Polybutylene Terephthalate Film in Motors and Capacitors: Dielectric Strength

Polybutylene terephthalate (PBT) film is a semicrystalline polyester dielectric evaluated for slot insulation, phase barriers, and motor-capacitor dielectric use where thermal, mechanical, and electrical stress overlap. In biaxially oriented film production, PBT resin is extruded through a flat die and stretched in the machine and transverse directions to increase crystallinity, reduce pinhole density, and raise short-time dielectric strength. The dielectric strength of PBT film is not an intrinsic material constant; it varies with film thickness, electrode geometry, voltage waveform, temperature, relative humidity, and defect population. Consequently, motor and capacitor qualification programs typically specify dielectric breakdown voltage per IEC 60243-1 or ASTM D149, with conditioning at 23 ± 2 °C and 50 ± 5 % RH, and with rising-voltage modes; supplementary tests for partial discharge initiation and tracking are required for inverter-fed electrical machines.

Dielectric Strength Measurement and the Thickness Effect

Dielectric strength values obtained on thin PBT film diverge significantly from those measured on moulded plaques because the statistical volume of stressed polymer is smaller. Under IEC 60243-1, short-time tests are performed using opposing cylindrical electrodes immersed in insulating liquid to suppress surface flashover; the voltage is ramped at 0.5–2.0 kV/s until breakdown. For unfilled PBT film in the 25–50 µm thickness range, supplier data sheets and laboratory round-robin data frequently report apparent field strengths of 100–250 kV/mm at room temperature, while moulded sections of 1.0–3.0 mm typically fall between 15–25 kV/mm. The increase in apparent strength at reduced thickness is partly an inverse-thickness effect and partly a consequence of reduced defect volume; it does not indicate a change in the fundamental polymer breakdown mechanism. However, published data for this specific configuration is limited across different manufacturers, so a single generic specification for capacitor-grade PBT film is not yet available.

Partial discharge and pre-breakdown current measurements add operational relevance. In a motor slot liner qualification, the film is placed between a grounded steel core and a copper conductor and subjected to 50 Hz or 60 Hz voltage with superimposed impulsive transients. Under these conditions, breakdown is often initiated at mechanical creases, weld spatter, or edge burrs rather than in the bulk film. Inverter-duty wave forms with dV/dt values above 5 kV/µs can lower the measured partial discharge inception voltage because space-charge accumulation and high-frequency field enhancement localize stress at the film–air interface.

Electrode geometry influences measured values. Parallel-plate electrodes conforming to IEC 60243-1 are used for flat film; for curved motor-slot specimens, cylindrical electrodes with a defined contact force are applied. The measured breakdown voltage is divided by the local thickness measured at the failure point; average film thickness is insufficient because thickness bands of ±1 µm can shift the apparent field strength by 5–10 %. High-reliability capacitor film is therefore measured with a contacting ultrasonic thickness gauge or beta-gauge before dielectric strength testing.

What Limits PBT Film Performance in Inverter-Duty Motor Windings?

PBT film used as phase insulation or slot liner stock in inverter-fed motors must survive not only sinusoidal withstand tests but also transient overvoltages generated by insulated gate bipolar transistor drives. In production-scale stator insertion, film thickness is commonly 125–250 µm, but the electrical stress is concentrated at slot edges and at the film–copper interface. Data from motor rewind shops indicates that failure frequently begins as partial discharge erosion in air gaps; the film may retain acceptable dielectric strength in flat-plate testing while showing discolouration, channel growth, or punctures under service conditions. Consequently, OEM specifications require partial discharge inception voltage measured with a surge generator with rise times of 0.1–0.5 µs and peak voltages up to 2.5–4.0 kV depending on the winding insulation class.

PBT film offers high dielectric strength and low moisture absorption relative to polyamide film; however, its comparative tracking index is grade-dependent. Unfilled PBT grades may reach CTI values above 400 V on IEC 60112, while filled or flame-retarded variants may drop below 250 V. For inverter-duty machines with high surface contamination, a slot liner material with CTI below 600 V requires creepage distance adjustment under IEC 60034-1 and may fail synthetic dust/fog tests. The dielectric strength of PBT film at 130 °C declines relative to room temperature because increased segmental mobility and ionic conduction lower the effective breakdown path; manufacturers therefore derate working voltage by at least 20–30 % above the thermal class of the insulation system.

