How is Metallized Film Capacitor Winding Done?

Capacitor winding made using metallized plastic film needs great care and precautions in manufacturing. Metallized film thickness can be as low as 1.2 microns for MPET or 2 microns for MPP film (PEN film can go as thin as 0.2 microns). For regular windings for most applications, thickness may vary from 3 to 12 microns. The thickness of metallization on this film is less than 100 nanometers.

As against this, a normal room air contains dust particles from 4 to 20 microns. These particles are comparable or thicker than the film used in capacitors. The particles are in hundreds of thousands in numbers per cubic meter of space. Further, the air contains moisture / humidity which can affect the metallized surface on film by oxidizing it. Then there are some chemical impurities present in atmosphere. Further, temperature of room keeps varying widely with season and time of day.

The film and metallization both get affected by these factors. Dust and suspended particles will certainly enter between film layers during winding, and damage or puncture the film. Atmospheric moisture and impurities damage and oxidize the thin metallization layer. These cause multiple short-circuit paths along the film, and also damaged metal layer, reducing the capacitance. Weak/ short circuit spots get cleared multiple times during tests and create further weak spots, increase loss angle (tan delta) ESR and result in element heating and early failures or even fires in service.

Therefore, all suspended particles and humidity have to be controlled within strict limits. Winding room has to be air conditioned, and made as clean a room as practical. Even outside the winding room, good air quality should be maintained. Humidity should not affect the metallized film during winding and storage, and humidity level of 45-55% is preferred. Utmost precaution is necessary in handling the film, wound elements and other process materials. Metallized plastic film capacitors for AC applications are usually wound on round cores. Flat windings are often used for electronic capacitors and some for fan and motor capacitors. Winding size, number of winding turns, width etc., are decided based on several factors. For a given capacitance and width, diameter depends on film thickness, dielectric material and winding core used.

Dielectric Film

Dielectric material choice depends on application, and desired properties and characteristics desired. Mostly polypropylene film is used for AC applications, while DC capacitors are commonly made from polyester. For AC power frequency applications, metallized polypropylene (MPP) films are normal choice, while majority of DC capacitors use metallized polyester (MPET) capacitors. Metallized PEN, polycarbonate and other films are used in special applications. While designing a winding for metallized film capacitor (MPP / MPET), first consideration is the metallization material to be used. Choice of metallization material depends on end use-  AC or DC, cost of film, ease of manufacturing, desired long term stability of capacitance and loss factor. The choice is between Aluminium, zinc, or zinc-aluminium alloy. Most DC application capacitors are made with Aluminium metallization, while AC capacitors are made with zinc, or more preferably zinc alloy. Among the latter, nowadays, heavy edge metallization is more prevalent. There are more varieties like tapered (sloped) metallization, segmented metallized films etc. One type uses aluminium metallization with heavy edge zinc metallization. 

Film Thickness

Thickness of film is decided by on working voltage of capacitor for a given application. What is ideal for motor capacitors may not be suitable for power capacitors. The criteria are different in these two cases. Generally, power capacitors use lower voltage stresses as they must have a much longer life with more reliability, and they have to operate continuously for days or years together without being switched off. Their duty requirements are different.  Thickness of film should be decided with considerations for any surges / over voltages expected in service, temperature, duty of capacitor, available standard film thicknesses, and long term capacitance stability desired.

Width of film

Width of film and capacitor value then decide the winding dimensions. There is a limit to which current density may be borne by the film, along the width and length, which depends on aspect ratio. Edge of metallized film must collect the current distributed over width of metallization at any given point. Current at non-metallized end is minimum, while current at metallized edge is maximum over the film width. The metallized edge should be capable of carrying this current.

Free Margin

Metallized layers on two films of a winding must remain electrically isolated from each other, and terminals are created by spraying the edges. A metallization-free margins are provided at their ends- one film on its left side, and the other on right side of it. Metallized films are supplied in pairs, with equal reels of left and right side margins. Often care has to be taken during winding to ensure that these pairs are consumed fully, without any excess reels being left out, since a reel on the inner side of winding will have slightly smaller length (and smaller weight). This can be done by interchanging of reels with left and right margins on machines at intervals.

