KBAC VFD controller tip(s)


KnifeDogs.com & USAknifemaker.com Owner
Staff member
These KBAC VFD speed controllers are pretty popular. They are called "drives".
These are used with 3 phase motors, which are cheaper than 1 phase and are found everywhere.
A VFD can take 1phase electric, like your house has and turns it into 3phase. It can also take in 3phase and work the motor but usually only commercial buildings have 3phase.

To control the speed, the drive adjusts the frequency of the electricity. There are DC motors that use varying voltage to control speed also. They are more expensive and when you drop the speed below half, they start to pulse and lose torque. VFD's maintain torque and smooth operating speeds across 90% of the speed range. When it gets to nearly 0 speed it acts up a bit but you wouldn't use it that slow anyway.

I have 5 VFD's in the shop. They are the bee's knees.
This particular one was brand new along with a new 3hp motor. It never worked right from day one. I worked around it and ignored it for some time.
I finally decided to get it fixed or replaced.

I called KBAC tech support and talked to Denise. She was positive and helpful and gave me some adjustments to try. It didn't work but I don't think it was her fault. The help script didn't cover this. It was a weird issue. My building is 3 phase with means my 30 amp circuits are 208v. I had a transformer put in to boost it to 240v across several outlets because the heat treat ovens didn't like 208v. I was convinced that was the problem. It wasn't.

I called back and we worked on it some more. She asked me to send in a video and she got the engineers to have a look and help troubleshoot. She pushed them for a resolution today, not tomorrow, next week. She gets it.


This is what the inside looks like. The red vertical things are jumpers that will change some things. Who knows what but apparently a lot of things. I'm sure some are for remote sensors and controls, voltage, frequency other than 60cycles, etc.

Along the very top you will see white circles. These are potentiometers that also adjust lot's of things.
These adjust with a little screwdriver. After a few attempts we did what the engineers said and the motor works perfectly now.
It seems the issue was the VFD (drive) was not properly synchronized with the motor and tweaking a couple settings got it all sync'd. The solution was to slow down the acceleration and deceleration of the VFD to better match what the motor was trying to do in response. We changed one other thing called COMP that does something also but I forget.

So, KBAC, great tech support and they have lot's of settings to work with to get your 3phase motor working. Recommended.

On a side note, when checking voltage across 240v leads make sure you have your multi meter set for AC and not DC voltage. The DC voltage setting creates a large arc with a blinding flash and a burnt up probe tip along with colorful profanity. It also causes Denise to laugh when she is on the phone as you do this.


Well-Known Member
I love my kbac, never had an issue with it. I also got the reverse switch on mine and I use reverse on every knife I make. I run the belt in reverse when putting my final edge on. That way I can see the burr forming! The reverse switch took me all of 2 min to install and it was like $17 from kbac


Well-Known Member
I've been looking at VFDs lately for the surface grinder I just bought, it'll be a while before I get it wired up so I've been looking at these.
I have one on my Bader grinder and it works great, never had any problems with it.
What I'm wondering is how to regulate the speed to be in spec with this machine, the chinese ones I've seen have a digital read out for the cycles so I figured I could just set it for 60 cycles and I should be good.

The one you show Boss is like the one I currently have but I don't know where it would be set so that the spindle doesn't over speed.
I guess I could read the amperage draw and that would dial it in close.


"The Montana Bladesmith"
All great info Tracy! I've been running VFDs in my shop since they first became readily available to ordinary consumers....prior to that, I was running 180V DC motors, with KBAC DC drives. Far less technical than the 220v single phase to 3 phase controllers.

I can't count the number of folks who have contacted me, asking advice about variable speed on their grinders.....and I always tell them to steer clear of the cheap stuff, and go straight to the KBACs & quality 3 phase motors.


Well-Known Member
I now have 4 KB Electronic VFD units in my shop and, as BossDog says, they are the Bee's Knees. For a while I considered making my own controller. The thing that pushed me towards KBAC units is the sealed case for the electronics, adjustable features and price. Having a tightly sealed electronic housing in a dirty environment is a must - and most everything in a metal shop is dirty. The toggle switches have a rubber cover to keep dirt out of the switch mechanism.
Having a hand controlled analog speed control is simple, functional and fool proof. Digital push buttons and read outs are nifty, but unnecessary in my opinion..
One thing that I really like is being able to power tap a thread with the forward and reverse switch, mostly done on my mill. For that purpose I reversed the switch's leads so the direction of the switch matches the direction of the tap travel - push the Fwd/Rev switch down and the tap is going down into the thread, position it in the up position and its moving up, extracting the tap - and it has very slow, precise control. The unit has a variable torque value setting which is good for low speed higher torque operations. That feature is really valuable on a mill or lathe.
Also, the units have a dynamic breaking feature that can be set to bring the motor to a rapid standstill when shutoff.
KB's service is superb. I had one issue on a controller and they sent me a new one, no questions.
You can tell, I'm a big fan of KB Electronics. Buy one, you'll never regret it!
One note about motors. Of course I changed out all my 120 volt single phase motors with 3 phase motors, 1.5 and 2 HP. I first bought a Marathon motor that made an awful howl. 3 phase motors that are not designed to be used with an electronic controller may do that. I then switched to Leeson motors and they work fine, no issues. When selecting a motor to be used on a single phase to 3 phase electronic VFD controller (which is what the KBAC units are), check to see if the motor is rated for that service.
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Sean Jones

Well-Known Member
I installed a kbac a few months back for my NWG and can't believe I waited as long as I did to do it. It was worth the cost to have the speed control. Just do it!
I've had mine for about a year now. I groaned out loud at the price when I bought it, but it makes grinding so much more precise.


