View Full Version : A.C.to D.C. conversion

George Hodge
09-24-2002, 10:14 PM
Just finished installing a 1/2hp.1750 rpm.D.C. motor on my 10 inch Atlas lathe.Removed the 1/3hp. motor,the original,bearings were starting to make some noise. The DC.motor came with a control box, with switches for on-off,forward-reverse and start-stop and a speed control. Haven't made a chip yet,what should I expect to be different from the original? The DC.motor and control were from a woodworking plant auction.

09-24-2002, 11:01 PM
I assume the control box is an AC-DC convertor already. If not, you got to find a way to get DC to the motor, as AC will destroy it (unless it's a universal motor, which I doubt). So you either need a huge bank of car batteries or an ac-dc convertor, called a rectifier. A bridge would be best along with a couple of large electrolytic capacitors for filtering. Also an isolation transformer to, uh, isolate the circuit from the AC line. A fuse can't hurt, either. The rectifier diodes, capacitors, and transformer will need to handle 150-200% of the motor's rated current, probably more to allow for extra current draw on a heavily loaded motor. Might be cheaper to just get an AC motor. Course if the control box is all that, then you'll notice no difference at all except for the extra power.

09-25-2002, 03:51 AM
You don't need an isolation transformer or
filtering caps... what you want is a simple
triac speed control circuit. If it has
back-emf sensing all the better. This
is pretty straightforward.

DC motors work very nicely for variable
speed drives; it's only been recently
(last 20 years) that variable speed AC
drives are practical.

Bart Smaalders

Ozarks Hermit
09-25-2002, 09:34 AM

Does the control box have any kind of label on it? The various commercial units vary in complexity (and features). I have used a DC motor on my 9" S.B. for several years, and really like it. I use a KBC speed control on it, all the features you describe plus constant torque. You must dial in the speed and sort of get the "feel" for rpm's (unluss you have a tach). The reverse feature is really nice for tapping, etc.

Ken aka Ozarks Hermit
Shell Knob, Mo

09-25-2002, 02:41 PM
My reply is actually a question. How would one go about building an AC to DC converter? I'm looking for a way that I could do it myself. Affordably, I must add, because money is a bit tight right now.

09-25-2002, 07:07 PM
Ok I was just about to post a query along these lines, so here is what I would like to ask...
I am thinking maybe instead of going to 3 phase VFD that DC might be the answer for my application. I am currently running a 2hp 3 phase motor through a phase convertor and it works fine except I don't have speed control... If I were to consider DC, what would I need to convert AC to DC, control the speed and this needs to have instant reversing, as this operates the table on a large knife grinder...
Thanks again guys for your expected excellent answers... Now to add to Thruds comments on this BBS, he is right, this is the best damn bulletin board on the internet for us gear heads, right?
Regards Jim http://bbs.homeshopmachinist.net//biggrin.gif

George Hodge
09-25-2002, 09:55 PM
The motor and control came as a unit,someone else did all the brain work. I just replaced burnt up toggle switches and replaced the speed adj. knob. Just took a look,the motor is a Reliance DC 1 Perm.Magnet,90 volt,Power matched/RPM. 9" long,5 3/4" dia. 5/8 shaft. The speed control does not completely stop the motor,but seems to have plenty of torque from the slowest to the top speed. The control box is an Emerson Focus 1. About 1/3 of the cover is a heat sink. The bearings on my grinder went out and I needed that,so I haven't tried the lathe yet.

George Hodge
09-25-2002, 10:02 PM
Shaque,for your blade grinder,have you considered a garage door opener motor? The newer ones have the instant reverseing and I don't believe they use brushes,so a router speed control might work. The motors might not be continous duty though.

09-25-2002, 10:23 PM
Yes I thought of that, but as I remember they are only 1/2 hp, and I need 2hp, as this traversing table is 10 ft long and is made out of cast iron. come to think of it don't garage door openers have plastic gears? Oh yeah the table is quite heavy and is about 10 ft long.

