Silica nanofilled varnish designed for electrical insulation of low voltage inverter-fed motors

The resistance to partial discharges of pure, microfilled and nanofilled Polyetherimide varnish for low voltage motors fed by inverters is presented. Micro and nano-sized Silica particles (1.5 % wt) are used as fillers. Experimental results of lifetime versus temperature (-55 to 180°C), frequency (5 to 15 kHz) and level of the applied square voltage (+/- 1 kV to +/- 3 kV) clearly demonstrate that nanofillers are more efficient than microfillers. The lifetime of nanofilled polyetherimide varnish is found to be up to thirty times higher than the standard one. Crucial properties for a varnish such as its electrical conductivity, its dc and ac dielectric strengths and its bonding strength are measured. Due to the presence of aggregates having a micrometric size, the microfilled varnish exhibits the worst characteristics, except for the ac dielectric strength which remains unchanged. These aggregates act as bridges for charge carriers, increasing the conduction current and reducing the resulting dc dielectric breakdown field. They also reduce the mechanical properties of the varnish.