VFD Amperage Draw

Colby L.

Colby L.

Joined
Feb 3, 2020
Messages
91
Hi All - I got my new grinder yesterday and need to build up an extension cord as I will be mostly grinding outdoors. I am trying to make sure I use the correct wire size in the cord. If I check the wire size calculators I am coming up with needing a 12 gage wire based on the specs on the face plate of the motor and the run I am dealing with +/-42'. I will be running it at 230 volts

I have a couple questions:
1 - I have gathered that there is an amperage increase on the incoming side of the VFD but I am having trouble figuring out how much. Does anyone have a rule of thumb or calculation I can run to determine how much increase in amps I should plan for? The VFD is a KBAC 27d.
2 - If I run the VFD at 2x to get more RPM out of the motor does that further increase the draw?

If there is anything else I should consider in my wire sizing please let me know. The motor faceplate is shown below. Thank you in advance!

ZthqXRM.jpg
 
The VFD doesn't draw much extra current. Your motor draws 6.8 amps per leg at 230VAC. 12 gauge will be fine for anything up to 50 feet. I would just make the cord from 10/3 and never look back. This will allow you to use the cord for running a welder or HT oven in the future. 75 feet or 100 feet would also be a good idea, as you never know when a longer cord may be needed.

There are lots of online charts for voltage drop per length and suggested wire size.
 
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Per the voltage drop calculator on this link: https://tinyurl.com/ya3lvyuw even with a 100 ft cable, using an 8 amp current draw the voltage drop is only about 1% (allowed is 3%), or about 2.5 volt drop when using 12 AWG size wire. So for a 50 ft cable and 8 amp current draw, even #14 AWG (rated at 15 amps) would be just fine per the NEC code book.

For a 20 amp load and 50 ft #12 AWG extension cord you'll only have around 2% voltage drop which is still fine. So, for once I'll disagree with Stacy and say #12 AWG is all you'll need not to exceed voltage drop for grinder or HT oven. "IF" you think you'll ever need an arc welder it might be good to have #10 AWG. #12 AWG rated at 20 amps, #10 AWG rated at 30 amps.

edit: I should mention that all the Chinese #12 extension cords I've checked have actually measured at #13 AWG, and I'm sure the Chinese #10 or #14 AWG would also run on the small size.
 
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Thanks Ken and Stacy. I ended up going with 12/3. I bought it at HD off of the cut per foot rack - hopefully its actually 12 gauge... My logic behind the 12 gauge was, like Ken, I saw that it was good for at least 50' at 20 amps. That and I put 20 amp heads on it, and a I wired up a 20 amp receptacle/breaker. So if I ever get a welder that requires 30 amps I couldn't use the cord anyway.

I got everything up and running and it all seems good! I did some messing around and the one thing I did notice is that when I was trying to flatten the knife I was working on when I really pushed into it I could tell the motor was bogging down. The piece of steel was about 1 1/4" wide by 6" long. This is my first time using a grinder so I wasn't sure if that was a scenario that could limit even a 2 hp motor. Does that seem normal? When I was just grinding the sides of the steel I couldn't sense any bogging. I figure grinding that much surface area was too much for even a 2 hp motor.
 
With a flat platen (especially bare steel platen) and a flat steel as you describe, really leaning in on it could for sure bog the motor. BTW, where you running the motor at 100%?
 
Ok good to know. Yes it was a flat bare steel platen. I honestly don't remember if I had it cranked all the way up. I would tend to guess no as I was being fairly cautious since it was my first go at grinding, but I may have turned it up when it statted bogging down. Would it make a difference if it was/wasn't?
 
The speed can make a difference. I know it's way easier to bog down mine when it's running slower.
One good way to get around this is to use a
Wheel to get most of your grinding done, then switch to the flat platen to finish up.
The wheel presents a smaller surface area, so you have lost friction and so bog down less. Makes for faster grinding!
 
A.McPherson said:
The speed can make a difference. I know it's way easier to bog down mine when it's running slower.
One good way to get around this is to use a
Wheel to get most of your grinding done, then switch to the flat platen to finish up.
The wheel presents a smaller surface area, so you have lost friction and so bog down less. Makes for faster grinding!
Click to expand...
Noted - I got a 10" contact wheel and have been reading it helps save belts when used for profiling so I'll try that moving forward. Flattening on the platen didn't work that great anyway. It just reinforced my theory that my next order of steel will be precision ground.
 
When running a 2hp motor at 50% speed you're only getting somewhere around 1hp. The only place you're getting the full 2hp is at 100% speed with a VFD. Note, I'm saying "somewhere around", not specific power. If you look at a power curve you'll see how speed affects power. You can see the dotted line for HP, it's almost a straight line from low speed to 1800 rpm (max motor speed) where it is 5hp (Baldor 5HP, 1750RPM, 3PH). When using a speed doubler to get 3600 rpm from motor you actually lose a good bit of torque from 1800 to 3600 rpm where you really need the extra torque.
HP-vs-RPM-curve.jpg
 
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That is a good chart Ken.

However, many times in these discussions the torque is given high priority, and in knifemaking torque is a low priority. For our needs RPM and HP are all that really matter.
A 2HP motor on a VFD will deliver whatever a knifemaker needs unless he is in a manufacturing setting where more horsepower is needed for rapid stock removal (hogging). The 1800 vs 3600RPM debate is always ongoing, but I feel that an 1800RPM motor with the VFD set to deliver 0 to 3600RPM is the best setup.
 
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