Hi,
In my current project I need to control a 24V, 350W bldc motor using arduino. I am thinking to use a variable resistor for this. which circuit should I use for this?

 

Details?
Do you mean reading the pot ( potentiometer or variable resistor ) with the Arduino ADC to set the speed or do you think to control the motor directly?
Do you have any idea how to drive a BLDC?

 

I don't have any idea to drive BLDC motor.

 

I don't have any idea to drive BLDC motor.

A good idea will be to Google BLDC motors to see how they work.
They typically need 3 phase switching to be generated in the correct timing and sequence to run. It is not just a process of applying power unless you get the correct controler/driver for them. The boards I made use a dsPIC30F14 (I think it was) chip driving a 3 phase "H" bridge and the switching is a PWM signal whose phasing is determined by the armature position that is sensed by a trio of Hall sensors.

 

Your quickest and cheapest solution is to buy a BLDC controller such as used for models. Here is one of 100's of examples: https://hobbyking.com/en_us/yep-40a-2-6s-sbec-brushless-speed-controller.html

It is rated for "6s" , which means the voltage of 6 lithium cells in series (about 22.8 V max). Your arduino would control it just like a model servo.

 

I wonder if those for models will be tough enough? It could be a way to go but it does depend on the final application. Is this "just for fun" or industrial use?
If the latter, you need to design or buy accordingly.
But those boards look like they could be fun to play with. It may be worth getting the programming card too. Does your motor have Hall sensors? That hobby controller does not appear to use them. For precise position control I think you will need that feature. Are you just after speed or position control too?
What is your application?

 

BLDC do not use 3phase switching, they only switch 2 of the 3 windings at any one time, commuted via hall or equivalent sensors, they represent a DC motor turned inside out, hence BLDC.
3 phase P.M. synchronous motors are constructed identically but the 3 windings are fed with true 3 phase.
Max.

 

3 phase P.M. synchronous motors are constructed identically but the 3 windings are fed with true 3 phase.
Max.

Do you mean to say that a 3-ph synch motor can be converted into a brushless dc type by just adding an encoder (or some other position-sensing technique) and a suitable controller?

 

Do you mean to say that a 3-ph synch motor can be converted into a brushless dc type by just adding an encoder (or some other position-sensing technique) and a suitable controller?

Yes, for example I have done this to the Fanuc (CNC) redcap (AC servo) motors, these have a regular quadrature encoder with a 4 bit binary code track that they use for their proprietary 3ph commutation.
You can either leave the encoder on and add descrete hall effects or do as I did and replaced the encoder with one that had BLDC tracks on it.
There is a poster on one forum that I believe decoded the 4 bits to hall.
Max.