We developed polyesterimide (PEI) nanocomposite enamell […]
We developed polyesterimide (PEI) nanocomposite enamelled wire using surface-modified silica nanoparticles with binary chemical compositions on the surface. The modification was done using silanes assisted by ultrasound, which facilitated high density modification. Two different trimethoxysilanes were chosen for the modification on the basis of resemblance of chemical compositions on the silica surface to PEI varnish. The surface-modified silica was well dispersed in PEI varnish, which was confirmed by optical observation and viscosity measurement. The glass transition temperature of the silica-PEI nanocomposite increased with the silica content. The silica-dispersed PEI varnish was then used for enamelled wire fabrication. The silica-PEI nanocomposite enamelled wire exhibited a much longer lifetime compared to that of neat PEI enamelled wire in partial discharge conditions.
Enamelled wire is composed of conductive copper or aluminum core surrounded by an insulation layer made of polymer. The lifespan of the electric motor is strongly dependent on thermal and electrical properties of the insulation layer. In a high speed switching inverter-fed motor, especially, when instantaneous overlapping voltage called surge voltage is higher than partial discharge inception voltage (PDIV), the insulation layer is degraded by partial discharge (PD), which leads to insulation breakdown. For this reason, surge-resistant enamelled wire has attracted attention for prolonging the lifespan of various inverter-fed motor installed electric devices such as electric vehicles. For protection from the degradation, nanosized inorganic fillers were incorporated in the insulation layer [5–8] where corona evolved by PD can be trapped on or reflected from the surface of the inorganic filler so that the degradation would be retarded.
Polyesterimide (PEI) was frequently chosen for the insulation layer of high temperature enamelled wires because it meets both requirements of high thermal index and flexibility sufficient to withstand severe stress while winding.
Because it is difficult to disperse nanoparticle in a viscous varnish solution, the sol-gel method, which has a great advantage for dispersion, has attracted attention. In the sol-gel method, nanoparticles are easily dispersed in the insulation layer as soon as they are synthesized from the precursors premixed in the varnish during the wire annealing process accompanied by a coating proces. However, serious problems were revealed; for example, the enamelled wire production cost increased because the process was too slow and the breakdown strength of the enamelled wire was reduced due to impurities remaining from the sol-gel process. In order to avoid such problems, nanoparticles need to be directly dispersed in the varnish and therefore should be properly surface-modified.
Here we propose a novel surface modification method in which silica nanoparticles are chemically modified using two kinds of silanes, selected based on the fact that their chemical composition on the silica surface matches that of the varnish, resulting in binary chemical compositions forming on the silica surface. Silanes are highly reactive to form chemical bond with silanol group on the silica.
During the silanation, ultrasound was applied so that the reaction rate was accelerated and the surface coverage was increased. We found that the surface-modified silica nanoparticles were uniformly dispersed in PEI varnish. The silica nanoparticle dispersed PEI varnish was then applied for the nanocomposite enamelled wires fabrication in commercial facilities. Surge resistance properties of the nanocomposite enamelled wire were investigated.