AC or DC Variable Speed Motor?

Matthew Gregory

Matthew Gregory

Chief Executive in charge of Entertainment
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Okay gang, I'm getting ready to take the plunge and swap out the motor for my Square Wheel to a variable speed unit. I've already wired for 220v, so that's not a problem -- as such, I'm guessing I'll be doing a 2HP motor. Does anyone have an input on which type they would prefer , and why?

I've already spoken to a few of you folks about this, and if you (read: Fitzgerald) wouldn't mind repeating yourselves, I was thinking this thread could be a real asset to those of us that fail miserably every time we try and search on BF!

So what do you think? AC or DC?
 
If $$ is not an issue..then the Ac three phase motor with variable frequency control is better, I think. You can, however, buy a DC mpotor on ebay for example and a variable speed controller for much less money. I have both and cannot tell difference in operation. I have variable speed AC on two machines and variable speed DC on one machine. Would not want to do without.
 
On DC: _honestly_ rated 2 HP TEFC DC motor will most more than a "VFD+2HP 3Phase AC" combo. You can always get one of treadmill DC motors - but these are NOT honestly rated, are NOT TEFC (instead, these are open frame) - so you'd need to fashion some dust/grit protection for the vents . If you can manage to protect such open frame DC motor, then going that route might make sense. Otherwise I most strongly advise going with 3phase AC + VFD.

Searching for VFD in this very forum should give you everything you ever wanted to know about the subject :)
 
AC + VFD is good. Getting one from Rob Frink with the proper enclosure is way better. I keep wondering when my VFD will give the dreaded POP and I'll have to give Rob a call.
 
Yes, a 2hp DC motor alone will run from $800-$1200 or so ... new. For camparison, I have a 2 hp VFD motor and controller package for Bader/sq wheel conversions for $849 in stock. This includes the motor, controller, electricall connectors, cord and hardware.

I like VFDs because they have more features ...but most importantly, they have programmable motor protection for current and overload. I like AC motors over DC because they do not have brushes and a communtator which wears and makes the DC motor somewhat perishable over time.

In a 2hp size, my choice is AC/VFD if I'm buying new. Otherwise, anything on the used or surplus market is fair game and the price will determine which set-up I choose. For example, I'd jump all over a $200 2HP DC package on Ebay..everytime.

Your square wheel motor is 3450 rpms...correct? DC motors typically come in either 1800 or 2500 rpm frames. Where as a AC motor will come in 3450 or 1725.

Have fun with your project!!

-Rob
 
The physics of 3-phase AC motors are such that they inherrently give constant torque over the entire speed range. Those of you who "really lean into it" will appreciate this. I believe that the torque of both single-phase AC motors and DC motors falls off as speed is reduced. Because VFD's have become an industrial commodity with little difference in technical specs, the various manufacturers are putting in all kinds of fancy features. Most of these features are not likely to be useful to a knifemaker (i.e. remote monitoring and control using a PC via a wireless network) but the protection functions that Rob mentioned will. Some give you the ability to select a number of preset speeds using external switches. The reliability of the AC drives should be pretty good regardless of the brand because the industrial users demand it. They all use the same basic parts; there are now only four companies that make all the switching devices used. There is also a lot of OEMing in this power range where one factory makes the same product, under contract, for several brands. The motor reliability related issues that Rob mentions are probably the reason that many more AC motors than DC motors are used in idustrial applications.

The most important thing is regardless of whether you use an AC or DC drive, mount the drive away from the grinder so that the metal dust is not being blown into it. Almost all of the drives manufactured are meant to reside in a clean environment. This is the same reason you use TEFC motors. The exception are the ones are designed to be dust proof, from the beginning. It is rather costly to make a NEMA1/IP20 enclosure dust proof so most industrial users (the primary customer base for VFD's) put them in a clean equipment room. I have only seen two dust proof drives on the market, the one that Rob Frink sells and the ones that PDL (now defunct) used to sell.

Phil
 
I just checked the current prices on the DC VS setup I put on my KMG when I got my grinder in 2002. (I wonder what "S/N grinder I got from you, Rob? It was an early one.) The 2HP Leeson (P/N 108266) is $1051. The Minarik MM23401C controller is $290.

I couldn't add anything to what's been said, Matt. :) I bought my DC VS because it was the "best recommendation" at the time from a number of people. A year or so later, everyone was switching to AC VFD. If I ever had to replace a motor/controller I'd call Rob and not mess around elsewhere. If he can help you get a fit to your Square Wheel, I recommend you call him, too. :)

The only thing I somewhat disagree with "current wisdom" is the oft heard "the DC motor has no torque at low speeds". I personally find it has all the torque I need, possibly because I have never been one of these power junkies who has to lean into it until the motor is strained. I have always been one to let a fresh belt do the work for me. OMMV. There's also the fact I spent some time twiddling trimpots to make it as happy as it could be. Small adjustments can make a difference, IMO.

I'd be hesitant to buy a used DC VS PM unless you know what it's been through. Like I told you, there are wear parts. Like Rob said, it better be a great deal.
 
if you really do not want to worry about metal dust, you should opt for the NEMA 4 enclosure for your VFD. It is more expensive, but you have the piece of mind that it is sealed.
 
gixxer said:
if you really do not want to worry about metal dust, you should opt for the NEMA 4 enclosure for your VFD. It is more expensive, but you have the piece of mind that it is sealed.
Click to expand...

Yep. That's what Rob sells and what I wish I'd bought.
 
Thanks for all the answers, guys!


...and Mike, it's always good to hear you reiterate something - I rely on you for info a lot, and it's greatly appreciated!



rfrink said:
.
In a 2hp size, my choice is AC/VFD if I'm buying new.

Your square wheel motor is 3450 rpms...correct? DC motors typically come in either 1800 or 2500 rpm frames. Where as a AC motor will come in 3450 or 1725.

-Rob
Click to expand...

Rob, the stock motor is 1725 rpm, rated at 1hp. Is the idea to stick to the stock rpm, or do I gain an advantage by increasing motor rpm and being able to retard speed using the variable speed control?

...or maybe I should be discussing this with you over the phone!
 
I picked up several 3-ph AC motors on ebay from 1hp thru 5hp none over $150. Two of them are washdown motors used in a car wash. each valued at over $1800 new, bought both delivered for $150. Buying a good motor on ebay and a controller new either on ebay or through a retailer will probably be less than $500. As pointed out previously a 2hp DC will be close to $1100 new and difficult to find (though not impossible) on ebay. Getting the DC controller is even harder to find on ebay. Looked for a couple of years before I went to AC with VFD. I now have both an AC and DC variable. I use the AC all the time and use the DC to travel to classes with.

Chuck
 
gixxer said:
if you really do not want to worry about metal dust, you should opt for the NEMA 4 enclosure for your VFD. It is more expensive, but you have the piece of mind that it is sealed.
Click to expand...

There are two ways to deal with the environment; increase the protection level of the enclosure or move the equipment to a more benign environment.

The drives sold by Rob and others take the first approach. It is tricky to engineer and invariably adds significant cost. The level of environmental protection can be compromised if the installation is not done correctly (depending on the design).

The second approach is the least expensive. The VFD is mounted somewhere "clean", say high on a wall or in the next room. Many VFD's have allow you you to mount the display remotely. Alternatively, one can also wire in a speed control pot and an ON/OFF switch (about $30-40 at the most) installed close to your grinder. The wire used would be the same type as your computer serial cable. This approach does require one to crack open the manual and do some reading. Let me know if you want to try this approach.

Phil
 
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