Well-Known Member
Here's the Lesson motor on my grinder that I like. Note on the label it says"Inverter Duty". The KBAC controllers are phase inverters and should be matched to a compatible motor like this one. Also the motor needs to be classified as TEFC "Totally Enclosed Fan Cooled", that assures no dirt will get into the windings.


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Well-Known Member
I'm in the UK and we don't see a lot of the KB Electronics drives over here, but I have used quite a lot of different drives.

As Bossdog says, a lot of drives don't play well at low frequencies, below about 10 Hz. These tend to be V/F drives, aka V/Hz drives. These vary the Voltage and the Frequency together using a fairly simple (usually linear) relationship. At very low frequencies, the response of the motor becomes non-linear and the motor starts to feel "coggy" (the best description I can come up with).

VF drives are relatively old technology. Current V/F drives tend to be either low-end units or, like the KBAC drives, drives that have been in continuous production for a long time.

There is a newer technology, Sensorless Vector, that the majority of newer drives can employ.

These measure the time difference between peak current and peak Voltage internally, calculate to determine the phase angle between them, then fine-tune the Voltage in real time to maintain the design angle (the motor Power Factor defines this angle, being its Cosine). These can keep the motor running smoothly well below 10 Hz and usually down to 1 or 2 Hz.

Looking at the specs, the KBDA series of drives appear to be improved KBAC drives with Sensorless Vector capability. They look to be worthy of consideration.

The KBAC drives are "Hybrid" drives. They use the potentiometers and jumpers shown in Bossdog's original post for setup. I suspect this is part of their appeal to many folk who actually do or make stuff: the analog controls are more easily understood by an analog mindset and all of us will have been deeply frustrated at some point by menu-driven electronics.

The KBDA drives are "Digital" drives. The digital interface is the price you pay for some useful extra features, including Sensorless Vector.

John Wilson

Well-Known Member
As Tim says, drives come in three flavors. The KB drives are referred to (on this side of the pond) as “variable torque” VFDs. That means the drive controls the output frequency by also varying the output voltage. (Volts per Hz). These drives are typically used in industry for fans and pumps because it doesn’t matter if the output frequency (motor speed) fluctuates a little as the load changes.

The next step up is Constant Torque (or Sensorless Vector). These are used for conveyors or machine motion where the speed has to be very tightly controlled, particularly during acceleration and deceleration of the load.

Then there are true Vector Drives which read an input from an external encoder device to approximate a servo motor (as much as an induction motor can). These are typically used to keep machine processes synchronized and these drives are very expensive and way outside the scope of running a grinder, or any single axis machine tool.

So if you do not go with a KB drive, you can use a VFD that is listed as “Variable Torque”. They are the least expensive models. Look for one with a 0-10v “analog input” for speed control. That way you can wire up a potentiometer as a speed control knob. You can put the VFD over on the wall somewhere and run a cable to your controls at the grinder if you wish. This isn’t a bad idea if you don’t have your VFD in a sealed enclosure.
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Well-Known Member
The less expensive Iron Horse motors that many of us use are also inverter rated motors. Most new 3 ph motors these days will be inverter rated. "IF" (big if):) I remember correctly the inverter rated motor has a tad thicker insulation to better withstand the SWR voltage spikes that can occur with VFD drives, as well as better cooling fan for low RPM operation.

The cheap Chinese drives that many of us use work really nice, and the digital display can be programed to show SFPM belt speed which I like. With a simple filter over the air inlets these drives tend to last several years, and for <$100 they are a good deal. Simple to setup and cheap.

John Wilson

Well-Known Member
Yep. Inverter duty motors have class I insulation. A typical motor has class F. It’s about the type of varnish used. Inverter duty motors can run slower without the varnish breaking down, which leads to grounds and shorts in the windings.

That doesn’t mean you can’t run a VFD on a non-inverter duty motor. But it does mean that if you like to run slow (Below 30 Hz or about half the rated RPMs) the motor’s fan isn’t doing a whole lot and your motor will begin breaking down faster.

I’ve had my Ironhorse motor hot enough to start smelling bad before and I was sure that I had cooked it. But she’s still kicking several years later. I have to say, these el cheapo Ironhorse motors are pretty darn good for what they cost. At this point that motor sure doesn’t owe me a dime.


Well-Known Member
There's something in bearing grounding that an inverter ready motor has as well. Apparently, there is a current created by the chopped signal that can/does go through the bearings, causing erosion of the bearing surfaces.

John Wilson

Well-Known Member
There's something in bearing grounding that an inverter ready motor has as well. Apparently, there is a current created by the chopped signal that can/does go through the bearings, causing erosion of the bearing surfaces.

I didn’t know that but it makes sense. That’s one nice feature on the more expensive drives is that you can change the carrier frequency, for those motors that like to sing- which I believe is the bearings ringing like you said. I went through a stack of motors on a job once where they all ground faulted. But if you meggered them they all passed. They would run fine on line voltage. But As soon as you put them back on a VFD they’d ground fault again. I want to say they were Baldors or US Motor, but it’s been a long time. Being a new line install, the motors were probably all from the same batch and had the same internal defect.


Well-Known Member
Worth noting that I don't think *all* inverter ready motors have the grounds. Just some.

I don't think there's anything wrong with gen purpose motors on belt grinders. Short lines and light loads really don't present many issues relative to industrial applications. The bigger part of potential issues (heat) can be mitigated by using 4 pole motors at 2X.