I am converting a desktop milling machine to cnc, I want to use 1 phase 1.5 HP 220 V AC motor for spindle motor. I use belt drive with 2:1 pulley ratio. What is the best one to use for spindle motor : 2 pole 3000 rpm or 4 pole 1500 rpm. Thanks.
It depends what you're intending to cut with it. Above ~8k RPM you will need a balanced spindle and motor, but generally speaking you can drive some fairly large tools in aluminum and plastic at a good feed rate with 1.5HP. You can also run small tools in steel, titanium, etc. With a VFD and a 2 pole motor you will max out around 10k RPM (theoretically), with a 4 pole motor you will max out around 5k RPM.
If you mainly want to cut harder materials and don't expect to use tools smaller than 1/8" or so, then you may prefer the extra torque you can get out of a 4 pole motor. Alternatively you can use a 2 pole motor and different pulleys.
I will mainly use tools smaller than 1/4", is it still OK to use 2 pole 3000 rpm. The motor will be strong enough and don't get hot to cut aluminum or steel that maybe take long time. Or it is a must to use 4 pole 1500 rpm.