motor power and chinese spindles power HELP

I am really confused. I see online Chinese spindles of 2.2Kw and they are really small and around 10Kg in weight. I have 3 phase squirrel cage motor that has 3000 RPM and it is only 0.75KW but it weights 15KG. now can someone explain me how those Chinese spindles are rated at 2.2KW and bigger motors are rated at only 0.75KW. Also I was not able to locate ANY squirrel cage motor like this http://image.made-in-china.com/2f0j0...tion-Motor.jpg that is rated at 2.2KW but in size similar to the Chinese spindles. I have found 2.2KW squirrel cage motor but it was 4 times bigger then the Chinese spindles. PLEASE SOMEONE CLEAR THIS RATINGS TO ME.

Thank you.

it's a different type of motor,

the Chinese spindles are brushless so they spin a lot faster and use a fair amount of energy. but they have less torque then the brushed motors.

No, the 3-phase 3000rpm 0.75kW motor is exactly the same type of motor as the spindle motor. Squirrel cage is just another name for an induction motor.
The short answer is making the motor higher speed gets higher power density (i.e. more power for same weight). Also, the water cooling extracts the heat from the spindle windings far more efficiently than air cooling, so the conductors can be run at a much higher current density, so less mass in copper/iron is required.

WOW great reply my friend thank you very much.
I have AC Squirrel cage motor 0.75Kw it is brand new 1HP. It weights 13KG. Do you think I will be able to use it direct drive with Er20 chuck and work on light passes on steel? I'll use VFD.

Another question. If I take 18.000RPM induction motor that is rated at 3KW, how can I mill steel with it? As far as I know for steel milling it is used max 3000RPM?

First question:
It depends on lots of factors, but mainly the diameter of the tool and how fast you want to cut with it. If that motor is 2-pole (just under 3000rpm) then 750W is enough power to cut steel but not quickly, depending on your definition of quickly. The maximum tool diameter will be limited since the motor will output (approximately, depends on the drive) constant torque over the speed range using a VFD. Bear in mind though that since the output power is simply torque multiplied by speed (angular velocity strictly speaking), your available power will reduce linearly to zero with reducing speed, so at 1500rpm you get about 375W, etc...
Also, if you need to use over 3000rpm then it's generally safe to run the motor on a higher frequency to get higher speed, maybe 5k-6k.rpm (do research this before trying as there's more to it), but again the power will drop linearly with increased speed. One way round this, which John S has done, is to get a company to rewind the motor for 6000rpm, then you will get more power. Or if you're keen, rewind it yourself ... although I've not tried it yet, I'm confident that it's not difficult to do.

2nd question:
You'd either have to use very small tools, or tools with a special coating (Cut-well 'blue' tools are allegedly good) to enable the spindle to still be run at fairly high speed. As above, your problem is the output power drops with speed, so at 3000rpm the motor might only output about a sixth of the rated power. That's still 500W though, so it may be enough. Have you got a link to the motor you have in mind?

Edit: Are you concerned about the weight of the spindle motor? I wouldn't be as for any machine able to do a good job of cutting steel the mass of the motor will be a tiny fraction of the machine as a whole.

Yes I am concerned about the weight. Can you please check my build log and tell me what do you think? THANK YOU.

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