Switching motor loads with a hybrid (TRIAC + mechanical) relay - Page 1
Forum section: electronics / projects

author: TimNJ
author_rank: Super Contributor
timestamp: August 07, 2024, 10:46:09 pm
content: Greetings,I'm toying around with a DC injection brake design for induction motors. I'm trying to keep it as small as possible. Intended application is for old DeWalt radial arm saws, up to 1.5HP/120Vac/1-ph. Above 1HP at 120Vac, you generally need to move to a big open-frame relays (e.g. something likethis.)1. If I roll my own hybrid relay with a suitable TRIAC and timing system, can I basically forget about the HP rating of the relay in parallel? I could probably reduce the overall relay footprint to about 1/3 this way. As far as I know, the HP rating is related to a motor's inrush current and with breaking the inductive load current.2. In my searching for off-the-shelf hybrid relays, they seem pretty rare. What gives? I guess industrial applications requiring a big relay are not usually too concerned about space, so a big ka-chunk-a-chunk relay is no big deal? Economics? Reliability?3. Any general comments about implementing a hybrid relay?Thanks,Tim----If you're curious, I've attached my rough simulation. Lot of things I need to work out with timing. Also want to figure a way to vary the braking current over the braking period to achieve roughly constant braking torque throughout. Providing continuous gate drive to the SCR and TRIAC, because...well, I 've never used a thyristor before and figure it might be more sure-fire. Maybe could be replaced with a pulse transformer, optotriac, etc. Simulation requires the Bordodynov library, and I turned down Gmin, Abstol, and Rtol a little to get it to run successfully.

author: moffy
author_rank: Super Contributor
timestamp: August 08, 2024, 02:15:15 am
content: I tried looking at your LTSpice file but it was complex and had a lot of library references that I didn't have. Maybe just a JPEG of the schematic might be easier to view. If you are already using an SCR as the controlled rectifier, I am unsure why you need a relay/contactor at all, apart for maybe safety reasons? The SCR can stay off until needed and as long as there is AC across it i.e. a negative cycle, it can be readily turned off. I designed and built previously a 3 phase SCR supply for DC traction motors and it had 6 reverse SCRs for regenerative braking that were permanently in circuit and only switched on for braking. It's surprising how fast and effective it is.

author: TimNJ
author_rank: Super Contributor
timestamp: August 08, 2024, 04:17:07 am
content: Hmm...Let me try to recap my thinking. You could be right...After all, I've only been brainstorming with myself.That said, currently, there is a freewheeling diode switched in during braking to get CCM stator current. I am undecided whether this is truly necessary, but it seems like the RMS braking current is somewhat limited without it, in DCM, at least for this half wave SCR configuration. Also, if the control switch is flipped back on while braking, I was planning to wait for the DC stator current to collapse down to zero before applying AC again. (Necessary? I don't know. I figured this would be rather stressful on the toggle to switch this directly.) So, the relay (SPDT, essentially) follows the SPDT control switch, but with some delay and/or condition(s) that needs to be met before switching.It does add a bit of complexity just to switch in a diode and to (maybe?) save a switch contact. In the current configuration, the toggle switch would only be subject to breaking the motor current and breaking the braking current, but not making the contact for either which would be handled by the relay(s). The DC braking current would be ramped up electronically anyway, and I figured, with a hybrid relay for the 'run' contact, the overall reliability for switches and relays should be pretty good.As far as safety and robustness, I guess that's another issue. I haven't given it the full DFMEA treatment.I attached a rather wonky print of the simulation. The relays in question would be U11 and U14, in the middle, above the box labeled "stator". Like I mentioned, it's only a very crude simulation to get a feel for it. Lots of current measurement resistors, and random things that only make the simulation run, but maybe you get the point.Thanks!

author: moffy
author_rank: Super Contributor
timestamp: August 08, 2024, 04:56:25 am
content: The one thing an isolating contactor does is protect the DC motor breaking circuit from the motor turn off transients.

author: TimNJ
author_rank: Super Contributor
timestamp: August 09, 2024, 04:10:57 pm
content: Thanks. I had based the basic concept off of the block diagram given for this 3-phase motor brake.https://www.peter-electronic.com/wp/en/wp-content/uploads/sites/2/electric-motor-brake-versibrake-l-lp-6-30a-data-sheet-en.pdfI concluded that, if they had decided to use a contactor there, they did it because they needed to. What I do not know is whether any of the applicable standardsrequiresuch a mechanical contactor? Goes back to my question about hybrid relays -- Is there safety standard that does not like the idea of a semiconductor switch for a safety-related applications? I will study the industry standards, but just wondering if anyone knew of the top of their head.

author: moffy
author_rank: Super Contributor
timestamp: August 10, 2024, 12:16:50 am
content: In the circuit it looks like the relay provides latching fault protection and isolation, isolation from the motor because it's on the output, failure in case of excessive current, probably to guard against an SCR shorting. The input side needs its own fusing/CBs. It makes a lot of sense, it's there to protect the motor as a fail safe. Sorry I'm not up on the relevant standards.