HY VFD and spindle at low speed

[Neale]

Bert - thanks for the comments.

I'm not worried about spindle overheating as I'm only a small amount below its "official" minimum speed and the water-cooling keeps the spindle body at more-or-less ambient temperature even after 30-40 mins running. However, there are quite a few VFD settings which do things like boost voltage at lower speeds, and there are some torque-sensing features that might do something similar. However, if 400Hz is 24K RPM, then 5K RPM is still over 70Hz which seems to be reasonable for the VFD. I do need to have a look at the "torque" control settings on the VFD.

The effect I'm seeing is a bit like a governor on an old gas engine - the speed slowly drops until the governor detects that it has fallen too far, then there's a bit of a surge to bring the speed up again. In my case, I can't really detect speed changes, just this kind of "thump" which is a bit like a scaled-down motor-start effect. The VFD readout flickers between two similar readings, but it always does that anyway at any speed, and the difference between readings is very small. The speed displayed on the VFD is within a couple of per cent of the Mach3-demanded speed, which again is normal on my machine.

I'm using the router and spindle for something that neither was really intended to do so I wasn't really expecting anyone else to have experienced this. But it seemed to be worth asking the question!

[m_c]

Torque doesn't decrease with speed, Power does. Torque remains constant (at least in an ideal world, but there may be a slight variance due to things never being ideal).
And torque is directly proportional to current.

The only way you're likely to kill an induction motor with adequate cooling at rated speeds, is by pushing too much current through the windings, which can cause them to overheat. Even though the spindle is water cooled, heat transfer won't be ideal, so by pushing too much current, you can still overheat the windings.
Torque boost options simply increase the current. For short boosts it's not likely to be a problem, but prolonged it is.

For this kind of application, I would be monitoring the motor current, as that will give you the best indication of just how much load you're putting on the motor.

[Neale]

I agree as far as the motor is concerned, but the VFD has various torque-related settings and corresponding parameters, including things like voltage-frequency linking which can boost torque at lower speeds by increasing applied voltage - useful during acceleration if the motor has load on it at startup. I think - I'm still trying to understand the manual and the various settings available. I'm also aware that there must be some kind of feedback system as there is a set of PID parameters that can be modified, which implies feedback. Given that the speed is notionally determined by VFD output frequency, modified to some small extent by slip, I assume that this feedback system will boost applied voltage and hence current to increase torque if the sensing system believes that the motor speed is too low due to applied load on the spindle. Maybe - it's guesswork on my part at this stage!

One thing that does seem to be true is that increasing depth of cut and hence spindle load by a small amount (0.4mm to 0.5mm) seems to smooth things out slightly, which agrees with my thinking that any feedback mechanism in place, governed by the PID parameters, is tuned on the basis of a more heavily-loaded motor than I am presenting, or at least the parameters are not really optimised for the speed/load combination I am using.

I know that the display can be switched to display motor current, so I'm going to have a look at that tomorrow, running the motor at various speeds. Assuming that the displayed value is actually an accurate representation of motor current!

Trouble is, the rate of cut is low enough that I am standing there vacuuming up chips for long enough that my mind starts speculating on what's going on; I need to keep my "engineer" hat on long enough to just get on with it while it's working!

[Jonathan]

Fossilised somewhere on this forum (4-5 yrs ago) there's a thread/post where I've discussed how to get more torque from the common 2.2kW spindle and HY VFD combination at low speed. Essentially you use the voltage vs frequency mapping settings to boost the voltage applied to the spindle at low speed. This will put more current through the stator windings, so in my case I had something like 2-3 amps read by the VFD at just 4800rpm, but it did give me enough extra torque to drill aluminium with a 4.2mm HSS drill at that speed.

Adding extra heat in the stator is not too big a deal - in theory you could go all the way to the current rating of the winding, and a bit more as the iron losses should stay fairly small at low speed. However this will induce extra current in the rotor, which is much harder to get the heat out as it's not directly water cooled. I did once characterise the 2.2kW spindle to find the equivalent circuit, so you can work out how much current is permissible in the stator to have the same rotor losses as it was designed for. How much torque you actually get out of this is harder to say though, as we don't know the saturation characteristic of the stator. Not sure where that spreadsheet is now though.... so I suggest you increase the voltage until you see a few amps with no load and see if it's enough.

Another approach is to use a cutter with the right coating to enable raising the spindle speed. These have worked well for me:
https://www.shop-apt.co.uk/carbide-e...ted-65hrc.html

[Neale]

Thanks, Jonathan. I'll have a search back through forum history - I hadn't remembered this one being discussed before. My own background is electronics rather than electrics so while I understand the basics, I'm a bit hazy as to exactly what happens inside these motors. However, my thinking had also been to tweak the VF curve at the bottom end - thanks for the suggestion of using motor current to get a bit of insight.

This is the first time I have ever seriously used carbide milling cutters. I've generally used HSS on the mill and carbide for routing wood but I've been driven to carbide cutters because of this minimum spindle speed issue. I bought the ones I am using after a quick bit of web browsing but along with the cutters I was sent an enormous catalogue and looking at that, I see that there are indeed cutters that might be more suitable - at a slightly higher price... My concern about those is breaking them, either due to running them into a clamp (it happens!) or vibration/resonance due to a less than optimally-rigid structure. Ok, my machine is all welded steel with profile rails and ballscrews, but I'm still surprised-but-pleased that I can cut steel at all.

Those APT cutters look like a good deal so I'll do a bit more research down that line as well.