[jackjr-123]

I took that off the manufacturers data that accompanies the spindle. The Torque/Speed curve clearly indicates 10,000rpm as rated speed.
It is four pole, ie 333Hz is 10,000rpm, 700Hz is 21,000rpm and 1333Hz=40,000rpm.

That sounds right. I missed the part where you said it's a 3.5kW motor. Mine is 5.5kW, hence the confusion.

Even if the torque is enough for some steel milling, you also have to keep in mind that the bearings stiffness in these spindles is lower due to the special geometry to allow these high speeds.

Just for reference, a small benchtop mill with gearbox and a 1.1kW/4 poles async motor has about 100Nm at 100rpm.

[joeavaerage]

Hi Jackjr,

In the end if the rated torque is 3.4 Nm (like mine) and it's a 4 pole motor the rated speed should be 16'350 rpm

I rather think you have made a mistake in the calculation.

You claim the rated speed is 16350rpm and the rated torque is 3.4Nm then the power is:
speed= 16350/60 x 2 x pi
=1711.8 rad/sec
Power = torque x speed
=3.4Nm x 1711.8 rad/s
=5820W....and that does not make sense.

Follow this calculation, speed=10,000rpm and torque =3.34Nm
speed= 10000/60 x 2 x pi
=1047rad/s

Power = 3.34 x 1047
=3496W or 3.5kW which does make sense.

The torque/speed diagram and the power/speed diagram that I posted are self consistent. What may look weird to you is the V/F diagram. It shows a 200V@333Hz inflection point while at rated max voltage
380V @700hz and at 380V thereafter up to 1333Hz. It is not uncommon for a V/F curve to have that sort of characteristic, but don't let the V/F curve detract from the simplicity of the Pn an Tn curves.

Craig

[jackjr-123]

I just didn't remember very well... My motor is rated at 3.2 Nm not 3.4 Nm, hence more confusion
I'll double-check next time.

I'm curious though what kind of VFD you'll be using. From my experience V/F control is not good enough for a spindle, and the quality of sensorless FOC implementations is very hit and miss.

[joeavaerage]

Hi,

I'm curious though what kind of VFD you'll be using. From my experience V/F control is not good enough for a spindle, and the quality of sensorless FOC implementations is very hit and miss.

I have always used Delta VFD's and on the basis that I've never ever had a fault once I will be buying another.

This is 5.5kW and up to 1500Hz:

https://www.ebay.com/itm/225643242854

I want a little extra headroom because depending on how much credence you give to the source this spindle is capable of over 6kW at overload. I don't know whether I
believe it, but a larger VFD never hurts. If I have the extra (money) I'll go with 7.5kW at up to 1500Hz:

https://www.ebay.com/itm/324169787221

Firstly irrespective of whether its V/F OR vector control the drives must be programmed with the correct V/F curve anyway. I've been using V/F control with my 800W spindle for over
ten years, and all the other VFDs I've supplied to customers over the years are all V/F as well. You are correct 'sensorless' vector control is pretty marginal, with a sensor it makes sense,
but remember these spindles are asynchronous, so you will never get genuine position control like a servo......so I've never bothered. If you want position control use a servo or similar
PMSM motor and be done with it.

In recent times many manufacturers have withdrawn from sale high frequency vector control drives from the market. They are apparently used extensively in the centrifuges for enriching uranium
and consequently the authorities are trying to restrict access to them. In this case it means I can buy a high frequency version of a Delta VFD but it does not have vector control. No skin off my nose.
Uranium gets stuck in my teeth anyway.

Craig

[joeavaerage]

Hi,
just as a matter of interest, what makes you say that V/F is no good for a spindle?

For many years prior to manufacturers having the ability to build in vector control V/F was the ONLY choice....and they seemed to work......both then and now.

