Inverter motor on milling machine for higher rpms

I have a German pattern mill, a Zimmermann FV-5V, which is near-identical to the Czech-made TOS FNK-25, for those who might be familiar with that machine.

The head on my machine has numerous problems relating to the quill, quill lock, brake, and tool holding system that has led me of late to consider swapping in a new head onto the machine. In investigating that idea a bit, i found a company in Canada, HH Roberts, which makes replacement milling machine heads:

Replacement and upgrade milling machine heads for Bridgeport mills and Taiwan clones.

Their larger head is the closest fit to my mill (still a bit undersize at the mounting plate), but after looking at the mounting situation a bit more I am starting to wonder if this option is not quite so 'bolt-on' as it may appear.

In looking at that site though i noted with interest that they fit their replacement heads with a vector drive type of inverter run motor, rated for 7.5 hp, and max rpm of 7200. I like the simplicity of it and the higher rpm (my mill maxes at 4500 rpm) it offers is also attractive. So, an option I am now considering is to obtain another of the same type of mill (there's a few used ones in better condition and of more recent manufacture (1980s) available out of Germany), with a better situation with the quill, and more modern electrics, and then retrofit onto that a inverter motor similar to what HH Roberts uses. I consider this option worth looking at as rebuilding my current head is likely to cost the same or more.

Trouble is, when I go looking at motors and inverters, I find the whole thing decidedly opaque. I cannot find motors, for one thing, rated for 7200 rpm. Obviously they exist, but how to find?

My machine is running on 460v. 60hz service. Can anyone point me in the right direction in terms of variable speed inverter motors? Ideally I would like to obtain 6000~10,000 rpm at the spindle.

You can’t find it because it can’t exist as a stand alone AC induction motor. Speed is determined by frequency and the number of poles in the windings, which are always pairs. So the smalles number of pairs is one, meaning 2 pole, and that results in a 3600RPM synchronous* speed.

(*actual shaft speed is between 3-5% lower, called “Slip Speed”, but we’ll just stick to synchronous speeds for this discussion.)

A VFD then can mess with the frequency component of that, so by giving that 2 pole motor 120Hz instead of 60Hz, it can make the motor spin at 7200RPM. But at the same time, the TORQUE that a motor produces is a function of the ratio of voltage and frequency applied to it, so increasing the frequency 2x without increasing the voltage results in less than half of the available torque (at rated current, because there are more losses in the motor). In addition, PEAK torque cspability, the short-term torque used to accelerate or RE-accelerate after a change in loading, drops at the SQUARE of that V/Hz ratio. So running at 2x speed makes it much easier to stall the motor. However if your machine only NEEDED the torque of a 3HP motor, you are probably OK because they started with a bigger motor than needed.

There is a trick as well, IF you have 480V available and a motor designed for 230/460V. You use a 480V VFD, then WIRE the motor for 230V and program the drive to reach 230V at 60 Hz, so the motor gets the V/Hz ratio it was designed for, 3.83 V/Hz. Then because you have 480V available, you can take the motor up to 120Hz at 460V and you are still giving it the same V/Hz ratio, 3.83 V/Hz, and you get full torque at 2x speed.

What you don’t know here is if they are using a 7-1/2HP motor for a load capability of a 3HP motor, or using this trick to get full torque at 120Hz. If the latter, then understand that you VANNOT duplicate this scenario with a 240V supply.

Wow! So nice to receive your post and I learned a bunch. That explains why the motors i kept finding were 3600 rpm max.

I do have 460v service, so your 'trick' could very well work for me.

you might also want to check the bearing size, speed rating and whether you need to upgrade to a higher precision to handle the additional speed. You may not run it long enough to matter but larger 63xx bearings in particular max out in the 5000 rpm range unless higher precision or bathed in oil. Dave

Chris Hall said:

I have a German pattern mill, a Zimmermann FV-5V, which is near-identical to the Czech-made TOS FNK-25, for those who might be familiar with that machine.

