I have a motor i want to use but inatumia 21V AC. Now tell me how to step down my power from 240V
just get a step down adapter ama kitu ka hio i had one kwa offisi step down to 110v
itafanya na 110?
ingia Aliexpress utafute voltage regulators ama stepdown
Look for guys who rewind transformers, they will like to know what current your motor requires.
Look at the wattage and then look for a corresponding step down transformer. A new one will cost you from 2500 depending on the power ratings.
But here’s the question:
Why not get a motor that does the same thing but runs on 240V AC?
I’m sure they’re there and it’ll be friendlier (hassle free, fewer points of failure) and efficient in the long run.
Unless it is a specialty application? 
Well am talking about a microwave motor.Basically i want it to do what it does there; ie move slowly while taking on a few kgs equivalent of load
Do you know of any that can do that? with low R.PM( révolutions per minute ie below 5 rpm
Stepper motor can work well , though you will need a controller but pia they are reading available and you can precisely control their rpm. Ama tafuta geared dc motors ziko mob aliexpress.
???!!!
This kind of Engineering we do offer solution.
If it runs inside a microwave I’m sure the circuitry to run it is already in place.
If you mean to run it outside of a microwave why not get that circuitry from the “donor” microwave it came from?
Failing that just get a little switchmode power supply with variable output and have fun with your project (ziko mingi with outputs that vary from 3-56 volts) 
Oh yes, there are motors that can do that - usually geared (but stepper motors and their controllers do that very well too!).
Must you use this motor.
Wait waache kuleta vitu without following the standards in kenya