4775 RPM From 1750 Motor with VFD?

T

The Mighty Ginsu

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Aug 11, 2016
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My Oregon Blade Maker arrived. I ordered a drive wheel with it. I thought I was getting a 5" drive wheel, but it's actually 4".

To get 5000 FPM, which I am guessing is a good goal, I will need to make my 2 HP 1725-RPM motor run at 4775 RPM (I think).

Is that an unreasonable goal? It's a good quality US 3-phase motor. It's old, and it was free, so it won't ruin my life if it explodes.

The grinder looks very good.
 
You are actually looking for the belt speed, expressed as Surface Feet Per Minute. The calculation is: (diameter of wheel x 3.14) x rpm of motor/ = inches per minute. Divide that number by 12 to get SFPM. So your calculation is: (4 x 3.14) x 1725, then divide by 12= 1805 SFPM. Most VFDs have a jumper that will allow you to double the speed of your motor, in your case you would get a max belt speed of 3611. I don't believe you can go faster then that without a larger drive wheel. This will work for everything except really hogging out metal. That's my speed on my 72x2 fixed speed and I have had no burning desire to go faster.
 
I appreciate the help. The figure I came up with (4775 RPM) is based on 5000 FPM. The VFD I have will definitely go to 120 Hz; not sure if I can do better. I have to endure the misery of reading the manual.

Now I have another issue. The motor is really big. The grinder has two arm slots, one over the other. The motor comes up past the lower slot and rises half an inch above the floor of the upper slot. If I try to push an arm all the way back in the lower slot, it will hit the motor.

I guess what I'm wondering is, will I ever want to push a tool arm all the way back?
 
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I found the maximum frequency, and it's 400 Hz. I believe I'll need 166 Hz to get 5000 FPM, so unless the motor vaporizes, I think it's doable.
 
The only way to tell is to put both arms on with a tensioned belt and see if you need to go that far back. If it's in the way, I guess you could cut a bit off the tool arm tube, but then if you ever want to use a contact wheel, it may not be long enough. Watch this video. It shows a guy with exactly your issue only his motor blocks even the top arm. he just cuts off a few inches. You can jump to 6:30 for the fun part.

[youtube]hJdRJYtpiVM[/youtube]
 
Thanks.

I'm learning a lot already. For one thing, I see why people use small drive wheels. There isn't a lot of room within the belt loop, so if I had a 6" wheel to make life easy on the motor, it would make it hard to get the motor close enough to the grinder.

If I put the grinder on a raised platform, the upper arm will not be obstructed. It looks like the best strategy is to install everything on the same level, use the grinder, and see what I should change.

I was asking someone how big a platform I needed. In case anyone else has the same question, it looks like you can get by with 23" by 15", and that's with a big fat motor with a wide base. The 23" dimension is back to front.

Not sure what to do about dust and the VFD. I might be able to make some kind of box for it, with a hole in the bottom to pass wiring under the platform.
 
Here is what I'm working with.

09%2002%2016%20oregon%20blade%20maker%20mocked%20up%20with%20motor%20small_zpsybhov4k2.jpg
 
I would install a remote start stop and speed pot then you can put the vfd in an enclosure. If your vfd has a remote, you're good to go. Just make sure you make the enclosure with filtered ventilation or it may overheat.
 
On mine, i have to keep the gas shock with less than 1/2" on the piston for the best tension. This will pull your tool arm out a bit from where you have it. If i leave the shock mostly extended like in your picture the belts track terribly bad. But near perfect with less than the 1/2".
 
I'm thinking I may build a box, put the motor in it, and put the grinder on the box. It would raise it to a comfortable height, it would reduce the footprint, and it would kill any issues with the tool arms hitting the motor.
 
I dont know about that machine...
But, cant the motor be fitted to the other side of the machine ?
 
I just did basically the same setup. I bought a new 2 hp 3450 rpm motor fir mine. I don't know all the calculations but I programmed my vfd to stop at 60 hertz and it is plenty fast. Let me know if I can help at all. I set mine up with a remote power switch, forward/reverse switch, and stored control. I really liked the setup I have so far.

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Trying to get your grinder to go faster may cause you more problems than you suspect. I have tried various combinations of motors and drive wheels and the fastest I need is about 4000 SPM. I now use a custom made 4 3/8 inch drive wheel for about 4000 SPM tops and it is more than enough for any task I want to perform. Just my opinion. It is very easy to burn your steel with fine grits and high speeds. Larry

ofqB79qm.jpg
 
400 Hz is used for high-speed spindle motors (around 24,000 RPM) and the motors are designed specifically to run at high frequency/speed. They are not standard industrial motors.

For several decades, the motor and drive manufacturers have recommended running standard industrial 4-pole motors ( designed for 1750 RPM or thereabouts at 60 Hz on the rating plate) between 10 Hz and 100 Hz. The motors are designed to be run at 50 or 60 Hz and the electrical losses start to rise as the frequency moves out of the design range. We tend to have no real problem with the increased losses when we run up to 120 Hz, largely because the operator incorporates a very sophisticated control system (eyes, hands, brain, etc) that reduces load if needed. I'd expect the losses to be very significant at 166 Hz; certainly noticeable in use.

The motor will almost certainly have been designed for a maximum mechanical speed of 3600 RPM; the speed of a 2-pole motor running at no load on 60 Hz mains. It is the windings that make yours a 4-pole motor and halve the speed at any given frequency, and these are in the casing which doesn't move. Once you get above 3600 RPM, you are eating into the manufacturers safety factor and will certainly shorten the life of the motor bearings, if nothing worse.

I'd advise running with a 10-120 Hz speed range for a while and see how it goes. If you find you need more top end speed, you can then get a 5" or 6" drive wheel
 
One thing to think about while you're still in the building phase. The left side of the platen doesn't leave you much clearance. I spaced my platen out with 1x2x8 aluminum. Gives clearance around the platen and I think it keeps it a little cooler.

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Here is what I have at the moment. I saved some room by removing an unneeded VFD from my mill. I have another source of 3-phase power for the mill, and the mill VFD is smaller than the one I planned to use originally.

I set the VFD for a maximum of 167 Hz, but it scared me at 90. Of course, part of the fear was related to the fact that nothing is screwed down yet. Just clamps.

I can figure out the dust protection issue later.

Try not to be too envious of my tidy garage.

 
Failed: I am envious of your tidy garage. You have some visible floor!

Figure out the dust protection issue before you grind ANYTHING. I know at least 2 guys who have decided to just give it a test run before enclosing the drive and fried their drives. There are some pretty strong electromagnetic fields around the power electronics and they will tend to attract any airborne steel dust over some distance.

That looks a lot like a TENV motor to me. If it is, it's better than a TEFC for use with a VFD. The TEFC needs the fan on the shaft to provide cooling air. If you slow the motor down, you also reduce the cooling air flow and need to derate at low speeds. It's actually no big deal for "us" as derating can be done either by reducing the current by using less pressure, or by reducing the duty cycle.

A TENV motor has no fan and is able to dissipate the heat produced at full load without one. It can therefore run at low speed and full rated current indefinitely, assuming it's a continuous-rated motor.
 
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