DC vs AC motors for shop equipment

[Stacy E. Apelt - Bladesmith]

I have been moving motors to the upstairs shelves in the new shop. This has made me realize what a motor hoarder I am, but also lead me to think about my plans for them. Many are destined to be disc grinders, knife grinders, specialized grinding and polishing tools, and machine shop tools.
I have some 1 phase motors for general use, like running simple bench tools.
I have 3Phase motors for equipment that needs speed control.
I have many new DC motors. They range from 1/10HP to 3HP. For those who aren't familiar with DC motors, they are usually a little thinner and much longer than AC motors. They also weigh considerably more. The 3HP motor has a lifting ring on it. It takes two people to move it around. Prices vary on DC motors, and new ones can be crazy expensive. However, great deals can be had on used or new-old-stock DC motors.
A good 1HP 90VDC motor costs less that $200. A suitable controller sells used for between $100 and $200. It can run easily on 115VAC.
DC motors have some advantages.
The main one is they are more efficient.
Fast start/stop times ( not really an issue one way or the other to knifemakers).
High torque in all speed ranges and at startup - good for our purposes.
Very good regulation and speed control at a set speed.
The weight, different size/shape of the frame, and bolt patterns are an issue sometimes. Many are 56C or have standard foot spacing, though.
DC motors are almost always sealed units.
Bader uses DC motors on the BIII( or at least they used to).
There are different winding types, with PM motors only needing a DC voltage from 0-90VDC or form 0-180VDC. These conform to 110VAC and 220VAC controllers.
Non permanent magnet types have four wires two for the field ( voltage varies) and two for the armature (the same voltages as the PM type)

3Phase AC motors have advantages/disadvantages.
They are cheap to build, start up pretty easily, can run up and down in speed quickly. VFD controllers are a bit more complex and finicky and there can be serious torque issues at lower speeds and at startup.
Power easily delivered by a VFD.
VFD controllers a bit more expensive than DC controllers.

So, what do I plan on doing with these DC motors?
Drill presses, lathes, milling machine, dedicated speed machines ( disc and other grinders, as well as polishers and laps. A 3/4HP DC motor will fit right on an old benchtop drill press ands it will be a high torque variable speed unit for well under $200. Add a tachometer for around $20 and you can set the spindle to any desired speed. The floor drill press will get a 1HP DC motor upgrade. I have one disc grinder that is already DC, and will build at least one more. I have a DC slow speed belt sander for delicate work and sharpening already built.
The big 3HP motor will run the kaiten toishi and another 2HP unit will run a 24" flat lapping wheel (somewhat like a Blanchard grinder). I have a huge Fincor controller that will run both of those.

Any of you chaps use DC motors? Any opinions on using them in shop equipment. Just to be clear here, we are talking about proper industrial motors, not salvaged treadmill motors.

 

[REK Knives]

Yeah I've wondered in the past why more knife makers don't use DC motors... The torque at low speed is what's very appealing to me

 

[Stacy E. Apelt - Bladesmith]

I did some ebay searching and you can get an unused 1HP DC motor and a suitable speed control for $200-$300 total.

Years back when I was buying lots of stuff from HGR surplus through ebay, they had DC motors for $10 because no one bid on them, and controllers for $30. That is where I got almost all these motors. They had some sort of shipping deal with UPS, and once you bought an item, they put it in a big box with any other items you bought until Friday, and ship it all as one lot for stupid low shipping fees. All boxes were foam filled and shipped with no damage. I would get 80# boxes of motors and tools that I had paid as low as $40 for the whole box of stuff ... and the shipping was like $20
At that time a 1HP 3 phase motor would get bid up to $100-150. VFDs were crazy high then, running a good $400. The 3Ph motors and VFDs have come down, but DC is still cheaper.

 

[J. Hoffman]

My one Burr King is 180v DC, my other grinders are 3 phase. Not much difference in end use for me. If my old Burr King ever dies I'll likely switch it over to 3 phase. I also have a few extra motors laying around. 3 phase and DC motors are dirt cheap on the marketplace.

 

[Tony Park]

I did some ebay searching and you can get an unused 1HP DC motor and a suitable speed control for $200-$300 total.

Years back when I was buying lots of stuff from HGR surplus through ebay, they had DC motors for $10 because no one bid on them, and controllers for $30. That is where I got almost all these motors. They had some sort of shipping deal with UPS, and once you bought an item, they put it in a big box with any other items you bought until Friday, and ship it all as one lot for stupid low shipping fees. All boxes were foam filled and shipped with no damage. I would get 80# boxes of motors and tools that I had paid as low as $40 for the whole box of stuff ... and the shipping was like $20
At that time a 1HP 3 phase motor would get bid up to $100-150. VFDs were crazy high then, running a good $400. The 3Ph motors and VFDs have come down, but DC is still cheaper.

This post makes me cry....so jealous.

 

[Jason Fry]

I occasionally ponder upgrading my Rong Fu 31 to a variable speed instead of the belts. Speed controller and tachometer sounds epic compared to what I do now. Need to look into the DC stuff more, as my disk and 2x72 are on a VFD.

 

[Mecha]

I used a DC motor and controller for my new grinder, specifically for the low speed torque as a direct-drive setup. The slower a DC motor goes, the harder it pushes, with the theoretical highest torque output at zero rpm, ha!

With a three-phase AC motor, the slower it goes the less torque it makes, but I think it's way better for running a grinder at high speed.

 

[Jesse Latham]

I have two Bader IIIs. One DC with a 6" drive wheel, the other 3ph with a 3" drive wheel. I much prefer to grind on the 3ph as it has a faster belt speed. The DC mostly has a small wheel on it for that reason. I've sent a fair amount of time on the phone with Bader trying to get more speed to no avail. I will eventually change it to 3ph. Interesting side note: I bought the DC Bader from HH Franks. If you don't know who he is look him up. He is an awesome guy and maker.

 

[Thom Moss]

Got lucky and picked up what looks like a brand new lesson 1/2hp tefc 56c 90v dc motor and minarik drive at a flea market over the summer. Plan was a dedicated horizontal grinder, but ended up using it on my atlas 618 lathe until I sold it recently. Plan is to install it on my new (to me) Sheldon lathe but worry about it being underpowered so might look for a 1hp. Or loot Stacy’s stash of motors...