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Reliability and availability for high voltage synchronous machines are analyzed. The presented study is based on
data from 34 motors at three paper mills in Sweden, where the motors have been running for approximately 15
years. The failure rate is approximated with constant values in one initial phase during the first 3 years and one
best phase after three years. Any aging phase is not noticed. Values on failure rates mean time between failures
and availability are presented and compared to other studies.

By : Erland Olsson, professor mechanical engineering Mälardalen University, Sweden

Summary

Maintenance data from 34 synchronous motors at three paper mills in Sweden has been collected and analyzed. The motors in the study have been running approximately 15 years. Reliability data in the form of failure rates and mean times to failures are calculated and discussed.
Repair times and stop times are analyzed and used to calculate availability, both design availability and operational availability.
The failure rates for the motors can be modeled by a constant value for the initial phase, the first three years. This value is 0,29 failure per year. The best phase, 3-15 years, can be modeled with lower constant value 0,074 failures per year. Expressed in terms of mean time between failures, MTBF, this correspond to 3,4 years and 13,5 year. The 95% confidence interval is +/- 2 years.
The repair and stop times are shown to fit well to lognormal distributions. The repair time has the mean value 36 hours and standard deviation 143 hours, while the stop time has mean value 298 hours and standard deviation 3749 hours. This gives the mean design availability Adm=0,9985. For 95% confidence we get instead Ad95=0,9931.

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