Dave wrote:

If anybody is interested in following this I asked on
uk.rec.models.engineering and there's been a good discussion

Yes sorry, your last post (here) made me realise that I'd completely
mis-read your original article and just though that you were trying to
run the 4kW motor from a converter.

I've no experience of these rotary converters and can't help much more,
although in the meantime I did come across
http://www.isomatic.co.uk/rotaryconverter.htm, which might be of interest.

Sorry for the confusion.
--
Andy