09-26-2002, 12:22 AM
Converting to dc is no big deal. The more power you want out of it the bigger the semiconductors(stud type) and heatsinks you have to use. Instant reversing is best done using power MOSFET's - they really have the balls the handle the back emf without complaining very much. The power MOSFETS can also handle the speed control. The only filtering you will really need is for the drive control electronics.

Jim Wilson
12-14-2002, 04:28 PM
Browsing through the message threads looking for this exact topic. Did anyone ever answer the question on how to make an AC to DC controller. Anyone have a URL that has the info. Please don't say just get this and that and through them together. My electrical expertise is limited however I am real good at following directions.


12-14-2002, 06:14 PM
Teaching you advanced power control electronics via a text based BBS is asking a lot. I would suggest that your best course of action might be to ask yout local high school electronics teacher, tech school, or university engineering department for assistance. You would be surprised how helpful these people are when you explain you situation and ask their help. Often the Tech School or Professor will have top students design one for you as a project. Get involved with your schools.

12-14-2002, 07:16 PM
I converted my south bend 9" to dc a few months ago with no real problems so far. I am using a 2 hp dc motor and the control/converter that it used in its first life. If any one is interested I usually have several of these motors in my shop available for almost nothing or trade and I also have the converters/controllers that use a simple potentiometer for controll. Being a newbie at this I do not know if they are Ideal, but the motors and controls are used in a pretty tough application as is, and they are cheap when available.

Jim Wilson
12-14-2002, 08:13 PM

I would be interested in buying a controller from you. My email is jwilson002@ameritech.net if you want to pursue this offline.


George Hodge
12-14-2002, 09:14 PM
I've been using my lathe quite a lot lately and am in love with the variable speed of the DC motor. Just finished cutting the welds holding sprockets on 6 different shafts,for a neighbor. Also turned and tapped 6 nuts for some threaded castors that I had. All done without shifting any belts ! I did use the back gear for tapping.

12-15-2002, 01:22 AM
The motor being as you posted a permanent magnet motor, is NOT universal, DO NOT use a triac type control, Since apparently you have the matching control, presumably you are in business. You might need to find a manual for the control, to be sure of voltages etc. But the 90 volt should work OK off "120V" Ac via the box, and is likely set up that way.

Differences are tough to predict, aside from the great speed control feature! The DC control may have a different stall characteristic than the AC motor, it may stop instead of just slowing and 'grunting", due to an overcurrent limit, etc. Might work BETTER than Ac. Depends how it's made.

Oh, and low control speeds won't have the torque of back gears, so you may still need to change belts or use back gears at times.

Per AC-DC controls, the easiest way is to get a controller from KB electronics, or others. These are SCR controllers, with integral rectification as part of them.
Usually all you need is a box, fuse, switch and control pot to get going.

No need to get into componentry unless you have to. I have used these controls (for generator fields in my case, not motors) and they are fine.

As a side note, only the "universal" motor will work from a triac control alone (and some conrollers "don't like" them, i.e. may fail).
The treadmill motors are permanent magnet, but they follow the triac with a full wave rectifier, hence deriving DC.

I would not presume to try to explain the making of power controls on the BBS because it does involve lethal voltages and fire hazards etc. Too much chance of mis-understandings and resulting problems.

A specific question is much less open-ended.

George Hodge
12-15-2002, 10:52 PM
OSO,I'm using the DC motor setup as it came to me from the auction.All I did was to replace several switches and a knob. I haven't checked any voltages,since I have no complaints and haven't gotten that curious yet. It does have a hefty heat sink built into the control box. At the slowest rpm,it doesn't have much torque,but that's where the back gear comes into play.

12-15-2002, 11:16 PM
The DC motors Oso is refering to are series-shunt wound DC motors - these you can use the Triac controllers on.

It should be noted that DC motors are designed for optimum operation in one direction only. To allow good operation in both forward and reverse requires comprimising the dwell on the brushes resulting in higher heat for the same work done.

