Every hot afternoon, millions of air conditioners, heat pumps, well pumps, and refrigeration systems restart against heat, pressure, and sagging voltage. Capacitors sit near the top of the repair list: one Arizona HVAC contractor documented capacitor failures in 50 of 100 repair calls during a single week in June 2026. Another reports that on a 110°F day, an outdoor unit's electrical compartment can exceed 150°F, above the 149°F (65°C) maximum rating of a standard electrolytic start capacitor.
When a motor needs extra starting torque, from a struggling compressor on a hard-start kit to the control box of a three-wire well pump, a start capacitor does the job. The part costs $9 to $25, and replacing a failed one runs $99 to $225 with labor. Aluminum electrolytic capacitors have held the start position for decades because they pack the most capacitance into a low-cost can, and BOPP film has held the run position because its low-loss, self-healing construction lasts through years of continuous duty.
This post looks at where start capacitors are used, where today’s materials limit their performance, and how designers can improve them. It explores how NanoPlex™ HDC, Peak Nano's high-dielectric-constant, nanolayered film, offers a path to new start-capacitor designs—from pumps and compressors to larger industrial motors—as well as related energy-discharge applications such as turbine ignition.
A start capacitor works for at most a few seconds, then rests until the next start. It sits in series with the motor's start (auxiliary) winding and shifts that winding's current 90° ahead of the main winding's current. Because the two windings also sit 90° apart around the stator, the offset currents produce a rotating field that turns the rotor. It drops out near full speed, when a centrifugal switch opens at about 75% of rated speed or a current relay, potential relay, or PTC starter cuts it out. That short burst gives a capacitor-start motor about 3 to 4.5 times its full-load torque, and the job takes large capacitance values:
The two positions reward opposite things. The start position rewards capacitance per dollar and per cubic inch, while the run position rewards low loss over years of continuous duty. Electrolytic capacitors won the first contest, and BOPP film won the second. A film with 1.7 to 2.1 times BOPP's capacitance and higher breakdown strength at higher temperatures narrows the gap, enabling a thin-film dielectric's thermal durability and lifetime performance to enter the electrolytic market.
A report on the Motor Start Capacitor Market expects growth of 5.67% per year through 2032. It suggests air conditioners are the largest application and powered gates are the fastest-growing, at 8.30% a year, with large fans and other uses making up the rest. A start capacitor passes through six links on its way to the field, and the dielectric chosen at the first link shows up as service calls at the last.
Dielectric materials - Start capacitors use etched aluminum foil and electrolyte-impregnated paper, and run capacitors use metalized BOPP film. For motor-start capacitors, one study finds raw polypropylene film from China and South Korea, winding in India and Vietnam, and final assembly in the U.S., Germany, and Mexico.
The economics run backward through the chain. Capacitor makers compete on price per part, while OEMs pay for warranty failures and owners pay for service calls and downtime. A start part built on a higher-rated dielectric with no liquid electrolyte gives a capacitor maker something to sell besides price, and gives OEMs a candidate to qualify against their warranty data.
The start position used to be easy duty: a few seconds of work, then rest. Five pressures from the field now hit the start capacitor at once, and each one is a requirement a capacitor maker can design for.
Each pressure lands on the same part. The start position now needs more thermal headroom, tolerance for more starts, and a more secure supply.
Four dielectric families could fill the start position, and each solves one problem while leaving another open.
Electrolytic start capacitors wind etched aluminum foil with electrolyte-impregnated paper. Etching multiplies the foil's surface area, packing large capacitance into a low-cost can. The electrolyte sets the limits. One maker's datasheet rates standard parts to +65°C for short starts at a duty cycle of 0.0167 and allows a power factor of up to 10% when new and 20% after life testing, measured on finished parts at 60 Hz. Based on those limits, a 100 µF start capacitor at 330 VAC carries about 4,100 volt-amperes reactive and can dissipate up to about 410 W, or about 410 joules per second it stays in the circuit. A continuous voltage from a stuck relay can destroy the part, and the datasheet also recommends mounting the capacitor to minimize damage if it explodes.
