Yes I did misunderstand you. I thought you wanted to make wort not filter it. Sorry.

But if the wort is temp controlled how will it get hot enough to scorch?

Well actually you are doing both. Making it and filtering it at the same time.
The wort can scorch since although the heat exchanger is temp controlled the point that the wort gets measured is as it reenters the grain. (i.e. the only point that we care about being a certain temperature. As you can imagine if the wort isn't flowing then that point isn't changing in temperature. Oh sure I could build in failsafes and so forth with extra temperature sensors but I think the general consensus is to just keep it flowing, at least slowly. Of course any time the hot sticky wort is kept in contact with an element for even a minute or so and it is on there is localised scorching.

This doesn't tend to happen when it is flowing as the wort doesn't stay in contact with the element for long enough.

 

i cannot find the circuit i used, all i have are assembly layouts, but from memory i believe the frequency is fixed and the mark-space ratio changes.
there is lots of info on controlling brushless motors on google, and lots of controllers on ebay. these are the pumps i used http://www.daviescraig.com.au/Elect...R_PUMP_12V_SHORT___PART_No__9002-details.aspx
and we get rotation from zero rpm to max with out any issues .

Looking at those pumps and the controllers that go with them at least. Doesn't really look like they use pwm to control them. More turn the pump on for 10sec off for 30sec. So kind of like it I guess just with a much slower frequency. I guess that would probably work. But not really driving the motor at a slower constant speed..