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VFD and motor for 2x72

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I'm in the process of building a new 2x72 grinder, and i like the idea of having a VFD instead of step pulleys. I'm currently looking at the KBAC 27D.


I'm having problems working out how to get the SFM i want. I'm after about 500sfm to about 5000-7000sfm.


I can get a 4 pole motor at 1400rpm or a 2 pole at 2800rpm.


My thinking is that i can get the 2800rpm motor and use the 2X jumper on the KBAC to double the speed. Would that mean i could then go from 0 - 5600 rpm? Resulting in 7330 SFM with a 5" drive wheel.


Does the kbac 27d go from 0 rpm to double the motor rpm? (not counting the 2X jumper option)


My other concern is reading about rather large loss of torque on the high rpm motors when they are dialed down to a slow speed, and also the lack of cooling due to different fan design on high rpm motors.


Also does anyone have a source for 8" or 10" crowned drive wheels with a 24mm bore?


Any advice or links much appreciated. Cheers

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KBAC 27D looks like a good drive. Lots of guys use it Stateside.


As far as I can tell, it has 2 big advantages there.


First is that it will take either a 110V or a 220V supply, though the motor is limited to 1 1/2 HP on 110V. This probably does not matter to you in Oz.


Second is that it is a sealed drive. This is essential for a grinder. However, there are lots of other manufacturers of sealed drives. Invertec are probably the European go-to sealed VFD manufacturer with their IP65 and IP66-rated drives and there are sealed drives available from most of the big manufacturers. I'm half a world away and don't know what's available to you. Look for something sensibly priced and supported locally.


Other points that I suspect are in the KBACs favour in the US knifemaker market: it is US manufactured, and setup is by jumpers and trimpots, rather than a menu-driven keypad/display.


The KBAC 27d seems to be a V/Hz drive and does not have Sensorless Vector capability as far as I can tell.


You are on 50 Hz mains in Oz. This means that your motor rating plate will show the rated speed at 50 Hz. With the VFD, you don't actually need to worry about the rated speed very much. The important thing is the number of pole pairs on the motor.


50 Hz is 50 cycles/second and there are 60 seconds in a minute, so 50 Hz is 3000 cycles/ min (60 Hz is 3600 cycles/min). Each pair of poles takes one cycle, so a 2-pole motor will only need one cycle per revolution and will turn at "about" 3000 RPM on 50 Hz. A 4-pole motor has 2 pairs of poles, so needs 2 cycles/rev and turns at "about" 1500 PRM on 50 Hz. 6 pole, "about" 1000 RPM and so on. On 60 Hz mains, as used in North America and some other places, the speeds would be 3600 RPM, 1800 RPM, 1200 RPM and so on.


An unloaded motor will run pretty close to the speeds given in the last paragraph, but as the motor is loaded, it slows down and produces "slip". This slowing down is not a bad thing: it is just part of the physics of electric motors, with the slip causing the motor to produce torque. The 2800 RPM motor you mentioned would have been slowed down from a bit under 3000 RPM at no load to 2800 RPM at its rated output: about a 7% speed reduction. The 1400 RPM motor sees a similar speed reduction in percentage terms.


Some of the NEMA 56-frame motors may be slightly different because the 56-frame was originally intended for Fractional HP motors and there has been a lot of work put into getting 2HP or more from that small a frame size, but generally it is the case that manufacturing economics have driven manufacturers to standardize their products as much as possible. You can be pretty sure that all the IEC 90-frame motors in a particular range from a particular manufacturer will use the same bearings, casing, rotor, etc. and that the rotating assembly will have been designed for 3600 RPM (a 2-pole motor running on a 60 Hz supply). The 90-frame has a 24mm shaft, so I assume that is the frame size you are looking at.


The only major change for the 4-pole, 6-pole, etc variants will be the windings, which are static and are fixed to the inside of the casing.


The design speed of 3600 RPM sets the upper end of the speed range (you can run faster, but you'll probably be outside the manufacturers design range unless you can find a spec for the motor you are intending to use that gives a higher maximum speed.


Most basic (V/Hz or V/F) drives will run OK down to about 10 Hz happily. Much below that, my experience is that they start to feel "coggy" by about 7 Hz and become noticeably less smooth. Sensorless Vector drives have some additional electronics that allows them to run a motor smoothly at lower frequencies: I've used them at 3 Hz with no apparent loss of smoothness and they may go lower still. I've never need to try.


You want a 10:1 speed range or better (500-5000+ SFM).


Running a 4-pole motor (rated for "about" 1500 RPM on 50 Hz) from 10 Hz (about 300 RPM) to 120 Hz (about 3600 RPM) will give you a 12:1 speed turndown and ticks that box.


If we do the belt speed calculation at the bottom end, where you are likely to be using very light pressure and not slowing the motor to generate much torque, we can get a drive wheel diameter.


500 SFM / 300 RPM = 1.67 feet per rev = 20" per rev.


Drive wheel diameter = 20" / Pi = 6.36"


Call it 6", which is 0.5'


At the top end of the speed range, you'll probably be leaning on the belt and hogging, so you want maximum torque.


At 50 Hz, the 4-pole (1500 RPM unloaded) motor is doing 1400 RPM.


1400/1500 = 0.93333


3600 x 0.93333 = 3360 RPM


3360 x 0.5' x Pi = 5277 SFM


The 500-5277 speed range on a 6" wheel is a safe bet. You could maybe go higher at the top end if you are happy to exceed 3600 RPM, and maybe go a tad lower at the bottom end even with a V/Hz drive and certainly with an SV drive. This is something you'll only really be able to determine once it is running.

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Have you ever run a grinder that fast? Have you ever seen one run that fast? If so do you even want to be in the same room with one again?

3600 rpm is as fast as you want to run a belt grinder. You don't want to get slapped with a broken belt running even 3600 rpm. The belt is more prone to break running overly fast.

Ann 1800 rpm motor is a better motor than a 3600 rpm motor and runs better at the slower speeds. So go with the 1800 rpm motor and set the J4 to 2X.

You want a TEFC (totally enclosed fan cooled) induction rated motor.

Wayne Coe
Artist Blacksmith
729 Peters Ford Road
Sunbright, Tennessee

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Thank you Tim for all the info. I've already learnt a lot from your posts in other threads and other forums. I really appreciate you taking the time. Quite a bit to get my head around. I definitely have a much better understanding of motors now, having read your post 4 or 5 times. You make a good point about trying to source a drive locally, i've found some sites that look promising, and will make some calls on Monday.


I phoned a company today, and the guy i spoke to seemed to think that i would be better off with a 2800 RPM motor as he said they are balanced more accurately than the 1400 rpm motors of the same model... Does it sound like he's just making assumptions, or could this be the case with certain companies?


I'm having trouble finding a crowned drive wheel anything over 5" with either a 5/8" or 24mm bore, but i'm thinking i'll just buy a 5" that will give me 4398 SFM at 3360 RPM, and if i find i want more speed i can look at having a wheel made.


Wayne, I have not run a grinder at 7000 SFM, but my current home built one runs at a bit over 4000 SFM (not RPM), and i think a little faster would be nice. I've also read in a few places that some of the ceramic belts are designed for those speeds or higher.


Thanks for all your help.

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