Hi to all!!!
I am designing 3-phase AC induction motor speed controller drive using PWM inverter method.
In that application I need to store sine look up table so that at every instant my 3 phases are seperated 120 degree with respect to each other,,,

I read a particular 256 value sine table , but I don't know how this value comes??

I am following microchip application note for this project..
http://ww1.microchip.com/downloads/e...tes/00843a.pdf

Can any one help out how to crack this??


Sine table example
// C 8-bit Sine Table
const unsigned char sinetable[256] = {
128,131,134,137,140,143,146,149,152,156,159,162,16 5,168,171,174,
176,179,182,185,188,191,193,196,199,201,204,206,20 9,211,213,216,
218,220,222,224,226,228,230,232,234,236,237,239,24 0,242,243,245,
246,247,248,249,250,251,252,252,253,254,254,255,25 5,255,255,255,
255,255,255,255,255,255,254,254,253,252,252,251,25 0,249,248,247,
246,245,243,242,240,239,237,236,234,232,230,228,22 6,224,222,220,
218,216,213,211,209,206,204,201,199,196,193,191,18 8,185,182,179,
176,174,171,168,165,162,159,156,152,149,146,143,14 0,137,134,131,
128,124,121,118,115,112,109,106,103,99, 96, 93, 90, 87, 84, 81,
79, 76, 73, 70, 67, 64, 62, 59, 56, 54, 51, 49, 46, 44, 42, 39,
37, 35, 33, 31, 29, 27, 25, 23, 21, 19, 18, 16, 15, 13, 12, 10,
9, 8, 7, 6, 5, 4, 3, 3, 2, 1, 1, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 1, 1, 2, 3, 3, 4, 5, 6, 7, 8,
9, 10, 12, 13, 15, 16, 18, 19, 21, 23, 25, 27, 29, 31, 33, 35,
37, 39, 42, 44, 46, 49, 51, 54, 56, 59, 62, 64, 67, 70, 73, 76,
79, 81, 84, 87, 90, 93, 96, 99, 103,106,109,112,115,118,121,124
};

What is the logic behind these resulting values??
How 128 refers to zero..???
And how this value can be used to decide pwm frequency in software program?

ya, its not d2a, but your interpretation sounds correct regarding 8 bit...however how can we say it is providing 120 degree phase shift?
I am not able to understand how to interpret equation 4 given in the datasheet??
timer reload value= [FFFFh- ({Fosc/4}/sine samples per cycle *ADC)]
I am not getting the logic behind this....

See, I understand the terms, but this derivation that I am not able to digest...see my aim is to produce these sine waves using similar concept..for that I need to program chip...as far as my understanding is concern , the sine table's respected values can be considered as variable for calculating on time of pwm...And at particular instant my 3 phase should be such that they should be having a on-time which lead them 120 degree phase shift.
the question remains , how the value of sine table associated with calculation of pwm on time??

ok...I read that before, but...lets take one value..
say value=128 and half value of ADC=512...
Fosc=20Mhz, prescalar=1,sine samples=256
now timer0 reload value= 38.14
this is the time period between two states...
now what about on time of pwm which will be applied during above time period..
After every Timer0 overflow interrupt, the value pointed
to by the offset registers on the sine table is read. The
value read from the table is scaled based on the motor
frequency input, by multiplying by the frequency input
value to find the ratio of PWM, with respect to the maximum
DC bus. This value is loaded to the corresponding
PWM duty cycle registers.

what is other input???Is that ADC value???

thanks to all!!!
I have to rethink this matter, feeling lost this time...anyway thanks for your co-operation...