Today is...
Sunday, December 17, 2017
The Motion Control Community Forum, sponsored by Control Technology Corporation.
DC Speed Control
DC speed control is irregular
0 out of 1 members thought this post was helpful...

I know this is probably very elementary, but it is a problem for me....

I have a DC motor, controlled via electronic speed control. There are two physical relays for forward/reverse, and a potentiometer for speed variance. My problem is that about half the time, pressing forward or reverse will not drive motor. Relays engage every time, but no DC voltage at main drive contactor. Twice, I have seen half the volts expected (about 50V DC instead of 92V), and once I saw the volts drop from 90 to 0 while driving, and corresponding speed dropped, too. All readings and observations done with a multi meter.

The DC board is all electronics, fuses are all OK (tested), so there isn't a lot I can check on the board (no scope, test bench etc). Do you have any suggestions I can further check, given limited equipment?

By Steve Myres on 30 May, 2016 - 7:54 pm
2 out of 2 members thought this post was helpful...

Your relay contacts are probably burned up to the degree where they're high resistance. Switching DC (if done under load) is really hard on contacts. Try replacing them.

Yes, in fact people often miss in a spec sheet for relays that the contact ratings can be VASTLY different for AC vs DC. I have seen in one case where contacts rated for 30A AC were only rated for less than 1/2 amp at DC. When switching AC, the arc quenches fast because the current is passing through zero 120x per second. In DC, that doesn't happen, so contacts have to endure that arc for longer as they separate, and if not designed for it, will vaporize quickly.