Disc grinder RPM advice

Matt Browning

Matt Browning

Joined
Oct 7, 2010
Messages
41
I'm planning on building a disc grinder pretty soon here, and have seen in previous threads that 3450RPM is way too fast a motor to get. I'm planning on hooking whatever motor I get up to an AC drive, so I was wondering, if it's on an AC drive, is 3450RPM still way too high? I know 1725-1750 seems to be the recommended RPM for both belt and disc sanders, but occationally I find pretty good deals on 3450RPM and 1150RPM motors. Any comments on why or why not the faster OR slower RPMd motors would work is appreciated.
 
From what I have been reading a disc grinder running too fast can be dangerous if even sightly out of balance. Also faster surface speed means faster mistakes. I have received some advice that 3450rpm can be used with step pulleys on the slower end effectively. I may build another one soon, though. I find running a 9" disc at 1560rpm feels about right to me as a beginner.


-Xander
 
If there's another way to effectively slow a high speed motor than a VFD, I sure haven't heard of it, and I've researched that one out pretty hard. You'll just fry your motor putting any load on it with an AC drive, and disc grinders are definitely high load applications. A full 2hp motor on a 12" disc is not too much, but there isn't any point to running a disc at 3450rpms ever. You'll cook your work piece in a heartbeat.

If you are going to have a tool like that, it's a lot better to build it right the first time than have issues to worry about every time you turn it on.

Step pullys on a 3450 rpm motor are ok, but certainly not ideal. You loose a lot of power cutting the speed in half right out of the chute. Even if you are running a 1750rpm motor, step pullys are nice.

Guess I haven't seen an 1150 rpm motor.
 
Ignoring the minor loss due to friction, the mechanical advantage of using pulleys to reduce 350RPM to 1725RPM is double...not a loss. The total power remains the same, but the torque increases by two as the speed decreases by the same factor.

1150 RPM motors are usually constant load fan motors, and would probably work, but are not the perfect motor for the job. They also usually have open frames, IIRC.
 
1150 rpm motors are fairly common in commercial and industrial applications. For instanse a low rpm pump (1150 rpm) is much quieter than a higher rpm pump. An 1150 rpm motor should work ok.
 
Thanks for the replies guys... yeah, after thinking about it, it seems weird/stupid to hook a motor up (3450RPM) that could only be turned up to a max of 50% on the AC drive. I'll likely go with the traditional 1750RPM.

As I browse ebay more, I'm beginning to think I might actually go with a DC setup. It looks like you can find NEMA enclosed DC drives for half or less of an equivalent AC drive, and though the DC motors are a little harder to come by, it seems like affordable ones can be found.

I appreciate all the input, I'll be posting pictures of the grinder once I get it built.
 
Matt Browning said:
Thanks for the replies guys... yeah, after thinking about it, it seems weird/stupid to hook a motor up (3450RPM) that could only be turned up to a max of 50% on the AC drive. I'll likely go with the traditional 1750RPM.

As I browse ebay more, I'm beginning to think I might actually go with a DC setup. It looks like you can find NEMA enclosed DC drives for half or less of an equivalent AC drive, and though the DC motors are a little harder to come by, it seems like affordable ones can be found.

I appreciate all the input, I'll be posting pictures of the grinder once I get it built.
Click to expand...


DC Drives are cheaper, DC motors are not cheap and not common.

If they are posted, the people who play with windmill generators snap them up.

Buy a package deal, or the motor first.
 
Thanks for the replies guys. I have decided to go with an AC setup, and while the going has been slow, I have managed to find a cheap Minarik VFD on ebay, and am getting closer to finding a motor. There is a Leeson on ebay I am eyeing right now, a very nice motor at a good price, but the RPM is 1140. It SEEMS like this might be fine, as the difference between 1140 and 1760 vs. 1760 to 3450 is much closer. Thanks in advance for any advice you're willing to share.
 
PS - I'm aware of Tru Grit's 1140 rpm machine, so I know it CAN be done. My question is more: does anyone have experience with a disc grinder running at 1140 rpm that they thought was too slow?
 
