Please help answer my VFD questions

Today I ordered a 5HP VFD from Dealers Electric. It's a Danfoss, I
think. New but surplus. $265.00. The best price for that size new that
I've seen. It goes as high as 1000 hertz . Boy, that would really spin
up the drive motor! It is constant torque. My questions are below:
1) How much faster can a normal motor be spun?
2) The mill has a mechanical vari-drive, Where should the pulleys be
3) Since the drive is constant torque does that normally mean it will
power the motor at rated HP at rated rpm but that when the frequency
goes up the torque goes down so the motor still produces it's rated
4) If the torque stays constant throughout the frequency range then
would it be desirable to adjust the mechanical drive so that the
original top spindle speed is reached when the motor is being driven
at about 120 hertz?
5) Since the motor will probably be running at 30 hertz or slower
some of the time it will have a cooling fan installed. What other
kinds of problems should I look for because of the changing frequency?
6) The mechanical vari-drive adds drag to the system so the power
available at the spindle is less than the power put out by the motor.
I have been told vari-drives of this general size waste about 1HP
through friction. But that would make the spindle drive a 750 watt
heater and it doesn't seem to put out that much heat. Do they really
have those kind of losses?
7) If the mechanical vari-drive does consume that kind of power I will
consider replacing it with a belt drive. Would it be worth it to use a
toothed belt instead of vee belts to reduce friction and the radial
load on the motor and spindle bearings?
8) Will a toothed belt tend to add vibration to the drive train?
Thank You,
Eric R Snow
Reply to
Eric R Snow
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Often, standard motors are called upon to run at ~ twice the rated speed, probly the better retrofits are balanced to a higher standard and higher grade bearings are installed.
A tall, skinny motor would probly be a better choice than one having a larger dia rotor, as centrifugal force could be an issue here---you dont want the rotor to fly apart.......
Remove any internal cooling fan within the motor, and install a large muffin fan on the shroud so as to keep airflow going through it at all times regardless of the shaft speed.
Suggest filter or at least screen off the cooling intakes and exhaust of any motor you would use on a machine tool so chips cannot enter it and cause shorting or other damage.
Reply to
Probably I would not take a 3600 RPM motor much above its rated speed, but I would risk taking an 1800 RPM motor of the same family up to 3600. The reason for this is that they tend to use the same rotor in the whole series, so you know that it is already rated for the 3600 RPM operation -- 120 Hz with a four-pole 1800 RPM motor, and perhaps even 180 Hz with a six-pole 1200 RPM motor -- though I would be more worried about that one. Though I would expect all of the rotors to be balanced for the worst-case speed, which is 3600 RPM on 60 Hz.
And note that your constant torque is *not* the same as constant horsepower. Ideally, you want o keep the motor running not too far below perhaps half speed, if you want to get serious work out of it. For this reason, plan on still using the variable speed pulley system some of the time, though you can probably set it for a comfortable range with the VFD when doing high-speed work down through light low-speed work.
The higher frequencies would be useful for things like driving aircraft electronics and motors -- which are designed to operate at 400 Hz. The motors *might* even survive up to 800 Hz, but I would put them in a blast shield before trying. I wonder what my old artificial horizon would do at 800 Hz -- and for how long? :-)
Amen! That is why you want to check whether the maker produces a 3600 RPM version of the same motor. If so, you have pretty good odds that it will survive the 3600 RPM operation from boosted frequency.
Agreed -- the internal fan will probably be nothing but a drag on the cooling air at the lower speeds, and perhaps an additional drag on the motor at the higher speeds.
Right! While there are not raw contacts in there like the centrifugal switch on a single-phase motor, a chip being dragged around by the rotor could cut through the enamel insulation on the wiring, and lead to shorts there.
Good Luck, DoN.
Reply to
DoN. Nichols
Hi, DoN.
My Basis lies in the Fadal machining centers, appears they are using a ( modified ) Baldor standard motor rated at 3600 and taking it up to 7500 reliably via better ( perhaps dual ) bearings at the driveshaft end.
It appears an entirely new bell is fabricated by Fadal at the drive end of the motor for this, and I am only assuming they hopefully have better balanced the rotor shaft.
I might give a rough check of the whole system again when I have time and at least estimate the belt ratio and give specs on the motor.
The OP sat a while, and rather than let it die altogether, it looked like a good place for me to set out some bait.....
Reply to
150 % (90 Hz) is pretty safe in any situation. I would start worrying at 200% (120 Hz). A 4 or 6 pole motor (1725 or 1140 RPM) can probably be revved up more (percentage-wise) than a 2-pole (3450 RPM).
VFDs typically provide rated torque from 0-60 Hz (voltage proportional to frequency), and then constant HP (voltage constant at nameplate value). You can program them for other settings, consistent with motor characteristics.
This is likely to burn up your belts and other varidrive parts a lot faster, if you do this a lot (running the motor at 200% RPM).
Yes, at high speed, I think you'll find it DOES get hot. A full HP may be exaggerating it, but under heavy load the losses might get that high. Idling, I don't think it will be that bad.
I know the step-pulleys and belt on my 1J head get pretty hot at high speed, too.
What model Danfoss drive is this? I am interested as I have an air-bearing drilling spindle and a milling spindle that can use power up to 1000 Hz and beyond. Right now I'm limited to 400 Hz with my current VFD.
Reply to
Jon Elson
DO NOT TRY THIS! A guy at the local Rockwell (I think) service depot was killed a couple of years ago. The newspaper article was not too clear, but I'm guessing he hooked a 12V gyro to 24V, and ran it up to 2x rated RPM. The gyro wheel exploded and killed him!