Surge testing of inserted stators follows IEC 60034-18-41, with impulse pairs of opposite polarity and rise times selected to simulate long-cable reflected waves. PBT film slot liners that pass sinusoidal withstand may fail during impulse testing because the voltage distribution across the winding is nonlinear and the first turn-to-turn section can see peaks far above the average. Production equipment includes surge generators with output impedance of 50 Ω, pulse repetition rates of 1–5 Hz, and peak voltage capabilities of 6–12 kV. Insulation systems containing PBT film are classified by partial discharge inception voltage, repetitive partial discharge inception voltage, and off-line single-impulse strength; the relationship between these values and long-term service life is validated through motorette tests.

The narrow processing window for PBT film affects dielectric uniformity. Melt temperature excursions above 270 °C accelerate thermal decomposition and gel formation; excursions below 245 °C produce insufficient stretching and higher pinhole density. Film line operators typically target a melt temperature of 250–265 °C and a transverse-direction thickness variation of less than ±5 %, measured with a non-contact capacitance gauge. Greater thickness variation produces local electric field intensification that cannot be corrected by increasing average film gauge.

In capacitor-grade PBT film, winding, metallisation, and impregnation govern the dielectric strength retained in a finished element. Unlike polypropylene, PBT film has a higher dielectric constant, typically 3.0–3.3 at 1 kHz, which allows higher volumetric capacitance but also increases dielectric heating at high ripple current. In a wound motor-run capacitor, the film is metallised with aluminium or zinc-aluminium via vacuum deposition, then segmented to create controlled fusing. The dielectric strength of PBT film in this format is assessed after winding because mechanical tension and winding curvature reduce the flat-sheet breakdown voltage; capacitor makers therefore specify minimum dielectric strength per IEC 60243-1 on film extracted from wound elements, not only on incoming rolls. Published data for PBT as a sole dielectric in high-voltage motor-run capacitors is limited, and most commercial high-voltage film capacitors use polypropylene or PET. PBT film is more often considered where the continuous operating temperature exceeds the 85 °C reference of metallised polypropylene and where PET's lower thermal capability is insufficient.

Loss characteristics also affect dielectric strength. Dielectric dissipation factor of unfilled PBT film is commonly reported between 0.001–0.005 at 1 kHz and may rise above 0.020 at 1 MHz depending on moisture and residual catalyst. For capacitor service, a dissipation factor above 0.010 at low frequency can produce thermal runaway if internal heat generation exceeds the thermal conductivity of the wound element. Thermal mapping on wound capacitor cores using embedded thermocouples has shown that the hottest region is normally located between the winding core and the outer margin, not at the case surface; this mismatch must be accounted for when setting voltage derating. Because PBT's dissipation factor increases with temperature and frequency, the maximum safe electric field in a capacitor at 10 kHz is lower than at 50 Hz.

Surface quality and pinhole density control are essential. Incoming PBT film for capacitor use is inspected with laser scanning or high-speed optical systems to flag gel lumps, carbon specks, thickness bands, and pinholes greater than 5 µm. A single conductive particle greater than the film thickness can reduce breakdown voltage by creating a field concentration point. Winding tension is typically maintained in the range of 3–10 N/mm depending on film width and machine configuration; excessive tension creates microcracking at the dielectric surface and reduces partial discharge inception voltage. Qualification for capacitor use includes dielectric strength after thermal endurance, self-healing performance, and insulation resistance after high-temperature storage. Film capacitors for motor-run applications are typically tested under IEC 60252-1 or IEC 61071, with endurance testing at 1.25–1.35 times rated voltage for 1000 h at the maximum operating temperature. PBT film must maintain a defined capacitance loss, usually less than 5 %, and a dielectric strength after endurance sufficient to survive the specification's overvoltage test.

