Hairpin technology is a winding technology for stators in electric motors and generators and is also used for traction applications in electric vehicles. In contrast to conventional winding technologies, the hairpin technology is based on solid, flat copper bars which are inserted into the stator stack. These copper bars, also known as hairpins, consist of enameled copper wire bent into a U-shape, similar to the geometry of hairpins.
In addition to hairpins with U-shape, there are two other variants of bar windings, the so-called I-pin technology and the concept of continuous hairpin windings.
I-Pins are straight copper wire elements that are inserted into the stator slots. Unlike Hairpins, these Pins are not bent prior to insertion into stack. However, contacting is necessary on both sides of the stator. In the concept of continuous hairpin windings, so-called winding mats are produced and afterwards inserted into the stack from the inner diameter.
Hairpin stators are most commonly used for synchronous machines.
Stator structure
The structure of a hairpin stator differs from conventional stators only in the type of winding system - other components of the stator are little changed. The stack of sheets consists of many layers of individual sheets, each insulated by a thin coating. The housing is another subcomponent that does not generally require modifications. The thin, round wire of the conventional winding technology is substituted by copper bars, which better fit the slot geometry and therefore provide a higher slot-filling degree than regular winding. To create the necessary winding scheme, the free ends of the hairpins are twisted before welding. In addition to the impregnation process for the entire stator, which is also necessary for conventionally wound stators, a layer of insulation resin is applied to the ends of the hairpins.
Manufacturing
The hairpin stator process chain is based on an indirect winding approach. Due to the solid cross section, hairpins can be shaped into their final geometry ahead of the actual assembly process. In contrast to conventional stator production, in which winding-based assembly processes predominate, a forming process is applied.
Production can be divided into 4 steps:
Hairpin
In the first process, a flat copper wire, which is usually already enameled, is continuously unwound and straightened in several stages to reduce residual curvature and stresses. In preparation for welding of the copper ends in a later process step, this insulation is partially removed. Laser-based and mechanical processes are feasible. The hairpin wire is cut to length and bent, in varying order. Hairpins are formed into a three-dimensional geometry either in a single-stage process using special CNC bending equipment or in multiple stages in which a die bending process follows a swivel bending process.
Assembly and twisting
Next the pins are inserted into the stator stack. The insertion process is limited by overlaps in the winding head geometry. The hairpins are usually pre-assembled in an assembly nest. Individual pins are arranged in accordance with the winding scheme. In general, a single hairpin stator uses 3-16 different hairpin geometries. The stator slots are lined with insulation paper to separate the winding system from the ground potential of the stator's sheet stack.
In the next assembly step, the hairpin basket is inserted axially into the stator stack. To support the insertion the hairpins are sometimes equipped with chamfers during the cutting process – grippers can improve positioning precision.
Each layer of hairpin ends is twisted in accordance with the winding scheme. During the associated rotation the tool has to be moved in an axial direction for height compensation. To ensure axial accessibility the hairpin ends must be radially exposed in a preparatory step.
Welding and interconnection
Next, hairpin ends are electrically contacted with each other to form the winding scheme. Using a laser, the hairpin ends are partially melted and joined. An optimal welding process is marked by homogenous weld geometries as well as minimal thermal input. Repeatable welding strategies require the stator to maintain a stable condition.