2hp 1800RPM or 3hp 3600RPM motor?

oldschool45

oldschool45

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Oct 15, 2007
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I was given a frame for a Jeremy Schmidt call it gen (1.5) 2x72 grinder. I've been reading threads for 2 days trying to figure out what motor to mount on it. In the garage there is a 20amp/240v single phase circuit with a sub-panel if I need to go to 30amps.
So do I go 2hp/1800rpm for more torque or 3hp/3600rpm? I can't seam to find a 1800rpm 3 phase 3hp motor. Which one do I bolt a VFD to and call it a day?
Most of my blades are kitchen knives and I have been getting along beautifully with the 1x42 Belsaw that JV Knives sold to me. But I have a 7# piece of 52100 that will be a Khukri Like Object before the end of the year and a few other Big choppers that will make use of the 27" Evenheat oven.
Thanks.
 
I have a 2hp/3600 motor on the Jer Schmidt Gen 2 grinder that I built, and it works great. Doesn't seem underpowered to me at all. It's controlled by a TECO( Westinghouse ) 510 VFD.
 
As Stacy says 2hp is more than enough. The 1800 RPM motor gives slightly more torque at low RPM, but you never do any heavy grinding at low RPM so the torque isn't really needed. I run a 3600 rpm motor with VFD tweaked to turn motor 4,000 RPM and with a 5" drive wheel gives me 5,000 belt speed which seems pretty good. You'll also like a motor to allow belt speed around 300 SFPM also for knife sharpening.
 
Stacy E. Apelt - Bladesmith said:
2Hp is more than enough.
I recommend an 1800RPM motor. With VFD settings and drive wheel size, high speed is not an issue. Low speed power is better on an 1800RPM motor.
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Stacy, what is better for torque all other things being equal ... An 1800rpm motor running 2x in the vfd or a 3600rpm motor running single speed?
 
Short answer - at rated speed, an 1800RPM at 1HP produces 3FtLb of torque. 3600RPM produces 1.5FtLb.
VFDs change that, so with 1800 set at 2X vs 3600 - the full speed rating is the same torque. At low speed, 1800 has twice the torque.

We really don't need torque on a grinder. It isn't pulling trailers or lifting things. We need speed control. With 10% being the bottom of most VFDs to run stable, an 1800RPM motor will bottom at around 180RPM vs 360RPM for a 3600RPM motor. Slow speed ability is why we use a VFD.
At the high end there is little difference because both are getting full power (and torque doesn't matter). The 2X setting will give you 3600RPM which is more than sufficient for most knifemakers.

If you have a dedicated hogging grinder, 3600RPM would be fine, but for a single grinder shop I feel that the versatility of 1800RPM is the best compromise.


Until it finally burned out the motor, I had a 3600RPM, 2HP, direct drive, two-wheel grinder, with a 6" drive wheel that belonged to Ron Frazier. It ran at a constant 5600SFPM. What was really nice was the 20" platen. It was also scarry to use with a 36-grit ceramic belt. You had to use strong grinding magnets to hold the steel because one bump would remove an entire knuckle. It would turn a blade red-hot when hogging a large knife. I got some so hot they fell off the magnet.
It was an open frame motor, semi-exposed to the weather, and finally died. It was pretty worn out all over, so I got rid of it once the motor died. I will build a new 3HP "hogger" in the smithy after rebuilding the entire forge area this spring. I'll probably make it VS with a 3600RPM motor.
 
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Stacy E. Apelt - Bladesmith said:
We really don't need torque on a grinder. It isn't pulling trailers or lifting things. We need speed control. With 10% being the bottom of most VFDs to run stable, an 1800RPM motor will bottom at around 180RPM vs 360RPM for a 3600RPM motor. Slow speed ability is why we use a VFD.
At the high end there is little difference because both are getting full power (and torque doesn't matter). The 2X setting will give you 3600RPM which is more than sufficient for most knifemakers.

