Recommendation? Any downside to 3 HP motor?

L

Lynn Westfall

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Aug 6, 2007
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I'm finally getting around to upgrading my 20 year old Wilton Square Wheel to variable speed.
I have not located the exact package in stock but am looking at something similar to this:
Complete 2hp 3600rpm AC motor and drive package.
KB Electronics KBAC-27D AC motor control 9520, and Leeson 119417.00 General Purpose AC motor. This package comes complete with motor, drive, forward-brake-reverse switch, On/Off switch, and wiring kit.
Leeson 119417.00 general purpose ENERGY EFFICIENT AC Motor:
Specs:2HP 3600RPM 56C TEFC 230/460V 3PH 60HZ CONT.NOT.40C.1.15SF.
RIGID C GENERAL PURPOSE C6T34FK132 WATTSAVER.AUTOMATIC $764.00
Out of stock at indsencon.com. (any other good sources would be appreciated)


The price for a 3 HP motor and upgraded to KBAC-29 is not that great a difference in price. Is there a downside to installing a 3 HP motor on my Wilton Square Wheel?
 
Only that you would need 240V and could not use regular 110
 
Only downsides are :
cost
weight
voltage
amount of electricity used
shaft and mount size

None of those are any real problem.
 
’Mo power is always better.
Just be willing to pay for more electricity. But, Probably wont notice if a homegamer.
 
Depending on how often you run your grinder, it may add up to costing a few more dollars in electricity at the end of the year, and the motor will be a little heavier, etc... but other than that, there's no real downside. Will you notice any difference in grinding performance, over running a 2hp motor? Probably not.
 
I have both a three HP and a 2 HP motor, both on KMG machines. There is no better HP set up for grinding bevels fast. There is a big difference on the ground between the two machines. When grinding on a machine, where you know the belt will not slow down under any pressure; you grind differently. And you grind faster. I like the two HP as well but it's not the same as a 3 HP machine.
 
I strongly recommend getting an 1800 rpm motor and doubling the hertz in your kbac. More and more guys are having problems getting 3600 rpm motors to run slow.
A few years ago the federal government mandated that low hp electric motors meet efficiency requirements. I suspect that these requirements affected the ability for some manufacturers motors to run smoothly at low hertz settings.
 
That its not 4HP!

I run a 3 horse on mine and I am keenly aware of this every time I take a training course and use their grinders. Its also a 3450RPM and I haven't noticed any issues running it slow but it was new old stock I picked up 4 years ago so maybe its not affected by what you mentioned above.
 
I've got 2 of Iron Horse 3600 rpm motors and both run just as smooth at very low rpm as the 1800 rpm Leeson motor I've got on the lathe. With no load, and at certain freqs there can be a slight "rough" to running, but not bad, and once there's a load it does just fine. I wouldn't worry about getting a 3600 rpm motor to run at low rpm.
 
That being said, as far as 3600 vs 1800 rpm, check the motor plates carefully. When I was shopping for a 2hp motor, I found a lot of 3600 motors that claimed to be 2hp but would have only been approaching 1.5hp (if that!) given the specs on the nameplate. The 1800s were far more accurate to their claimed power (as well as cheaper). There's no downside to running an 1800 with a KBAC drive versus a 3600, because you can set a jumper on the KBAC to run the motor at double speed.
 
Stacy E. Apelt - Bladesmith said:
Only downsides are :
cost
weight
voltage
amount of electricity used
shaft and mount size

None of those are any real problem.
Click to expand...


You can find them in 56C configuration, they're just not as common, this'll obviously negate the shaft and mount size consideration, although it may still be cheaper to buy a 145tc which is more common, same flange bolt pattern for C-Face direct mount, but larger shaft size (7/8").

Used to be difficult to find drive wheels with bores larger than 5/8", but not anymore.


Weight/size can definitely be a factor for flipping hinges or horizontal mount grinders, but otherwise, yeah, 3hp is a big upgrade over 2hp, especially since most people with 2hp motors are running them with 110v input and not producing full power on their motors.


More expensive VFD, and if you're using a KB Electronics 29, you have to make sure and get the model that supplies 3hp with 220V *Single Phase Input*, as there are two models of 29s one which accepts single or 3 phase input but doesn't produce 3hp with 220v single phase input, at least, on paper. I've run them both with 3hp motors, and couldn't stall either.


Definitely go with a ~1800 rpm motor and use the 2X jumper. Torque is more important than HP at lower speeds.
 
Can anyone explain in technical detail why a motor designed to run at 3500RPM and 60hz has less torque at low speed than a motor designed to run at 1700RPM running at 120hz? I've heard folks say this a lot but I don't recall ever seeing a technical reason as to why. Not looking for a debate or anything, genuinely curious here.

-Clint
 
Augus7us said:
Can anyone explain in technical detail why a motor designed to run at 3500RPM and 60hz has less torque at low speed than a motor designed to run at 1700RPM running at 120hz? I've heard folks say this a lot but I don't recall ever seeing a technical reason as to why. Not looking for a debate or anything, genuinely curious here.

-Clint
Click to expand...


The quick answer is; the more poles, the more torque, and the greater efficiency, but also, lower static motor speed. A 3600 rpm motor only has 2 poles, where an 1800 rpm motor has 4. By doubling the frequency, you gain the ability to over-drive the motor to a higher speed, without losing the torque advantage.

This is an oversimplification, without going into detail, and of course, making assumptions, and generalizations.
 
I just realized there was something I should have added to the first post:
From the article:
The 1800 RPM, 1HP motor produces 3 ft. lbs of torque at 1800 RPM; at 3600 rpm this same motor has only 1.5 ft-lb torque
Click to expand...
The big advantage of a 1800 rpm motor is at 1800 rpm (which is half speed of max with doubling jumper) it has 3 ft-lb torque where you can really put the pressure. BUT - at the lower rpm usually is when finer grits and light pressure are used. If the plan is to hog at half speed, then it will be helpful.

Hogging/profiling will take place at 3600 rpm where both 1800 the motor (with doubling jumper) and 3600 rpm motors will have about the same torque since both motors are running at 3600 rpm. I max the 3600 rpm motor at 4,000 rpm with a 3.9" diameter drive wheel, for 4,000 SFPM belt speed.
 
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Thanks Ken, this info is good to know and Javan's comment about the poles made a lot of sense. I'll break down and take some time to read up on it before i buy another grinder. I must have an NRT grinder! So I'll pick up a 1700RPM for the next one.

That said, for the record I have no issues with my 3HP 3450RPM motor and KBAC 29. The only thing I've noticed was at the speed of 0 on the dial it will stutter a bit but I would never grind anything that slow, I can hand sand faster!

-Clint
 
I am going to look for new sources with the 1800 RPM motor. I was about to purchase a bundle at a good price but 3600 RPM. I only want to do this once. Thanks for the info.
 
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