1750 RPM vs 3500 RPM

1750 RPM or 3500 RPM

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jkm628

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Feb 6, 2019
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Can I get some opinions/advice on whether I should get a 2hp 1750 RPM motor or a 2hp 3500 RPM motor?

About to purchase an OBM chassis and trying to decide on motor and VFD setup.

Going to get a KBAC-24D VFD.

I'm just confused on what RPM to go with on the motor.

Thanks in advance!
 
The slower motor will reduce the chances of overheating at higher grits. While lower grit ceramics should be run above 4000sfpm, if you have to pick one, I'd go with the 1750. Keep in mind, the larger the drive wheel, the faster the belt goes as well.
 
That is a discussion where it seems "most" folks tend to choose the 1725 rpm because max torque available in the lower rpm ranges - exactly where you DON'T need max torque. Here is the torque vs rpm curve for a Baldor 1750 rpm motor.
HP-vs-RPM-curve.jpg


As you can see the torque available up at the 3450 rpm range is way down. This is the rpm you get with the 2X jumper on the KBAC-24D VFD. I can't put my finger on the link right now, but seems like I remember the 3450 rpm motor has just a small tad more torque at 3450 rpm than does the 1725 motor that's been oversped to 3450, but doesn't really make sense because the 1725 motor has 4 poles while the 3450 rpm motor is a 2 pole motor. That's why the 3450 rpm motor is less expensive and not so heavy as the 1725 rpm motor.

What you're looking for is a max belt speed around 4,000 SFPM (or more). It takes 4,000 SFPM as a minimum for the ceramic belts to work, and perhaps even better is 5,000 or 6,000 SFPM for ceramics. To me, anything much over 4,000 SFPM is scary fast. (edit (2023) to update, I'm now comfortable with 5,000 SFPM max speed and use that speed for hogging.)

With a 4" drive wheel it takes around 3800 RPM to get 4,000 SFPM.
With a 5" drive wheel it takes around 3800 RPM to get almost 5,000 SFPM. (edit to update, I now use a 5" wheel)

With a 4" drive wheel 1725 RPM to get 1800 SFPM.
With a 6" drive wheel 1725 RPM to get 2700 SFPM, that is why most folks using a 1725 rpm motor will put max freq at 120 to double motor speed to 3450 rpm. With a 6" drive wheel 2450 RPM to get 5400 SFPM (all belt speeds are approx)

For my work it seems a 3450 rpm motor works good with max freq set to 70 hz for a max motor rpm around 3800 rpm. This allows the motor to run down around 300 SFPM speeds for those slow delicate grinds.

Once the motor is wired up and grinder in service, you'll never know the difference between the 1725 vs 3450 rpm motors, except the 1725 motor is heavier and more expensive to purchase.

Oh, there is no problem with overspeeding the 1725 rpm motor with the 2x jumper. I would hold max motor rpm below 4,000 rpm.
 
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So basically, Ken got the tough assignment. If you are going with a single speed/ single phase motor, read my post. If you are going 3 phase with vfd, read Ken's.
 
jkm628 said:
Can I get some opinions/advice on whether I should get a 2hp 1750 RPM motor or a 2hp 3500 RPM motor?

About to purchase an OBM chassis and trying to decide on motor and VFD setup.

Going to get a KBAC-24D VFD.

I'm just confused on what RPM to go with on the motor.

Thanks in advance!
Click to expand...
The KBAC 24D Won't run a 2hp 3 phase motor.....it is rated from 1/8-1 hp...you will need the KBAC 27D for 2 hp.
 
Good catch Busto - I don't know the KBAC drives at all so didn't catch that one. I use the cheap ($62 for 2hp last check) Chinese drives with a filter over the air inlets.
 
Ken H> said:
Good catch Busto - I don't know the KBAC drives at all so didn't catch that one. I use the cheap ($62 for 2hp last check) Chinese drives with a filter over the air inlets.
Click to expand...
I run 4 KBAC 27's in my shop and 3 of them run multiple pieces of equipment through a Switch so I don't have to unplug to change which machine they operate one is stand alone on a modified drill press.
 
Spend a little bit more and get the 27d. It will run on 110 or 220 volts. Get a 1 1/2-2hp 1750 rpm motor and double the hertz setting in the kbac. Run a 6" or bigger drive wheel.
 
mike-E said:
Spend a little bit more and get the 27d. It will run on 110 or 220 volts. Get a 1 1/2-2hp 1750 rpm motor and double the hertz setting in the kbac. Run a 6" or bigger drive wheel.
Click to expand...
The OP was planning on 2 hp motor so he really has no choice the 24D will only handle up to 1hp they will both run on 110/220v...Not a matter of spending a little more it's a matter of NEED to Run 2 hp YOU NEED 27D.
 