A high performance DC controller is more expensive than a VFD for the same horsepower. The current trend in the Canadian MRO field is replacing DC drives with VFD drives because of reduced maintence costs.

12-16-2002, 01:47 AM
Wouldn't expect a problem, you got the whole setup together, Ouggta work.

I guess I thought you hadn't run it yet and were asking more than you were.


Not necessarily series-shunt, or did I misunderstnad you?. If you rectify the AC from the triac you can use the triac type on a PM motor. The treadmill motor I saw a schematic on is just that deal.

Triac is in series with a PM motor with full wave bridge rectifier. Works great.

Universal motors are AC DC as you know. Most are straight series. Might be some other types.

12-16-2002, 12:29 PM
OSO is right about PM motors working ok with triacs (at least I hanve had no problems). Thrud is right about DC motors having a "sweet spot where sparking in minimal at the brushes and power is best, but for smaller stuff, they seem to reverse with no problems. and most motrs are not loaded at optimal loads so they give the sparks etc but do fine in the real world.

Kind of like the bumble bee that can't fly?

12-16-2002, 01:18 PM

I looked at George's new setup Sunday, looks good to me excepting one little item. Needs a drum switch, but thats just me, I like drum switches.

He's been busy with that old shaper I traded to him this year. Don't think that a loose chunk of big bar stock is safe now, he'll be making a tool block or such out of it.

Forrest Addy
12-16-2002, 03:10 PM
I can't stand it! While some guys like DC and SCR drives I like three phase motors and VFD's. You get essentially the same performance for about the same cost but with a more robust motor, a far more comprehensive list of control options and parameter settings, and better modes of electrical protection built into the drive.

Unless very low price and instant availability were driving the choice, I'd reccommend a three phase motor/VFD setup for any machine tool spindle drive and strongly suggest leaving the DC motors and the SCR drives for other applications.

A new 1 HP VFD/TEFC motor single phase input package is available from Dealer's Electric for $240. I put my money where my mouth is: I have VFD's on most of my machine tool spindle drives - even the hydraulic pump of my 20 HP Rockford hydraulic planer.

Yes there are limitiations to any motor drive for running a machine tool. I solve them by oversizing the motor drive a bit and get in return a smooth, quiet, reliable, stable, flexible source of mechanical power.

[This message has been edited by Forrest Addy (edited 12-16-2002).]

12-17-2002, 12:13 AM

Yes, A series-shunt universal AC/DC motor.

I agree, the VFD is the cheapest, easiest, and most effective way to roll. It is possible if you have the right motor to McGuyver a usable setup with DC motors - they both have their places.

Here in Canada most DC drives are being replaced by VFD/AC motor setups to reduce MRO cost and problems inherrent in the DC drives

12-17-2002, 11:51 AM
I say: why not help a man use what he has? Advise him of other stuff, but help answer the question. Man wants a variable speed set has a water wheel, i say use avalve or flood gate on the dam. To do other wise is like sticking yu hands in anothers pocket and spending his money, time and Nuff ranting

A dc motor with a triac will have same torque at low speeds as high. But the back gears multiply the torque, so you dont have the torque of a set of back gears. But several other ways of reducing motor speed you lose torque at low speeds, VFD i have no hands on experience. Just dont try to control a universal motor with series resistance. You lose torque, speed control and effieicieny all at same time.
For a dc motor with PM's (NOT PMS) all you need to reverese is to swap leads. Cheap way to do that is to hook up a common house hold lighting "Four way switch". Drawings every where!!!!! basicaly run two cable (4 Wires) to switch. Wires will be white and black. put the white wires under the light colored screws, the balck wires under the dark colored ones (or vicy versy http://bbs.homeshopmachinist.net//smile.gif ) and it should reverse. Won't work on "series wound", "universal" just works on motors that spin different directions when you swap the wires. That includes 3 phase stuff, drum switches are better for 3 phase. (but for less money use the sinle ploe lamp switch pluse a four way switch to get the "ass out of the ditch".

