Ok, here is a bit of a dopey question, but I can't get it right in my head.

My CNC lathe (denford orac) has a manufacturer declared spindle speed of 2000rpm in the literature.

The motor is a 4 pole, 3 phase motor rated at 1/2hp

The motor end pulley is 35mm diameter, and the spindle pulley is about 63mm diameter.

The variable frequency drive was a Parametrics FHP Parajust motor speed controller. (Long since sold to ABB and now brand no longer used)

I need a bit of help with my math here....

The motor is rated at 1/2hp on a 50hz supply with 4 poles gives a motor speed of about 1480rpm allowing for slip. The Parametrics drive seemed to be adjustable to 100hz.... So the motor speed could be ramped up to ~2960rpm....

But the motor has a 35mm diameter pulley on it and the spindle has a 63mm diameter spindle on it (10rib j profile v-belt)... So the pulley ratio is ~ 1.8:1 is it not...

So if the motor was running at 100hz from the vfd (2960rpm) the best the spindle could have done was 1644 rpm... Am I missing something? ... Again manufacturer stated spindle speed adfustable from 0-2000rpm

Ok now onto torque, given I'm metric 1/2hp ~ 373watts.

At 1480rpm (rated motor rpm) the motor will develop 2.41 n.m of torque (21.3lb.in)

Now onto servo motors....

The DST-880 listed earlier has a rated output of 2.39n.m of torque and 750w and it's rated at 3000rpm.

Servo motor torque is flat up to the rated output, so doing the math, 750w at 3,000rpm does in fact equal 2.39n.m. of torque....

http://www.wentec.com/unipower/calculat ... torque.aspNow the catch....

Given the servo is rated to 3000rpm, should I design / select my pulley speed according to this?

Because the spindle speed is supposed to be able to go up to 2000rpm, that would mean I should select a 1.2:1 drive....

Problem is the old drive was ~1.8:1..... So the torque was multiplied by 1.8 so even though the motor was only rated at 1/2hp and developed 2.41n.m. at ~1480rpm, the torque at the spindle (cutting force) was 4.3n.m.

Where as if I use a 750w servo motor rated at 2.39n.m. of torque running at 3000rpm and want the spindle to do 2000rpm with a 1.5:1 pulley arrangement the cutting torque would only be 3.6n.m. of torque at the spindle (83% below that of the vfd 3phase 1/2hp motor)

Should I not be looking at a higher rated servo motor because of its rpm limitation?

Should I select a similar to the existing1.8:1 pulley ratio and overdrive the servo motor to 3600rpm which is within the "intermittent" region of the servo motor rating where the torque drops off a bit (it will be way less than what it ever dropped off with a 3-phase induction motor)

Should I add heat sinks to the servo motor?

I am guessing I will only use anything between 1600 to 2000rpm spindle speed very occasionally (if ever!) given 2mm diameter at 1600rpm is a surface speed of 10,000 mm/min (395"/min), and 6mm dia (~1/4") would be 3x this at 30,000mm/min (1187"/min)

Been looking at the Delta ASDA-A2-0721-M drive & Delta ECMA-C20807RS motor (750w/ 2.39nm / rated 3000rpm, maximum 5000rpm / 80mm frame size, 19mm spindle / 17bit encoder).... Basically very similar to the DMM servo and drive