It must be noted that this distance between sprayed metal and metallization end of other film must be able to withstand the applied voltage, as also any high voltages and surges. Film thickness is not in play in this area, and any failure of capacitor can happen along the surface. Creepage distance available must be sufficient for this purpose. The non-metallized free margin could be as low as 0.5 mm for low voltages and can be over 5 mm for high voltage capacitors.

Aspect ratio

Aspect ratio (winding diameter / winding width) is an important parameter from manufacturing point of view, and decides the current distribution over the width and length of film. Wider the film, longer is the winding (and lesser number of winding turns) and smaller is the diameter, and vice versa. So there is a compromise between film width and its length. Often smaller number of turns means more winding speed and some more production. Further, smaller diameter and longer size means more surface area available for cooling and ventilation of capacitors.

Stagger

Ends of films are projected out by a small margin (stagger) on each end.  Stagger serves number of functions.

• It increases the separation of metallized end (or current carrying surface) of one winding from the non-metallized end of the other winding over film surface (creepage distance), thereby increasing the insulation level between winding electrodes.
• Stagger serves to enable metal spray to grip firmly on to the film.
• In case of impregnated capacitors, oil (or impregnating medium) occupies this space and excludes any air from this space.
• Stagger depth is decided by voltage and purpose of capacitor. It could be as low as 0.3 mm for low voltages, to 1 mm (or even more) for high voltages.

Offset of films

Metallized ends of films are projected outside of metallized end of other film on one side, creating a gap from the other film metallization. Zinc spray done on this end makes contact with base metal of electrode, and also grips the plastic edges by slight melting, as it enters a small part of this free space. The gap created between the metallization end and zinc spray should be sufficient to withstand high voltages / over voltages between the electrode surfaces and spray. This is taken care of by free margin and offset. Modern winding machines can adjust offsets very accurately, often by vernier adjustments. 

The figure above shows essential construction details of metallized plastic film capacitor assembly. Outer edges of metallized ends of film are sprayed with zinc (or tin / zinc) and are soldered to external terminals of capacitor. It can be seen that solder metal occupies space in the offset area, and enables to bind it with base plastic film.

Round winding could be made on plastic cores, or they may be wound directly on machine mandrel for small core diameters. Winding with or without cores is done on high speed automatic winding machines. The machines have fine adjustments for individual film reels. Winding is usually finished with few turns of plain plastic film, generally of a higher thickness, and heat-sealed on machine.

For flat windings of capacitor elements, winding is made without core, and then flattened under pressure. Windings for flat capacitors is made on split mandrels of suitable size, and then pressed on automatic machines.

Considering that films and elements get easily affected by surroundings, it is best to reduce storage time of film and elements at each stage till elements are fully encapsulated or hermetically sealed. The film should be opened from packages only when needed, and used quickly. Zinc gets more easily oxidized than aluminium, and needs more care.

Wound elements are then masked before spraying, or extra length of plain film is provided for protection of elements during spray. Ends of windings are later sprayed with zinc or other metal / alloy to collect material from all over the film length. Zinc is the most common metal used for end spray on element, while a second layer of tin may be sprayed for better solderability where needed. Spray is done mostly by arc spray nowadays, as this is easier, safer and more amenable to automation. Flame spray using oxy-acetylene gas was common in old days, which is now almost entirely replaced by arc spray. Both ends of element are sprayed one after another.

The zinc spray on ends (scooping) must be thick enough to carry required currents and surges, and to ease soldering of wires or leads to the spray. The later becomes important as soldering heat should not melt the base film or affect the metallization. Spray thickness may vary anywhere from 0.3 mm to 1 mm or more depending on capacitor design or end use. It must able to carry very high currents in some capacitors.

The sprayed ends are in turn soldered to leads for external terminals of capacitor by soldering. Soldering is a critical operation, as the plastic film should not get damaged, while the ends must be firmly soldered to lead wires or lugs.