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Power above 60hz, is it really constant HP? Or not?

mmurray70

Stainless
Joined
Jan 11, 2003
Hi guys, I seem to be missing some power at high rpms on my Fadal 4020 and trying to get a better understanding of what HP I have available at what rpms. I've been doing some reading and many places say constant HP above 60hz, but ive found some other graphs that suggest much less then this, especially after 90hz or so it tends to drop down fast. Seems to be conflicting information on this, see attached pics. So whats the real story on power above 60hz?

My 10hp Fadal runs at 140hz at 8000 rpm, and seems like I cant get anywhere close to a 10hp cut at that speed. What should I realistically expect for power at that speed?
 

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It depends totally on motor design. I do not think there is any hard and fast rule of "exactly this graph and no other"..

What you DO know, is that at 50Hz, you probably have all the voltage that the VFD can produce. That suggests that at higher frequencies, motor impedance is going to limit current, and hold back power. But the motor may or may not have a steep drop off, depending n how it was made.

It almost certainly WILL drop off, it's exactly where and how much that changes.

Also, a drive may go from sine to a square wave and actually put more volt-seconds on the motor at higher frequencies, which has some of the same effect as increasing voltage.
 
Below are some points regarding your question:

- 60 hz is not a magic number - it is really the rated base speed frequency of the motor - many times in the USA it's 60 hz
- It's all about the formula - hp=(torque x rpm)/5250 - can't break that rule
- Below base speed you are in constant torque range and above base speed you are in constant hp range - this is true for all practical purposes, even though there are additional losses for your purpose you can ignore them
- The biggest issue in running above base speed is that the breakdown torque comes down quickly (off the top of my head I believe it is a square function) above base speed. If you reach the breakdown torque limit, the motor stalls. DC motors don't have this limitation and can be oversped a lot more than AC motors because of this. The breakdown torque curve is custom for each motor and can only be obtained from the motor manufacturer.
- Is the drive set up properly?

Question I don't know the answer to. For cutting, do you need torque or horsepower? Although might not be said technically totally correct, work is done by torque and power is a function of work done over time.

Are you out of torque?
 
.....
- Below base speed you are in constant torque range and above base speed you are in constant hp range - .....

That is true IF YOU HAVE NO INCREASE OF VOLTAGE.

You can run at 120Hz, with double the nameplate HP, if, for instance you have a 230/460 motor, and you run it on 460 with 60 Hz if you wire it for 230V and set the VFD to have 60Hz/230V as the motor rated (base) speed/voltage, going up to 460V at 120Hz.

The reason the drop happens in general is that you "run out of voltage", so there is no more current available, the volts/Hz is getting lower as frequency increases. So current, and thus torque, drops off as frequency increases, but rpm goes up. As long as that is true, then the motor is in the constant HP range, because the two are generally proportional, and the product of them stays the same once you are well beyond 60 Hz. In the range just above 60 Hz, the motor characteristics control how fast torque drops, how much slip, etc, etc.. It probably will not be constant HP until a higher frequency.

Eventually, you drop below even constant HP, because the motor impedance is high enough that it becomes a factor.

Exactly where and especially how the current drops off depends on how the motor is made.
 
Hi guys, I seem to be missing some power at high rpms on my Fadal 4020 and trying to get a better understanding of what HP I have available at what rpms. I've been doing some reading and many places say constant HP above 60hz, but ive found some other graphs that suggest much less then this, especially after 90hz or so it tends to drop down fast. Seems to be conflicting information on this, see attached pics. So whats the real story on power above 60hz?

My 10hp Fadal runs at 140hz at 8000 rpm, and seems like I cant get anywhere close to a 10hp cut at that speed. What should I realistically expect for power at that speed?

Take your first graph. The one that tops-out at 120 Hz. Extrapolate to the 140 Hz you are attempting to work with.

If the red "torque" line drops to 50% AKA 5 HP, for the sake of comparison to your "10 HP" expectations? That is where you still have "stability".

Enough power left to sustain and regulate acceptably in the cut. Anything above that is a dice-roll. Some tasking will be fine. Other tasking will not even be at all acceptable.

