On Sunday, 28 June 2020 19:36:13 UTC+1, wrote:

I need to control the speed of a kitchen extractor fan so have been
investigating how best to do it.
Using a phase-control light dimmer seems to be a no-no but I can't see
why except, perhaps, for starting torque - can anyone here explain?
Switched caps in series seems to be a fairly common technique. There are
lots of cheap(ish) variable speed controllers but I doubt they're
inverter VFDs.
How do the cheap continuously-variable gizzmos like
https://www.ebay.co.uk/itm/Variispee...s/252403438896
work?

Motor laminations avoid turning a bunch of 50Hz power into heat. When you greatly raise the frequency by chopping the waveform, that no longer holds true. Also motors depend on magnetic phase shift, which with some types is frequency dependant. When it is, messing with the frequency messes with the phase shift thus efficiency plummets, or it doesn't work at all.


NT