How to slow the dc motor ..what frequencies for 100 dutycycle
Please read this post before asking anything else.
The question in itself makes no sense.
A 100% duty cycle is not slow unless you’re underdriving the voltage - which in turn is the worst possible way to drive a DC motor under load.
After reading @ballsrewbob’s link, restate your problem, and maybe we can help fed the answer you’re looking for.