VFD's, AC motors and a belt grinder

Hello,

Have read through a few threads here and it seems there's some quite knowledgeable folks when it comes to VFD's and motors, so I have question(s):

I endeavor to build a 2x72 belt grinder for my knife making. Of course, I want to make it speed controlled with a 3-phase motor and a VFD (3HP). What I'm not sure on is the "mode" to run it. I've done a fair bit of reading about VFD's and motors and I understand things for the most part, but that reading also states that grinders are best suited for a "constant HP" mode. That means the motor has to run at or above base RPM if I understand VFD operating principles correctly? Of course, by default, if you go below base RPM, then you are operating in a constant torque mode. I also understand that most "decent" VFD's (I'm looking at a KB Electronics KBAC series) will run down as low as around 30% to around 90% above normal operating frequency (60Hz in my case).

I guess my question is, would it even be feasible and desirable to set up the grinder to run exclusively in a constant HP regime, or would it be better to run things in the given range of whichever VFD I get and not worry about it?

I'm mainly concerned with two things: heat build up at lower RPMs over extended use periods and not bogging down the grinder under heavy use.

Your concerns - if you run the motor at low RPM, there isn't enough fan cooling and you could overheat. Size your pulleys to get the proper belt speed and keep the motor at higher RPM. As for not bogging it down, that pretty much comes down to having enough motor HP.

My questions:
Do you really need adjustable speed on a belt sander?
Why not just buy a grinder and add a VFD to it?

I have no idea why a grinder would prefer constant hp vs constant torque.

Adjustable speed is very handy, if not required for a belt sander in knife making. When you have to grind bevels on a large knife that is made from carbon steels that have been hardened, power and speed come in handy. When rough shaping, speed is handy. But, when you need to do fine work, slowing things down is important.

If we just want to be frankly or brutally honest, one doesn't even need a grinder to make a knife, but a stout one, with variable speed, is handy and time saving.

I didn't either, I kind of equated constant torque as close enough, but after reading about it and looking at the loading graphs, it makes sense in my head, I just have a hard time explaining it. The best I could tell you is that below base RPM, you are not getting full power output, sure, you get the same torque, but the RPM's are dropping, and HP is a relationship of torque and RPM. Lower one or the other and power drops too.

I guess there two options:

1. Get a lower speed motor (say, an 1800 RPM one), size the drive wheel such that the base RPM is the "speed" you want for your slowest setting, then only run at or above base RPM. That guarantees full power to the belt at all times, but limits the speed range by almost 70% (or whatever range above base frequency your VFD can send and the motor can handle).

2. Get a 3600RPM motor and run it via the range available from the VFD. That gets the slow end on target and puts the high end up there.

The rpm is going to be determined by the range of belt speed you need and pulley diameter. how much speed range do you need? That's going to determine where you need to be on the VFD.

If you take it down to 40-50hz on the low end and up to 90hz on the high end, most motors can handle that without complaining. 30-100hz could still be ok. I wouldn't see an issue with losing power at lower speed since you're not taking big cuts with it anyway.

another option if you need more speed range is to belt drive it and have a high and low speed pulley. two speed ranges, plus variable power would cover a wide span of rpm.

Understood, but turning the VFD down to 30Hz, is that going to overheat the motor after, say an hours use at that speed? I can't say that all slow speed work won't entail a heavier load on the machine. Certainly not full on hogging at high speed, but if I am trying to do fine work over a large surface area, that could still entail a fair load.

The speed range between 30-90Hz is plenty, but it's the power output and the heat that's the concern. I don't know, I keep reading and I start getting conflicting information. For example, I just read that both 2-pole and 4-pole motors are usually balanced to 25% over base speed of the 2-pole motors. Essentially, a 2-pole (3600RPM motor) is only good up to 75Hz (4500RPM). Still, 4500 RPM with a 4" drive wheel is a tad over 4700 surface feet per minute on the belt. That's plenty fast for the high end. The 1800RPM motor will run the same top speed (it can go to 135Hz), but will also have more torque on the lower end. It doesn't say, however, how well the 4-pole motor behaves below 60Hz. I would guess more torque equates to more heat, but is it smoother at sub-60Hz frequencies?