Ken, To make it apples to apples, do you have a torque/speed chart with the same motor type rated for 3500RPM and the same controller.
 
HP is a function of torque and RPM. As rpm drops so does the HP, but torque remains constant. This is because the vfd has to keep the proper voltage to Hz ratio to prevent magnetic saturation. As you increase the speed above 100% (60hz) torque and HP flip flop. This means HP stays at 100% but torque drops. HP= (torque x RPM)/5253
This means if you drop the motor speed by half your cutting the HP in half as well. Torque is a measurement of work over time so if you wanted to keep HP constant then cutting the speed in half would require the motor to do twice the amount of work.

this all comes down to where you want your torque curve and how you grind. I personally need to keep my torque in the lower rpm range and I can afford to loose some at the higher speeds. This is because high speed is for hogging and low speed is for finish grinding. This might seam counterproductive but I will explain. Hogging with fresh belts at high speed requires less torque as there is overall less surface area contact with larger grits. But when I go to a finishing belt I slow down as there is a lot more friction and surface area contact which requires as much torque as I can provide.

Also 17XX rpm motors generally are happey to go over 60hz. I think Industury standard is double to 120hz. I have a 1750rpm motor that’s rated to 220hz. And it runs just fine at thoes speeds. But the 3450rpm motors generally are not rated to go over there rated speed. Also the torque on a 1750 motor is higher then a 3450 motor. If you drop the 3450rpm motor to half hz at 30 then you have half the torque as a 1750rpm motor. On a 1750rpm you kinda have the best of both worlds as you keep the torque where you want it.

But a 3450rpm has you locked in at one set torque. You could have the 3450rpm as your max rpm but then your allrrady handicapped in the torque department by half compared to a 1750. You could bump the torque up but reducing the rpm with belts but you will loose efficiency there. Remember as you drop your rpm the torque remains at 100% of its rated torque at 60hz. So it’s just a numbers game and you have to decide where you want what and how much of it you need.
 
That is how I understand it, too. I hope Ken can post a chart of the torque for a 3500RPM motor. I think the difference should be interesting.

Just to be clear about something:
In the normal use of a grinder with a 2HP motor, there will be more than sufficient power/torque/speed to do all of our knife grinding tasks regardless of which RPM motor you choose. We aren't running a winch or a truck … where torque matters. Our uses are merely a matter of power ( 2HP provides plenty) and speed (The VFD provides the needed speeds). Which motor you use will not matter in most builds. Torque is not an realistic issue in the 1750 vs 3500 debate. However, if the question is "Which is better", then 1750 is the answer ( for several different reasons).
 
Here's a comparison of two 2.2 kW (3 HP) electric motors at 50 Hertz (I'm in Europe). The left one is a 4 pole (1440 rpm) and the right one is a 2 pole (2880 rpm).

8iiXbY4.png
 
Thanks for the info - looks like it pretty much confirms what I was thinking about 4 pole (1725 rpm) vs 2 pole (3450 rpm) motors. Unless a person is wishing to "hog a blade" at low rpm, there's really no real difference in use between a 2 pole or 4 pole motor.
 
It's not unusual to run a motor really slow down at a crawl. A four pole will do this with less cogging I think. Whereas the speed limit imposed by weight and bearings is pretty similar between the two. I run a 4 pole motor at triple speed without any problem, which would be 1 1/2 speed on a 2 Pole. that's more than enough speed for my uses, and the smooth running at low speed make the 4-pole a better fit for me.
 
Thanks Stromberg. That is what I was saying. The 3500RPM motor will have less HP (Nm) than the same size 1750RPM motor. The low speed power of the 1750 is also a big advantage for us slower speed grinders who grind after HT ( 50% less power from a 3500RPM motor).

As I said before, this isn't a GOOD/NO GOOD situation. either type motor will run a grinder in the 2HP range.
 
In my purchase of a motor I was thinking more about mid to lower end torque.. I bought a 1750 with 7” drive wheel. Where I can reach higher speeds and hog material well enough (I don’t hog much if any). Most of my shaping is at 60hz, bevels done at 60-80hz and the other half of my grinder work is at 15-30 hz shaping wood and blade finishing.
 
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