I need to generate the variable frequency sinusoidal wave for V/F control of induction motor. i am using ROM to generate sin wave but it is capable of generating sin wave of fixed frequency.
u can use ROM approach where u can store all the sine value and update the address
using up counter
Usina a simple table and counter may lack the flexibility you will need to change frequencies. The standard solution is using a phase accumulator and changing the phase step. Since you can think of frequency as a rate of phase increase this will allow you to change frequency. Output will be in a table mapping phase to the respective valeu, and you can use only a quarter of a cycle using some logic to take advantage of simetries. However it is important to note that there are some distortions in a straightforward implementation. Dithering and other forms of compensation are used to ímprove output SFRD. You can find a good initial reference in this Analog Devices paper
http://www.analog.com/library/analogdialogue/archives/38-08/dds.pdf
With NCO(numeric controlled oscillator)+SINE Lookup Table, only need to change frequency word to change frequency.
You can implement a NCO by Cordic Algorithm, which can be found in my paper
A digitalized silicon microgyroscope based on embedded FPGA.
Frequency is controlled by NCO frequency word. NCO isn't only counter, it's counter + adder for phase/frequency control. More info you can find in wikipedia.
http://en.wikipedia.org/wiki/Numerically_controlled_oscillator
V/F in induction motors is a complex function. It must be done by ROM table in front of NCO. For every voltage(address of that ROM) you must have corresponding frequency (output of that ROM) that goes to the NCO control word. This is simplest way for production purposes. For research you must do math function instead ROM that can change slope of the V/F function.
A NCO can be devceloped in following way:
1. ROM
2. CORDIC
3. Multiplier Based
Advantages and disadvatages of all the schemes should be kept in mind while chooosing one.
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