Matt Browning said:
Thanks for the replies guys. I have decided to go with an AC setup, and while the going has been slow, I have managed to find a cheap Minarik VFD on ebay, and am getting closer to finding a motor. There is a Leeson on ebay I am eyeing right now, a very nice motor at a good price, but the RPM is 1140. It SEEMS like this might be fine, as the difference between 1140 and 1760 vs. 1760 to 3450 is much closer. Thanks in advance for any advice you're willing to share.
Click to expand...



I see discussion here of rpm, but not of Horsepower

In a quick search, this is the only one I find by that name.

Is this it?

MINARIK VFD VFD01-230AC
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=350424592670


If you read the manual that model # is a 1/4 hp motor rating
http://www.kinecor.com/documents/catalogues/minarik/MDManuals/250-0348.pdf


I think you may find that too easy to stall.
Maybe 3/4 or 1 hp will be better & of course more $


Are there any other opinions on which HP is appropriate?
 
I've got a 3450 rpm AC motor turning a disc on a belt/pulley. The drive pulley is 1.75" and the disc pulley is 6" If my math is right 3450X1.75 / 6 = 1006.25 rpm. I like that speed for most stuff. I may get a 10-12" pulley for the disc side so I can get in the 500-600 rpm for some stuff.

I'm not the engineer here, but like Stacy said, when you slow down the speed by pulleys, the torque increases. Think of your 10-speed bicycle and climbing hills.
 
The specs on the Minarik I recently purchased is here: http://www.alstron.com.sg/PDF/Minarik/250-0379r1-VFD-PCM.pdf

The unit is nearly identical to the KBAC-24D, the AC drive that Beaumont Metal Works and Wayne Coe sell. It's rated to 1hp and is in a NEMA waterproof housing, plus it has a reverse switch. It's a nice VFD because like the KB drive, this drive inputs 115 single phase power. The price was right too: $100 including shipping, quite a bit less than the KB drive.

Since the Minarik drive can accomodate up to 1hp motors, and I'm not interested in having an underpowered setup, I'm planning on buying a 1hp motor. In other disc build threads I've seen guys using washdown motors, so I thought to myself, if I could find a decent deal on one, I'd go for it. The motor I am currently looking at is a stainless washdown 1hp Leeson, which like I said earlier, is 1140 rpm.

The nice thing about the motor, in addition to being washable, is that it has a footed base and a C-face as well. Ken Coats kindly shared some pictures of his disc setup in a previous thread where he has mounted the motor to a grinding pedestal using the footed base, and had used the C-face to mount a bracket that a grinding table was bolted to. I plan on mimicking his design. I will fabricate the table myself and buy a disc from RW Wilson per Ken's recommendation.

So because I am using a VFD, I will not be setting up the disc grinder on any pulley setup. This definitely means that for this particular way of building a disc grinder, the max rpms of the motor I buy will dictate the max rpms of the machine. That's why I'm checking and rechecking my logic behind each decision I make. Alb1k, thanks for the math and for confirmation that ~1000rpm is a good operating speed. It seems plenty fast.

Thanks again for all everyone's input. Like I mentioned earlier in this thread, it's my intent to post a build thread of this project pictures and build costs for anyone else who might embark on it in the future.
 
That's a nice looking drive at a great price, I've not see one of those before.

I'm looking forward to seeing your build.
 
I have been running a 1/3hp , 1140 rpm disc since 1992 , even with an open motor . For the main grinding it could be a little faster ( not too much , heat build up ) but I would like slower for the finish work after H.T.
 
Thanks again for the input everyone. I went ahead and bought the motor of ebay last night... the link to the item is here: http://www.ebay.com/itm/26075728937...NX:IT&_trksid=p3984.m1439.l2649#ht_500wt_1129

Even though the seller has the motor listed as a .75hp, the data sticker on the box indicates that it's actually a 1hp motor. This motor sells for around $1100 online, so I'm very happy with the $112.50 I payed including shipping.

Another link to the motor's specs http://www.leeson.com/leeson/search...ProductDetails&motorNo=116299.00&productType=
 
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