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Unformatted text preview: January/February 2005 — Vol. 21, No. 1 5 F E A T U R E A R T I C L E F E A T U R E A R T I C L E F E A T U R E A R T I C L E F E A T U R E A R T I C L E F E A T U R E A R T I C L E B The PDIV under surge voltages was 1.6 to 1.8 and 2.3 to 2.7 times higher than those under AC voltage application for the used and virgin samples, respectively. Introduction ecause conventional inverter-fed motors for electric vehicles and hybrid vehicles operate under low-voltage stresses – , their electrical insulation performance has not been critical for their basic design. However, the operating voltage of inverter- fed motors is being increased for higher performance, e.g., high power output and compactness, which requires a rational electri- cal insulation design. Especially, inverter-fed motors utilize power electronic devices, such as IGBTs with high-speed switching abil- ity , and are then exposed to transient surge voltage stresses with steep wave fronts as shown in Figure 1. The surge voltage generation is attributed to the difference in surge impedances of the inverter (Z inverter ), cable (Z cable ), and motor (Z motor ). Z motor for a 20- to 50-kW motor is of the order of k Ω , as can be seen in Figure 2 , whereas Z cable is of the order of 10 to 30 Ω , and Z inverter is generally very low and ~ 0 Ω , i.e., Z motor >> Z cable > Z inverter . Such a surge voltage stress can cause partial discharges in the motor in- sulation system and can deteriorate the electrical insulation, lead- ing to breakdown. From this background, in this article, we have investigated the partial discharge (PD) inception characteristics of a twisted pair as a simplified inverter-fed motor-coil sample. We discuss the difference in the PD inception characteristics of a twisted pair under 60-Hz voltage and surge voltages with a steep wave front. Experimental Set-Up Figure 3 shows the experimental set-up for the measurement of the PD characteristics of the twisted pair. The twisted pair con- sists of two enamel-coated wires with the following specifica- tions: conductor diameter: 0.845 mm, coating thickness: 0.03 mm, twisted pitch: 7 mm, and total length: 130 mm. The enamel coat- ing has two layers: an inner layer (thickness, 0.023 mm; dielec- tric constant, 3.8) and an outer layer (thickness, 0.007 mm; di- electric constant, 4.2). One of the wires was grounded, and high- voltage AC or surges were applied to the other wire. The fre- quency of the AC voltage was 60 Hz, and the surge voltage had a rise time of 20 ns, a duration of 1 μs for a single shot, and a repetitive shot frequency of 6 or 60 pps. The PD inception voltage (PDIV) was detected by PD light intensity under both AC and surge voltage applications. The PD light intensity signal was observed using a photo multiplier tube (PMT). The PD light emission images were taken by a still cam- Partial Discharge Characteristics of Inverter-Fed Motor Coil Samples Under AC and Surge Voltage Conditions 1 Key Words: Motors, twisted pairs, partial discharges, inception voltage, periodic surge...
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