On Sat, 23 Mar 2013 10:48:27 +0000, Clive
wrote:

In message
,
" writes
Instead of just a simple
fan motor, it's a fan that's variable speed, driven by a PWM
signal.
If by PWM you mean pulse width modulation, then it would allow for
variable speed, but a DC motor is an inductive load and is not sensibly
controlled by such a system unless there is something in the circuit to
allow the peak voltage generated by the motor at pulse cut of to be
shunted to earth.
PWM is the most common method of controlling the speed of DC motors -
a flywheel diode is part of the "system" to handle the inductive
kick-back. Virtually all battery operated variable speed power tools
use PWM. So do virtually all electric bicycles with brush motors and
the vast majority of electric forklifts.

In fact, just about any application of a brush type DC motor that
requires reasonable speed control has switched to PWM control of some
sort over the last 20 years, including power wheel chairs (except
those using 3 phase brushless motors)