[j3dprints]

Can anyone help me out with recommended pulley ratios for a belt drive conversion?I'm looking at buying a 2 pole 3 phase 2800rpm motor (50Hz in Aus) and a VFD (vectorless is apparently the way to go). I am just not sure on what ratios to use, when you take into account milling requirements, torque curves, and the effect of the VFD. I've even heard you can overspeed the motor with a VFD.Thanks.

[ninefinger]

Not uncommon to use a step up and step down at ~ 1:2 and 2:1 ratio to cover the full range of high and low speeds. So to get high torque use the 2:1 reduction and double the motors torque or to get high speed use 1:2 and double its speed.
That way only 1 belt is needed as it fits both positions. You will still loose lots of low end torque compared to the standard setup - again you can gain some of that back by going to a larger horsepower motor than stock and using CNC typically doesn't need that high a torque - you can program your way around torque requirements typically (i.e. thread milling versus large diameter taps, circular interpolate holes versus large diameter drills).
You can overspeed the motor with a VFD - just be sure that

1. The motor is capable of the speed (100hz = 5600rpm) and
2. The rest of the system can handle it - with 1:2 you could end up with 11200rpm at the spindle and bearings become an issue.

for the pulley size - looks like a 4" diameter should fit nice for the large and 2" would be the small. I've drawn up a 3 stack with a 3" middle pulley as well, just for fun but will likely go with a 2 step pulley when I get around to building mine....

Mike

[j3dprints]

Thanks is it better to get a 4 pole or 2 pole motor? I have seen people using 4 pole at like 100Hz+ but wouldnt it be better to go 2 pole and run at lower Hz?

[ninefinger]

I'm no expert on the subject but my take is that you'll probably have more torque / hp than needed at the higher RPM's (because you'll be running a small cutter) and less torque / hp than desired at the low RPM's. That means in my mind you would want a 4 pole (1500 / 1750 rpm) motor as it has its full rated torque at that rpm where as a 2 pole motor will be half as much toque at the same RPM. The VFD is electronics and couldn't care less what frequency its output is as long as its within the specs of the unit. The only caveat is to get an "inverter duty" motor. As I understand it operating the motor below its design rpm can result in increased heating so an inverter duty motor has better temperature rating on its windings.