Need motor advice for surface grinder

I don't claim any working knowledge of VFDs. I did look into them a few years ago when I was hooking up my 3 phase horizontal mill. I eventually decided on a static phase converter, which meets my needs.

One of the claims made about the VFDs was that the current they supply is not true 3 phase, but has some harmonics or secondary oscillations which can cause problems with some older motors. This seems to be rare, but might be your problem.

Before you dump the motor, you might try driving it with a static or rotary converter.

John Martin

John - that thought hadn't occurred to me. The noise on startup seems indicative of bearing problems, though, so I'll check that first as soon as I get the current project off the work bench so the motor can be looked at.

The motor installed (GE K163) still seems to be in production so it may be OK with a VFD. It looks like it was a recent addition on the grinder, though, and that is part of the reason why I'm asking about brands for potential replacements. Maybe it's just not a good motor for a surface grinder.

Mike

This is indeed quite true. Until I got my 5hp rotary..I was running all my (then, very few) machines on a single 5hp VFD. Oddly enough the old Clausing 6525 would leave a slight herringbone pattern on one direct drive speed setting. I had assumed it was something worn in the headstock. When I went to the RPC..the pattern vanished. I then swapped back and forth and indeed, it was the VFD. None of the other machines exhibited this phenom, including the surface grinder, which gives the same finish either way.

A cautionary note...if you were to power up your surface grinder via vfd,..be damned sure..that you set the maximum rpm rating to less than that of the max rpms of wheels. IRRC 3600 rpm. It would be very possible to over spin a wheel and have it catastrophicly disassemble itself..which will result in a grenade going off on your grinder.

Gunner

"To be civilized is to restrain the ability to commit mayhem. To be incapable of committing mayhem is not the mark of the civilized, merely the domesticated." - Trefor Thomas

Odd - I haven't seen that on my VFD-powered 5914, but then I rarely use the VFD for speed control and it's almost always at 60 Hz. Does the 6525 have the hydraulic vari-speed system (like the 5914) or step pulleys?

Good point - I leave the VFD at 60 Hz and only use it to convert single phase power to 3-phase.

Mike

Actually, VFD's do provide true 3-phase. While the phase voltage waveform has high harmonic distortion, the phase current waveform is sinusoidal, with a little ripple(not enough to matter); the current waveform is what counts. I think what you saw was a mechanical resonant condition excited by a small unbalance. Because motor rpm can be infinitely varied using a VFD, resonant conditions can show up when the speed is just right. Using a step pulley or even a mechanical variable speed drive can completely miss those resonances, especially if a single-phase motor is used. I have one on my

13x36 lathe (at about 600 rpm(spindle) that was invisible using the original single-phase motor because the none of pulley steps were precisely the right speed. When I switched to VFD drive, it appeared on one particular pulley step @ 40 Hz drive output.

Randy

To me, it sounds like a mixture of problems. The standing waves are most likely from an imperfect balance of the combination of the motor and the pulley.

The noise during startup could be bad bearings -- or it could be the vibration from the imbalance passing through resonance with some structural feature of the grinder.

Since you have a VFD, try starting the motor very slow, and stepping the speed up a little at a time. If it is a problem with balance and resonance, you will find one speed at which it is particularly bad.

When you find this speed, start touching various parts of the machine to see where you can damp the vibration. If you can't damp it with pressure anywhere on the machine, then it is more likely to be the bearings.

However, if you do find such a speed, you might try removing the pulley (and its key, to keep that from flying away at speed). If you have similar vibration, the motor's rotor is poorly balanced. (Though the absence of the key will introduce a bit of imbalance.) However, if it is much better without the pulley, then the pulley itself is the primary candidate for balancing. Ideally, the pulley and motor rotor should be balanced as a unit, probably after doing as good a job as possible on the motor's rotor first. I believe that a proper job of balancing calls for specialized equipment.

It is bad news to run a motor under load at low speeds from a VFD for very long (without augmented cooling from an external fan), but for the time of this test that should be within reason.

I suspect that you can get many grades of motor from *any* maker. The trick is finding someplace to get ones of the quality which you need from whatever maker you choose. Most GE motors are sold for appliance applications, or machine tools where the balance is less of an important factor, so you may have to search more to get what you need.

Trace down where the vibration comes from. It may be that the pulley was balanced for a different motor, and simply needs to be re-balanced for this one.

Since vibration in the surface grinder can negatively impact surface finish, I think that I would consider it to be more important than for a big horizontal mill.

Run the tests which I suggested, and go to them if balance of the motor/pulley assembly turns out to be the problem.

Good Luck, DoN.

Nope..its a gear head with a two speed motor and 4 speed gear box. I believe the top speed is direct drive.

The 1501 I replaced it with is hydraulic vari-drive, with 3 gear box speeds, the top speed being direct drive.

Gunner

"To be civilized is to restrain the ability to commit mayhem. To be incapable of committing mayhem is not the mark of the civilized, merely the domesticated." - Trefor Thomas

I have a Rockwell "toolmakers" grinder which is a cross between a surface grinder and a tool/cutter grinder. The manual states that the motor is a special balance and is also dynamically balanced with the pulley in place. They stress that changing the motor requires the motor and pulley be rebalanced together.

Now, does that still apply to motors made today? I thought I read here that today's motors are much better balanced than the motors of yesteryears; even to the point that production motors are as good as specially balanced motors of the past. I have no idea if this is true or NOT.

