FAQ:
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1. What is your MOQ?
Normally our MOQ for each size is 1ton. If you think it is too much, we can discuss according to your requirement in details.
2. Can you offer free sample?
Yes, A4 sample for free.
3. What is your delivery time?
We deliver in 10-20days
4. What is your payment?
TT or LC at sight.
5. Can you accept mixed products in one container?
Yes, we accept for sure.
Contact Information:
Susan Fu
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Metallized capacitor films have a thin coating of metal (commonly aluminium and zinc) deposited on them by vacuum deposition process. Several types and patterns are available to choose for metallization, depending on application and usage environment.
This was the first metal used when metallized films were developed back in s. It continues to be a major part of metallization material even today because of its advantages in many applications. Aluminium is particularly suited for robust capacitor designs, as it is very little affected by atmosphere as compared to zinc.
Aluminium metallization layer, when stored in air, gets oxidized on surface, forming extremely thin Al2O3, which protects the inner volume of metal layer. No further oxidation takes place and bulk of aluminium layer is unaffected by oxidation. This makes aluminium metallization safe from atmospheric oxidation during storage and processing. Hence storage life of aluminium metallized films is good, a couple of months before being unpacked for use. It is also less affected by atmospheric moisture and oxidation during storage between production processes.
However, during service, aluminium metallized PP film capacitors working at high voltage and current densities, generation of Al2O3 leads to rapid decline in capacitance value, with consequent increase in losses. Resistivity of metallized layer is 2-4 Ω/square, and thickness being significant, this means a good amount of energy needed for self-healing.
Aluminium is easy to metallize, and thicknesses from 1 Ohm per square to 200 Ohms per square can be managed (higher resistivity means thinner metal deposit). The metallization can be done as plain metal deposit or with desired profile.
One limitation of aluminium is energy required to clear a fault. Short clearing a fault needs high amount of energy compared to zinc. This puts a limitation in its use in high value AC capacitors for fan, motors and large KVAR capacitors. Another factor is the contact reliability of zinc spray with ends of metallized surface.
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Fan and motor capacitors, most power frequency devices and power capacitors were made with aluminium metallized films in early days. These are all nowadays made from zinc (or zinc alloy) metallized polypropylene films for greater stability over time and in all weather conditions.
Zinc removes the limitation in aluminium metallization in AC capacitors made from PP film, working at high voltages and currents. It has low clearing energy requirement, and resistivity can be controlled more closely during manufacture. It is best suited for low frequency AC applications. Zinc was found better suited for these applications by a series of experiments and field trials. Further, end spray metal also being zinc, it readily bonds with zinc spray on ends of metallized PP edge.
Resistivity of zinc is higher (5-10 Ω/Cm) and metal thickness smaller, and its vaporizing temperature lower than aluminium. This makes for much lower energy requirement for self-healing.
When a zinc layer is exposed to atmosphere even for a short time, it gets easily oxidized and the ZnO formation is not limited to just the surface, but the entire thickness is affected. Metal coating is destroyed within days by atmospheric air and moisture, and extreme care is needed in handling the film. Even then, storage time of unpacked metallized film cannot be long. A reel, once opened, has to be consumed fast, and processed fast to avoid undue damage to zinc layer and consequently the metallized film. Otherwise the capacitor can fail prematurely, and also develop high loss factor and large number of uncontrolled clearings, degrading the capacitor.
An improvement developed over time has become universally acceptable and gives much better properties and stability than zinc metallization. Zinc alloy metallization has proved ideal for most low frequency AC applications.
Since zinc does not adhere to plastic film readily, an extremely thin layer of aluminium is laid before depositing zinc. This layer adheres well with film, and zinc is then coated on this layer. Metallization process is not as easy as plain aluminium. The layer of aluminium is just around 5-10% of metallization thickness. Resistivity of surface is 5-10 Ω/square, and is same as zinc metallization. Aluminium diffuses with zinc, and there is no distinct boundary between the two. The structure gives advantages of both zinc and aluminium. First, aluminium ensures good adhesion with base film. Secondly, aluminium layer acts as protection, as the two layers mingle with each other and prevent early degradation.