Not sure that even matters since a 2-pole motor at 30Hz is the same speed as a 4-pole at 60Hz (1800RPM). Looking at it from that angle, the 4-pole motor would seem to be the choice since when run at nominal 60Hz is the same speed as the 2-pole at 30Hz and both will achieve the same top speed. Effectively, the 4-pole can be run in the "constant HP" mode (at or above base RPM) and provide the same speed range. Arguably, it could also be dialed back even further into the sub-60Hz range for even slower, albeit constant torque range (and probably subject to overheating). Well, at least, if I understand everything correctly...my brain hurts.

I doubt you'd have an issue of overheating at 30hz. Are you going to be running for an hour non-stop?

135hz on the motor seems kinda high, but if it's rated for it at 100 duty cycle I guess it'll do.

If you're worried about not having enough power, oversize the motor. 3hp seems like a crap-ton of hp for a belt sander since most of the commercial ones are 1/2 hp.

Another option would be to call your motor supplier, tell them what you intend to do and let them recommend a product. They could explain all your tech questions as well.

Oh, 2 to 3HP on a 2x72 knife grinder is the norm. Commercial grinders run the same power too, at least those that are designed specifically for knife making versus a generic all purpose grinder.

I am building my own simply because I can use thicker steel (beefier) and get the power I want, the design features I want, for about 1/4 the price. The upper end commercial units run anywhere from $3,500 to over $5,000.

If I understand the 75Hz vs 135Hz deal, that's based on mechanical balancing of the armature and vibration (like balancing your wheel/tires on your car). I suppose being able to run the motor at 135Hz will depend on other things as well.

One problem with running an AC motor too slow is cooling. Fan efficiency goes way
down with decreasing speed. If you want to get full torque at reduced speed then
you might want to add an external fan that will add cooling when the motor is running
slow.

There are VFD rated motors that should have all the acceptable operating parameters
listed in the spec sheet.

Watch out with really old motors. Sometimes they have problems with running on a
VFD. The old insulation is not as robust as modern motors. The pulses that a
VFD uses to drive the motor can break down the insulation.

Understood, this will be brand new motor (inverter rated) and VFD.

Now to decide on brand/model of motor. The prices vary considerably across brands and models. I don't want the most expensive thing, but not necessarily the cheapest either.

I think I'm going to go for a 3600RPM motor and KB Electronics KBAC series VFD. This seems to be a popular recipe and by all accounts it works well.

I agree that variable speed is not really needed for a belt sander. But its yours, so you can do as you like.
Here is the way to look at the poles, a 2 pole runs with 60hz runs at 3450rpm, a 4 pole runs at 1750rpm, and a 6 pole at 1125. if each motor is the same HP say 2hp. then the torque for each would be 3.04ftlbs for the 2 pole, 6ftlbe for the 4 pole, 9.3ftlbs.

Say you want 1000 fpm belt grinder speed. the grinding belt pulley is 4"D, so the grinder shaft needs to turn a 1000rpm. so the motor drive needs to be 1:3.4 for the 2 pole motor, which increases the torque to the ginder shaft by a factor of 3.4, so motor torque of 3.04 times pulley ratio of 3.4 equals torque to the grinder shaft of 10.3ftlbs. and each motor when set to the same grinder speed, will be the same input torque to the grinder shaft.

if you purchase a inverter rated motor like a Marathon Black Max(have one) rated at 5hp 1800, but with a max speed of 5000. That gives you a huge range for speeds.

t3.04*3.4

In knife making, the 2x72 is the generally preferred grinder, 2-3HP with variable speed. It's not absolutely required, per se, but the power and variable speed makes things much easier and better control over the work piece.

Remember, this is for grinding and hogging off steel, both soft and hardened high carbon. It's not so much a "sander", but sure, put a high grit belt on it and turn the speed down and you shape the wood handle with it.

as i understand it, running the motor above the rated rpm where torque drops with speed can be beneficial, because you get more torque when slowing the motor with load. and a belt grinder needs variable speed for sure, up to 10'000 ft/min for a 36 grit belt and under 1'000 ft/min for a polishing belt.