VFD at low RPM

[FarmerDK]

Hello guys,

I'm building a vertical mill XYZ 500/300/300, and was planning on using 2 universal motors with feedback (straight from the washing machine).
The thing is that I would prefer a vfd and a asyncron motor, and I have a contact that buys this equipment at decent prices (although the original plan was for free). Now Ive been looking into some vfd litterature, and its a jungle for me, but I found a worrying statement that the slip at low rpm is high and jumpy, is this correct?
I made some calculations, and for milling steel with HM I would need around 2 kW, and I would like to being able to adjust the speed between 0 and 5000RPM. So can I mill with a big hss endmil at 200rpm stable with a standard vfd, or do I need some kind of feedback for this?

[neilw20]

1. VFD torque at low speed drops of dramatically.
2. VFD requires a 3 phase motor.
3. If you want a lot of torque at low speed you need a brushless dc (BLDC) motor and controller, or a servo motor.
4. Bothe VFDs and BLDs, can be run on single phase power at the power you require.

[RotarySMP]

with or without gearing?

[Al_The_Man]

As with commercial CNC spindles, they all practically have a pulse generator or encoder to enable high torque at low rpm.
Also some like WEG offer 100% torque down to zero rpm with their vector rated motor without fan or pulse generator fitted, but the cost tis not cheap.
Al.

[FarmerDK]

Thanks for the quick feedback!
I was hoping for a gear-free solution 0-5000, but if its not possible on a normal budget, then I would settle for 2-3 gears with 2000 intervals, for example.
I have 3phase in my shop, so no problem.

"1. VFD torque at low speed drops of dramatically." I've heard that somewhere before, and thats what worries me, but here is a document (in danish), where a figure on page 111 shows that the torque is 100% from f=0 to f=100%, and then drops when runnning faster. Thats how I got the idea to use a 2 pole motor n=2800@f=100%, as that would give me 5600rpm@f=200%. But what about the torque at low speeds, can it be done?

[RotarySMP]

Torque is constant, but HP= Torque x RPM, so if you use a 2 pole motor rated at 2hp at 2800 rpm, you are not going to spin a big face mill in steel at 200rpm. The VMC do this but they run at about 15hp ratings.

Edit
Before I get corrected, Power = torque x rps x 2Pi.

For any users of a base 12 system still out there, HP = torque (in something like footslugs) x rpm x some non recurring irrational number as a constant

[Al_The_Man]

Although a 3 phase induction motor is capable of high torque at low rpm, the problem is that the controlling element (VFD) does not really know where the best vector angle is WRT the rotor, especially when the rpm gets down low.
If a feedback device is fitted then the VFD can switch accordingly as it now knows the rpm (rotor position) and the rotor current (torque) and will adjust accordingly to maintain constant torque, an ordinary non-vector motor may need a fan if used in this manner.
Al.

[FarmerDK]

RotarySMP: Yes ofcourse, P=T*omega. I seem to forget the basic stuff, when trying to apply my knowledge in the real world! 15hp rating, Ill just go for that then! no maybe I could try and calculate the exact speed and power for the various job that I have in mind, and come up with a rating that fits my needs. If I do this, will I be able to go as low as 200rpm?

Al_The_Man: Feedback is always the way to go, but I can understand that its not the way my wallet wants to go!

With your help so far, Ill try and figure out a decent compromise between power and price, and if it doesnt match up Ill just have to make a gearing. While I type on my calculator, feel free to give me your tips! Thanks in advance.