When Hydrolysis and Thermal Aging Reduce Dielectric Strength

PBT is a polyester; its dielectric performance is tied to molecular weight retention, which is threatened by hydrolytic chain scission at elevated temperature and humidity. In film production, resin is dried before extrusion with hot-air desiccant dryers at 120–130 °C until moisture content falls below 0.02 %, measured by Karl Fischer titration. Extruder barrels are typically operated with a 30:1 L/D ratio and zone temperatures from 240 °C to 265 °C; residence time at melt temperature is limited because excess residence can produce tetrahydrofuran, carboxylic acid end groups, and gel formation. The film is then cast onto a chilled roll at 15–30 °C and biaxially stretched at ratios between 2.5 × 2.5 and 3.5 × 3.5 to orient crystallites and reduce the number of large spherulites that would lower dielectric strength. At relative humidity above 60 %, pellet drying before extrusion is mandatory.

In humid motor environments, absorbed water accelerates hydrolysis of the butylene ester linkage. The consequence is not a single abrupt failure but a statistical shift in breakdown distribution: tensile elongation and fold endurance decline before dielectric strength collapses. Qualification under IEC 60216 uses elongation retention as a thermal-endurance criterion, but dielectric strength after thermal ageing is a separate test under IEC 60243-1. A PBT film that retains 80 % of its original elongation may still exhibit a 10–20 % reduction in short-time dielectric strength depending on thickness and electrode geometry; at advanced degradation, brittle fracture at crease points produces direct puncture paths. Therefore motor repair facilities must not re-use PBT slot liners from motors that have operated above 85 °C in condensing humidity unless dielectric integrity is re-tested.

Compatibility with impregnating varnishes is another operational boundary. Some epoxy and unsaturated polyester varnishes use amine curing agents; residual amines can attack ester linkages and accelerate hydrolytic degradation. PBT film should be evaluated with the complete insulation system, including varnish, tie cord, and brazing flux, under IEC 60034-18-1 or motor OEM specifications. In capacitor use, liquid impregnants such as aromatic hydrocarbons or vegetable-oil derivatives can plasticize the PBT surface if the film is not biaxially annealed; this reduces stiffness and can lower partial discharge inception voltage at elevated temperature.

Resin and Film Grade Selection for Dielectric Reliability

Selection of PBT film for motors and capacitors begins with the resin's intrinsic viscosity, usually in the range of 0.85–1.20 dL/g for film grades when measured in a phenol/tetrachloroethane mixture at 30 °C. Lower molecular weight resin extrudes more easily but produces brittle film with lower mechanical endurance and reduced dielectric strength after thermal ageing. High-purity film grades require low catalyst residue and low gel count, so resin producers optimise condensation catalysts and stabiliser packages to minimise conductive defects.

Flame-retarded or glass-fibre-reinforced PBT grades are not used for thin dielectric film because fillers create local field enhancement and reduce dielectric strength to moulded-plaque values around 12–18 kV/mm; even short glass fibres present in recycled resin can generate pinholes during stretching. For this reason, film lines use dedicated unfilled resin feedstock and closed-loop scrap handling to prevent contamination by engineering compound regrind. Periodic melt filtration through screen packs of 20–40 µm removes carbonised particles and agglomerates, but screen pressure fluctuation can indicate gel build-up and trigger lot quarantine.

The dielectric strength of PBT film is also sensitive to electrode material in very thin sections. Breakdown tests in air with sharp-edge electrodes produce lower values than tests under oil with spherical electrodes; therefore motor/capacitor specifications should state electrode geometry and immersion medium. In biaxially oriented PBT film, measured breakdown voltage may fit a two-parameter Weibull distribution with shape parameter between 5 and 15, reflecting defect-controlled breakdown; acceptance criteria based on minimum breakdown rather than mean values are used for high-reliability capacitor lots. When batching film for motor slot liners, conversion processes often perform a 100 % dielectric roll test with a voltage derived from the minimum acceptable breakdown, but such testing must be applied below the discharge inception voltage to avoid latent damage.

Unmodified PBT film is not corona-resistant. In inverter-fed machines with high partial discharge inception demands, PBT film may serve as a base layer beneath mica paper or as a component of a composite slot liner; the mica layer absorbs partial discharge and protects the polyester from direct erosion. Where PBT film is used alone, operating voltage must remain below the discharge inception threshold throughout the expected temperature and humidity envelope.