atmega64 adc problem

hi every one.

i am using atmega64 microcontoller and i want to know how adc works....i am trying to run a 3 phase motor with input supply 415v ..due to power fluctuation my motor gets heated up...so i need to protect the motor from high voltage as well as low voltage...so can any one tell me how i compare voltages and give it to adc pins of microcontroller...?

regards
rimpul

You would need to scale the voltage down, using an isolation transformer would probably be best as you could drop the voltage and isolate at the same time, then I'd suggest feeding it to an opamp setup to provide an absolute output voltage along with a sign indicator as the ADC can only handle positive voltages. With a practical ADC resolution of 8 bits you'll be able to sense the mains power to within 2-3 volts, since you're talking about surges though you'll need to even with isolation transformer make sure that whatever fluctuations are produced can't damage the circuit.