If you have a dedicated hogging grinder, 3600RPM would be fine, but for a single grinder shop I feel that the versatility of 1800RPM is the best compromise.


Until it finally burned out the motor, I had a 3600RPM, 2HP, direct drive, two-wheel grinder, with a 6" drive wheel that belonged to Ron Frazier. It ran at a constant 5600SFPM. What was really nice was the 20" platen. It was also scarry to use with a 36-grit ceramic belt. You had to use strong grinding magnets to hold the steel because one bump would remove an entire knuckle. It would turn a blade red-hot when hogging a large knife. I got some so hot they fell off the magnet.
It was an open frame motor, semi-exposed to the weather, and finally died. It was pretty worn out all over, so I got rid of it once the motor died. I will build a new 3HP "hogger" in the smithy after rebuilding the entire forge area this spring. I'll probably make it VS with a 3600RPM motor.
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Ah thanks was just wondering... Sometimes I bog down my 2hp 1800 rpm motor at 2x in the vfd running at 60-70%...but it's not something that would normally be a problem (I'm grinding under flood coolant with jig so I can really push into it)
 
If you are bogging a motor down, you probably need more HP, not more torque. I understand that they are realted by HP is a bigger factor when you reach the point of overload.

BTW, there are settings on most VFDs that will stabilize torque.
 
Everything can be made slow. Not all can be made fast. And there is a nice thing about noise. Slow is quiet. 1800 and 3600rpm has the same bearings, so going double speed with a 1800 motor is in it's margins. Choosing a wheel is probably more important.
 
A decade or two ago, the smart advice would have been to use the 2 HP, 1800 RPM motor and to run it to 100 Hz, maybe even 120 Hz, maximum, to get the widest operating speed range. The "best" choice of VFD back then would likely have been a KBAC, primarily because it was one of the few sealed drives that was available. Now, Sensorless Vector drives have muddied the waters somewhat and the 3 HP, 3600 RPM motor is possibly a better choice, if used with an SV drive.

I "think" the KBDA drives may be SV drives, but I've not used one personally and can't be sure. The KBAC drives are not SV.

"Old school" VFDs are "V/F" aka "V/Hz": the Voltage and Frequency (Hz) both vary linearly below the rated motor Frequency. Above the rated Frequency, the Voltage is constant and only the frequency increases.

This is fine for frequencies reasonably close to the design Frequency (60 Hz for North America, 50 Hz for most of the rest of the world): between about 10 Hz and 100 Hz, everything is usually pretty happy. Above about 100 Hz, things start to get less linear and losses within the motor start to become disproportionately high. You can still increase Frequency and motor speed, but the output power will start to drop off significantly. By 120 Hz, power is likely to be down by a few percent: not really a problem with a grinder that has a 2 HP motor, being operated by a reasonably skilled operator, as the small reduction in power is barely noticeable. Try to go to 140 Hz, though, and it'll be pretty obvious power is dropping off quite rapidly.

At low frequencies, below "about" 10 Hz, the linear relationship between Voltage and Frequency also suffers and motors tend to become "coggy", rather than smooth-running. I did some testing of several different drives running a metalworking lathe. All of the non-SV drives showed poor finish as a result of "cogginess" by 7 Hz. With SV drives, I could get a smooth finish down to 2 Hz or less.

There are usually some settings that can be adjusted to boost torque at low speeds, but they are something of a black art. To get down to the 7 Hz, I'd spent a bit of time playing with them, but I'm no expert.

A grinder will be much more forgiving of any lack of smoothness than a lathe, but it's nice to have the ability to run smoothly at low speed. This used to mean running a 4-pole (1800 RPM) motor to give a minimum smooth speed of 300 RPM at 10 Hz and a 10:1 speed range with 100 Hz maximum Frequency or 12:1 with 120 Hz maximum Frequency (and the small reduction in power at the top end).

