Flow electrification phenomenon in forced air-cooled rotating electric machines

The factors affecting air-flow electrification under both HVDC and HVAC of variable frequency have been investigated experimentally on air flow between coaxial-cylinder and point-to-cylinder systems in a synthetic open cycle. Instantaneous measurements of the air flow electrification (AFE) 'streaming current' and corona current 'radial current' are presented as a function of mass flow rate, air temperature, pressure and relative humidity, electrode configurations, and voltage amplitude, polarity and frequency. The results reveal that both streaming and radial currents augment with voltage, relative humidity, mass flow rate, and temperature. On the other hand, both the streaming and the radial currents do not vary with pressure. Increasing the frequency of the applied HVAC significantly reduces the streaming current, while the radial current increases linearly. Similar trends of the results are observed for both electrode systems. For the point-to-cylinder system under negative HVDC, increasing the mass flow rate drastically reduces both corona inception and breakdown voltages, while positive HVDC gives only a slight effect. The present work confirms the significant role of the AFE on the field failures occurring inside forced air-cooled rotating electric machines. Therefore, such phenomena should be taken into consideration in the design of such machines.