VFD recommendations

[Sunset Machine]

Timely thread- I'd like to put a VFD on an old metal planer. It runs at 30 FPM and I'd like to slow it down to a more grandfatherly-like speed of 20 FPM, yet retain the higher speed. The load is sudden; cutting air for a few inches and then wham, into the work. The VFD should respond to it and maintain a steady speed.

240 volt, 2HP, 3 phase motor and a 240 v single phase supply, currently running an RPC. Is this the one? http://dealerselectric.com/FM50-202-C.asp

[BadDog]

Just guessing, but if all the original mechanism is still in use, the flywheel effect of the big iron wheels should be more than enough to keep things smooth. Mass and inertia, just like it did when driven by line shaft (I expect?). But if you've converted to a minimal direct drive, that could be a problem. If so, my first thought would be to try to get the flywheel effect back. Beyond that, cheap VFDs usually don't respond to load, so will not keep a constant speed under increasing load. The next step up would be a "sensorless vector" VFD, which uses current feedback to try to address the issue. Then there are paths beyond that you get into servos and complex drives that run CNC spindles and designed specifically for those type of concerns. But I think that is far an away beyond the needs to run a planer.

[Sunset Machine]

It's the inertia that's causing the problem, right when the 1000 lb table changes direction and begins the cut. It returns at 60 fpm and slows to 30 for the cut. The simplest solution seems to be just slowing the machine down, and it sounds like a VFD isn't the way to that without some sort of feedback. Sticker shock will probably have me looking at pulley's instead... Thanks for the heads-up.

[BadDog]

Mechanical advantage might provide the solution, but what I was talking about regarding mass/inertia was in the old drive system. I know they made hydraulics and others, but every planer I've seen ran off a line shaft system with a belt throwing reversing mechanism that would give OSHA inspectors an outright heart attack on the spot. Those relatively slack flat belts could handle the reversal and cut loads with ease mainly (I think) because of the massive iron wheels and the provided flywheel effect. And that's what I was getting at here. If you put a VFD/moto running the original driven wheel (appropriately geared to match original input speed) that took power from the line shaft wheel, then the rest of the mechanical system, including energy storage, should keep things working as smoothly as ever.

But most of the conversions (on lathes, shapers, etc) I see attempt to get rid of that whole bulky complex system putting the drive as close to the final drive mechanism as possible. But that eliminates a lot of benefits, and may be the source of your problem? From a different angle, on my old lathe which had a "Reeves Drive" that needed rebuilding, I initially wanted to get rid of all that mess and save the rebuild by putting in a direct drive (but keeping back gear) using a VFD for speed control. I knew I would need a larger motor than OEM, but was surprised when I did the calculations and it turned out I needed to replace a 1 HP motor with a 5 HP motor just to break even on capability (with a little more rpm range). And if you look into the cost of 5+ HP VFDs you'll see why that plan ended quicker than it began. Losing the mechanical advantage can be more than you might expect, and losing the mass of the original planer drive may be doing something similar here.

[Sunset Machine]

Yes, the motor is mounted up high and the machine driven through a single belt that shifts from pulley to pulley. The problem occurs when the table reverses from its "high-speed" return. The inertia causes a BANG! that makes the whole machine shudder (it's on legs) and can damage the work piece if the cut begins too soon. I should have explained this. Inertia won't carry it through the 5 foot cut, and all those gears/pulleys reverse too.

Without feedback, the table will return at a VFD-reduced speed but when the cut begins I'm afraid the load will slow the cutting speed more than it does the problematic return speed and accomplish little else but slowing down the job. Unfortunately I have no experience that would predict the effect and would have to simply toss money at it to see if it works. I'm too cheap for that.

[BadDog]

I understand, good luck getting it sorted.

But I would keep coming back to it having worked acceptably (one would expect) back when it did exactly the same while running on a line shaft. If everything is the same except for a dedicated motor that replaces the line shaft, then my focus would remain on what caused it to change for the worse. I really don't see this as a technology problem. If nothing else changed, then why can't it work at the same speeds it once did? Is there wear or something else contributing? Maybe the line shaft just had so much inertia within it's own system that it carried the day? If so, then a larger motor (perhaps run by RPC) might get you there? My focus would be in those directions. Once it works in general at the speeds it was designed to work at, only then would I be looking to see what I might gain by adding a variable speed drive.

