24P147-P
Characteristics of partial discharges and electrical insulation of inverter-fed
random wound motor coils
インバータサージによる低圧乱巻モータコイルの絶縁劣化と部分放電特性
Masanori Miyamae, Keita Fukui, Yuki Fukuda, Yusuke Kikuchi,
Naoyuki Fukumoto, Masayoshi Nagata, Shinichiro Yamada* and Tetsuo Yoshimitsu*
宮前 公準，福井 経太, 福田 有貴, 菊池 祐介, 福本 直之, 永田 正義，
山田慎一郎*，吉満 哲夫*
Graduate School of Engineering, University of Hyogo
2167 Shosha, Himeji, Hyogo 671-2280, Japan
兵庫県立大学 工学研究科 〒671-2280 兵庫県姫路市書写2167*Toshiba Mitsubishi-Electric Industrial Systems Corporation
2-4 Suehiro-cho, Tsurumi-ku, Yokohama, 230-0045, Japan*東芝三菱電機産業システム（株） 〒230-0045 神奈川県横浜市鶴見区末広町2-4
Repetitive partial discharge inception voltage (RPDIV) of twisted enameled magnet wires was investigatedin order to clarify insulation capabilities of inverter-fed random wound electrical motors. A power supply
which produces bipolar repetitive impulse voltage pulses was used in this study. The measured data ofRPDIV has a relatively large dispersion in the early stage of the voltage sequence. After that, it reached tothe steady-state value.

1. Introduction
It is recently known that a transient surge voltage
induced by the switching of insulated gate bipolartransistors (IGBTs) is a critical issue for electricalinsulation of inverter-fed random wound motor
coils [1]. Partial discharge (PD) activities in motor
windings due to the surge voltage have been
identified as one of the main factors of insulation
failures of inverter-fed motors. Then, theIEC60034-18-41TS requires no PD during the
lifetime of motor.
A PD inception voltage (PDIV) is defined by avoltage at which PDs start to occur. A lot of studies
of PDIV measurements under ac voltage
applications have been performed. In particular,
dependences of PDIV on environmental humidity
and temperature have been investigated [2, 3]. On
the other hand, a voltage waveform of
inverter-surge is repetitive transient voltage
oscillations, so that PDIV measurements underrepetitive impulse voltages have to be studied.
In this study, results of PDIV measurements
under repetitive impulse voltage applications arepresented.
2. Experimental setup
The twisted pair consists of two enameledmagnet wires to simulate random wound motor
coils, as shown in Fig. 1. The enameled wire with adiameter of 1 mm is used in this study.
Polyamide-imide is used as insulation organic
material for the enameled wire.

Temperature andhumidity chamberPatchantenna
Impulse voltage
power supply
PMT
Twisted pairDigital
oscilloscope

Fig. 1 Experimental setup.

Ten twisted pairs are simultaneously installed ina temperature and humidity chamber (ESPEC Co.,
type: LHU-113), as shown in Fig. 1. The
temperature and relative humidity fluctuation levels
at the control sensor are 0.5 °C and 3 %,respectively. Here, the experiment was carried out
in atmospheric air. Bipolar repetitive impulse
voltage pulses were generated by an impulse
winding wire testing machine (ECG-KOKUSAICo., type: DWX-05). The unipolar (negative) power
supply has been used as a common impulse
generator with a fixed impulse sequence patternspecific to round-robin test (RRT) for repetitive PD
inception voltage (RPDIV) measurement in Japan[4]. The waveform of single pulse voltage istriangular with 140 ns in rise time. The waveformof repetitive impulse voltage pulses are shown inImpulse voltage

PD pulse

20 ms/div

Fig. 2 Time evolutions of the applied impuse voltage
pulses and the PD pulses.

Fig. 2.
The PD was detected by a photomultiplier tube
(PMT, Hamamatsu Co., type: R960, 185-650 nm)
and narrowband antenna (patch antenna, centerfrequency 1.8 GHz) [5]. The repetitive impulse
voltage pulses were measured by a high voltage
probe (Tektronix Co., type: P6015A, 75 MHz). The
measured data was recorded by a digital
oscilloscope (Tektronix Co., type: DPO7104).
3. Experimental results
The ten twisted pair samples are tested thirty
times in sequence. The sequence is as follows: Ten
voltage impulses with the same voltage peak
amplitude were applied with a period of 20 ms.
After 50 ms pause, the next ten voltage pulses withan increase of 10 V follow. If PD pulses exceed fiveagainst ten impulse voltages, the averaged voltageof the impulse was defined as RPDIV. Thus, thedefinition of PPDIV is one of statistical data
analysis methods, because the PDIV under impulse
voltage pulses has a relatively large degree ofdispersion in comparison with the traditional
measurements under ac voltages.
Figure 3 shows the result of measurements of
RPDIV, where the horizontal axis is number of thevoltage sequence. Here, the temperature andhumidity were set 30 deg. and 40%. It is found that
there is relatively large dispersion of the measured
RPDIV. After the 20 sequences, the RPDIV reaches
to the steady state. It is considered that surface
charges on the enameled wire due to the PDs affectthe next PD. On the other hand, the behavior of theRPDIV was not observed in the experiment with theunipolar power supply [4]. The surface charge
distribution on the enameled coated copper plate
due to PDs will be performed in future.
4. Summary
The RPDIV of the twisted pair samples under theFig. 3 The history of the RPDIV with respect to thepulse number.

bipolar repetitive impulse voltage were measured inorder to clarify electrical insulation capabilities of
inverter-fed random wound motors. The RPDIV has
fluctuates with large dispersion in the early stage ofthe voltage sequence, and it reaches to the certain
steady-state value. It could be considered that
surface charge accumulation on the enameled wirecauses the fluctuation of the RPDIV data.
Acknowledgments
The authors thank to TEIKOKU ELECTRIC
MFG Co., Ltd. for their assistances to ourexperiment.
References[1] G.C. Stone, Proc. of Petroleum and Chemical
Industry Conference, (2000) 149.
[2] Y. Kikuchi et al., IEEE Transactions on Dielectrics
and Electrical Insulation 15 (2008) 1617.
[3] Y. Kikuchi et al., IEEE Transactions on Dielectrics
and Electrical Insulation 17 (2010) 839.
[4] K. Kimura et al., Proc. of the Conference onElectrical Insulation and Dielectric Phenomena(CEIDP) 2010, 5B-18.
[5] K. Kimura et al. Proc. of International Symposium
on Electrical Insulating Materials (ISEIM2008), 196.