Sometimes the triac actuator doesn't turn off

The TRIAC actuator of a DC motor through a bridge rectifier.

I activate a DC motor from the mains through a TRIAC (BTA16) in series with a 10 Ohms resistor for the inrush current limitation, in series with a PTC thermal protection for the motor jamming protection and in series with a bridge rectifier needed to supply the dc motor with dc voltage, filtered by a 100 uF/400 V capacitor.

The TRIAC is triggered by a zero-crossing optotriac (MOC3043). The TRIAC is provided with a snubber of 100 Ohms and 100 nF. Hundreds of motors having this actuator have worked very well. It happened in some particular place that the motor once started didn't stop, but changing the place it keep working properly. It is the influence of the network of this particular place but I don't know which influence. I have added in series an inductance shifting the current with almost 30 degrees and the TRIAC worked very well.

Could some other inductive loads on this particular network have such a bad influence on the behaviour of the TRIAC? I mean a weak electrical network and some asynchronous motors around. Could you please give me any advice?

PS: I have attached a drawing but I was not able to post it. I hope that this description is good enough.

• Add a link of the drawing in your question and we'll post it for you. (Nice to see another fellow Romanian around here) Sep 27, 2012 at 19:57
• The zero-crossing in a weak grid is not such a good idea, because it isn't always "strong enough" and the sudden current may cause "islanding", which is problematic for zero-crossing detection. The simplest of solutions could be an RC low-pass with a fairly small time-constant (depending on the actual effect and the desired cure) to smoothen out just a bit the waveform while not adding too much delay and fire the triac too late. Of course, there may be other problems, but this one seems like a cause for it.
Sep 27, 2012 at 20:23
• I have uploaded the drawing at Google+. Sep 27, 2012 at 21:24

• @Mircea - Sorry, still nothing. "Mircea hasn't shared anything with you". I'm not really an active Google+ user, but I guess we'll have to share a circle. I have added you to my EE.SE circle. I guess you have to do something similar :-/, or make it public somehow. Sep 28, 2012 at 12:47