Craig

[peteeng]

Hi All - I usually get mechanicals from BST automation. They have the attached servos. Thinking about the 110ST 1.2kW 6Nm 2000rpm with a 1:2 upspeed. This will give me 3Nm torque and 0-4000rpm speed. I looked at small mills at Hare & Forbes and they have:

BF-16 100-3000rpm 0.5kW
BF-20 90-3000rpm 0.85kW
HM-32 100-2080rpm 1.5kW
HM-46 95-1650 and 1.5kW

In the small CNC range
F80TC 50-9000rpm 2.2kW
F150 80-10000rpm 7.5kW

Or maybe a direct drive 110ST 3000rpm and 6Nm... Hmmmm 3000rpm at 12mm tool is Vc= 113m/min OK for steel Peter

[peteeng]

Hi all - My current thoughts are to use an AC servo with a pulley to a Dia80mm spindle. I'll buy a suitable power head and make a bespoke D80 housing. Then it can be swapped for a high speed motor easily.... I'll start drawing that up.... I maybe able to get a power head with a drawbar as well.... The grail is out there... Peter

[jackjr-123]

Hi,
just as a matter of interest, what makes you say that V/F is no good for a spindle?

Well there is simply no speed regulation. The actual spindle speed will vary with the load. And also the low torque below rated speed.

Peter,
I recommend avoiding these Chinese servos. For little more money you can buy Delta servos which are much better quality and have understandable documentation. I'm sure BST Automation can provide them.
Also be warned that these powerhead spindles are very low quality.

[peteeng]

Hi Jack - Thanks for the info. I have found that BST have good quality parts for the price. And Fred has always looked after me if something goes wrong or is poor. I'll look at Delta, they have come up often in the forum. It will be my first foray into servos. I realise the asian power heads could be a can of worms. But I'll try to find good ones or make my own... Its always a bit of an adventure when I need new parts. eg harmonic drives. Still have not found a source for small ones yet... Peter

[jackjr-123]

It will be my first foray into servos

This is exactly why you should NOT buy the cheapest Chinese servos, they are an exercise in frustration. The Delta ones have decent software and documentation.

[joeavaerage]

Hi Jackjr,

Well there is simply no speed regulation. The actual spindle speed will vary with the load. And also the low torque below rated speed.

That is not really correct, or not as dramatically as you suggest.

Any asynchronous motor when fed with a constant frequency voltage will rotate not at synchronous frequency but somewhat less due to slip. The slip is what causes the rotor to be magnetized
and an induction motor would not work without it. Should the motor experience an increase in load its speed will drop and thus the slip increases, which in turn increases the rotor magnetization
which in turn increases the torque delivery to the load. A typical induction motor with have a no load speed, and a speed at rated output and they seldom differ by more than a few percent.
If I dial up 20,000 rpm on my spindle, at no load I might expect 19,750 rpm given slip, but even at full load I would expect at least 19,000rpm. Does that 1000rpm make any difference?.
Remember also this is exactly how many millions of induction motors work around the planet as the load increases the speed drops....a little. If the motor is overloaded, say by 50% of rated torque it may
well slow enough that it will stall....but clearly that is just a case of overload and you'd be silly to operate any motor that way, a spindle included.

For how many years have we operated manual lathes, mills, drills etc.....and they had a gearbox at best with a number of fixed speeds. Lets say you would ideally want a spindle speed
of 300rpm on your lathe. The nearest gearbox setting is 340rpm....so you use it. Does it wreck the job? No. Why is everyone so fixated on spindle speed on a CNC machine. I certainly like to think that
if I dial up a speed with my spindle I'll get within 10%, but mostly I get within 5% which is much better than many a manual machine I've used. Likewise manual machines slow a little as the torque of the
load increases, usually not by much, and does that wreck the job? No. Provided the demanded torque does not stall the spindle I would expect a slow down of 5% or less. Is that a problem?

As for low torque at low speed.....that's not correct either. When you first turn on an induction motor it exhibits its starting torque, and typically is 50% more the its rated torque. At that time
the slip is very high and consequently the rotor currents are high which in turn means the applied current to the stator will be high. This will cause an overheat should it continue for long periods
of time. So I argue that it is not that you get low torque at low rpm but rather you risk high rotor currents and consequent overheating of the motor.

