How does an electronic speed controller work?

[thebluser]

i know what it's supposed to do but what are it's electric components for an AC motor?

 

[Mordred]

Speed controllers for AC drives such as soft starters or VFD. Utilize what's referred to as pulse width modulation. PWM for short.
Here is a starting place for you. Keep in mind their are different circuits that can generate PWM.

http://en.m.wikipedia.org/wiki/Pulse-width_modulation

 

[Windadct]

As for what are the components - that depends on how detailed of a list you want- the quick summary I would categorize by the specific engineering efforts:
System Control ( HMI or other "higher" level interface)
Real Time Control ( PWM Generation)
Drivers - Sensors and Isolation
Power Section ( DC Link, Switches typically MOSFET or IGBT, Cooling)
Filtering ( Basically trying to remove the PWM pulses to output an ideal sine to the motor)