Grinder Motor Question

[Bühlmann]

If 4-pole 1800 RPM motors are superior with a VFD at lower speeds, why are most grinders supplied with 2-pole 3600 RPM motors?

 

[A.McPherson]

Generally speaking you're not going to be hogging material at lower speeds, so you don't need to have all the torque. And higher speeds are better for hogging material with lower grit belts.

so high torque at high speeds is generally more desirable, than high torque at low speeds.

at least that's my take on it.

 

[CallumRD1]

If 4-pole 1800 RPM motors are superior with a VFD at lower speeds, why are most grinders supplied with 2-pole 3600 RPM motors?

Short answer: 3600 rpm motors tend to be cheaper, lighter, and have less copper in them.

Longer answer: An 1800 rpm motor running at 2x on a VFD is superior in every possible respect to a 3600 rpm motor running at 1x on a vfd. The 1800 rpm motor will maintain its rated horsepower from 1800 rpm to 3600 rpm while the 3600 rpm motor will be putting out half its rated power at 1800 rpm. In practice this doesn't tend to matter as much as it may seem as hogging (highly power dependent) is typically done at full speed (where both motors put out full power) and finishing work (not power dependent) is typically done at lower speeds where the power output may be different but typically still sufficient in either case. The final difference is that when running below about a 5:1 reduction in speed many motor/vfd combinations will tend to cog or run a bit roughly. All else equal, the lower speed motor will thus give a lower minimum speed before cogging sets in. (Caveat on this last point: some combinations will run smoothly down to stall speed, some combinations will cog at only half speed. YMMV)

 

[Bühlmann]

I figured it was the difference in cost that drives the decision for 2-pole on stock motors. Is the functional difference one that is glaringly apparent to knifemakers in general, or is it more of a "it's better, but you're unlikely to ever notice" type better?

 

[Ken H>]

It's more like you'll never really notice the difference between the two. I tend to prefer 2 pole motors since they weight less and less expense. In "theory" the 1800 rpm motor will fall a slight tad at 3600 rpm with 2X, but you'll never know the difference in use.

While the 1800 rpm motor might have a tad better low speed rpm, my 3600 rpm motors will run at 360 rpm just fine.

 

[Stacy E. Apelt - Bladesmith]

In most grinders it doesn't make much difference.

I use this comparison:
What is the difference in a four-cylinder motor and an 8-cylinder motor in a car if both will hit 80MPH?
1)The 4cyl is cheaper, and lighter, but less powerful for going uphill or pulling heavy loads.
2) The 8cyl has more power, cost more, and is heavier. It is better for heavy duty tasks.
For around town transportation, it doesn't make much difference which motor is used. For hauling a trailer through the hills, it does.

 

[12345678910]

I figured it was the difference in cost that drives the decision for 2-pole on stock motors. Is the functional difference one that is glaringly apparent to knifemakers in general, or is it more of a "it's better, but you're unlikely to ever notice" type better?

What's the price differernce ? Usually same price when I look.

 

[Ken H>]

Count: That's interesting, most every time I've checked the 2 pole is less expensive. Currently the Iron Horse 2hp motor on Automation is 1800 rpm (MTRP-002-3BD18) = $352, and 3600 rpm (MTRP-002-3BD36) = $262. Wow, that's $90 difference in price? AND WOW!!!! Motors sure have increased in price since I purchased the last motor a yr or so ago. Then I think the 3600 rpm was less than $200.

edit: Ooops, that's a link I posted - hidden in the part number). I removed the link. Now just do a search for the part number