12-17-2002, 12:09 PM

Calm down, George's setup is neither high tech nor expensive. He's retired and doesn't have to get in any kind of a hurry. He's also a world class scrounger, probably has less than 20 bucks in the complete setup.

I sure would like to have a VFD unit on the new asian made lathe at work, not so much for the variable speed but for control of the starting speed. This machine doesn't have a clutch, uses motor, that 7 1/2hp motor starts with quite a jerk. Variable speed sure would help fill in the gaps in the speeds though. I might manage to get one installed sometime.

We use lots of VFD units in the plant anymore, we have 20+ acres of woodworking factory. Used to be lots of rotary frequency converters sitting about, they are being replaced by VFD units. Conveyor drives are also going the VFD route. Noticed a couple of motors in the motor room which had a VFD fastened right on the motor.

They must be lasting better or the electrical engineers are watching voltage supply better. Couple years ago these VFD units were dying like flies, ground fault problems the electricians said. Still see a dead one in the scrap bin once in a while, but not often.

Forrest Addy
12-17-2002, 12:30 PM
I guess that was aimed at me. You're right, Doc, Halfnut, but it was a question of holding my eager unasked-for helpfulness in check like a full bladder. I lasted for 20 messages before I blurted out "VFD".

There was plenty of advice in this thread on the subject of DC drives both practical and informational. I wanted to suggest what I thought was a needed and practical alternative more for the benefit of people following the thread than George who seems to be on top of his problem. I'm a heavy duty scrounger myself and there's no greater asset to a guy with a bad machinery habit than a talent for sniffing out solutions in a pile of industrial salvage.

A well adapted DC spindle drive is hard to beat. Problem is some low intial cost expedients can be heart breakers. The fact is some bargains aren't worth exploiting: those open frame treadmill motors and controllers can be one example.

For a while treadmill drives were plentiful and cheap and a lot of home shop machinists snapped them up to run their small lathes and rill presses. So long as the users' power requirements were modest and the drive was well protected from chips these motors worked well but when run to capacity they could be quirky and poor in speed regulation.

Since I often involve myself in discussions of machine tool drives I'm frequently asked via email for advise on the how-to and how-to-fix them. Other people's sad experience in machine dool spindle drives has given me a good idea of what not to do. I haven't kept score but it shouldn't be remarkable if I reported more spindle drive problems are associated very low cost spindle drives made with stuff on hand and with fewer problens associated with bargain VFD's (I found the 30 HP VFD that runs my planer under a refrigerator at a local dump) and motors obtained through persistant and vigilant scrounging.

[This message has been edited by Forrest Addy (edited 12-17-2002).]

12-17-2002, 05:03 PM
Naw Addy, Like you I had held it in a long time.
Seems to me as though lots of times, a man with a chevy and leaking radiaotr is told that that model chevvy was never any good, go buy a rolls. Audrey pops to mind- she is gonna get a dozer running and do it as I would- mimimum investment. She may just be in Vendetta Mode, short of cash, wants experience. What ever- and I have no good advice.

Some here are expert at cobbling something up, working around etc. I used to go on tech assists (mostly electronincs) and would cobble up something from the stuff on hand to measure and get them back in operation- using what they had on hand. Then leave a authorization to get them the good stuff they needed. In effect, fix the radiator, let them order something that is not worn out.

I consider my self an expert scrounger too http://bbs.homeshopmachinist.net//smile.gif. And deliberatly taught the techniques to others. Nearly cried when I came to USA, worked at a Navy base and saw what went out in trash. Guys in VietNam trying to trouble shoot with a thousnad ohm per volt meter and these state siders people throwing away Hewlitt Packards oscilloscopes. The world just is not JUST!!!! You, Forrest, have passed out lots of good advice, befitting the needs of the person involved. I was just musing when I wrote. I delete most of my postings- cause upon further thought, I relaize I have nothing to give for what I suggest taking away. That last one should have been deleted also.
But it was aimed at no one, just thinking aloud.
Peace and happy holidays to all

George Hodge
12-17-2002, 09:37 PM
Hey,I appreciate all the insights into my first post !! Thank you all !!! Being a first class scrounger,I feel I'm among friends. Making something broken work has been my lifelong hobby,(so far).Haven't repaired many hearts,but enjoy the shop and garage. I admit,Forrest,when I see a VFC at a garage sale or auction,it's going on my mill/drill. George PS Happy Holidays !

