Need help on DC motor control

I need some information on DC motor controls.

I got 3 controls that don't work. They are Dayton (Grainger) 2M171E controls for a 90 volt armature. They control speed and direction. The controls have 2 power SCRs and 2 power diodes in a bridge configuration. Several of the SCRs have broken leads (TO-220 packages and the leads broke at the entry into the epoxy).

The boards are relatively simple, with only one IC on it - an LM324, and one resistor array made by Dale, a bunch of various Rs and Cs and

4 other signal transistors and one programmable unijunction. I doubt either of these parts are bad. Also the boards show no signs of smoke or anyone screwing with them (thank God for that - 3 virgin boards). One unit has been somewhat cannibalized but not that badly. I can pick up the switch, pot and other minor stuff for it.

I did talk with a tech service fella at Grainger who was quite willing to talk with me but he, also, didn't know anything about these SCRs. He did mention that the controls cost about $450 so they are worth fixing tho he did mention that that if the defect is in the high power control side of the circuit, the cost of repair might be prohibitive. As we talked, I did find out that he's a ham (so am I) and that I might be able to sub another SCR to make it work if he can't find out what generic part number it actually is. The part numbers on the SCRs are 'house numbers' (I hate that) so I will have to start with the manufacturer for them, unless a garden variety SCR will work without too much noticeable degradation. Get one of them running, I'll experiment with substitution on the others. The Grainger-ham told me who makes the control so that's progress and he's willing to query them on part numbers.

Opening the case, I found that there is a terminal strip marked for connecting to the armature and another one to connect to the field. Must be a DC motor but I don't know anything about DC motors and not all that much about other motors, either. Apparently, I have to use a motor that I normally don't seem to have handy here at home. And I don't plan to hook the thing up to just any other motor.

Can any of you shed light on this kind of speed control ?

The fella who wants me to fix these things didn't seem to know much about them (makes 2 of us) so I'm pretty much at a loss to start working on them. The reward for me if I can fix them is that he is willing to give me Hobart engine driven welder he no longer uses. He says that the motor needs rings but that's not a big deal. Sounds like a fairly simple rebuild.



Reply to
John Waverly
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Sure sounds like it is set up for a shunt DC motor, but with armature control only (nothing connected to the field) you should be able to test using a simple Permanent magnet 90 vold DC treadmill motor. Available everywhere pretty cheap these days.

Reply to
clare at

This document from KB is the troubleshooting procedure for their KBIC drive.

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The Grainger drives have some features that the KBIC doesn't, but I'll bet the power circuit is similar. KB also has component level schematics in at least some of their manuals, which may be helpful.

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Over half the price of your drives is in the enclosures. Worst case is find a board level drive with the features you need and pop them into your boxes.

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Ned Simmons

Reply to
Ned Simmons


You can take many different approaches to your problem.

Now, ninnies, hide your eyes and realize that not everytime can things be made to work wearing a white smoke while sequestered in a 'clean room' with 'massaged' air.

John, if you were in a remote location, away from clean rooms and tea and crumpets and just had to make this/these device/s work, Dremel is your friend on the s.c.r.s that have broken off at their to-2oo pkgs. Simply grind away the composite plastic part of the to-220 near the broken lead until you have enough of the terminal to solder and then clean it fastidiously and, well, you know the rest.

Ninnies: that is how to get it done when it can't be done. When you get back in your Perrier and pedicure environment, you can take other avenues of approach.


Postscript: Ninnies are those who can't do a damned thing unless it is in the 'book' o.s.h.a. approved and won't kill a bacterium.

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Btw, think out of the box. That is what made this country what it used to be.


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Ned, John, what Ned says is good. Those controls are full wave SCR drives for

90-100VDC permag motors. Dart, Minaric, KBC and others make various versions based on a similar basic design. Most are almost bullet proof. I have never blown one but they are tough. I made some serious sparks with the output on one and it shrugged it off. The nice ones have the speed and feedback controls to custom tune a drive. The feedback design gives you full torgue at any speed. My old round ram Bridgeport has a Minaric drive on a 3/4 HP field wound DC motor. You talk about grunting if I hog a little too much metal! I can't help on the parts. The SCR's may in fact be DIACS since these are full wave rectifying. RichD

Ned Simm> > > I need some information on DC motor controls.


Reply to

The actual silicon chip is likely not near the leads which is why Buffalo's suggestion is the DING DING DING we got a winner cheapest shot at fixing it.

Do not over heat when soldering.

Maybe look at encapsulating in epoxy if vibration is a problem. I have been using a Dayton/Grainger scr motor control on our band mill to run the elevation screws that uncle and I use to turn trees into lumber.