"Reliably"? About HALF what you seek as far as "10 HP" once you push the RPM that far up, IOW.

One could "instrument" all this, as the motor makers assuredly DO in their R&D labs, lo a hundred fifty years and counting.

All that will gain you is a more specific graph. That is not the same as "free magic".

If you need a more powerful spindle?

That's part of why they make and sell such things. The only figures "on paper" (or 'puter screen) that will alter that are not generally classed as an "Engineering Exercise".

We tend to call them a "purchase order", rather.

JFDWT Everyone else does.
 
Thanks for the help guys. I dont really need the extra power or a larger spindle, just a little surprised to be bogging spindle when I am. I was sort of thinking I had 10hp continuous, 15 hp intermittent and according to Kennametals calculations, Im actually bogging down spindle with a 7hp cut at 8000 rpm. But Im thinking now this is probably all I can expect. Couple more factors too, the machine is belt drive so theres obviously some losses there, especially at high rpm with a fairly small driven pulley. And the voltage at the VFD is only like 220, not a full 240 and voltage seems like a huge factor at high rpm. And finally, im on a phase converter too, voltage is probably dropping some under load, making things even worse.

The first time I bogged the spindle with a 1.25" 3fl insert endmill in aluminum. Tried to run 8000 rpm, 150 ipm, at 0.200 depth. Backed down to 6500 rpm and it ran fine, even at same feed. Second time, I was doing some high speed machining with mild steel using 3/8 endmill, 15% stepover, 1" DOC, 8000 rpm and 128 IPM. And amazingly I bogged spindle with a 3/8 endmill! Slowing rpm just a touch back to 7500 and it ran fine.

I couldnt believe I bogged spindle with 3/8 endmill, this is when I started to look into this. But kennametals novo software says this is a 6.9hp cut. Looking at it now, and considering the other factors I mentioned above I guess this is all I can expect. And If i really want to get max HP out of my machine I should be running a little lower RPM, maybe in the 6000-7000 rpm range. What do you guys think? Sound about right?
 
Looking at it now, and considering the other factors I mentioned above I guess this is all I can expect. And If i really want to get max HP out of my machine I should be running a little lower RPM, maybe in the 6000-7000 rpm range. What do you guys think? Sound about right?

Yup. These are the real-world realities.

The "nameplate" figures on any motor or servo are good at the FULL SET of nameplate figures. They are essentially a "snapshot" taken at convenient parameters, usually a standard distribution Voltage - and are actually always off some sort of curve.

Said curve is only "reasonably linear" over part of its range anyway. That's all we usually need - or get - out where the goods are put to actual work.

The MAKERS have "the rest of" those curves for all their product line(s), and generally far more yet that they have already experimented with when they made the choice to settle on a given design so it became a "product".

Most have - or COULD - build to special order or make different choices out of their accumulated "library" of potential solutions, and will do where new business opportunities arise that need different solutions.

Meanwhile, we chikn's simply take the "understand what yah have, then run what yah have" approach.

Good on yah for making the experiments and sharing. I'm sure you've also kept the info in your "shop notes" to continue to do the best you can with what you have, not what you might WISH you had!

T'was ever thus, yah?

:)
 
Part of your issue MAY be the phase converter, which I assume is a rotary..... They DO tend to drop the voltage unless well oversized.

Fadal has some information about that, they recommend a Phase Perfect converter, which definitely will work well. Some of their info concerning rotary phase converters is so wrong that I am surprised they would put it up on their site, but it is stuff that does not matter for the larger picture.

You might, if you can, check the voltage going to the machine from the converter during one of the cuts. it may be dropping. Fadal does have a single phase machine option for some models at least, I do not recall if yours is one of them.

I was hoping the manual would give some info on the sort of cuts that could be expected, some curves such as you showed, but for the exact machine. But there was no such thing when I looked, in any of the available manuals..
 
I gotta ask. Are you expecting too much of that 4020? 750 SFM with .004" chipload at 1" DOC with a 3/8 EM in steel seems pretty aggressive to me. I'm sure there are machines that are capable of that but even if the motor on your 4020 was up to it, does the rest of the machine have the rigidity to not be wearing out tools at an unreasonable rate?