BOPP owns the run position for good reasons: a dissipation factor of 0.05% at 1 kHz and wound, metalized construction that heals itself around small faults. Its limit is permittivity. BOPP's dielectric constant is 2.2, and its breakdown strength is high: 650 V/µm DC at 23°C in one film capacitor maker's data, and 792 to 794 V/µm at room temperature, falling to about 607 to 633 V/µm at 100°C in one study. Start capacitors need hundreds of microfarads, and at a dielectric constant of 2.2, that requires a large film area. One patent notes that 300 µF capacitors with typical film thicknesses would be bulky, and answers with zinc-metalized polypropylene only 2 to 4 µm thick for about a second of work per start. Thin film keeps size down, but at any given thickness, BOPP's permittivity caps capacitance per unit area, and biaxial orientation, the process that raised BOPP's breakdown strength, leaves little room for further gains. Film start capacitors exist, but electrolytic cans still hold most of the start position.
Polyester raises the dielectric constant to 3.3, with a DC dielectric strength of 580 V/µm. On one film capacitor maker's published values for both films, PET stores only about 20% more energy per volume than BOPP at the material level. It pays for that small gain in loss: PET's dissipation factor is 0.8% at 1 kHz, 16 times polypropylene's, and pulse power literature rates PET unsuitable for high-repetition pulse power. PET earns its place in compact DC circuits rather than line-voltage AC start duty.
High-temperature films solve the heat problem but worsen the density problem. PPS runs to +140°C with a dielectric constant of 3.0, per one film capacitor maker's comparison, but its breakdown strength of about 400 V/µm leaves it with half or less of BOPP's energy per volume at the material level. PEN offers thermal and dielectric performance similar to PET with a higher glass transition temperature. Neither film shrinks a start capacitor, and neither targets line-voltage AC duty.
The start position needs high capacitance density, film-class durability, and temperature headroom in one part. Electrolytics bring the first, and films bring the second and third. A film that narrows the density gap without sacrificing the rest changes the trade-off.
NanoPlex HDC gives a capacitor designer a starting point: 1.7 to 2.1 times BOPP's capacitance at the same thickness. NanoPlex coextrusion can build hundreds to thousands of layers, each as thin as 25 nanometers, and HDC's published properties translate into six design levers:
HDC suits burst duty, not run duty. Its loss fits seconds of work rather than steady AC current, so run capacitors and power factor correction stages stay on low-loss film, which is serviced by NanoPlex LDF film. Inside that envelope, each system environment opens a different value.
Most residential scroll compressors start unloaded and need no start components, so HVAC/R start capacitors live where systems need extra torque. Hard-start kits rescue compressors on weak supplies, where low-voltage conditions can cause hard starts or locked-rotor conditions, and high-starting-torque refrigeration compressors use a start capacitor on every start. Technicians generally limit electrolytic start parts to 20 starts per hour. Film already has a foothold in this channel: a metalized film hard-start capacitor rated 324 µF serves systems up to 5 tons, and IEC 60252-2 covers metalized start capacitors with paper or plastic film dielectrics.
Three-wire submersible pumps depend on a control box above ground, and the start capacitor inside it decides whether a home has water. Control box values run from 43 to 53 µF at 220 V for a 1/3 HP motor to 350 to 420 µF at 330 V for a 15 HP motor. The same manual allows 300 starts a day for motors up to 0.75 HP and 100 for motors from 1 to 5.5 HP, warns that excessive cycling reduces the life of control components, and keeps boxes between 14°F and 122°F, out of direct sun. The electrolytic qualification test calls for 40,000 starts at 165 to 330 V, so a small pump at its 300-start daily allowance would pass that count in under five months.
