DC Drive Control Problem

[jcbvet]

We have a very old DC drive (Dodge-Mishawaka Ind. 300A circa. 1970)that has started eating up the external Speed pots.. They sometimes last months, others only a few weeks. 1K 2 Watt potentiometers.
There is a resister in series with the Speed pot. It's a ceramic 10 Ohm 1 Watt.
Could this ceramic resistor be going bad or a bad connection cause the Pot to burn out?

I read that it could also be attributed to a low resistance ground in the circuit but, I have no idea how I would find that.

Looking for ideas about this dinosaur.

 

[itsmoked]

Lots of possibilities. I'd solidly interpose an ammeter in that circuit to see what's happening.

Has anything changed to thermally insulate the pot? It needs to dump a whole lot of power and anything what-so-ever that reduces its ability to shed waste heat will cause it to fail quickly. Adding a knob to it, changing it's location, exposing it to external heat, a reduction in air flow such as a bad fan will all hasten its death.

Also a changed operating point can cause problems. As the pot is turned in one direction less and less of it's structure is available for heat distribution. If recently your process changed the setting that can be contributing.

I doubt the ceramic res is the problem but measure its resistance with the entire system depowered.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jcbvet]

This pot is used regularly for this machine. I get's adjusted from 0-25% every 5 or 10 minutes as part of a manual speed control for starting the process.
Maybe we are just wearing the darn thing out. Could it be they just don't make them like they used to?
this is a 2 watt pot, would getting one that might handle more wattage help?

 

[djs]

I would check how much current draw you have on the center or movable contact of the pot.

 

[Skogsgurra]

"Could it be they just don't make them like they used to?"

Yes, that is certainly possible. I bought some cheap pots a couple of years ago. Several failed in a short time and I couldn't afford to test them all (customer upset). So I bought Bourns single turn cermet potentiometers, the blue ones, and they have worked ever since.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jcbvet]

Ok, well maybe they aren't burning out. The range has changed. Instead of 0-100% on the speed dial, the pot operation is between 15% and 40%, so as you turn up the speed pot, nothing happens until it reaches 15% on the dial, then the drive responds. As you ramp up the speed, it will reach full speed at 40% of the dial. Beyond 40%, there is no speed change.
I would love to replace this drive with a new one but before I spend the money, I want to figure out this issue.

1K, 2 Watt.

 

[Crowbar1]

That sounds like you have an audio taper pot and not a linear taper pot.

 

[jcbvet]

Crowbar1,
Ah ha moment. Thanks......

 

[jraef]

As to the "don't make 'em like they used to" issue, I've found that to be true, in a sense. I had to do a little project that needed a pot last year and just went to the local supply house and bought one, cost me around $8, didn't think anything of it because it's been almost 2 decades since I've bought one. It was utter junk, lasted less than a week. I took it back and asked if they could get me a good brand, like Ohmite. They found me one, it was 5x the price. I bought it because I needed it, and it has worked fine for over a year now. But I Googled it afterward to see how bad the distributor had gouged me, they didn't. So the lesson was, they DO make 'em like they used to, you just have to be willing to pay for them.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[jcbvet]

All pots are Linear. I just replaced it with a new 5K Ohm 3 watt. Same issue.
I can't imagine the commutator being dirty enough to cause this.
There is a 10 Ohm ceramic resister in series with the wiper on this pot. I'm going to take a look at it this evening when the shift ends.
It may be time to just replace the drive and be done with it. The drive is so old there is nobody who can support it. Plus the company got bought a couple times since this was made. The Dodge name is no reflection of the Reliance Dodge products.