The head on my machine has numerous problems relating to the quill, quill lock, brake, and tool holding system that has led me of late to consider swapping in a new head onto the machine. In investigating that idea a bit, i found a company in Canada, HH Roberts, which makes replacement milling machine heads:

Replacement and upgrade milling machine heads for Bridgeport mills and Taiwan clones.

Their larger head is the closest fit to my mill (still a bit undersize at the mounting plate), but after looking at the mounting situation a bit more I am starting to wonder if this option is not quite so 'bolt-on' as it may appear.

In looking at that site though i noted with interest that they fit their replacement heads with a vector drive type of inverter run motor, rated for 7.5 hp, and max rpm of 7200. I like the simplicity of it and the higher rpm (my mill maxes at 4500 rpm) it offers is also attractive. So, an option I am now considering is to obtain another of the same type of mill (there's a few used ones in better condition and of more recent manufacture (1980s) available out of Germany), with a better situation with the quill, and more modern electrics, and then retrofit onto that a inverter motor similar to what HH Roberts uses. I consider this option worth looking at as rebuilding my current head is likely to cost the same or more.

Trouble is, when I go looking at motors and inverters, I find the whole thing decidedly opaque. I cannot find motors, for one thing, rated for 7200 rpm. Obviously they exist, but how to find?

My machine is running on 460v. 60hz service. Can anyone point me in the right direction in terms of variable speed inverter motors? Ideally I would like to obtain 6000~10,000 rpm at the spindle.

Click to expand...

The Roberts website has a view that shows the belt and implies 1:1 ratio "direct" drive.

True "direct" drive is not actually happening because of the drawbar's needs, power drawbar or not.

Page two:

What Jraef said but also. 400 Hz motors are not so uncommon Roberts could not have used such. Same again, VFD. I just binned one that had 400 Hz capability as well as "normal" 50/60 Hz and its variations.

Page Three:

Belting. Study-up on what happens when one runs a belt at very high speeds. Presuming it can even stand that, it pulls the shafts together with significant force as it TRIES to make a perfect circle under the forces acting on its length.

10,000 RPM - may want a different approach. Even more is possible, but I don't think the drive system, overall, is up for it, even if you find a motor that is.

thermite said:

Belting. Study-up on what happens when one runs a belt at very high speeds. Presuming it can even stand that, it pulls the shafts together with significant force as it TRIES to make a perfect circle under the forces acting on its length.

10,000 RPM - may want a different approach. Even more is possible, but I don't think the drive system, overall, is up for it, even if you find a motor that is.

Click to expand...

10000rpm and 7.5HP should be well within the limits of poly-v belt.

Automotive 6 rib PK profile is good for up to 3.3kW PER rib with 100mm pulley and 10000rpm so that is almost 30 hp for the 6 rib belt.
If you want to go faster than that Optibelt recommends you to consult factory beforehand...

Even the smallest PH profile "washing machine belt" is good for about 12 000 rpm and 7hp.


http://www.optibelt-usa.com/fileadmin/files/Catalogs_and_Manuals/TECH_MANUAL_RIBBED_BELTS.pdf

Jraef said:

What you don’t know here is if they are using a 7-1/2HP motor for a load capability of a 3HP motor, or using this trick to get full torque at 120Hz. If the latter, then understand that you VANNOT duplicate this scenario with a 240V supply.

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They claim " full Hp from about 325 rpm with the 6,000 RPM range."
That would call for seriously oversized motor and inverter. For the 4.5hp model that would be like 25hp 1800rpm motor and inverter combo.

MattiJ said:

They claim " full Hp from about 325 rpm with the 6,000 RPM range."
That would call for seriously oversized motor and inverter. For the 4.5hp model that would be like 25hp 1800rpm motor and inverter combo.

Click to expand...