12-18-2002, 08:27 AM
Unlike you guys, I am just a cheap bastard at heart. Why buy material when you can scrounge it? I have hundreds of pounds of small diameter shafting from faxes, Typwriters, main frames. Bought a Main frame at an auction once to get several hundred feet of 64 conductor twisted pair 3m Spectral cable. It cost me almost two hundred to move all of it by flat deck (for a $40 purchase). It was well worth it. I had an Hitachi electron microscope for awhile - it had a 10 HP vacuum pump in it! So much fun...

Go ahead, call me a pack rat. http://bbs.homeshopmachinist.net//biggrin.gif

12-18-2002, 12:11 PM
Thrud: You ARE a pack rat! I think most of us are. The un-imaginative see a piece of junk. Us pack rats see some thing we can't get (with ease) that COULD be be used to do something we might just find time to build some day when we get around to it.

Also you a a pack rat about information. I tell the son in law that he tries to forget what ever he has no immediate need for, thus every thing is a NEW problem for him. I never see a piece of information that needs to be forgotten.

Heres onefor you: a while back you asked about "cold fusion". Yesterday my "Analog " magazine came in. The science fact this month is a two parter discussing the state of cold fusion research. No real info on coldfusion this time- he just discusses how he was involved in measureing "electrovoltage" in rare earths and how hard it wouldbe to duplicate the findings. I think he is going to (next month) say there is some thing to cold fusion, and that thefact that some can not duplicate the work of Pons and company is not proof that their work was totaly flawed. You ,as a pack rat of info might read the artical next time you are at a news stand. Not worth buying unless you are a science fiction reader also.

"You can build anything if you have big nuff pile of junk"


12-18-2002, 12:17 PM

I've enjoyed reading your VFD posts, it's like you are preaching the gospel of VFD. Rotary and static phase converters work but they have their drawbacks which are overcome by VFD's.

How many years did you put up with a phase converter anyway. I'm sure you were sure happy with your first VFD unit.

I've been using a homemade rotary for about 5 years now, sure miss the 3ph line coming off that pole to my old shop. One of the reasons that I bought the place where I live is the 3ph line which runs just south of the shop. Just one more wire to run and another transformer and I could have wild leg 3ph. But they want more to run it in than I'm willing to spend right now. Would cost less than a thousand but I've got other things to spend it on right now. If I was trying to make a living out of that shop it would be installed as quickly as I could hang a meter base, even if I had to borrow money to do it. Electric bills in my old shop, and I had everything I could running on 3ph, were very low.

Keep preaching to us Forrest, and keep up the scrounging.

Happy Holiday's all.

[This message has been edited by halfnut (edited 12-18-2002).]

12-18-2002, 01:28 PM
JAYMO: you ask how to make a AC/DC converter. Go to radio shack and ask for a bridge rectifer, it will be a square block with four place to connect wires. two are marked AC, one is marked plus one is marked minus.

Tie your AC to the AC places (either wire can go to either place) and Woo Laaa!!!! you have DC availalble at the + - leads.

its that simple. and again, it can kill you dead if the AC could kill you. Play with the low voltage stuff unless you know how to handle the higher voltage (wall outlets etc).


12-19-2002, 03:26 AM
Can't get that here. At least none of the porn shops handle it. http://bbs.homeshopmachinist.net//tongue.gif (just kidding)

I agree about the reproducable results. It still does not mean they did not experience it. Conditions may have had some uncertainties they are still unaware of. History has shown these accidental discoveries all along the way in many fields.