I will say that the old Rockwell motors on my mill definately have annoying resonances when ran at certain speeds with a VFD. The worst one is the 1/2HP 1140RPM motor on the rockwell vertical head. This motor has numerous resonances with just the pulley. However it runs very smooth at high RPM.

My new VFD rated 2HP Leeson motor is perfectly smooth at all frequencys with or without the belt running the spindle of the mill/drill. However I have also have a 2HP leeson motor on my rockwell lathe and it is smooth except below 15Hz where the drive system has some kind of resonance. I believe this is a resonance in the big flat drive belts used for the mechanical variable speed and it not directly related to the motor. The drive belt is difficult to remove so I have never tested this theory.

I have also noticed that older motors seem sensitive to carrier frequency. Some of mine are quite noisy with low carrier frequencies. I have not experimented to determine if the resonances are different with different carrier frequencies. I try to use a low carrier freq with older motors because I THINK high carrier freqs can cause motor heating and I think the high freqs stress the winding harder. The leeson motors run very smooth and quite with a high carrier freq (7.5-10K) There is more noise from the motor with lower freqs but I don't remember any resonances when I was experimenting in the past.

Mike, I have a static phase converter if you want to try running your grinder with it. chuck

Well, I tried your suggestions and am a bit perplexed. The gradual ramp up revealed that the vibration (or rather, wave amplitude) was worst at about

51 Hz and somewhat smoother at 60 Hz. Applying force at various locations didn't seem to make a difference until I started to fiddle with the motor mount. The mount has a rigid plate cantilevered off the back of the column to which a hinged plate is attached. The motor mounts to the bottom of the hinged plate (motor is upside down) and there is a spring-loaded bolt to adjust the angle of the hinge, and presumably the belt tension between the motor and spindle pulleys. The standing waves all but disappeared as soon as the adjusting bolt was, er, adjusted. No amount of re-fiddling could bring the standing waves back.

Put the belts back on and cranked up the grinder to 60 Hz - no standing waves. A test grind with some 12L14 and a blue 5sG46-IVS Norton wheel produced passable results.

Tried to call KO Lee today but got put on automated hold and gave up before a live voice responded. I'll try them tomorrow and see what they say about motors. I also need to find out what sort of tension the belts are supposed to have.

That makes sense, but it will give me a good excuse to call and see what they have to say about motors and balanceing.

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Hmm ... perhaps the belt was too loose at first, and flapping around at the speed in question. Can you find a new resonance point?

Or perhaps there was just enough give to make for resonance at that speed, and clamping a weight on the plate would shift the resonance point.

It sounds good to me.

FWIW -- the manual for my Sanford grinder, which uses a flat rubber belt says the following:

"Belt tension should be such that the belt between pulleys may be compressed to within one inch of touching. This is rather loose, and should be so."

I don't know how this may apply to your machine, as it is a different size and a different manufacturer.

Good Luck, DoN.

I've no clue on what changed unless maybe the grease in the motor bearings suddenly re-arranged itself for more complete lubrication. The grinder was bought used from a dealer and I don't know how long it sat before it arrived here. It's had about 10 hours of run time since arrival.

KO Lee tells me that this motor (GE K163) is the one that they installed on surface grinders (although they now specify 2 HP instead of the 1 HP on mine), that they do not specify any special sort of balancing and that the (2) belts should be adjusted so that there is about 1/8" "give" when pressed from the side. The tension was a lot less in my tests the other night so I'll play with the tension a bit to see how that affects things.

The tech support guy seemed pressed for time and was not interested in any kind of detailed answers, so I'm not feeling any better informed than before.

Mike

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Or perhaps some swarf was cushioning the support, and that was displaced when you adjusted it.

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Well ... at least you now know what the tension should be for your machine. Are those flat belts, V-belts, or something else, like perhaps poly-V belts? It sounds like V-belts just offhand.

Good Luck DoN.

After going over lots of machine tool manuals over the years..the

1 1/8 - 1 1/4" gap seems to be pretty common.

Gunner

"Gunner, you are the same ridiculous liberal f--k you ever where." Scipio

Didn't mean to imply that the output from a VFD is not true three phase in the sense that the output from a static converter is not, but just that the distortion or ripples cause - in the opinion of some experts - problems with some motors. No more, no less.

John Martin

I'm not sure what to call them - they look like a truncated V in cross section and the periphery is serrated. There are two individual belts so I guess that they are V-belts. A brief Google search indicates that Poly-V belts are like two or more V-belts joined along there edges into a single belt. Is that right?

While checking the belts I noticed that there is some light rust on both the motor and spindle pulley grooves. That's probably not good for a grinder. I wonder if cleaning them up with some fine steel wool or a 3M abrasive pad would help or hurt?

Mike

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Yep! Some are notched and some are not. The notched ones do a better job bending around very small pulleys -- but might contribute more vibration in your application. Check how flexible they are. It might be that they have taken a permanent set from aging unused. In that case, new belts might be a good idea.

Correct -- and the grooves are a lot smaller. These are common as fan belts in modern vehicles, while the V-belts are found on older ones.

Flat sides (not sloped) along with the serrations are timing belts, and engage pulleys like gears.

As long as you don't remove more than the rust, it will probably not do much one way or the other. Running for a short while will remove the rust where it matters.

Good luck, DoN.