24P148-P
Development of insulation deterioration diagnosis of motor coils
モータ巻線の絶縁劣化診断技術の開発
Yuki Fukuda, Masanori Miyamae, Yusuke Kikuchi, Masayoshi Nagata
Hikaru Miyazaki*, Tadashi Ito*
福田 有貴，宮前 公準，菊池 祐介, 永田 正義，宮崎 光*，伊藤 整*
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* Nishishiba Electric Co. Ltd., Japan1000 Hamada, Aboshi-ku, Himeji 671-1280, Japan*西芝電機（株） 〒671-1280 兵庫県姫路市網干区浜田1000
The relation between the aging conditions and the partial discharge (PD) characteristics in twisted
enameled magnet wires has been investigated for early detection of deterioration of electrical insulation ofmotor windings. In particular, an application of repetitive impulse voltage pulses was used for the PD
measurements in order to simulate inverter surge. The aging temperature was varied from 220 to 300 °C,
and the exposure time was changed from 1 to 1000 hours. As the results, the small deterioration due to thethermal accelerated aging, which cannot be detected by conventional methods such as a withstand voltage
test, was clearly found as a decrease of repetitive PD inception voltage (RPDIV).

1. Introduction
For motor manufactures, it is very important todetect deterioration of insulation capabilities ofmotor coil windings due to manufacturing defects,
mechanical stresses, environmental ones such astemperature and humidity, and electrical discharge
activities. In particular, recent inverter-fed randomwound motors are suffered from inverter-surge
voltage pulses [1]. Failure of motor operations due
to insulation breakdown causes a heavy cost ofrepair and loss of service. Thus, it is necessary to
make a method of early detection of deterioration ofinsulation capabilities of motor windings. Recently,
we have started to develop a new diagnostic method
to detect insulation failures of random woundmotors [2]. In this study, a thermal stress on motor
coil windings was considered as one of degradation
processes, and measurements of partial discharge oftwisted pair samples simulated for random woundmotor windings with thermal accelerated stresses
were carried out. Especially, repetitive impulse
voltage pulses were applied to the twisted pairs inorder to simulate inverter-surge voltage.
2. Experimental setup
The twisted pair consists of two enameledmagnet wires to simulate random wound motorcoils. The total length and the number of the twisted
turns are 120 mm and 10, respectively. The
diameter of conductor is 1.4 mm. mm. Ester-imide(EI) is used as insulation organic material, andamide-imide (AI) is coated on the EI layer in orderto protect the surface from mechanical damage

during manufacturing processes of motor windings.
The temperature class of the insulation material isclass F standardized by the IEC (InternationalElectrotechnical Commission) which withstands atemperature of 155 °C. The twisted pairs were
heated in a temperature-controlled oven to simulate
thermal aged motor windings. The aging
temperature was varied from 220 to 300 °C, and theexposure time was changed from 1 to 1000 hours.
The experimental procedure in this study is shownin Fig. 1. Five twisted pairs for each kind of aging
conditions were used for PD measurements. Here,the measurement was carried out in atmospheric air,and the ambient temperature and humidity were25 °C and 30 %, respectively. Repetitive impulse
voltage pulses were generated by an impulse
winding wire testing machine (ECG-KOKUSAICo., type: DWX-05). This power supply has beenused as a common impulse generator with a fixed
impulse sequence pattern specific to round-robintest (RRT) for repetitive PD inception voltage(RPDIV) measurement in Japan [3].
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) [4]. 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). Ten
voltage impulses with the same negative voltage
amplitude were applied to the twisted pairs with aperiod of 20 ms. The peak value of the impulses

Twisted pair(Virgin)

Twisted pair(240 °C, 300 h)

Thermal accelerated aging(controlled oven)
Temperature: 220 ~ 300 °C
Exposure time: 1 ~ 500 hour

RPDIV measurement
Repetitive impulse voltage
5 twisted pairs

Fig. 1 Experimental setup

was increased from 1 kV to 2 kV.
3. Experimental results
Firstly, the RPDIV of the virgin twisted pair was
about 1.9 kV. Figure 2 shows the RPDIV of thethermally degraded twisted pairs as a function ofthe thermal aging time at each temperature. Eachdata point was obtained by averaging over themeasured RPDIV of 5 twisted pairs. Here, thehorizontal axis of Fig. 2 is logarithm of ta. The
breakdown voltage of the twisted pairs degraded
thermally at 240 °C (10 h) from the withstandvoltage test was same as that of the virgin sample,however, a significant decrease of the measured
RPDIV was observed. The RPDIV of the twisted
pair at ta = 10 h decreased into 90 % of PDIV of thevirgin sample. This decrease of RPDIV wasreproducibly measured in this study. In addition, thePDIV was linearly decreased by plotting (PDIV-logta) with the increase in ta. Therefore, it could beconsidered that insulation capability of randomwound motors could be estimated by the decreaseof the RPDIV. The sensitivity of the RPDIV for
detection of thermal deterioration of enameledwires is higher than that of the PDIV under avoltage application of ac 60 Hz [2].
4. Summary
The RPDIV measurements of the twisted pairs
with thermal stresses have been performed in orderto develop a new evaluation method of degradationof insulation capabilities in electrical motors. From
the results in this study, it is found that the small
degradation of the enamel layer gives rises to theFig. 2 Effects of thermal stresses on the RPDIV

significant reduction of RPDIV.
The other
deterioration processes such as salt damage forelectrical motors in a ship will be considered in thefuture. In addition, this method will be applied tonot only twisted pair samples but also actualmotors.
Acknowledgments
The authors thank to TEIKOKU ELECTRIC
MFG Co., Ltd. for their assistances to ourexperiment.
References[1] G.C. Stone, Proc. of Petroleum and ChemicalIndustry Conference, (2000) 149.
[2] Y. Kikuchi et al., Proc. of 16th International
Conference on Electrical Engineering (2010)
PS-HV&ED-17.
[3] K. Kimura et al., Proc. of the Conference onElectrical Insulation and Dielectric Phenomena(CEIDP) 2010, 5B-18.
[4] K. Kimura et al. Proc. International Symposium onElectrical Insulating Materials (ISEIM2008), 196.5.