AC/DC?

J. Hoffman

J. Hoffman

Knifemaker / Craftsman / Service Provider
Joined
Jan 1, 2011
Messages
1,784
I'm putting together another grinder after Blade and have two motors laying around. I can't decide which one I want, but I'm leaning towards the AC. The grinder will be a dedicated flat platen.
The AC is a brand new Leeson 1.5 HP but only runs 1140 RPM, the VFD will run it at 2280.
The DC is a used PM 2 HP 1750 RPM. I don't think a DC drive can double the speed like a VFD, but I'm not sure. The DC motor is much larger and weighs a ton. What are everyone's thoughts?
 
Well first you have to flick the switch and make sure you have some high voltage. Take a knife, get it hot and make it hard as a rock. Look at the motors and decide who made who and with the right choice your blade will be on the razors edge in no time.

Sorry that was of zero help for you but I couldn't resist. One of my favorite bands and still the best live show I've ever seen.
 
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Since a dedicated horizontal grinder is more used at lower speeds than high, both will work. I agree that those huge DC motors are a bit large for a grinder. In horizontal, it would need a custom stand built to hold it, as the motor would extend far below the benchtop.
 
scott kozub said:
Well first you have to flick the switch and make sure you have some high voltage. Take a knife, get it hot and make it hard as a rock. Look at the motors and decide who made who and with the right choice your blade will be on the razors edge in no time.

Sorry that was of zero help for you but I couldn't resist. One of my favorite bands and still the best live show I've ever seen.
Click to expand...
I think your references will be missed, I appreciate them tho.
 
Well first you have to flick the switch and make sure you have some high voltage. Take a knife, get it hot and make it hard as a rock. Look at the motors and decide who made who and with the right choice your blade will be on the razors edge in no time.
Click to expand...

I came here hoping for something like your reference. Thank you.
 
Yes it is 3 phase. I'm guessing 6 pole, but not sure.
 
I have two Bader 3s. One is a factory DC variable, the other is 3ph VFD. I much prefer the 3ph. It runs much faster than the DC. The DC has a small wheel on it 90% of the time.
 
Sorry, I misread "dedicated flat (platen)" as meaning dedicated horizontal. Reading too fast.

As the others said, 3Ph has less issues and is a lot smaller. I have a 150 pound-plus 3HP DC unit that I may use someday on a large grinder setup, but I also have a 5HP 3Ph that is half the size.

Where DC shines is low speed torque .. which isn't a big concern for most knifemakers.
 
AC is the way to go, unless you have plenty of the DC stuff around.

DC is old-school and obsolete in all but a tiny number of extremely niche industrial applications.

DC varies the Voltage supplied to brushed motors. Speed varies with Voltage on these motors, as does available power, and maximum rated Voltage is reached at Maximum rated speed.

AC varies the Voltage and Frequency together up to the rated Frequency and Rated Voltage. However, AC can hold the Voltage at the rated value whilst increasing the Frequency further. The speed varies with Frequency. The available power varies with the Voltage. This is how a VFD can run an 1800 RPM-rated motor to 3600 RPM.

If you happened to be around when an industrial plant got refitted from DC to AC and have tons of suitable DC drives and motors to hand, using DC makes sense. Otherwise, you need to accept that something will fail at some point and you'll need access to replacements. Replacement industrial-grade AC stuff will continue to be available off-the-shelf for a good while yet. Industrial-grade DC stuff is already obsolete.

Note that there are VFDs that have "Sensorless Vector" capability. Without getting too nerdy, non-SV drives tend to run motors smoothly down to 10 Hz or perhaps a little less. SV drives will run down to 1 or 2 Hz smoothly. SV effectively brings the low-speed capability of AC drive/motor systems up to the same sort of level as DC.

All else being equal, the premium for an SV drive is now very small indeed. The only non-SV drives I see recommended are the KBAC (which, in electronics terms, were designed shortly before Noah made landfall) and some of the ultra-low-cost drives on ebay, etc. I think the KBDA range of drives are SV and they have the same environmental protection as the KBAC drives. I have used a few of the ebay drives and actually been quite impressed by the drives. Much less so by the documentation.
 
timgunn1962 said:
Note that there are VFDs that have "Sensorless Vector" capability. Without getting too nerdy, non-SV drives tend to run motors smoothly down to 10 Hz or perhaps a little less. SV drives will run down to 1 or 2 Hz smoothly. SV effectively brings the low-speed capability of AC drive/motor systems up to the same sort of level as DC.
Click to expand...

Will a SV driven AC motor produce the low-speed torque of a DC setup? If so, that's impressive. When I was using an AC system, the slower it was ran the lower its torque was. With the DC system it gains more torque as it goes slower. At low speed the AC's torque was incomparable to the power of the DC setup.
 
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