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Message-ID: <37A679095FA44A62A6E423E17CB5DC63@lindavideo>
From: "Ken" <ken.h.wright@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
References: <7.0.1.0.1.20090715193731.01a75230@xtra.co.nz>
Date: Wed, 15 Jul 2009 14:06:19 +0100
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Subject: LF: Re: Microwave oven transformers
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Hello All.
This confirms my original posting that the magnatising current is greatly 
increased when the core shunts are removed, as previously stated use two 
with primaries in series, then lifting the earth connections on the seconary 
windings connect in series and make up a full wave bridge, not forgeting to 
put a resistor across each diode to equalise the voltages.

Ken

73deM0KHW







----- Original Message ----- 
From: "Brenda Hayes" <bjhayes@xtra.co.nz>
To: <rsgb_lf_group@blacksheep.org>
Sent: Wednesday, July 15, 2009 11:35 AM
Subject: LF: Microwave oven transformers


>  Hello all,
>
> I have recently done some tests on a pair of identical transformers 
> removed from two identical ovens.
> Perhaps unfortunately, I removed the magnetic shunts before doing any 
> no-load testing. After half an hour, with 235V applied to the primary 
> windings and no secondary load, the iron was far too hot for comfort..
> At this stage, I measured the iron losses with an accurate wattmeter that 
> certainly takes account of the crummy power factor..
> At 50 watts apiece, for no-load losses, I had visions of cooling fans etc.
>
> My solution was to wire a double-wound 230/32V [150VA] transformer in 
> step-down "auto" style to feed the ex-microwave transformer primaries. 
> This step-down transformer showed negligible iron loss of 4W or so.  With 
> a new primary voltage of 200, the standing iron loss dropped to around 30W 
> per ex-microwave transformer. Naturally, the secondary voltage has also 
> dropped. I now have about
> 1600 - 0 - 1600V from the two transformers when the primaries are 
> correctly phased.
>
> While the earthed ends of  each HT winding could be lifted and FW bridge 
> rectification employed, I was reluctant to follow this path due to 
> insulation considerations.
>
> There is another trick that I found useful for safely matching 
> non-identical  transformers, provided they are much the same size:
> Apply 230V to the HV secondary windings and compare output voltages - 
> usually 23/26 volts or so.
>
> Watch where you put your fingers.
>
> Kevin    ZL4MD
>
>
>