Nitin,
You will be controlling the firing angle of the triac using the uC,just as you would do it using an analog circuit. That means instead of a using a triac+diac+pot+ capacitor combo to provide the pulses, you will be using one of the timers provided on the uC to give the pulses on a pin.

You will need a zero detector circuit to notify your uC when to start timing... I think you will need to act only on the positive-going zero-crossing. Once you detect a positive zero crossing, you start the timer for a certain value, and when the timer gives the interrupt, you send a pulse on a pin which strobes the gate of the Triac, putting it in conduction mode. The timer value corresponds to the firing angle.

Thats the basic working...the kicker will be in the control algorithm. You will need to modify the timing of the firing pulses on the basis of the timing of the input pulses. Hence you will need two timers: one each to time the O/P and I/P pulses.

so your basic algorithm in pseudocode will be...
org 0000
ljmp start:

T0_int:
x = calculate_input()

T1_int:
give_pulse_2_triac()
stop_TI()

Ext_int_from_Zero_Detector:
;positive going zero-crossing
start_T1(alpha)

start:
init_everything()
do_soft_start()

main_loop:
alpha = calculate_firing_angle(ip_t)
do_other_functions()
ljmp main_loop

calculate_firing_angle(ip_t) will contain the control algorithm to value of 'alpha' based on the load on the motor as implied by the input pulses. I cant say which algorithm you will need...

do_soft_start() will provide the soft start routine, which will probably be just another algorithm to vary the firing angle 'alpha' over a period of time to start the motor properly.

check out this link:
http://www.ece.uvic.ca/499/2001a...sides.html

I suggest you use provide a LOT of isolation between the uC and the power circuit ;-D Optoisolators should be best. Infact you have opto-triacs and what not too...

Kundi