I am fairly new to CNC Routers. So these questions are basic(ish).
What I have for a CNC machine.
1. Very generic small China CNC machine. The decal shows Eagle II. The spindle drive is 200v and 1.5kw. There is no documentation or manuals with any of it. I got it 2nd-3rd hand, but still new in box for $800.00. Not bad for a beginner machine, I think.
2. I changed the control interface from pendant based to PC based so I can use Vectric and Mach4 software. This part works very well.
3. The machine works well, but as I got more familiar with it I found it a bit lacking.

Thinking that I may not be using it right I started looking into its functioning. What I found is that no matter how I saved the Vectric files, they only set the spindle RPM to 2000 RPM. Much of the info I have found says that the spindle speed can and should be different depending on the bit, cut speed (inches per minute) cut depth etc..
I have found how to change that in the G Code file, I am working on changing it in the Vectric program. But I do not know what the allowable speeds for the spindle motor can be, or should be. IE minimum speed, maximum speed, best speed etc. With no documents or manuals for the spindle motor, I am guessing at the allowable spindle speeds. Rather than over speed or or under speed the spindle (which will most likely ruin the motor), I am looking for how to determine the allowable spindle speeds without bricking the motor.

So, any suggestions on how I determine the allowable spindle speeds for this motor? From manuals or similar machine, or experience.
Also, how to tell if I am over speeding the motor. By how it sounds, or looks etc..