There seems to be some confusion. Pulse width modulation is a common way of altering the speed of DC motors. Varying the frequency is applicable to AC motors.
__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856