Which motor replacement?

[dazp1976]

Which way would you go?
1kw Induction motor & VFD
Or
1kw servo motor & amplifier/controller

Looking at it more the servo unit looks better but the wiring for the induction VFD to get full spindle control with Mach3 is a damn sight easier than on the servo unit. Pro's and cons eh?
The induction motor also spins at lower rpm's so I assume it would have a lot of torque to compensate.

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[skrubol]

Induction is usually heavier for the same power, so if that's a concern, servo gets an advantage there. Depending on the induction motor, you may be able to overspeed it a decent amount. For example, 4-pole Baldor inverter duty motors are rated to run up to 6000 RPM. So that means constant torque up to 1750ish RPM, constant power from 1750 to 2625 RPM (150% rated speed,) and then power drops off as you get to 6000 RPM (not sure how fast.) Those higher RPM's are handy for running smaller tools that can't use all the power anyway.
Some servos also have a max speed that's higher than their rated speed. 3000RPM rated and 5000RPM max are common. So you get some constant power range, but not as high as some induction motors.
With a servo you could add rigid tapping if you use a timing belt as well (if you aren't going to do rigid tapping, use a V or poly-V belt, they run smoother than timing belts.)
Are there any particular models you're looking at?

[Al_The_Man]

Other advantages of a servo are C axis positioning etc and CSF on a lathe etc.
The servo should operate down to 0 rpm, and all the servo's I use have maximum torque at zero rpm, I think you are getting confused with H.P.?
Al.

[skrubol]

I was thinking mill, I guess the OP didn't state whether mill or lathe.
CSF can be done with a VFD can't it?
No confusion, the servo of my example would have constant torque from 0-3000 and constant HP from 3000-5000.
I did forget that non-inverter rated induction motors (usually TEFC,) have a fairly narrow constant torque band, sometimes as low as 2:1. Inverter rated TEFC motors typically have at least a 5:1 constant torque range and TENV or TEBC typically have constant torque to 0 RPM. Reason for the limited range on TEFC motors is the fan is driven by the shaft, so your cooling drops with your RPM's.