Hello AAC Group Wisdom,

To start out, I have tried the typical arc suppression methods (MOVs, RC Snubber, and TVSS Diodes) across the contacts and across the motor, with no perceived change. This feels like something different.

I'm working on a system that utilizes DC injection to brake AC induction motors. My circuit uses a phase controlled TRIAC circuit which is then full-wave-rectified to produce a phase controlled full-wave-rectified "DC" signal. Unfortunately, I have begun to have intermittent issues with relay contact arcing that I'm trying to get to the bottom of. I simplified the circuit to do some experimentation and have described the simplified circuit below.

Simplified circuit: 110 VAC mains, going through a VARIAC, then through a bridge rectifier to make full-wave-rectified "DC". I then connect this "DC" signal through a 15A relay contact to a 1HP capacitor-start-capacitor-run AC induction motor. At low voltages (< 15 Vrms), I get a small spark when opening the contact but nothing unexpected. There is then a magical threshold around ~20 Vrms at which contact arcing becomes SIGNIFICANT. Often, the full displacement of the relay mechanism isn't enough to quench the arc and the arc will remain on until circuit protection kicks in, quickly ruining the relay.

At one point, I even removed the start capacitor (as an experiment) and perceived no change.

I'm wondering if this is more than the typical inductive voltage spike trying to keep current flowing. Could this be some kind of resonance between the 60HZ DC pulses coming in and the LC (windings and run capacitor) of the motor? Can anybody help me understand (a) what is happening, and (b) how I might prevent this crazy arcing?

All help is much appreciated.

 

Why not switch the DC using the Triac instead of a relay after the fact?
Max.

 

Did you look at the schematic shown in your link? Why "full wave rectification" instead of the half wave shown? Is your brake relay DC rated contacts? Max's idea makes better sense.

 

Why not switch the DC using the Triac instead of a relay after the fact?
Max.

Hi Max,

Thanks for the question! I left out the larger context. The contactor in question is part of a 3PDT relay with the common of two contacts connected to hot and neutral of the motor, thus allows the relay to switch the motor between an AC (running) and DC (braking) source. All logic in the circuit is electromechanical through a few relays (which also prevent accidental startup and some other features) so I don't have a lot of maneuverability around what switches when (other than coordinating relay latch times). If I could find a way to turn off DC upstream at the TRIAC instead of at the relay, wouldn't the triac be exposed to the same voltage spikes? My understanding is that a triac responds to those spikes by just turning full on so I'd have the same issue there.

Getting back to the original question, I really want to understand what's producing this issue as I work to remedy it. Your continued help would be much appreciated.

 

Did you look at the schematic shown in your link? Why "full wave rectification" instead of the half wave shown? Is your brake relay DC rated contacts? Max's idea makes better sense.

Hi ShortBus,

I originally experimented with half wave with the but the lower frequency of the pulse (60hz vs 120hz) was audible and a bit scary. The torque was also insufficient for braking.

 

Except a triac turns off at the zero crossing point, and can also be made to switch on at the same point if needed.
Max..