Pros and cons switching 40A ac  Triac vs. Relay - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: February 27, 2023, 02:07:04 pm
content: The back story for this question is here...https://www.eevblog.com/forum/projects/sensing-stalled-ac-induction-motor/I have not captured any current waveforms yet, however the nominal rated current for the 1/2HP motor is 8A. Hopefully I can trip before it gets to locked rotor current that is probably in the ballpark of 40A?Important to note that whatever device breaks this current, it will be a rare event, only when the blade jams and this only happens a few times a year at most, so forget about umpteen thousands of cycles.I'm leaning towards this relayhttps://www.digikey.ca/en/products/detail/cit-relay-and-switch/J115F31A120VACS/12418782Release time is 10msShould at least consider a Triac, smaller and simple if isolated tab variety.....One possibility might behttps://www.digikey.ca/en/products/detail/littelfuse-inc/Q6030LH5TP/3997470Need to wait for the zero crossing so about the same speedWill be mounted in an aluminum enclosure, so that should be adequate for heatsinkSurge current is ~200A resistive for 10 cycles (might be a bit iffy)Thoughts?

author: Siwastaja
author_rank: Super Contributor
timestamp: February 27, 2023, 02:50:01 pm
content: Both a relay and a TRIAC could fail as short-circuit when stressed beyond maximum current or voltage ratings, but semiconductor switches do that more easily. With a relay, you might get away with theQuote from: Jester on February 27, 2023, 02:07:04 pmmindset; with a semiconductor switch, not so.I would do it with a relay. Still, make sure the interrupt rating is clearly over your expected stall current. And use a normal fuse as added fire protection.

author: bdunham7
author_rank: Super Contributor
timestamp: February 27, 2023, 04:33:33 pm
content: That relay has a 120VAC coil--how were you planning on operating it?The triac has the edge on speed, as your maximum interrupt time is ~8ms, while the relay may be 10ms plus whatever delay results from its driving circuit and snubbing, if any. Those alternistors are made for running things like inductive motors and if the circuit is set to go off whenever the current rises to a point well under 40A, I doubt they'll be very stressed. Your circuit should probably be sensitive enough that you need to bypass it to start the motor, as well as cause nuisance trips if you overload the saw a little. That would be the price of having quick enough protection to save your $100 bronze gear.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 27, 2023, 04:39:45 pm
content: The DIY Arduino way, on the other hand, allows precision adjustment of not only delay, but reaction curve shape as well, as well as adding special edge cases into mix (like startup mode which ignores the limit for half a second or something like that).I would, of course, recommend a VFD to drive the motor, so you could actually control the torque. But that could be way overkill.

author: Jester
author_rank: Frequent Contributor
timestamp: March 01, 2023, 12:59:58 am
content: This is what the startup current looks like, about 7 cycles, not too bad, I would imagine the locked rotor current is similar. I really like VFD's for machine tools, I was actually thinking with a larger motor, and some reduction via pulleys and a VFD might be able to make it direct drive.Drive the relay coil with a MOC OPTOLooking forward to playing with this, too many projects and not enough time (and I'm retired 10 years).

author: james_s
author_rank: Super Contributor
timestamp: March 01, 2023, 01:56:53 am
content: The biggest advantage of a triac is they don't wear out, you can cycle them rapidly and there are no contacts to burn out like in a relay, they are also quieter. The biggest disadvantages I can think of is they are more fragile, while it's true that a relay can weld its contacts and fail on, it is much easier to do kill a triac and they almost always fail shorted. They also have higher dissipation, a SSR dissipates a significant amount of power roughly proportional to the load current, they require heatsinking.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 01, 2023, 02:52:06 am
content: You can consider a pair of SCRs, more robust than a triac.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 01, 2023, 06:45:28 am
content: Quote from: james_s on March 01, 2023, 01:56:53 amAlso:* Relays rated to 50 000 - 100 000 cycles are easily available. That's dozens of cycles every day for decades. Only if you need to switch every minute non-stop 24/7, then SSR is the answer* More disadvantages for TRIACs:* Very significant leakage current. You can't turn some loads like sensitive indicator lights off at all!* Troubles to turn on, off or both with poor power factor loads, necessitating a snubber design.* Way more expensive per switched powerAll in all, I would not recommend TRIAC-based SSRs unless you absolutely must operate the switch over 50k times during the expected lifetime of the thing. (Phase angle dimming being an obvious example, but heater process control with PWM frequencies around 0.1-1Hz, too.)

author: james_s
author_rank: Super Contributor
timestamp: March 01, 2023, 08:13:19 pm
content: Quote from: Siwastaja on March 01, 2023, 06:45:28 amI was thinking in particular of PID controller applications, where switching is often done at around 1Hz.Other SSR advantages, there is no contact arcing so there is less chance of an explosion or fire in an environment with flammable vapors. The control power is far lower, especially compared to larger relays and contactors, a microcontroller IO pin can easily drive a SSR directly. SSRs with zero cross switching are readily available, that can be an advantage if you need it. The control voltage can be a wide range, 3V to >30V is common. Silent operation could also be an advantage.

author: CosteC
author_rank: Regular Contributor
timestamp: March 02, 2023, 05:01:28 pm
content: Motor protection topic is very old, much older than modern semiconductors, so I suppose you can start or example here:https://www.tme.eu/gb/details/3rv1011-0ka10/motor-protection-breakers/siemens/literature on topic is rather extensive, manufacturers usually provide guidelines how to use and configure their products.Regarding triacs or thyristors it is not so easy to by yourself: to scale it properly, protect against overcurrent, overvoltage etc.I would rather buy module, something likehttps://www.tme.eu/gb/details/3rf2410-1ac45/three-phase-solid-state-relays/siemens/(I did not search for module appropriate for your motor)I is nice packaged and ready to go. Obviously far east equivalents are cheaper and it is reflected in parameters too.