Speed control of asynchronous and synchronous random wound motors by using frequency converters is commonly used. However, the thyristors in such a frequency convertor may give short transient pulses which may cause problems. The transients may lead to electrical ageing of the insulation by partial discharges, resulting in a decreased lifetime of the motor. Accelerated ageing was performed on a chromium oxide coated enameled round wire and compared with a reference enameled wire. In order to accelerate the ageing, frequency, voltage and temperature was used. The results show that the enameled wire containing chromium oxide had a much longer lifetime compared to the standard wire when exposed to partial discharges.
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