Kbac 27d 60 vs 120 Hz

H

hugeo

Joined
Jan 8, 2009
Messages
53
Hello all,

Looking to put together a 2x72 for part time hobby use probably no more that a few hrs a week.

Running on 110

Probably this motor which seems to have the specs most recommend


2HP LEESON 1800RPM 56HC TEFC 3PH WATTSAVER MOTOR 116747.00

reading the Kbac 27 manual on page 19 figure 25

https://cdn.shopify.com/s/files/1/1...-3e64-4653-aeee-e99875e1054b.pdf?v=1595963014

it shows that torque at 120 hz is 50% of what it would be at 60 hz

I’m don’t completely understand the torque vs HP and all that. At higher speeds are you gonna have less torque regardless of 1800 vs 3600 motor? Planning 6 inch drive wheel.

Side question why is 56c the standard vs 145tc seems like bigger shaft better and getting either size drive wheels ez these days

Thanks
 
The simplest answer is that if you've already picked a drive wheel size, then go for 120 hz. If you set the speed control knob at 1/2 speed then it will behave exactly the same as if it was at full speed but configured for 60 hz. But now you have access to higher speeds which is great when hogging with ceramic belts.

Aside: It's true that you get reduced torque at speeds over 60 hz, but that doesn't matter much. Between 60 and 120 hz you're running at constant power. What really matters is power, effectively torque times rpm, which determines the amount of work that can be done. Increase rpm, and torque decreases, but your capacity for work remains constant. That being said, sometimes your situation requires more torque and can accommodate a lower belt speed, so turn it down to get that. But below 60 hz you're running in constant torque mode so you don't get additional torque by going slower.

Edit: 56c vs 145tc doesn't matter much at all. The difference in drive shaft diameter doesn't matter from a performance standpoint. As long as you can get the wheel you want for the motor you want, you're good. Conveniently, 56c and 145tc have the same flange and bolt pattern for face mount motors so they're effectively interchangeable. 56c motors are commonly available up to 2 hp, and less commonly at 3 hp. 145tc motors are rarely seen below 1.5 hp and are easy to find at 3 hp, less commonly at 5 hp. The flange on 182tc motors (the next size up) is massive in comparison and makes little sense on a belt grinder.
 
Thanks for the quick (and understandable) response. I am not 100% set on 6 inch drive wheel but it seems like the way to go from what I’ve read .
 
My preference for a 2 hp grinder is a 5" wheel running on an 1800 rpm class motor at 120 hz. I think it's a good balance of high belt speed and torque. That being said, I'm in the process of shopping for a 6" wheel to upgrade my current grinder to because it now has a 3 hp motor and I have torque to spare! (Going from a 5" wheel to a 6" wheel will increase belt speed by 20% but reduce the force applied to the belt by the same amount.)
 
The KBAC-27D also has a trimpot that let's you control the maximum speed between 70% and 110% of the frequency. Assuming your motor makes 3,500rpm at 120Hz, these are the approximate speed ranges in feet/min that you will get with different size wheels:
  • 5": 3,200-5,000
  • 6": 3,850-6,050
  • 7": 4,500-7,050
I think it is more likely that you will want to go faster than 5,000 ft/min than it is that you will want a max. speed less than 3,850 feet/min, so I would go for at least a 6" wheel. I have one grinder with a 6" wheel and one with a 7" wheel both running about the same top end speed of a bit over 5,000 ft/min give or take and there is no performance difference that I can notice.
 
hugeo said:
I’m don’t completely understand the torque vs HP and all that. At higher speeds are you gonna have less torque regardless of 1800 vs 3600 motor?
Click to expand...

HP is proportional to Torque x RPM. For an induction motor like this, throughout most of the range HP is constant. Therefore, for example, 2x less torque = 2x more speed.
 
The bottom line of all these speed/motor/wheel threads is this:

1) In knifemaking torque is not a factor we deal with. The grinder is already running when we apply the blade. How fast and strong it starts up is inconsequential ... and appears nearly instantaneous to us. About 2 seconds after flipping the switch the grinder is at full speed and power.

2) HP is rather over-rated in knifemaking. Any grinder over 1HP will work. 3 HP is really overkill unless you are a production shop hogging blades all day. Certainly 1.5HP is sufficient for any hobby maker. 2HP is great for most full shop folks. Higher HP does not make it run faster or the belt cut better. It merely determines how much force it will take to slow down or stop the belt. Few people will press a blade hard enough against a belt to slow down or stop a 1.5HP motor.

3) Motor RPM is a factor of the number of poles. A 1750RPM (four pole) motor will run fine at 120Hz and turn at 3500RPM. Technically, it has more power at low speed than a 3500RPM (two pole) motor ... but the difference is insignificant with any motor over 1.5HP. Yes, you will have more power at 750RPM on the four pole 1750RPM motor, but you probably won't be able to tell. If the motor you have is 3500RPM, use it.
(Personally, I prefer a 1750 RPM motor running at 120Hz. This is because I need a controlled slower speed much more than a screaming fast speed.)

4) Drive Wheel size simply determines the max belt Surface Feet per Minute. Pick the drive wheel that gives you the desired SFM. Most folks find 5000SFM a good top range. Some folks who hog lots of blades like 7000SFM. A simple calculation method for a 6" wheel is 1.5XRPM =SFM 1750 = 2625SFM and 3500=5250RPM. By running the 1750 motor at 120Hz it gets the same 5250SFM speed as the 3500RPM motor. A 3500RPM motor ate 120Hz would be running at 10,500SFM ... which is crazy fast and probably damaging to the bearings and belt.
 
Thanks for all the responses!

The whole power HP torque discussion reminds me a lot of chainsaws when in everyday use rpm and how sharp your chain/belt is really determines how fast you’re gonna cut.

Does $500-550 sound about right for that Leeson motor I was looking at? Any input on Marathon motors they look to be a bit cheaper ?
 
If i make grinder with 1800 RPM motor i will use drive wheel size to get max . belt speed i want at 1800 RPM.
 
You must log in or register to reply here.
Back
Top