Sent from my XT1710-02 using Tapatalk
 
I gotta ask. Are you expecting too much of that 4020? 750 SFM with .004" chipload at 1" DOC with a 3/8 EM in steel seems pretty aggressive to me. I'm sure there are machines that are capable of that but even if the motor on your 4020 was up to it, does the rest of the machine have the rigidity to not be wearing out tools at an unreasonable rate?

Sent from my XT1710-02 using Tapatalk

I've been fairly impressed with the rigidity of the fadal in all honestly. I tried using the high speed machining with a 3/8 tool in this case because the setup wasn't wonderful and wanted to reduce forces. But my favorite tool in steel is the sandvik R390, and I find the fadal handles it fine, much better then I expected. The 3/8 endmill cut great other then not being able to hold the programmed rpm lol

At my last shop we had a Mazak 510c and a fairly heavy Taiwan 4020 sized machine, and I almost find the fadal is somehow able to take a heavier cut without vibration then either of these. But the motor does seem to be the limit. Heres a video cutting steel with the R390, 2" wide, 0.100 deep at 80 IPM. 217412_193928_646739356768183.mp4 - Google Drive

Thermite, Yes I looked for the graph in the manual and didnt find it. I know they are included in manual with some machines. My mazak lathe has one that shows exactly what torque is available throughout the rpm range. I will make some notes for sure, thanks again
 
Thermite, Yes I looked for the graph in the manual and didnt find it. I know they are included in manual with some machines. My mazak lathe has one that shows exactly what torque is available throughout the rpm range. I will make some notes for sure, thanks again

The world seems to have gone through a phase from where the curves were published as part of enabling design choices, then sort of pulled-back behind the fences - even "NDA" - as product was more closely integrated into a system or function and the "integrator" chose to apply the motating goods in ways of their own and USE some of those "special" configurations.

Time was, one took a paper feeder on a copier or a can or bottle feeder on a vending machine apart, found a Bodine motor, that motor was what it was, and Bodine published ALL the specs, curves included, whomever had bought their motors.

Current era, you MAY - or may NOT - be able to find "white papers" further back up the chain among the OEM motor or servo maker's pubs. Even if you DO, the drive system and controls it is powered by and answers to may have a great deal more to do with what happens at the tool-tip than the raw nameplate, one application to another.

The final-assemblers have gotten THEIR Engineers into the act and engaged in altering how they motivate the motators as it is all a competitive business and each player seeks an edge - or several - at the most favourable cross of performance, cost, and service life - even physical size and shape.

The "good news", of course, is that after a time, it became fair-obvious whose machines were good at what tasking, were durable or not, economical to run, or less-so, and we could - and still can - mostly just go off and put them to work.
 
Once when trying to explain this to a customer who was over speeding his motor and was upset by the lack of torque, I did a demo for him wherein we ran his 1/2HP motor up to 400Hz with the VFD, a donned a welding glove and stopped the shaft with one hand. At that speed, the shaft torque was nearly zip (burned the glove though...).
 
Once when trying to explain this to a customer who was over speeding his motor and was upset by the lack of torque, I did a demo for him wherein we ran his 1/2HP motor up to 400Hz with the VFD, a donned a welding glove and stopped the shaft with one hand. At that speed, the shaft torque was nearly zip (burned the glove though...).

LOL! And so liveth the high-RPM zone of an induction motor.

Dare yah to try that on a "nominal" 3 HP large-frame Reliance shunt-wound creeping along at six minutes to the revolution, 300 poised-to-pounce volts at 24 Amps lying in wait if the DC drive sees the revs try to drop and "responds accordingly".

Or a comparably well-fed series-wound DC motor already seeking exit velocity to solar explorations .. ELSE a glorious death to splash its DNA about, whichever it can reach FIRST .... if it hasn't been well-tamed!

Motors, heaters, shaft-twisters, sirens, hand-grenades... must have all gone to different schools together?

:D
 








 
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