Stator insulation covers all insulating layers and formed parts that separate the winding of an electric motor in the stator core from the laminations, from adjacent conductors and from the phases. It determines whether a motor holds the required thermal class, whether partial discharge erodes the insulation over the years and whether the winding sits securely in the slot. In the traction motor of an EV, in the industrial drive and in the generator, the insulation system is safety-critical. A breakdown takes the machine out of service. GOBA has been producing the stamped and cut parts of this system since 1959, from Nomex slot insulation to the slot wedge.
Structure of the stator insulation system
A stator insulation system consists of several functional layers that work together as a coordinated composite. Each layer separates a different voltage difference. Anyone designing a stator insulation thinks in four electrical levels plus the mechanical retention of the winding in the slot.
- The slot insulation lines the slot and separates the entire winding from the earthed core. It is the main barrier to ground and carries the full phase-to-ground voltage.
- The layer insulation sits between two winding layers and takes up the voltage between stacked groups of turns.
- The turn insulation is the enamel of the copper wire itself and separates turn from turn, where the voltage difference is smallest but the number of interfaces is largest.
- The phase insulation separates the three strands in the end winding, where conductors of different phases lie close together and the full line-to-line voltage is present.
At the top the slot wedge closes the slot, holds the winding in place and absorbs centrifugal forces and vibration. Only this mechanical closure makes the electrical system durable. How the wires lie in the core is described in more detail in the article on the motor winding.
The slot cell is usually made with protruding edges, the cuffs. These folded-over rims stand out of the slot, protect the wire from the sharp lamination edge during insertion and extend the creepage distance to the core. Typical wall thicknesses of the slot insulation lie between 0.18 and 0.35 mm, depending on voltage and available slot cross section. A thinner sheet lets more copper into the slot and raises efficiency, but lowers the breakdown reserve. This trade-off between slot fill factor and dielectric strength is the core decision when building the system.
Materials and thermal classes
The choice of material depends on the thermal class and the electrical load. Aramid paper dominates in traction motors, polyester laminates in more economical drives. The common thermal classes of insulation to IEC 60085 set the permissible continuous temperature: class B 130 °C, F 155 °C, H 180 °C and the higher classes up to 200 and 220 °C. The class refers to the winding temperature in continuous operation, not to short peaks.
- Nomex 410, an aramid paper from DuPont in class H, withstands continuous temperatures up to 220 °C and reaches around 18 to 40 kV/mm dielectric strength depending on thickness. It is the standard for slot insulation and phase separation in the demanding motor.
- DMD, a laminate of polyester nonwoven, polyester film and polyester nonwoven, covers class B and is the economical choice for standard motors. The nonwoven absorbs impregnating resin, the film carries the dielectric strength.
- NMN, the composite of Nomex, polyester film, Nomex, combines the impregnability of the aramid with the high dielectric strength of the PET film and reaches class F to H.
- Plain PET film such as Mylar or Hostaphan gives good characteristics on a small budget but stays limited to class B.
- Kapton, a polyimide film, goes beyond 220 °C and is used where the end-winding temperature stays high in continuous operation.
- Mica products carry the partial discharge resistance in high-voltage machines and generators, where films alone do not withstand the corona load.
For slot insulation in the motor, GOBA usually reaches for aramid paper or NMN. The background on grades and thicknesses is covered in the article on insulation paper for electric motors.
The match with the impregnating resin matters. A nonwoven laminate like DMD soaks up the resin and forms a solid bond with it, a smooth PET film does not. Anyone who later impregnates the winding by the VPI process chooses the insulation material to suit the resin and does not decide afterwards. Otherwise the sheet delaminates during curing or the resin does not reach the bottom of the slot.
Manufacturing the stamped and cut parts at GOBA
Slot insulation, phase papers, slot wedges and coverings are produced from roll or sheet material as finished formed parts. The path from material to ready-to-assemble part runs in clear steps.
- Cutting the web to the required width by roll or rewind slitting.
- Stamping the contour and the fold lines, tool-free for small lots and prototypes, with a stamping tool in series.
- Creasing and folding into the finished slot cell or the folded phase sheet, so the part holds its shape in the slot.
- Visual inspection for edge cracks and delamination, dimensional check, batch documentation.
With laminates, the cutting determines the quality. A frayed edge or delamination at the cut becomes a weak point in operation, where the field strength rises locally. GOBA produces contours within DIN ISO 2768 and places fold and crease lines so the Nomex does not crack. With thin films we run the knife shaft slower on start-up, otherwise the edge frays.
