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This study aims to evaluate the impact of the bending radius of edgewise insulated wires on dielectric properties such as partial discharge inception voltage (PDIV), partial discharge extinction voltage (PDEV), dielectric dissipation factor (DDF), and insulation capacitance (IC) through thermal aging. The tests are performed at room temperature (20 °C) and atmospheric pressure (1013 mbar) on unaged and thermally aged polytetrafluoroethylene (PTFE)-wrapped pairs of edgewise insulated wires, models of the turn-to-turn insulation. The accelerated thermal aging is carried out at 250 °C (i.e., 50 °C higher than thermal class) for two different aging periods: 156 and 312 h. To manufacture a coil, it is generally demanded to shape 90° bends out of edgewise enameled winding wires. Therefore, the obtained experimental results are helpful for the coil manufacturers, providing a clue how the bending radius can impact the dielectric properties of turn-to-turn insulation and introducing an optimum radius, which can present better insulation performance.

This article models the partial discharge inception voltage (PDIV) as a function of ambient humidity ( ${H}{)}$ at various temperatures (Ts) using Schumann’s streamer inception criterion (SCSIC) for turn-to-turn insulation, which is the most vulnerable part in inverter-fed motors’ insulation system. The Schumann constant (i.e., the natural logarithm of the critical electron number defining the Townsend-to-streamer discharge transition, ${K}{)}$ varies with ${H}$ , showing distinct patterns at low and high Ts. The ${K}$ equations are derived across a wide range of relative humidity (RH) levels (20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90%) at four Ts (25 °C, 40 °C, 60 °C, and 90 °C). These equations can be used in finite element analysis software to predict PDIV under varying ${H}$ with outstanding accuracy. Additionally, a novel approach is presented for partial discharge (PD) phenomenology under ${H}$ variations at different Ts using SCSIC-derived streamer inception parameters (SIPs): critical field line length (CFLL), air effective ionization coefficient ( $\alpha _{\text {eff}}{)}$ , PD inception field ( ${E}_{\text {inc}}{)}$ and firing voltage ( ${V}_{\text {firing}}{)}$ . Notably, at high Ts (e.g., 90 °C), a transition phase emerges concerning RH, leading to significant SIP changes due to the disappearance of a critical region (CritR) in $\alpha _{\text {eff}}$ at specific electric field intensities. The developed humidity-dependent PDIV model supports insulation designers in achieving PD-free designs that account for ${H}$ variations and sheds light on SIP variations concerning ${H}$ changes.