A vector controlled induction motor is still an induction motor. The vector control does not suddenly and magically increase the torque of the motor. What vector control does is control
the applied stator current to maximize the torque with the minimum of current and reduce its propensity to overheat. Note the qualifier 'reduce' , not 'eliminate' overheating.

My spindle is air cooled and I don't like to run it below half speed, ie 12000rpm. When I use a slitting saw I do however run it down to 1/4 speed, ie 6000 rpm. When I use it in such a manner I am
very careful NOT TO DEMAND high torque from the spindle for the certainty that it would overheat. So I am in no way concerned about the lack of torque but rather that I might overheat my spindle.

Craig

[joeavaerage]

Hi peteeng,
jackjr is right, do not buy el-cheapo Chinese servos. Its not because they don't work but they typically have atrocious documentation, even worse technical support and no set up and tuning software.
A modern AC servo has many hundreds of parameters, many of which you'll never need or touch but there are many that you will need. In the case of my Delta B2's there are 270 parameters.
You do not want o program those by pushing buttons like a microwave, its just too error prone. Do not do that to yourself.

If you buy a cheap Chinese spindle you'll have a nuclear meltdown guaranteed, and you are only 3000km away.....I don't want to be caught up in the blast!

Most AC servos are very similar, if one manufacturer has a feature all manufacturers follow suit. So within the difference in terminology of each manufacturer if you can program one then its highly probable
you can program another. Your problem is that you have not set up and/or tuned ANY servo....choose a make and model that gives you the best probability of success.

I use Delta, but there are many good brands. The same company I buy my servos from sell ToAuto servos, their own brand. I have no reason to doubt they work and I have good history with
the supplier and they are approx $100USD to $150USD cheaper than the same size Delta B2. Given I've programmed Delta, Allen Bradley, Omron, Yaskawa, Mitsubishi and Schneider
servos I could in all probability handle ToAuto but I vote with my wallet and buy the Delta's.

Craig

[peteeng]

Hi all- Here's the documentation for the BST servos. Looks pretty good to me. Peter

[joeavaerage]

Hi peteeng,
yes, looks great, but looks can be deceptive. Does it have tuning software? If not walk away, in fact don't walk....run away.

Craig

[peteeng]

Hi Craig - I'll ask. Peter

[G-Spot]

Did the FEA thing on your machine. Here are the results I got.
Do your deflection results correlate with mine?

I did the analysis with the force applied between the face where your spindle shaft intersects the motor face and the test pad on the base.

[peteeng]

Hi G - Convert your results to N/um so we have common units to discuss. What load did you use? I'm sure all commercial FE systems will give the same result if given the same data and geometry. Peter

In the optimisation image the parts seem to have a double mesh. Check you don't have two bodies superimposed on each other for those parts. If they are then the part will be twice as stiff as it should be... Peter

[G-Spot]

Hi G - Convert your results to N/um so we have common units to discuss. What load did you use? I'm sure all commercial FE systems will give the same result if given the same data and geometry. Peter

In the optimisation image the parts seem to have a double mesh. Check you don't have two bodies superimposed on each other for those parts. If they are then the part will be twice as stiff as it should be... Peter

I am doing all my tests with 1000N, there is no option to display relative stiffness, I am sure your brainy brain can do the conversion without a calculator|

[peteeng]

Hi G - You can also apply 1um deflection to your part and the FE will figure out what the reaction is to achieve that deflection then no brains needed as then you know its N/um immediately... Peter

[peteeng]

Hi all you brains out there - I'm designing a 3mx1.5m rack and pinion router with helical rack and pinion drives. Any one done R&P before and have any words of wisdom? I'm nailing down the final drive details at the moment. First time through the movie is always interesting. I'm going to try and use as many "optimised" and organic designs as possible. Peter