Phase insulation is the insulating separation between winding strands of different phases in the stator of a three-phase motor. It sits mainly in the end winding, where conductors of the three phases lie close together and the full line-to-line voltage is present, around 690 volts between two phases on a 400-volt supply. If this separation fails, the motor breaks down between two phases and stops at once. Phase insulation is usually made as a stamped and folded phase paper of aramid paper or laminate. GOBA has been producing these parts since 1959.
Function of phase insulation in the stator
Three electrical separation tasks meet in the stator, and phase insulation is the most demanding of them. The slot insulation separates the whole winding from the earthed core, the layer insulation separates stacked winding layers. Phase insulation, by contrast, separates two strands that carry the highest voltage in the whole system, the line-to-line voltage.
The end winding is the critical zone. There the wires leave the protected slot and bend over, conductors of different phases cross and almost touch. An inserted phase sheet keeps them at a safe distance. Without this barrier, an abrasion point on the wire enamel and the vibration in operation are enough to short two phases. The full system is described in the article on stator insulation, of which phase insulation is one of four layers.
The separation happens in two ways: through the clearance and creepage distance between the strands, and through the inserted phase sheet where the distance alone is not enough. In the densely packed end winding the distance is almost always too small. This is why the phase paper is the rule and not the exception. In compact servo and traction motors with a high slot fill factor this holds all the more, because there is hardly any room left for air gaps.
Materials for phase insulation
The materials match those of the slot insulation, because both need the same thermal class and impregnability. The choice depends on temperature and budget.
- Aramid paper such as Nomex 410, class H up to 220 °C, is the standard for demanding motors and holds around 18 to 40 kV/mm depending on thickness.
- NMN, the composite of Nomex, polyester film, Nomex, combines the impregnability of the aramid paper with the high dielectric strength of the PET film and covers class F to H.
- DMD, the laminate of polyester nonwoven and polyester film, is the economical choice for standard class B motors up to 130 °C.
- Plain PET film gives good characteristics on a small budget but stays limited to class B and absorbs no impregnating resin.
For most traction and industrial motors we recommend aramid paper or NMN. The few cents that DMD saves are recovered expensively by a phase breakdown in the field.
Typical thicknesses of phase papers lie between 0.18 and 0.30 mm. A thicker sheet gives more reserve against breakdown, but needs room in the tight end winding that the winding would otherwise use. The thickness is therefore fixed together with the slot fill factor, not on its own. Where the winding is later impregnated, the resin uptake counts as well: aramid paper and NMN bind the resin and form a solid bond with it, a smooth PET film does not.
Forming as a stamped and folded part
A phase paper is rarely a flat cut. So it holds its shape in the end winding and follows the contour of the strands, it is stamped, creased and folded. Typical forms are the U or V folded sheet that wraps around the strands, and cuts with stamped tabs that hold the sheet in place during winding. The crease line has to lie so that the aramid paper does not crack at the fold and no fibre tears. A broken fold becomes a weak point where the field strength rises locally.
The stamped tabs grip around the strands and hold the sheet in place during impregnation and in operation. Without this fixing, the paper wanders out of the separation point under vibration and exposes the conductors. In multi-layer end windings we often stamp the sheet in several zones, so it follows the contour without buckling.
Electrical and thermal requirements
Phase insulation carries the highest voltage in the system and at the same time has to hold the thermal class of the winding. Three points decide the design.
- Dielectric strength against the line-to-line voltage, with a reserve for voltage peaks from the inverter.
- Partial discharge resistance in the switched drive, because the steep edges of pulse-width modulation erode film-based insulation over years.
- Continuous temperature of the thermal class plus ageing reserve, together with compatibility with the impregnating resin, so the sheet does not delaminate during impregnation.
On top of this comes the mechanical side. The phase sheet has to withstand insertion and the vibration in operation without slipping or wearing through. This is why, besides the material, the edge quality of the cut counts.
Making at GOBA
The finished phase paper is produced from roll or sheet material in a few steps. For small lots and prototypes GOBA works tool-free, in series with a stamping tool.
- Cutting the web to width by roll or rewind slitting.
- Stamping the contour including tabs and cut-outs within DIN ISO 2768.
- Creasing and folding into the U or V profile, so the sheet holds shape in the end winding.
- Visual inspection for fold cracks and delamination, dimensional check, batch documentation.
With thin films we run the knife shaft slower on start-up, otherwise the edge frays. With laminates we place the fold line parallel to the fibre direction of the aramid, so the fold stays clean. So phase paper and slot insulation match, we prefer to stamp both from the same material stock. That way slot cell and phase sheet have the same thickness, the same thermal class and the same impregnation behaviour, which simplifies assembly and impregnation. Send us your drawing with material, thermal class and quantity, and you will receive a technical quote.
GOBA Takeaway
Phase insulation separates the strands that carry the highest voltage in the stator, and it sits exactly where conductors of different phases almost touch in the end winding. The right material, usually aramid paper or NMN, and a clean fold without a crack decide the service life. GOBA has produced phase papers as stamped and folded formed parts since 1959. Send us your requirement through our insulation and molded parts or directly through stamping and forming. You will receive a technical quote.