For the folded-over cuffs we crease and fold the edge at a defined angle so it does not spring open in the slot. Slot wedges and slot closure keys we stamp from the same material stock, so slot cell and closure fit together. For coverings and thin phase sheets we use the kiss-cut process where needed, which cuts only the top layer of a laminate and leaves the carrier web intact.
Electrical, thermal and mechanical requirements
The stator insulation has to withstand three stresses at once. Electrically, what counts is the dielectric strength to ground and between phases, plus limited partial discharge. Partial discharge is the critical point in the fast-switched inverter drive: the steep voltage edges of pulse-width modulation create field peaks that erode a film-based insulation over years. This is why the requirements for corona resistance rise sharply in high-voltage traction motors.
- Electrical: sufficient dielectric strength, defined creepage and clearance distances in the end winding, low dielectric losses at high switching frequency.
- Thermal: continuous temperature of the thermal class plus ageing reserve, resistance to thermal cycling and compatibility with the impregnating resin.
- Mechanical: resistance to the pulling-in of the winding, abrasion resistance at the lamination edge, dimensional stability under vibration and centrifugal force.
These three levels are linked. A material that ages thermally also loses its mechanical strength and then its electrical strength. This is why the thermal class is chosen with a reserve.
Insulation resistance and polarization index serve as condition indicators, measured before and after impregnation. If the value drops, moisture or a delamination is at play. The line-to-line voltage on a 400-volt drive is around 690 volts, while high-voltage traction motors run on 400 to 800 volts DC-link voltage and demand correspondingly larger creepage and clearance distances in the end winding. Where the distance alone is not enough, an inserted phase sheet takes over the separation.
Conventional winding versus hairpin
The type of winding changes the demands on the insulation. In the conventional random or pull-in winding, round wires lie loosely bundled in the slot. Here the slot insulation mainly has to survive the pulling-in and reliably cover the irregular wire position against the core. In the hairpin stator, preformed rectangular conductors are inserted into the slot as hairpins, twisted at the end and welded.
For the insulation this means three things. The slot fill factor rises because rectangular conductors use the slot better, which improves heat dissipation but leaves less room for the insulation. The turn insulation is loaded more heavily by the inverter edges because adjacent conductors lie in a defined position and the voltage distribution is more uneven. And the slot insulation needs a precise fit, because the stiff hairpin does not forgive a creased slot cell. The higher slot fill factor brings noticeably more copper into the same slot and lowers the ohmic loss, which improves efficiency especially in the part-load range. GOBA produces for both designs, but the tolerances for hairpin slot insulation are tighter.
The welding heat adds to this. When the hairpin ends are welded, local heat arises that reaches the insulation in the upper slot region. The material there has to withstand short temperature peaks above the continuous class as well, without becoming brittle. Aramid paper has a clear advantage here over plain polyester laminates.
Selection criteria for purchasing and design
Anyone specifying a stator insulation fixes the following points before cutting begins. The more precise the specification, the more reliable the quote and the safer the series.
- Thermal class and maximum end-winding temperature in continuous operation.
- Voltage level, inverter switching and the resulting partial discharge requirement.
- Winding design, conventional or hairpin, and the available slot cross section.
- Material system: aramid paper, NMN, DMD or PET, matched to the impregnating resin.
- Tolerances of the slot cell and the fold lines, documented process capability.
- Quantity and variant diversity, from which follows a stamping tool or tool-free cutting.
A common mistake is to choose the thermal class too tightly and save on the material. The cents saved per part later cost a whole winding. We advise designing the class with a reserve and treating the insulation system as a whole, not as the sum of cheap single layers.
On economics, the quantity decides the process. For prototypes and small series we cut and stamp tool-free, which saves the tooling cost. From a stable series onwards a stamping tool pays off over the volume and delivers tighter, reproducible tolerances. With high variant diversity, tool-free cutting often stays the cheaper choice. Before delivery, the motor manufacturer tests the finished winding with a high-voltage test to ground and between phases. The stator insulation has to pass this piece test with a reserve over the operating voltage.
GOBA Takeaway
Stator insulation is a coordinated system of slot, layer, turn and phase insulation plus slot wedge, not an interchangeable single part. The service life of the machine is decided by the right material mix of aramid paper, NMN or DMD, the correct thermal class and a clean cut without edge cracks. For hairpin stators in high-voltage drives, the demands on partial discharge and fit rise noticeably. GOBA has produced the stamped and cut parts since 1959 and advises from the first drawing to the series. Send us your requirement through our insulation and molded parts, and for traction and high-voltage motors use our industry page on electromobility and automotive. You will receive a technical quote.