Comparing motors with different RPMs

[Keith Weber]

At some point in the hopefully near future, I'm going to be converting my 16" jointer from direct drive to belt drive. The reason is due to space constraints, so the decision to do that has already been researched to death, made and is final, so I'm not looking to open a discussion on direct drive vs. belt drive. My question is about the choice of replacement motor.

I've been looking for a new, old-stock, Baldor, 7.5hp, 3ph, 3450rpm motor, but I haven't found the supply (of reasonably-priced motors) to be good right now. There is, however, a fair amount of similar motors available in 1725rpm. The jointer cutter speed will be stepped up to around 4000-4500rpm. My question is if there any difference between running a 3450rpm motor with say, a 4" drive pulley, and running a 1725prm motor with an 8" drive pulley? The belt speed will be the same, but I'm curious if there would be any significant differences in torque, spool up speed, starting amp draw, etc., that would affect the operation of the jointer. The cutter is 4" in diameter, so it has some mass to it.

I'm leaning towards waiting for a 3450rpm motor, if for no other reason than to just keep the belt cover size down, but I was curious about the 1725/3450rpm tradeoffs.

Keith

[Rod Sheridan]

Hi, the torque for the low speed will be twice that of the high speed motor, however that's irrelevant as the different drive ratios will make the torque at the head the same, and therefore the power at the head.

Acceleration and starting current will be similar if they're the same class motor.

That's a very large motor for a jointer, I wouldn't go higher than 5 HP...............Rod.

[Mike Henderson]

I'll add to Rod's comment that the 1725 motor will probably be physically larger than the 3450 motor (for the same HP). But HP and torque wise, I agree with Rod.

Mike

[Dennis Ford]

Other things being equal; 1725 rpm motors are normally more expensive than similar sized 3450 rpm motors. No worries about torque, amp draw etc specific to the nominal speed.

[David Kumm]

For what it is worth, I'd go with the slower speed ( four pole ) motor and appropriate pulleys. Biggest frame and shaft size I could find. Bigger bearings running slower are always better on the motor end. Larger frame are also easier to rewind if that become necessary. Dave

[Tony Haukap]

That's a very large motor for a jointer, I wouldn't go higher than 5 HP.

Another thing to keep in mind is how much power you can transfer with a single V-belt - with a motor speed of 1750 and a pulley of 8" diameter, to get 4000 RPM you're going to need a 3.5" pulley on the cutter head and with that arrangement the belt won't have enough surface contact area to deliver 7.5 HP... maybe with dual drive belts, but that's still a very small amount of contact for so much horsepower. A 3450 RPM motor would be a much better choice in your situation because the pulleys will be nearly identical in size.

[Keith Weber]

Another thing to keep in mind is how much power you can transfer with a single V-belt.

Yeah, I'd already planned on a dual-belt drive, for sure. I was initially thinking about a triple, but my 10hp planer seems to do alright with double, 5" or so pulleys, so the jointer should be fine with a dual-belt drive.

That's a very large motor for a jointer, I wouldn't go higher than 5 HP...............Rod.

The old motor was 7.5hp, so that's where the 7.5hp plan came from. I've thought about using a 5hp motor - they're a bit cheaper. I just didn't want to wish later that I'd gone with the 7.5hp. Northfield 16" HD jointers came with 3, 5, 7.5, and 10hp motors. I'm sure I could bog a 3hp, and 10hp seems like overkill (they even offered 10hp on a 12-inch jointer - Wow!) So that leaves 5hp and 7.5hp. It's like a table saw. A 3hp will be fine for 98% of what you do, but when you bury the blade deep into some thick hardwood while ripping, you see the need for more hp. I'm still open to 5hp. I just know that if I installed a 7.5hp, I'd never regret it.

What got me thinking about torque, hp, etc. with the different speeds was my Woodcraft 3/4hp, dual-speed, 8" grinder. I have a wire wheel on one side. At 1725rpm, it really bogs with heavy use, and I have to stop and let it spool up way too much. At 3450rpm, it's much better. I just don't like pulling the flung wire bristles out of my skin, so I run it at low speed and curse at it all the time.

Keith

[David Kumm]

A good 5 hp would work for a 16" jointer. I still like the heavy old motors but 5 hp is more than adequate but I never take off more than 1/16 with wide boards. A VX belt will handle the power if you go with the slower motor. They are rated for high HP. My Martin shaper uses one belt with big pulleys but you want the proper pulley and belts. Dave

PS. I have not computed the pulley sizes so if they get too big to work well on the machine, a 3450 has no drawbacks. Not quite as easy to find used. Dave

[Rick Lizek]

http://www.emotorstore.com has good prices. Searchtempest.com expands on craigslist searches.