With an SV drive, a 2-pole (3600 RPM) motor can run smoothly down to 2 Hz to give 120 RPM for a 60:1 speed range. Some motor manufacturers will allow operation of their 2-pole motors above 60 Hz. The metric TEC (Techtop) motors I've been using lately will allow operation to 70 Hz (4200 cycles/min) for an actual motor speed of 4000 RPM without invalidating the warranty. This gives a 35:1 speed range of 2 Hz to 70 Hz. You'll need to check the manufacturers specs for the actual motor you use.

An 8" drive wheel at 4000 actual RPM will give a belt speed of 8377 SFM, which is around the maximum recommended speed for some of the newer ceramic belts. Probably faster than most knifemakers will ever use, but it doesn't hurt to have the capability. A 4-pole motor isn't realistically going to run above 3500 RPM.

If you've no restrictions on the power supply, the 3 HP 2-pole, combined with a sealed Sensorless Vector VFD, would seem to be the obvious choice to me. Torque is not normally a priority at low speeds and the 3 HP motor will be ahead on torque from around 2400 RPM upwards.
 
Thanks all. I went with the 2hp 1800RPM motor a few days ago. The 1x42 Belsaw I'm currently using runs around 1750 RPM on a 1/3hp motor. I don't need more speed but being able to lean into it without it stalling is going to be a big advantage.

T timgunn1962 and others recommendations on budget VFD sensorless vector or traditional would be great! Also how easy and important is it to wire in a start/stop switch.
 
Just to add a benefit of very slow. My motor is 3HP 4 pole@1440rpm with a 10cm (4") drive (20cm (8") replacement when I want high speed). Belt speed @6Hz it's less than 1m/s. That's hand sharpening speed. And one get's a nice convex edge.
 
Agreed, my 4-pole 1800RPM motors give a very slow and smooth crawl for sharpening.

A charged leather belt will create insane sharpness as a final step. Many years ago, I built an arm with a 5" wheel at the end and a flat platen between the tracking and the 5" wheel. This creates a flat and firm stropping area where the strop moves, and I hold the blade. The 4" wheel keeps from bending the leather belt too much. This setup has to be run very slow to avoid heat buildup and the risk of catching/snagging and removing fingers and hand parts.

Many years ago, I was sharpening fillet knives and forgot to turn the speed dial to dead slow when I switched from 400 grit to the leather strop belt. It was on slow, but not dead-slow. The first several knives went fine but then one snagged at the tip as I drew the blade across the strop. It flipped across my palm and wrist. It cut my glove and a 1" long piece off the heel of my left palm. Just a few inches further and it would have severed my wrist veins. It was so fast and clean I never felt the cut. I have a nice scar to keep me reminded to sharpen and strop at dead-slow.

After my hand healed, I built a dedicated two-wheel sharpening machine that takes 2"X36" belts. It has a 3"drive wheel and a 3" tracking wheel, with a graphite lined flat platen between them. I made it with a 3/4HP DC motor that has a built-in speed control in the end of the case. It came off a medical surplus heart by-pass pump. Everything was sealed and stainless. After removing the pump, the motor had a 3/4" shaft, so it was perfect for a slow speed grinder. Best was that it runs between 0 and 700RPM. At full speed it will set and edge bevel nicely. At 100RPM it runs at 15 inches per second, which is about the speed of hand stropping or sharpening a knife. At 10RPM it moves the strop at 1.5 inches a second for polishing the edge to remove the wire. Since there is no real resistance in sharpening and stropping, I don't notice any slowing of the belt even at the lowest speed (a plus with DC).
This sharpener is a self-contained flat countertop style that has the belt/strop parallel to the benchtop. It's heavy, but not too heavy to be moved to where you need it and can be used sitting down at a desk. I still have to finish attaching an angle adjustable clamp arm similar to a Work-Sharp Pro system. The arm will clamp onto the knife and guide it as you move it across the slow-moving belt/strop. Flip it over and do the other side at the exact same angle. The next phase will be an electromagnetic clamp that allows for easy changing of the knives and even lower edge angles.
 
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