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From: John Woodgate
Subject: Re: 2.5kW-3kW-SMPS-(cheap)-suggestions?
Date: Fri, 20 Dec 2002 07:04:05 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Fri, 20 Dec 2002 08:30:31 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that John Jardine
wrote (in ) about '2.5kW-3kW-SMPS-(cheap)-suggestions?', on Fri, 20 Dec 2002:
>I'm rather confused by the wire slicing and dicing as all I'm seeing is an
>illustration of maintaining constant power. I may be missing some
>interesting physical layout here in which case I'm talking 'across' you. If
>I am, could you illustrate it differently?.
I think you are determined by tradition to paint yourself into a corner,
but since I can see a mistake in your examples, I'll have one more try.
>Say existing manufactured item is 50V at 1A.
>That's 50ohms of a particular length of resistance wire. 50W. (The 50W of
>heat is of course a key factor.
>Task ... redesign to a much more usable 100V.
>So, simply change R to 200ohms to maintain the same 50W dissipation. Select
>a suitable resistance wire and et voila' Sorted!.
>No it isn't ...
>1) We cannot change the length of the wire used.
>(Imagine a flat or circular inductor coil). Extra wire would change the
>heating surface area or wiring pitch. These are rigidly defined by the
>particular heat flow dynamics and final joint strength (1mm wire, 1mm pitch
>results in 0% joint, as melting material must flow -through- seperated
'Rigidly defined' - I just don't believe zero flexibility in real life.
>Increasing this wire length also increases the linear wire
>expansion.v.temperature. As the wire can be 'moulded in' it is restrained at
>low temperatures by solid plastic. Put power in and wire temp' ramps up, it
>can't expand, so internal stresses increase. At some temp' the plastic will
>soften sufficient for all the built up stresses to start releasing. Too long
>a piece of wire (or too intense power input) means the internal stresses can
>release in an extremely fast and localised area such that all the extra
>expansion length can suddenly appear as as a few cm's of 'chicane' in one of
>the windings sufficient to short across many of the others. Or if pitch
>spacing is too low due to extra length then the windings can expand sideways
>(quite normal) and clash into each other
Can you not conceive of any way of relieving this expansion? We're
talking about a few cm at most, not metres.
>2) So as our wire length is immutably fixed we simply need to replace it by
>a different diameter (x0.5) or use a different rho factor and diameter.
Not x 0.5. See below.
>Not that easy ...
>(For demo I'm showing some numbers here from an old prog I wrote.)
>Assume existing 50V design uses 15mtrs of wire at a 2mm pitch winding (or
>spiral) rate. Resistance wire type is .52mm dia stainless steel. 50ohm at
>23degC. 60.5ohm at 200degC. Max linear expansion 35mm. Cost say 22pence.
>First option ... go for a lesser diameter of existing wire so ...
>0.26mm SS gives 200ohms 50Watt, same material, same length, same tempco, and
>available at 1/2 the price!.
>Useless! ... The machines can only handle wire down to 0.4mm . £15k to alter
>and then problems start turning up with wire stressing, requiring a further
>expensive stress relief operation .
No, you want half the AREA to go with double length. That gives you 0.37
mm diameter. Oh, dismal failure; still below 0.4 mm. And, of course,
it's unthinkable to use 32.5 m of 0.4 mm wire.
>Second option ... go for a different material that will work with the
>machines .4mm min, yet still give the 50Watts at 100V using same length.
>Useless! ... Out of 18 resistance wires none can get down to 50W at 0.4mm
Oh, yes, dismal failure. Just 2.5 m extra works with the SS material.
>We're buggered, so lets pretend we made a mistake and the example problem is
>really 50V, 2ohm, 25amps, 1250watts. Stainless would have been impossible to
>use for this design (>£5) and the >2.5mm dia is too big for the machines. so
>copper (or a copper mix) *would* have been used for size and cost.
>So assume a particular copper mix (K3) heating wire ... gives 0.54mm dia,
>2.8ohm at 200degC, cost would be 27pence. 38mm max expansion.
>So at 100V can't use K3 again as dia would be too small.
0.54 mm divided by ROOT 2 = 0.38 mm. Once again, a very small increase
in length allows you to use the same material in 0.4 mm dia.
I can't tell you in detail how to accommodate the longer wiring, because
it depends entirely on the exact dimensions of each particular heater.
But I don't believe it's universally impossible.
>So look for
>another. Most are u/s but a cheap copper (K10) mix looks promising ...
>0.5mm, 9.1ohms hot, cost 25pence, a saving!.
>Useless!. ... This time we suffer from a temco problem. The tempco is only
>9.1/8= +13% compared to previous tempco of 2.8/2=+40%. This will mean that
>too much power is being dumped into the plastics at the tail end of the
>cycle. Heat flows and material degradation will suffer badly. (too many
>total Joules per mm^2).
But if these things are driven at present from generators through
transformers, the voltage is not closely controlled, so it's difficult
to see that tempco is a major factor. And if it is, your switch mode
solution wants to be a **constant power** source, not a constant voltage
>Stainless would work nicely in this 50V to 100V upgrade as it's tempco is
>useable but the 1.31mm wire costs £1.42 a go, pitch spacing problems turn up
>and the accountants would (and should) veto the idea simply on cost
I hope they include the cost of your transformers or switchers,
including the cost of meeting EMC requirements. You can't avoid the
requirements on radiated emissions with a label, and there must be at
least some doubt that you can avoid the conducted emission requirements
that simply, because the uncorrected emissions would be horrendous and
would create havoc if the switcher *were* ever connected to a public
Just as a reminder, here is what you originally asked for:
The suggestion is for a piece of portable kit that will be powered from
an on-site 110Vac generator (95V-140V building site type!) and will give
as o/p (unsorted yet) 40-50V DC (say 55amps max). The power will be
dumped into a resistive heating element (one shot) for up to maybe 30
minutes a time. Operating duty cycle about 60%. ('tool' rating).
The kit needs eventually engineering to a best cost, as it would be
compared to existing equipment using mains transformers
It also must make best use of the power available from crappy site
generators. So a 95% or better PF is wanted. Safety 'galvanic' o/p
isolation is needed but other than that all other spec' aspects would be
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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