Troubleshooting an old DC variable speed drive

[ALVIN DAVIS]

As DickDV suggested change that sucker out. If you do, look at the CAROTRON drives. I have used them many times and they are good stuff.

 

[rootboy]

Dan,

The reason I asked about FF is that as the motor accelerates up to speed, (during the period that 1A-4A are being activated) is that it will click on/off several times, always accompanied by a really nasty, long-burning arc. An arc that sometimes seems to want to keep burning until as small draft extinguishs it. I've seen it burn as long as 5 seconds.

DC will do that, I've seem them arc across until the contacts melted down. Try cleaning the arc chutes.

If you replace them, be sure that the ones you get are designed for DC (and preferably come with a magnetic blowout coil).

Or, if a small draft is what you need, install a muffin fan next to the contactor...

For now, the work-around is to whack the 4A with a broom handle every time they start the motor.

That'll learn it.


BTW, check out:

http://www.dccontactors.com/projects.html

 

[leitmotif]

Good drawing - dont understand all your symbols but your drawing is for you to comprehend so not a big deal.

Relay FF allows for full shunt field strength during start. From your drawing it appears to sense armature current such that when the motor is up to speed (out of starting resistor sequence) and armature current is low it then cuts in shunt field resistance ie reostat to allow for speed control with reostat. The only problem with this setup is when someone forgets to reset reostat to zero (low??) resistance then when the resistance is "cut in" the motor will speed up draw a whole bunch of current and either cause FF to short out shunt field resistance again - I have seen the breaker trip on a large multi stage centrifugal pump when the reostat was not reset to the "start / low??" position.

I suppose your effort and time repairing relays means I cannot count on you as source for DC relays - dang and double dang.

Dan Bentler

I should have gone into a little more detail on FF actuating during starting. As a starting resistor is shorted out out
1. Volt to motor goes up
2. motor current goes up
3. When current drops FF activates
4. When next resistor is shorted 1 2 and 3 repeat.

This is assuming FF is a current sensing relay which I am assuming from your drawing. Since you are not completely opening the line ie you still have the reostat connected in parallel I do not fully understand why the relay is arcing and would have to see to evaluate severity of arcing. Opening shunt field or any winding on DC (and sometimes AC) will give you arcing on contacts.

Double check your wiring a lot of guys have been in that cabinet. The FF contacts should be in parallel with reostat.

FF should not be activating during start. Again long distance diagnose is difficult. If you can see if there is some kind of adjustment on current sensing.

It should have an arc chute and maybe a blowout coil to suppress arc.

It it is that big a deal you may have to replace it. Ballpark high estimate 500?

Dan Bentler