and are listed in Table 1. That is to say, the shape parameter could be obtained from the slope, meaning that the data distribution and the scale parameter were obtained from the AC insulation breakdown voltage by which 63.2% of the cumulative probability was expected to fail. The B10 value represented the AC insulation breakdown voltage with a 10% cumulative probability. The insulation breakdown voltages at φ5, φ15, and φ25 mm were 7.30, 6.58, and 5.95 kV, respectively, and thos values decreased as the winding coil diameter increased.

Table 1.Weibull parameters for the insulation breakdown voltage of the original PAI wires with various winding coil diameters at 60 Hz, as obtained from Fig. 3.

Figure 4 shows the Weibull statistical analyses for the insulation breakdown voltage of Type_1 wires with various winding coil diameters at 60 Hz, Type_1 was made of double layers with flexural PAI and anti-corona PAI (nanosilica 15 wt%), and the Weibull parameters were obtained from each Weibull plot and are listed in Table 2. The insulation breakdown voltages of Type_1 wires at φ5, φ15 and φ25 mm were 7.09, 6.44 and 5.77 kV, respectively, and those values decreased as the winding coil diameter increased. The insulation breakdown voltages for the Type_1 wires were slightly lower than those of the original PAI wire, and this means that the nanosilica in the anti-corona PAI (nanosilica 15 wt%) layer did not play a significant role in the electrical insulation.

Fig. 4.Weibull statistical analyses for the insulation breakdown voltage of Type_1 wires with various winding coil diameters at 60 Hz.

Table 2.Weibull parameters for the insulation breakdown voltage of Type_1 wires with various winding coil diameters at 60 Hz obtained from Fig. 4.

Figure 5 shows the Weibull statistical analyses for the insulation breakdown voltage for wires of various types with a coil diameter of φ5 mm at 60 Hz, The Weibull parameters were obtained from each Weibull plot and are listed in Table 3. The Weibull parameters for wires of various types with a coil diameter of φ15 mm and φ25 mm at 60 Hz are also listed in Table 3. The insulation breakdown voltage decreased as the winding coil diameter increased, regardless of the wire type, and the values for the wires of the third type were slightly lower than those for wires of the second type while those for the second type were slightly lower than those for the first type. This meant that nanosilica in the flexural PAI (nanosilica 5 wt%) layer and in the anti-corona PAI (nanosilica 15 wt%) layer did not play a significant role in electrical insulation.

Fig. 5.Weibull statistical analysis for insulation breakdown voltage for wires of various types with a coil diameter of φ5 mm at 60 Hz.

Table 3.Weibull statistical analysis for insulation breakdown voltage for wires of various types with coil diameters of φ5 mm, φ15 mm and φ25 mm at 60 Hz.

 

4. CONCLUSIONS

The AC insulation breakdown voltage was investigated in winding coils of various diameters, including φ5, φ15, and φ25 mm. The winding coil was made of PAI/nanosilica enameled wires with an enamel thickness of 30~ to 50 μm. The enamel has two layers made from flexural PAI or flexural PAI (nanosilica 5 wt%) and anti-corona PAI (nanosilica 15 wt%). The insulation breakdown voltages of the original PAI coils with φ5, φ15, and φ25 mm were 7.30, 6.58, and 5.95 kV, respectively, and those values decreased as the winding coil diameter increased regardless of the type of wire used. The order of the insulation breakdown strength was as follows: original PAI > flexural PAI/anti-corona PAI (nanosilica 15 wt%) > flexural PAI (nanosilica 5 wt%)/anticorona PAI (nanosilica 15 wt%). This means that the nanosilica did not play a significant role in providing electrical insulation.