General-purpose capacitor-start motors drive compressors, pumps, conveyors, and blowers, as well as farm loads such as poultry feed augers. Larger single-phase motors gang their start capacitors: one 10 HP farm-duty motor carries four 216 to 259 µF, 250 V start capacitors. Makers design electrolytic start cans for short-duty start cycles, and the electrolytic datasheet allows on-times up to 6 seconds only with lower duty, cooler air, forced cooling, or a higher voltage rating. Regulation is not pushing small capacitor-start motors out: in February 2026, the U.S. Department of Energy left small electric motor standards unchanged, finding further efficiency gains not cost-effective and, for capacitor-start induction-run motors, already at the maximum technologically feasible level.
Three-phase motors need no start capacitor to turn, but a large one can pull its supply down hard while it starts which damages the motor’s mechanical parts and leads to slow starts and failure. A big motor's inrush can reach as much as 10 times its normal reactive power and 3 times its real power, and the voltage sag hits hardest at sites on the end of distribution lines or on local diesel generator power. A switched capacitor bank supplies that reactive power during the start, in at least two and preferably three or four stages, and one bank supports the start of every motor on the same feed. In one case, a soft starter cut a 2,000 HP compressor's starting demand from 9 MVA to under 6 MVA, still above the utility's 4 MVA limit, and a 4 MVAR bank built from two 2 MVAR units would hold it to 4 MVA. In a capacitor bank maker's study of a 5,000 HP motor, capacitor-assisted starting cut the start time from 9.1 to 3.7 seconds and the voltage drop from 17% and 27% to 4% and 6%.
Engine starting carries the same burst duty. A turbine ignition exciter powers the ignition system: it charges a storage capacitor and dumps it into an igniter plug. Older designs charge a mica storage capacitor to about 3,000 volts and fire about 2 to 6 or more times per second, and an industrial gas turbine exciter delivers 2 J minimum at 1.8 sparks per second across a -20 to +55°C ambient range.
NanoPlex HDC pairs film construction with 1.7 to 2.1 times BOPP's capacitance at the same thickness. The values below are as each source publishes them, and test conditions differ.
|
Property |
Aluminum electrolytic |
BOPP film |
PET film |
NanoPlex HDC |
|---|---|---|---|---|
|
Construction |
Etched aluminum foil and electrolyte-soaked paper |
Metalized polypropylene film |
Metalized polyester film |
Nanolayered film for metalized and film-and-foil capacitors |
|
Capacitance per volume |
Highest, from etched foil |
Baseline film, dielectric constant 2.2 |
Dielectric constant 3.3 |
Dielectric constant 3.7 to 4.7 (ASTM D150), 1.7 to 2.1 times BOPP at equal thickness |
|
Loss, as published |
Power factor up to 10% new and 20% after life test, finished part at 60 Hz |
0.05% at 1 kHz |
0.8% at 1 kHz |
0.5 to 1.0% at 1 kHz and 25°C (ASTM D150) |
|
Temperature rating |
+65°C for standard start parts |
Up to 105°C in one maker's data |
105°C, up to 125°C in some designs |
105°C film rating without derating |
|
Liquid electrolyte |
Yes, which can dry out or vent |
No |
No |
No |
|
Duty profile |
Short, infrequent starts |
Continuous AC run duty |
DC circuits |
Target: burst duty, from starts to sparks |
Electrolytic values come from a start capacitor maker's datasheet, and BOPP and PET values from a film capacitor maker's published film properties. HDC values come from Peak Nano's published specifications.
An HDC evaluation should test a part against the field, not just against a datasheet. Five steps cover it:
The fastest path to market runs through parts a capacitor maker controls. Hard-start kits and universal replacement start capacitors reach contractors without waiting on an OEM redesign, while well pump control boxes, farm and shop motors, and refrigeration compressors follow on their own design cycles.
Across those markets, HDC gives a capacitor maker a new set of design options: film start parts on a 105°C film with no liquid electrolyte and 1.7 to 2.1 times BOPP's capacitance at the same thickness. Qualification testing on real duty profiles decides where each option earns its place, from hard-start kits and pump control boxes to motor and medium-voltage start stages.
Capacitor makers can request a NanoPlex HDC sample kit for qualification testing at peaknano.com/nanoplex/hdc.