Dielectric strenght of vulcanized paper, parchment paper, and other papery mater - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: October 26, 2022, 12:34:52 am
content: are there specs for dielectric strength of parchment paper, and vulcanized paper?also other cellulose materials like cotton.I can just find 15-16MV/m for paper for it's probably different for other papers

author: jonpaul
author_rank: Super Contributor
timestamp: October 26, 2022, 05:49:44 am
content: Most papers are unsuitable for electrical insulation, due to poor texture, flammability etc.Use only so called "fish paper" or similar, vulcanized fiber.https://www.americanmicroinc.com/wp-content/uploads/2021/07/Fishpaper-Datasheet.pdfWe use mylar plastic tape, for electrical insulation eg 3M56Jon

author: PartialDischarge
author_rank: Super Contributor
timestamp: October 26, 2022, 07:02:16 am
content: 1) When talking about dielectric strength *always* mention what frequency you are talking about cause even from DC to 50Hz it changes *a lot*2) Paper is used together with oil insulated systems as a filter for dirt not to move around freely in the oil, or in transformers to have some physical separation.3) It is best to just test whatever material you have for a particular application, it is cheap and specs are not to be trusted sometimes.

author: jonpaul
author_rank: Super Contributor
timestamp: October 26, 2022, 08:08:40 am
content: Early caps were indeed metalized or foil wrap with paper, the oil or epoxy impregnated.Transformers can be paper insulated but need vacuum varnish impregnation.A porous paper could absorb the varnish, oil or epoxy in the spaces.Like an open weave filter paper.We use thick fish paper (vulcanized) in various PSU apps eg between closely spaced PCBs.Besides the caps mentioned and fish paper we do not know of any paper insulation.Jon

author: mag_therm
author_rank: Frequent Contributor
timestamp: October 26, 2022, 12:47:10 pm
content: Some notes for making one-off power and audio push pull plate transformers transformers using kraft paper and press-board.Paper insulation in storage will take moisture according to relative humidity which will reduce dielectric strengthto essentially zero kV/mm and increase dielectric loss angle (tan_delta)At 6 % water content , tan_delta is about 0.2 and that reading can be used to back calculate moisture content.The insulation can not be used without drying then impregnating after the winding is wound.For a small LV rectangular concentric windings, that was usually done by oven bake, vacuum down, vac-impregnate, then oven bake.As paper structure deteriorates above 100 C, the windings will be limited to Class A service temperature rise.Typical varnish was thermosetting like the well used Isonel 51 and 31. Although these alone have a dielectric strength of more than 100kV/mm,the combination of impregnated paper will be a lot less, as it is impossible to remove all the moisture before impregnation.For a bobbin made out of pressboard, it has to be strong enough to withstand the winding process, and not distort in service.The core of the bobbin will be at winding temperature ( Class A).In such a transformer, interlayer insulation (within each winding) was usually not used, with enamelled wire insulation supporting the voltage. ( These transformers do not have impulse voltage ratings, nor short circuit forces.)So,for the plate windinges, the enamelled wire insulation grade should be selected for more than say, 3 times the plate DC voltageThe kraft paper would be used between windings, that is, between primary and secondaries.Usually at least two wraps would be used. Excessive insulation leads to higher leakage reactance.A possible failure mode of push pull transformers is when the secondary is accidentally run open circuit by disconnected speakers.This can cause interlayer flashover within the audio push-pull plate winding.Maybe a final note is that by changing to a fibreglass bobbin and Nomex interlayer, most of the above difficulties can be avoided for the hobby builder.