Reply to
Jon Elson
Since no one answered this one..I will. The vari Drive should be set to as close to 1:1 as visually possible. Mid range..the belt should be in the same place in both pulleys.
I repair and install CNC retrofits and machines for a living. Typically the motors are a 1725 rpm and they are regularly taken to 200% or 3450 rpm. Many are taken to the full 4000 rpm. In 8 yrs, Ive yet to see one fail due to over speed. I have replaced 2, that were run too slow and overheated for years on end, and finally failed.
These are standard well made industrial motors, some over 40 yrs old.
You have a huge advantage with your vari-drive over a vfd operated with with fixed pulleys. You can dial the vari-drive down to very low speeds, while still retaining the mid range power rating of your vfd.
Example..if you run the motor at 100% (60 htz) but want to tap at say..25 rpm, simply reduce the speed with the vari-drive to as low as it will go, and if its not slow enough..THEN drop the VFD freq to whatever you want the spindle to turn. And of course..the same applies to high speed. If you you run the VFD at 200%..and the lathe with the vari-drive at 1:1 turns at 2000 rpm, you can raise the speed with the Vari-drive. Just be advised that most lathe bearings in modern lathes are not rated for more than 3000-4000 rpm, and will die much keep the high end at the max 3000 rpm and save your vari-drive usage for ultra low speeds. This allows the VFD to still turn the motor at a reasonable speed with adequate torque and cooling.
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Reply to
A number of the inverter duty motors I have used lately have maximum speeds on the nameplate. (I've spec'd, purchased, programmed, and operated over 30 vfd's in the last 3 years, mostly lower power, 2 to 25 hp.) Some motors were as high as 4300 rpm, some lower. Some of the pumps and mixers I use have stated maximums as well. Generally, I won't program the vfd's for above 120 Hz, often less.
The electrical engineer with whom I consulted asked me to program the plc emergency shutdowns for a maximum of 4000 rpm. This engineer has worked in the plant for over 30 years, and said he had seen a failed rotor due to overspeed.
Pete Keillor
Reply to
Peter T. Keillor III
Thanks to all who have responded so fast. And Jon, when the drive arrives I'll let you know the model number. Maybe Dealers Electric has another one of the same model. The salesman said this particular drive had just come in. Eric R Snow
Reply to
Eric R Snow
Don sez:
Yeah, right on! It sounds like a firecracker going off inside the motor. Bob Swinney
Reply to
Robert Swinney
Usually, about twice the nameplate RPM, but a lot depends on the size, balance, and construction of the particular motor.
Dead in the middle. That lets you reach the original top RPM with the motor at double speed, and lets the motor still turn at a good speed at lower RPMs. (But see below.)
I think that the constant torque characteristic only applies when you slow the motor below its nameplate RPM. If this weren't the case, you'd get twice the nameplate hp at twice the rated RPM, and that would cook the VFD and/or the motor windings. Note that a constant torque characteristic means the hp will decline with declining RPM too. So you won't get 5 hp at lower RPM.
At higher than nameplate RPM, the motor should exhibit constant *power*. That's fine, and so yes, you want to set the vari-drive to the middle of its range.
Increased hysteresis losses. In other words, the iron poles will get hotter.
I wouldn't replace the vari-drive unless you intend to constantly be using the machine right at its hp limit (unusual in a hobby shop), or unless the vari-drive is shot. Especially for very low speed work, it'll still make sense to adjust the vari-drive rather than run the motor at very low RPM. Start thinking of the vari-speed as a *coarse* speed adjustment for your machine, and use the VFD as a "fine" speed adjust.
Note that you should change the vari-speed setting every few hours of operation. Otherwise it will tend to freeze up, or wear excessively at the one setting.
Reply to
Gary Coffman
If this is a manual mill(nonCNC) in a hobby shop, the duty cycle will be so low that you need not worry about low speed motor cooling. Even though you run below 30 Hz, there will likely be sufficient cooling anyway because of low actual motor loading. I have a J head that has the original pancake style, 1 hp, motor running on a VFD for 9 years with zero problems.
Reply to
Randal O'Brian
This machine is not in a hobby shop. I make my living as a machinist. At least that's what I call myself. So this mill gets worked a lot. And changing the speed every so often is a pain. It does have a low gear so if the speed needs to be really low. ERS
Reply to
Eric R Snow
Note the smiley following the "and for how long". I have *no* intention of destroying this instrument by running it at double speed -- and if I *were* to consider it, I would want a good blast shield around it.
Enjoy, DoN.
Reply to
DoN. Nichols
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This is a factory-modified motor, not a stock motor, and I'm sure that they have tested it at the higher speeds before putting it out in service.
And have tested the rotor at well above the top-end speed, just to make sure that it won't fly apart.
O.K. But *don't* try a plain un-modified 3600 RPM motor at above frequency -- unless it is sold for VFD duty, and certified to handle the higher speed without problems.
Enjoy, DoN.
Reply to
DoN. Nichols
Ok, if you want run the machine near hp capacity, and get rid of the vari-drive, then you need to install a bigger motor. That's because the constant torque characteristic of the VFD means effective motor hp, and thus ability to move metal, will be reduced at reduced speeds.
Reply to
Gary Coffman
I have a rockwell 11 inch lathe with a VS drive and VFD. I leave the VS set to about 500 RPM which is a little below 1/2 and use the VFD for most of my speed changes. I can go down to 100 and up to about 1000. Like gunner says, use the VFD if you need very low speeds, other wise I leave it about mid range.
Reply to
Charles A. Sherwood

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