Theoretical benefits of powder-coating based insulation layers regarding copper fill factor in electric drives

Power density is a crucial requirement factor regarding high power traction drives for automotive application. As battery capacity is one of the most limiting factors regarding range of current electric vehicles, output efficiency in relation to package size is one of several levers to extend existing limitations. Within electric drives, efficiency is linked to the copper mass being implemented into the active components. The presented paper evaluates the possibility of expanding copper fill factor by the use of higher grade - and thus thinner - electric insulation materials. By using powder coatings with breakdown voltages superior to existing NMN and NKN laminates usually used as groundwall insulation system in electric drives, insulation layers can be reduced in size leaving more room for magnet wires. With regard to existing stator layouts and topologies, multiple slot cross sections are evaluated. Results show an increase of the electric copper fill factor by up to 13%, which leads to higher output or the possibility to reduce package size and mass without sacrificing output power.

I. Introduction

Groundwall insulations form a critical layer separating live wires from the lamination stack of an electric drive system. Besides this main aspect regarding their functionality, other properties and material behavior are of interest as well. As these insulation layers are also essential to an effective heat dissipation, thermal conductivity of these materials are also within focus of developers and engineers. Aramid-based slot-liners in combination with layers of Polyethylene (NMN) or Polyimides (NKN) as shown in Fig. 1 persistently form an industrial standard regarding groundwall insulation in electric machines regardless their size and power output, mostly due to excellent automation ability and low price level. Powder coatings already form an alternative to slot liners, yet economic application is restricted to high-volume production similar to thermoplastic overmoulding of laminations [1]. This type of production volume is mostly seen within small electric drives (as shown in Fig. 1) for auxiliary propulsion needs in vehicles or similar application, exceeding 100 k units per year. Using induction heating as thermal source for polymerization, overall process time is comparable to slot liner insertion [2], [3]. Fig. 1:

Protruding grundwall insulation slot-liners (white) in a random-wound stator (a); powder coated rotor (b)