Wes S

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  1. attempt to repair leads as others have suggested, I've done this a lot. if that works, declare victory.
  2. if not, remove a good "SCR" and make sure it's really an SCR and not a TRIAC. Then, locate a generic replacement (e.g. NTE) with an adequate current rating and the same form factor. you may even find the exact part listed in the NTE substitution handbook (you do have one of those, right? if not, they are under the last time I got one, from your handy electronics supplier)
  3. if new SCR/Triac doesn't fix it, the driver circuit may have bit the dust - trace it out and fix

Reply to
William Noble

Reply to
Don Foreman

How was that MOSES said ? Something like: I LOVE YOU, MAN ! in super huge capital letters, that is.


Thanks for the tip on DigiKey

I looked on the DK site for that 'house number' I alluded to. I FOUND IT !!!!! Turns out it isn't a house number after all. Not being familiar with Teccor numbers, I just surmised that Dayton (actually it was built for them by FINCOR) used proprietary numbers so they could lock up any repair market with confiscatorily high labor rates and outrageously high replacement parts prices.

The numbers in the circuit are D6015L and S6015L (2 of each in the bridge configuration plus one more diode across the output of the bridge). I didn't recognize the Teccor logo (screwey-looking T) but DK showed the parts as being Teccor and showed the Teccor logo. They have the SCRs in stock but not the diodes. So I went to Teccor.home, which turns out to be Littlefuse this week, and they said that Newark has all that I need of diodes and SCRs.


Now along the lines of adding to the leads, I HAD thought about doing that but wasn't THAT desperate. yet....

If necessary, I would have resorted to that (yes, every well equipped shop has a Dremel). Now, it seems that the parts sell for about a buck-thirty each. I'll order what I need and make the shop-owner happy that I got him on the air again. And he will make we happy with that Hobart.

One more chapter to this story Whilst I was there, I asked him what's the story on "THAT Miller welder" sitting in the trash pile (400 Amp DC welder).

He said that "it has a bad circuit board in it. They wanted too much to fix it, like $900".

So I asked him if he'd take ten bucks for it. YUP. So we loaded it onto my truck, along with some tubing I bought. I got it home and took the cover off. Guess what - NO circuit boards in it. It's simply a transformer and smoothing choke and feedback loop to regulate the output current. I don't have 3 phase here (at the house) to run it, but I've been toying with hooking it up to my 5HP rotary converter to see if it will do anything. (I've never done anything other than my Bridgeport on the rotary) I don't need to run it an anything near full capacity, just enough power to test it. If I can get it running (I suspect it's nothing more than either a bad switch or maybe a bad selenium rectifier) I'll take it back to him and "ask for my money back". I'm sure he'd be happy to have it back in his lineup of equipment. Plus, this should earn me some credibility and respect from him.

Does anyone here know of any usual failure mechanisms in Miller DC welders ? (Gunner ?)

So much for now John

Cut,gr>On Fri, 05 Jan 2007 21:42:40 -0500, John Waverly mail dawt calm> wrote: >

Reply to
John Waverly

It may be a blown transformer core. Thats the only usual thing that really goes bad, so far as my limited experience so far shows up, that costs that kind of money.

Btw..make damned sure you are on low settings when running it off the

5hp RPC and you have it fused low enough not to take out some windings in your RPC. That happened to me when running my Airco PhaseArc 300 mig from my 5hp commercial RPC. Fortunately the burned wiring was close to the top of the windings and I could solder it back together.

Also check the diode stack first off before powering it up. They can to to ground, but easy if not necessarily cheap to get replacement diodes.


"Deep in her heart, every moslem woman yearns to show us her t*ts" John Griffin

Reply to

I assume it is a normal bridge configuration with two SCRs replacing two diodes, so it will require normal phase-control SCRs (the least demanding kind of application). You might be able to get replacement devices at Radio Shack, certainly at large suppliers like

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First, though, it sounds like mechanical stress sheared off the pins; you'll want to straighten the sheet metal or remount the loose heatsink, or otherwise do the mechanical part of the fix. Then trace the circuit wiring, to check polarity. SCR stands for 'controlled rectifier', it's a rectifier with an ON/OFF switch built in, and your controller just does a light-dimmer process.

If your input is 120VAC, consider a 400V rating generous. For 220VAC, up that to 600V. And for a 1 HP output, ten amperes current rating (the two SCRs share the load, so that 10A each is inclusive of a safety factor). At DigiKey, 2N6404 is about a buck and a half, rated 16A/600V.

Test your controller on light bulbs as a dummy load, 'cuz rotating machinery would be distracting.

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