What kind of motor?

[NealebParticipant @nealeb]

I have a Wabeco mill fitted with a nominal 2KW (a bit under 3HP?) motor. Spindle speed range is nominally 140-7500 rpm. The spindle is driven via a toothed belt with a small step-down reduction, so the actual motor speed must be around 200-10000 rpm. That's a big range! It's electronically controlled with speed set by a 0-10V analogue input and is reversible. The motor casing does not, at first glance, seem to have any access to brushes but looking under the speed controller cover, there are plausibly three motor connections plus, perhaps, a tacho feedback signal. I have seen it suggested that at least the smaller motor fitted to many of these mills is a repurposed washing machine motor, and maybe this one is as well. It is fan-cooled by a separate fan blowing over the ribbed outer motor casing.

I have no plans to change anything on the "if it ain't broke…" basis but I am curious about the motor and control technology. So-called "brushless DC", which always looks to me like an electronically-commutated permanent-magnet rotor motor? Or something else? Partly I'm asking out of sheer curiosity but another bit of me would like to be prepared for the day that something goes wrong!

[NealebParticipant @nealeb]

[noel shelleyParticipant @noelshelley55608]

DO YOU REALLY MEAN THE SPINDLE SPINS AT 7500rpm ? ? Noel.

[mgnbukParticipant @mgnbuk]

I am curious about the motor and control technology. So-called "brushless DC", which always looks to me like an electronically-commutated permanent-magnet rotor motor?

The motor configuration you describe is the standard type of spindle motor / drive arrangement used in pretty well all modern CNC machines. Not a recent development by any means, as I was installing / commissioning such drives over 20 years ago (from Fanuc & Indramat mainly) & the company I worked for then were not at the "cutting edge" of the industry – brushed DC drives were cheaper & frequently had performance characteristics better suited to the older machines that we retrofitted.

The brushless syncronous motors tend to have a higher base speed (the cross-over point from constant torque to constant power operation) than brushed DC equivalents, but also a greated constant power speed range (not subject to brush commutation limits). From memory, DC brushed motors with field weaking drives had a constant power / base speed ratio of around 3 or 4:1, where syncronous motors were 6/8:1. So for a syncronous motor with a base speed of 1500 rpm (typical), that would give a maxiumum speed of 9000 rpm (6:1 ratio).

The brushless motors also have the advantage that the windings are in the stator & can be cooled externally (air or liquid) , where brushed DC motors have both stator (field ) windings and a wound armature & require the cooling air to be blown through the motor – always a problem where coolant or oil mist is present. They are also cheaper to make – wound armatures with commutators are expensive.

Nigel B

[SillyOldDufferModerator @sillyoldduffer]

Posted by noel shelley on 06/01/2023 10:35:23:

DO YOU REALLY MEAN THE SPINDLE SPINS AT 7500rpm ? ? Noel.

He does!

[NealebParticipant @nealeb]

Given that we are just in the Christmas season still – oh yes I do!

It's a really useful range, clearly, with no belt-changing involved. I've used cutters from 1mm carbide up to a 50mm inserted-tooth face mill. BT30 spindle taper with self-ejecting drawbar. Keep looking at it and thinking, "ATC…"

Wabeco do a wide range of options and this machine started out with most of their high-end extras – Hiwin profile rails, ballscrews, full enclosure, coolant – but was then finished off by a US company with servo drives and SmoothStepper/Mach3 electronics. I've scrapped the SS/Mach3 bit due to data buffering issues that I never managed to resolve and updated to a UC300/UCCNC combination to match my home-built CNC router. Add in wireless MPG and it's a very nice little machine. Not as sturdy, maybe, as my Warco VMC but having CNC means that a few extra lighter cuts are effortless and I can be doing something else at the same time. Very lucky to pick this one up off eBay, only 25 miles up the road from me.

Thanks for comments on motor technology, Nigel. Is this motor likely to be using tacho feedback, or just running synchronously? It could be that the "feedback" connections I mentioned are from an embedded temp sensor in the motor – I did have a issue a while back after managing to stall it when it would not restart for quite a while. I was starting to worry but found that eventually (motor had cooled?) it restarted. Again, out of curiosity, if I wanted to replace something like this, where would you suggest I look?

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