Zinc alloy metallized film has much longer storage life than plain zinc. Experience shows the metallized film can be stored in sealed plastic bags for six months without degradation. Capacitors made from this film also has much better stability. Capacitance loss over time is also very low.
Metallization can be done as plain uniform layer over the film surface, or can be varied in predetermined pattern. Current density on film is maximum at current collecting spray edge, and goes on reducing to a minimum at other extreme near free margin. Current from all over the width flows to contact edge, which has to bear maximum current density.
Aluminium or zinc metal is uniformly deposited over the active film surface, with constant resistivity. This was the method developed when metallized film was introduced initially. It is also widely in use for general DC applications.
Limitation in this method is the current density goes on reducing from free margin end to spray metal contact edge. This means in case of fault, lower energy availability at this extreme than near the spray end. This is of no concern for DC, but can make a difference in AC as also in surge / impulse applications.
Current at spray end of film is higher, as it collects current from entire surface of film. The spray metal has to make full contact with edge of metallization. Contact thickness of base metal is very thin, and low melting point of plastic film makes things difficult to get firm adhesion of spray to metallization. This can create points of discontinuity, or non-uniformity in contact along the film length. To avoid this and to get firm contact, an additional layer of metal is deposited towards the contact edge of film (edge is made heavy). This is made such that its resistivity is half of that of main surface metal.
This gives increased metal thickness for spray metal contact throughout the length of wound element. The heavy edge metallization could be of same uniform thickness, or it can be taper ended towards inner edge. Its width can vary if needed. Often, an aluminium metallized film may even be coated with zinc later at heavy edge for better spray contact.
In order to optimize utilization of metal, and to get uniform energy of discharge for short clearing at weak spot, the deposit thickness is varied uniformly over the width of film, the spray end being thickest, and free margin end having thinnest metallization. This ensures that current density is evenly distributed thickness-wise on the entire width of film, since current magnitude tapers from contact edge to the other end.
Often metallization is done in segments of specific shapes, joined together by thin links, which serve as fuses. Metallization on film is divided by lines or patterns by masking during metallization process. In case of a self-healing incident, these fuse links melt away, isolating a small controlled finite area. These segment patterns can be from several choices as desired, some of which are diamond (or mosaic), T-segments, etc.
Two common segmented film patterns are shown in above figure. The fuse links connect individual segments to current collector (sprayed) edge. Each short clearing clears one segment with better efficiency, and avoiding any dangerous short circuit. Capacitance is reduced by a predictable amount.
High voltage ratings of capacitor can be attained by using several capacitors in series, made possible by integral construction of series elements by specially made films. The metallization can be done as a series of isolated metallized segments, isolated by a sequence of free margins during metallization process. This creates a series of overlapping dielectric area one after the other, creating a number of capacitors in series.
In the metallization pattern above, two films have been wound to create seven capacitors in series. If each capacitor can stand 200 V, the total element can be rated as V. This is a very effective way to wind high voltage capacitor elements.
Edges of films are normally straight cut. This can create a problem with precise contact of zinc spray with film all along the length by way of weak or improper contact. Contact between spray metal and film coating is extremely important for good loss factor and high pulse capacity.
Edge of film, instead of usual straight cut, can be cut in wave shape. This creates a pattern at the edge to get space for spray metal to occupy and make good contact with metal coating on film. This is particularly useful for thin films. The wave cut shape can be at metallized end at at free margin end.
Two wave cut films are wound in arrangement as shown, and the advantage of this profile for zinc spray can be clearly seen. Increased edge contact makes for a much superior contact, with high stability and pulse or surge capability. Wave shape is usually made with an amplitude varying from 0.75 mm to 0.4 mm (from wave center line), and with wavelength of 1.0 mm to 5 mm. Total depth amplitude for spray metal is double of wave amplitude.
Mr. Deshpande is a tech pioneer, a published author, and a mentor to many. He is professionally active since and his depth of experience leads the Capacitor Connect project.
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