I'm very envious of your planer. Somewhat like a shaper, i love watching those things work, particularly the reversing belt throw.

[Sunset Machine]

It originally ran High Carbon tools at 20 fpm, which I'm now calling Plan A (smaller pulley). Someone has "hot-rodded" it to 30, which caused the 4000 pound contraption to walk across the floor when I first fired it up.. The legs sit on a stack of shims.

[BadDog]

Ok, now I get it, that's the piece I was missing. Since it's (likely) never worked in this configuration, all of my assumptions and suggestions are unfounded.

In that light, assuming configured to allow a little extra float time at the ends of travel, I wonder if VFD ramping could deal with some of it? For instance, if I'm tracking on all this, you could configure it to handle the desired cut speed using mechanical gearing to provide the desired/required characteristics, and allow the motor to drive it at 60hz. Most of these old machines used mechanical gearing systems to provide differentially faster return than the cutting speed. But if for whatever reason that is not getting you where you want to be (for instance, you needed to eliminate the mechanical reversing system, just for discussion) , you can overdrive the return up to around 100% on most VFD compatible motors, but when over driven power is constant, which means torque drops off. But you've still got to accelerate the mass of the table which may be a problem trying to accelerate (positive or negative) too fast. Braking resistors can be used to help stop quicker, but ramping reduces the power demands as well. So what you do is configure it to stop over N seconds, and start by accelerating through M seconds. It would require enough over-run to allow for N and M seconds of acceleration, but once you've got it moving, the mass in that table should keep things moving smoothly in the cut as long as you have enough power to manage the (calculated) metal removal rate. There will be some small amount of bog down, but probably not enough to matter. Of that turns out not to be the case, sensorless-vector drives aren't that expensive, but their importance is revealed most often when trying to slow down below 60hz. When slowing down, you loose torque as frequency and speed drop, so obviously that presents real serious problems when trying to maintain speed. The smoother ramping should also have a dramatic improvement in the tendency to walk. And I suppose you could work out a way to keep the existing reversing mechanism AND integrate the VFD with ramping using micro switches to remotely control the VFD, assuming suitably complex control circuits are available (basically allow frequency switching with ramping on transitions without reversing the motor).

I have no idea if that adequately addresses your concerns, but perhaps something in that mental tail chasing might produce an idea (or subsequent post) that proves useful.

[Magicniner]

If you use a VFD with the facility for a 0-10v potentiometer speed control you would be able to use a cam on the table, actuating the pot, to decelerate the table at the ends of travel to render reversal very mild whatever speed cut or reverse is set at

- Nick

[Sunset Machine]

Clever. You guys also have me thinking that the 30 could stay, but just reverse the motor and return at 30 fpm too. For that matter, up it to 40 fpm (the designed return speed) and have an even greater "hot-rod" speed on the cut. My math says that it won't harm production and the machine will behave better:

1 foot travel. 30 fpm one way, 60 on reverse = 3 seconds
1 foot travel. 40 fpm and 40 fpm = 3 seconds

[Patio]

I VFD can be setup to run at several different speeds. One only needs trip a switch to change between the different speeds. Travel, trip a switch, go through reverse, trip the switch back to high on the return, back to cutting. Just a thought.
I use a vectorless Hitachi drive on my mill. Cazenuve HBX 360 lathe with a VFD and have installed a few on conveyor belts for speed control.

VFD are becoming more prevalent it industry. We even install a VFD controlled well controller for a residence.

I do work at a tree nursery that uses one on a 100hp well, they no longer break lines and only water the fields they need to now.

Good luck

[Sunset Machine]

Good to know! Do you know if this Teco unit is suitable for doing that? I'm having trouble sorting wheat from the chafe. http://dealerselectric.com/FM50-202-C.asp