Look at the idiot that won a Noble prize for IBM that "discovered" high temperature superconductors. For over a year he told his students (who did all the work) to "throw away any samples that were not (this one color)". What kind of sanctimonious asshole presumes something he has never seen before will be a certain colour? Were it not for a bore student that tested a "defective sample" this dipstick would still be looking. (the student never got mention in his speach - it was me, me, me)

I have had many "mental realizations" over the years that some have received noble prizes for at a later date - I never considered them of any importance to what I was up to and brushed them off. Now that I try to be more open minded - nothing!

I also come to the understanding that even unrelated ideas can have impact on your own course of actions - such as the further development of technologies that help bring about a break through in your own area of investigation. I think science and technologie would greatly benefit if the investigators had a wider knowledge base. Look at Edison. Not really brilliant, but an opportunist willing to apply different ideas to acheive his goal - still making him a genius in his own right. (a good thing!)

05-28-2005, 05:05 PM
Old treadmills are a GREAT place to get DC motors and power supplies. Most treadmills older than 1996 use SCR speed control. This is just a potentiometer varied voltage fed into the controller... easy to adapt. Lots of the Proform treadmills use PWM... a more efficient means of providing power to the DC motor. The PWM controllers are capable of being controlled with a computer. You can find used treadmill DIRT CHEAP. I recently picked up four of them for $150 for the lot! That was a steal, but you can definitely find them for $100. These tend to range from 1.5 to 2.5 HP. Note the difference between CHP (Continuous HP) and HP. A motor that is rated at 2.5 HP is probably about 1.5 CHP. These also have RPM ranges of up to 7100rpm... therefore, if you use a 2:1 gear ratio, you will double your torque and HP (just fyi... HP is torque * rpm). So if you are using a 2.5 HP motor, you are getting 3 CHP and 5 HP peak. Impressive.

05-28-2005, 10:06 PM
Sorry to nitpic but "HP= Torque X RPM" WRONG.
HP = torque X RPM Divided by 5250 (at least when I was in school).


06-01-2005, 11:02 PM
router speed controls are for universal type motors only
A bridge rectifier and heat sink shold serve as a DC source I oncesoldabunch of old dC fans by installing bridge rectifiers into the line cords Steinmetz

06-01-2005, 11:46 PM
The Rat Shack rectifier bridges are not too bad for smaller projects, but they have some physical limitations for connectors and need to be really heat sinked well if you are going to use higher amperage power thru them.

I used to build dc power supplies for the repair bench where I worked a long time ago. Plus I repair large 60 volt AC power supplies the type that you use a big bank of light bulbs for your load. Zener Diodes were big time for us, the brain trust was worrying about the frequency being passed thru the diode and they never thought of measuring the voltage.

That was back when my cable bill was $6.00 a month, so that will tell some of you how long ago that was, and it was was the premium package.


[This message has been edited by jfsmith (edited 06-01-2005).]

J Tiers
06-02-2005, 12:07 AM
If by "rat shack" you mean "radio shack", watch out.

When I was at the in-laws, I needed a rectifier bridge for an old Hougen magnetic mount drill I was fixing for one of the myriad hangers-on around there.

Went to RS, and found that all their bridge rectifiers were 50V or 200V. You need 400V to be sure on plain old AC, particularly if no capacitor follows it.

200V would probably have worked just long enough for me to leave town, then kaboom. And I have to go back there......

Your Old Dog
06-02-2005, 07:01 PM
George, are you happy? See what you started? If you need to remove any rust from it just please don't mention anything about using dirty old diesel oil on this contraption or we might have some bloodshed http://bbs.homeshopmachinist.net//biggrin.gif

BTW, good luck with your project and the sidewalk full of superintendents lined up to way in !! Think I'll just go wipe the oil off my hands.........

[This message has been edited by Your Old Dog (edited 06-02-2005).]

George Hodge
06-02-2005, 10:31 PM
After using the DC setup for about 2+yrs,I have to grin every time I hit the switch! Never have had any problems. No heating while running slow,no lack of power.Nice to be able to adjust the speed at any time.