Subject: Re: Transformers
From: i_hopley-at-wintermute.co.uk (Ian Hopley)
Date: Wed, 28 Feb 1996 20:20:08 GMT
>Received: from oberon.wintermute.co.uk (root-at-oberon.wintermute.co.uk [18.104.22.168]) by uucp-1.csn-dot-net (8.6.12/8.6.12) with ESMTP id NAA08269 for <tesla-at-grendel.objinc-dot-com>; Wed, 28 Feb 1996 13:18:37 -0700
>From: i_hopley-at-wintermute.co.uk (Ian Hopley)
>Subject: Re: Transformers
>origional to P.L.Mason-at-bath.ac.uk
>copy to tesla-at-grendel.objinc-dot-com
>>I've just read your posting on transformers. I am interested to know why you
>>consider the microwave transformers to be a safety liability. I have a whole
>>bunch here, and want to connect them up. Potentially I could do a 4kW supply,
>>for about 12KW input power! The input impedance when the output is shorted is
>>sufficiently low to draw 12A from the line! This means that I have 4 electric
>>kettles running, without any heatsink!
>>Have you got any ideas?
>Could be a question for the group here. I was under the impression that for
>optimum performance the primary voltage needed was between 8-15KV for reliable
>firing of a spark gap triggered primary. The microwave transformers i looked at
>were around 2.5KV and had shunt plates fitted for current limiting, i think
>they origionated from a Matsui 800W oven. The problem was that in order to get
>10KV required four transformers in series. Now, normally when a transformer is
>wound, the end of the winding which will operate at the lowest potential to
>the core is wound closest to it. In the case of microwave transformers, this
>end is in fact earth and is riveted to the core. The voltage then builds up as
>the number of turns increase, and you are moving away from the core.
>In order to connect four primarys in series, the earth end of two of the
>must be removed from the core and connected to the outside of the preceeding
>transformer. This places 2.5KV at a point very close to the core and probably
>very near the limit of the insulation.
>Another problem i can see using only two transformers with the cores bonded
>as a center earth tap, assuming both transformers are wound with their
>primarys and secondarys in the same sense, is that in order to maintain
>the correct phase between the two secondarys one of the primarys will need
to be reversed. This will mean that the "hot end" of one of the primarys
>closest to the core, and as the core is usually bonded to the tesla r.f. earth
>there is the possibility of a kickback breaking down the insulation and
>the tesla primary to the mains.
>Unfortunately i dont have any practical experience of using microwave
>and had relagated them to the bottom of my list for the above reasons, but
>will welcome any advice regarding their use if my assumptions are wrong.
>Regarding the current measurements you took, because the shorted
>acting as a big inductor a proportion of the current flowing is phase shifted
>to lag behind the voltage. Only current which flows in phase with the voltage
>can do any work i.e as in a d.c. current flowing in a resistor, or the amount
>of heating effort measured in Watts. The lagging current is just swinging
backwards and forwards as the magnetic flux rises and colapses in the core.
>measures the vectorial sum of these two currents which will be larger than
>the actual current doing any work. Hence the difference between Watts or the
>amount of power doing work and Volt-Amperes or the total Volts x Amps flowing
>in the circuit. I did a post on this and powerfactor on Tesla-at-usa-dot-net and could
>posibly find a copy if you would like to know more.
>Hope this answers a few questions and look forwrd hearing of your progress
>with the coil.
Ian Hopley ----> i_hopley-at-wintermute.co.uk