Hi,
Controlling the spindle speed has little to do with the motor type, ac,dc,brushless, etc, and everything to to with the motor controller driving spindle motor.
It is the motor controller that determines how you will drive your spindle motor.
What motor controller do you have? If all you have is a DC motor, what are the motors specifications, that is what is the rated voltage and current for the motor?
Cheers,
Peter