Actual NEED of that range could even bring Dee Cee back to life. But around 500 lbs avoir of Type T would surely stress the living s**t out of the attach-point to the ram, even if one had 20-plus large to fund it and a 25% jump in utility costs, what with sub 75% motor efficiency? Smooth ain't free, y'see!

Servo motor surely, some belted ratios, maybe? Now THAT just might make much better economic sense.

Restoring the original TO original, then scouting a speeder sub-spindle, more sense, yet. 10K RPM ain't even the half of what those can add.

thermite said:

Actual NEED of that range could even bring Dee Cee back to life. But around 500 lbs avoir of Type T would surely stress the living s**t out of the attach-point to the ram, even if one had 20-plus large to fund it and a 25% jump in utility costs, what with sub 75% motor efficiency? Smooth ain't free, y'see!

Servo motor surely, some belted ratios, maybe? Now THAT just might make much better economic sense.

Restoring the original TO original, then scouting a speeder sub-spindle, more sense, yet. 10K RPM ain't even the half of what those can add.

Click to expand...

Yes, I've been looking also at different spindle speeders to accomplish the same end result, like the Big Kaiser product (Multiply Existing Spindle Speed - High Spindle | BIG KAISER Precision Tooling Inc.), however I haven't looked at the price on that yet. I'm guessing it is expensive....
Another option is to replace my mill altogether with another of the same, but newer (there's one in Germany that is 12 years newer and in better overall shape, for 6000 Euros plus shipping and crating, etc.), which *hopefully* will not have, or develop, the same quill issues. Talking with the guy at HH Roberts however, gave me pause. They've sold more than 100 TOS FNK-25 mills, after refurbishing, and in his estimation, the head/spindle design is not a strong point, his comment being "not at good even as a Bridgeport".

MattiJ said:

They claim " full Hp from about 325 rpm with the 6,000 RPM range."
That would call for seriously oversized motor and inverter. For the 4.5hp model that would be like 25hp 1800rpm motor and inverter combo.

Click to expand...

It all depends on the number of poles. I couldn't see the motor specs on the HH website and the catalogues have no mention of detailed specs.
Yaskawa drives can switch from 2 poles to 3 or 4 poles on the fly , so maybe for higher torque low RPM they use 4 poles and as the speed increases they drop them all while varying?increasing the Frequency.

Hard to say , can't see motor plate well enough.

Rigor said:

It all depends on the number of poles. I couldn't see the motor specs on the HH website and the catalogues have no mention of detailed specs.
Yaskawa drives can switch from 2 poles to 3 or 4 poles on the fly , so maybe for higher torque low RPM they use 4 poles and as the speed increases they drop them all while varying?increasing the Frequency.

Hard to say , can't see motor plate well enough.

Click to expand...

Switching from 2 poles to 4 poles on the fly? Care to provide link for some extra information on that?

It's still rather sizeable motor no matter what pole count if it can develop 4.5hp at 300rpm.

I'd have to do the work for you, and being at work I'll have to do this on my break.
Mitsubishi also has them and pretty sure Yaskawa was first to come out with it. It's not new technology. Been around for a while. Look into it.

Rigor said:

I'd have to do the work for you, and being at work I'll have to do this on my break.
Mitsubishi also has them and pretty sure Yaskawa was first to come out with it. It's not new technology. Been around for a while. Look into it.

Click to expand...

No luck with google-fu and I don't even understand how such a trick would be possible or why.
Does it need 2-speed motor or what? (2 speed induction motors are typically lot more expensive)

In this case I doubt they use a synchronous motor, but yes Yaskawa or Mitsubishi have VFDs that can switch pole count for increase in torque at lower RPM. Look into Mitsubishi Built in spindle motors. Lot's of info there.

Rigor said:

In this case I doubt they use a synchronous motor, but yes Yaskawa or Mitsubishi have VFDs that can switch pole count for increase in torque at lower RPM. Look into Mitsubishi Built in spindle motors. Lot's of info there.

