Metal paper capacitor

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MP capacitor, paper capacitor with paper metallized on one side, winding impregnated with insulating oil

A metal paper capacitor (MP capacitor) is an electrical capacitor and mostly belongs to the field of power capacitors or interference suppression capacitors . It consists of two paper foils of the appropriate width metallized with a very thin layer of aluminum or zinc , which are rolled up with another paper foil to form a roll. The coil is impregnated with an insulating oil, the metallizations protruding from the coil are contacted with the connections and installed in a cup.

Metal paper capacitors are a further development of paper capacitors , the first industrially manufactured capacitors with a roll of two layers of oil-soaked paper ( insulating paper ) and two metal foils as a capacitor cell. In contrast to these, metal paper capacitors have self-healing properties, because in the event of a breakdown the material around the breakdown point evaporates, the cause of the breakdown is eliminated and the defect is automatically healed. This prevents a permanent short circuit between the pads. However, its capacity is also slightly reduced with each breakdown.

MP interference suppression capacitors (X2) with the corresponding safety symbols

The pulp paper has a dielectric strength of around 100 V / µm, an insulation time constant of around 1000 s and a loss factor of around 70 · 10 −4 at 50 Hz . MP capacitors thus have a relatively low dielectric strength and insulation strength and a relatively high loss factor. Since paper is also highly hygroscopic, these values ​​can also deteriorate. To reduce the sensitivity to moisture and to increase the dielectric strength, the condenser winding is therefore removed from the moisture before it is installed in a vacuum drying system and the winding is impregnated with an insulating oil. According to a ZVEI leaflet (as at September 2000), capacitors marked with 'MP' are basically to be classified as PCB-free. If the manufacturing date is after 1982, insulating oils for all high-voltage and power capacitors manufactured in West Germany are free of chemicals containing PCBs, including the oils from other types of capacitors manufactured in West Germany. In the GDR, on the other hand, oils containing PCB were used until the collapse of the state.

Metal-paper capacitors are almost exclusively used today as interference suppression capacitors and as motor operating capacitors for AC motors. As interference suppression capacitors, they offer certain advantages in terms of flammability, caused by transient overvoltages. Compared to MK types, MPs when used as motor capacitors do not usually show the creeping loss of capacitance that occurs more often with MKs, which results in a poorer start-up of the motor, and later no longer at all. The efficiency of the motor also decreases, as the approximately elliptical (almost) rotating field of the motor increasingly becomes an alternating field due to the loss of capacity. A cooling device with energy efficiency class 'A' when new can drop to class 'B' or even 'C' after just a few years without the user noticing.

Environmental aspect

Until 1982, other capacitor types than MP and MK often used polychlorinated biphenyls (PCB) to impregnate the paper. MP and MK, on ​​the other hand, were always manufactured without PCBs (Bosch, Siemens, SEL). These can therefore remain in operation without hesitation, caution is advised with other types of older date.


  1. a b PCB leaflet of the Central Association of Electrical and Electronics Industry eV (ZVEI) ( Memento of the original from September 27, 2007 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot /
  2. Wima, Flammability [1]