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#59249 - 12/01/05 07:38 AM DC motor control cheap fix?
Jps1006 Offline

Registered: 01/22/04
Posts: 609
Loc: Northern IL
I normally don't entertain situations like this, but I've got an older friend who has not yet adapted the modern day mentality of throw it out and buy new. Plus I like him and owe him one.

He's got a fancy used tread mill that stopped working. It has two circuit boards and neither one of us is qualified to troubleshoot. He doen't need all the fancy intgrated controls, just a reostat. The motor is DC. Now if it was AC I'd just get a suitable control (3-speed or variable) and mount it to the unit by-passing the boards. Can I expect to be able to find, for less than the cost a working used treadmill, an inverter and speed control, or are we talking industrial control = BIG $$?

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#59250 - 12/01/05 08:24 AM Re: DC motor control cheap fix?
gfretwell Offline


Registered: 07/20/04
Posts: 9026
Loc: Estero,Fl,usa
These things probably use pulse width modulation (similar principle as a dimmer)
That is sure what it sounds like when I use mine. What is on the motor plate, volts amps? That will give you a better idea of your options.
Greg Fretwell

#59251 - 12/01/05 09:25 AM Re: DC motor control cheap fix?
gideonr Offline

Registered: 12/18/04
Posts: 152
Loc: Edinburgh, Scotland, UK
I've fixed variable dc drives before where the dc motor voltage was like mains level. They tend to be just like ac SCR variable angle dimmer controls, but with a bridge rectifier in front. I'm not suggesting any old dimmer with a bridge rectifier will work, they're just very similar. If the treadmill is for the tip, it might be worth a try though.

#59252 - 12/01/05 04:23 PM Re: DC motor control cheap fix?
JoeTestingEngr Offline

Registered: 11/04/05
Posts: 786
Loc: Chicago, Il.
It's funny that you mention this. I have a control/drive board on my desk that one of the electricians asked me to check out for him. It has a couple RJ-xx connectors on it. Pretty fancy but I haven't had a chance to TS it yet. I fixed one for a foreman last winter though.

Do the smarts still work? Is it that the get up and go, got up and went??? The final drive is the most common culprit in those cases. You will typically see a large, square bridge with a thru-hole mount, one or more large electrolytic caps of at least 200WV.(Working Volts) On the edge of the board, expect to see 1 or 2 stand up power tab devices with 3 leads and #s such as IRF640 or IRF9640. There will usually be similar looking device(s) beside them, only with 2 leads and FES16GT or similar #s. The 3 lead devices are enhancement mode power mosfets that PWM the high DC voltage on the electrolytic(s), to drive the motor. The 2 lead device(s) is/are fast recovery rectifiers that deal with the counter-emf from the motor.
The rheostat idea isn't too hot but the rheostat would be, at least at low speeds. Any power not dissipated in the motor would be dissipated in the rheostat. Plus, the only feedback would be human, with your friend providing the velocity feedback reference. This system will have a servo loop that is continuously creating an error reference of the speed you want Vs the current speed. It will reduce the duty cycle of the pulse train to the point required to maintain desired speed.
It would be better to shotgun the power devices than to hope that your friend was quick enough on the control to keep from launching himself. I just checked my invoice from the parts purchase and the mosfets and rectifiers were less than $1.00 each. You would pay alot more than that for a rheostat.
If you tried to use a dimmer type phase control, the fet(s) would need to be bypassed and the control applied to the input of the bridge. The avg. voltage on the caps would provide the motor drive and the speed would still be all over creation.
This might all be moot though if you don't have a decent, low to medium wattage soldering iron and a solder-sucker or solder wick. Feel free to email with any specific questions because I think I hear the DIY police knocking on my door.
Good luck,

#59253 - 12/01/05 04:56 PM Re: DC motor control cheap fix?
NJwirenut Offline

Registered: 09/15/01
Posts: 816
Loc: Bergen County, NJ
If the motor is a 180VDC type, you can get simple speed control with a variac and a bridge rectifier, with a filter capacitor if needed.

#59254 - 12/03/05 12:02 PM Re: DC motor control cheap fix?
mxslick Offline

Registered: 10/24/04
Posts: 785
Loc: Atomic City, ID USA
IMHO, you're talking about something here which could potentially cause serious injury if this "homebrew" controller should fail.

Jps, do yourself and your friend a big favor...check around in your area for a true electronics store (not Fry's or Radio Shack) and talk to the counter guys. Bring the control boards in with you. A lot of those folks are quite sharp and can get you the parts or maybe even repair the problem for you at a reasonable cost.

And email Joe, he's on the right track!

Best of luck and feel free to email me if you need more help, I have a local e-store with the right people too.
Stupid should be painful.

#59255 - 12/07/05 11:03 AM Re: DC motor control cheap fix?
Eng_Dave Offline
Junior Member

Registered: 09/02/05
Posts: 4
Loc: Rochester,NY
Check out the small / low cost DC speed controllers by KB Electronics:

Industrial grade stuff.

#59256 - 12/07/05 02:41 PM Re: DC motor control cheap fix?
Jps1006 Offline

Registered: 01/22/04
Posts: 609
Loc: Northern IL
I appreciate all the responses. I originally posted this question because I have no experience with DC motors. I thought I'd throw it out there to see if you guys would tell me "It's really quite simple, just....."

Which it sounds like some of you are saying, but you've confirmed my suspicions, if I'm going to get myself involved, I's gots some learnin' to do. I think I'll stick with the simple AC circuits for now and hunt down one of those counter guys MX is talking about. Besides why rig it up (and your right, it could be dangerous, you don't know who's fragile body will be using this thing) when it's quite possible for less money have the whole thing up and fully functioning.

Thanks again.


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