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Oh, yes, multi-speed sidetracked and more complex bespoke windings yet do come into the game. Cost is in money, physical size, and overall energy efficiency as well.

For that matter, easily 20 years since the trick of "printing" poles as required onto/into a featureless rotor was added to a generator set's bag of tricks so cheaper motors could throttle-down when loads were light and hope to last a few years longer.

At the end of the day? On economics if not also fiddle-f***k time wasted, most folks just plunk down a second and subsequent machine tool, each economically optimized to do what it does best.

Ask any five-star chef if he'd trade his arsenal of sharps for a single 100-blade Swiss Army knife?

Be very damned well shod aforehand!

You might have to try to outrun those keen blades...

Rigor said:

It all depends on the number of poles. I couldn't see the motor specs on the HH website and the catalogues have no mention of detailed specs.
Yaskawa drives can switch from 2 poles to 3 or 4 poles on the fly , so maybe for higher torque low RPM they use 4 poles and as the speed increases they drop them all while varying?increasing the Frequency.

Hard to say , can't see motor plate well enough.

Click to expand...

I asked the company if they would sell me just the motor and drive, but no reply.

The stock motor on my mill is Czech made, by Mez Mohelnice, and is 2-speed, 1440/2860 rpm. I gather that the speed change is effected by switching the number of poles, correct?

Chris Hall said:

The stock motor on my mill is Czech made, by Mez Mohelnice, and is 2-speed, 1440/2860 rpm. I gather that the speed change is effected by switching the number of poles, correct?

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Seems so. The RPM are just enough off an exact 2:1 to allow for a tad more slip.

So...

I was reading a page on lathes.uk (Machine Tool Electrical Matters), and the following remark was of interest:

"Advanced Vector 240V Inverters
This latest development offers all the functions of the basic unit and are known as "Advanced Vector Inverters" They have the advantage of allowing control of the motor speed without loss of power. On a "basic" inverter the power of the motor reduces somewhat as the speed falls, but with the latest vector type this does not happen - and that's good news for applications like lathes etc., where torque will be maintained down to very slow speeds. In addition the Advanced type is able to interface to 3-wire control systems, for example normal green "on" and red "off" push button switches - and this also makes it possible to connect the inverter to a machine's existing controls. Advanced inverters are also suitable for use with computer control on CNC machines and can be bought in versions from 0.5 to 5 h.p. and are available with a 240V 1-phase input and a 415V 3-phase output."

I'm guessing that this is the type of system fitted to the HH Robert's milling heads.

Would there be any problem, therefore, in simply fitting an inverter to my current machine (or the newer one I could buy to replace it) and obtain a bit higher speed that way? The mill currently tops out at 4500 rpm, though I am feeding it 60 hz instead of 50 hz, so the top speed is 20% higher, i.e., 5400. If this is workable, how can one determine the maximum speed that could be obtained?

This goes a bit back to the original post referring to the HH Roberts site, in which they state that the motor they supply on those replacement milling heads are rated to 7200 rpm - - how does one determine or know what the rated top speed of a motor is? In my online searches for different motors, all I ever see is the operating speed (3600 rpm, for example) but no mention of what a maximum speed rating might be for those motors designed for inverter control. And if my machine's motor was never designed for inverter control specifically, is it less than ideal or otherwise unsuited to having an inverter drive it and push the rpm a bit higher?

Chris Hall said:

how can one determine the maximum speed that could be obtained?

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Before chasing TOO far down that rabbit-hole, if it is not already on the data plate, go and dig the maker's data for your motor's maximum or "do not exceed" RPM.

They tend to physically come apart at some reasonable reserve much above that speed, and even the most over-hyped of VFD is sore pressed to reassemble the shrapnel.



I'd look harder at speeder sub-spindles. ELSE another mill purpose-built for HSM.

Just CANNOT bolt-on supersonic supercruise to a Learjet with bigger engines. See "mach tuck".

Your older-generation mill will have its own limitations, too, motor not the only one.