Looking for feedback on a 120V AC motor control circuit - Page 1
Forum section: electronics / beginners

author: tidnull
author_rank: Newbie
timestamp: January 11, 2023, 09:50:38 pm
content: Inexperienced circuit design beginner here. I would like some feedback on this circuit to switch a 1/2HP 120 VAC motor under the control of an ESP32 dev board. Have I done this right?Please ignore the pin numbers on the ESP32 schematic -- they're for bare ESP32 modules not for ESP32 dev boards.Beginner's stupid questions:I'm planning to use a Mean Well RD-35A dual-output power supply to provide both 5v and 12v power. Will I need decoupling caps to prevent relay coil transients on the 12v rail from disrupting the 5v rail that powers the ESP?Is it reasonable to expect the Omron G7L relay to cope with on-times of several hours? It's not explicitly rated for continuous duty but the datasheet doesn't specify a maximum duty cycle, ether.Any other pitfalls I haven't thought of?Thanks in advance.

author: Benta
author_rank: Super Contributor
timestamp: January 11, 2023, 10:04:59 pm
content: Why the SSR? A simple transistor would suffice.

author: tidnull
author_rank: Newbie
timestamp: January 11, 2023, 10:43:05 pm
content: Two reasons:It's optoisolated, so the ESP32 is less likely to be destroyed if the flyback diode fails shorted.The SSR can switch AC, so if I need to upgrade from a 12 VDC relay to a 24 VAC contactor I don't need to change the low voltage circuit.

author: Benta
author_rank: Super Contributor
timestamp: January 11, 2023, 10:56:28 pm
content: 1: I think you mean "open"2: I'll buy that, but there was no mention of it in your OP. Do we need to play detectives here?I really dislike "You need a crystal ball" postings, sorry.

author: tidnull
author_rank: Newbie
timestamp: January 12, 2023, 05:06:55 am
content: Quote from: Benta on January 11, 2023, 10:56:28 pmI don't see where you're coming from. Knowing why I chose that part doesn't impact whether the schematic, as drawn, will work or not. You don't need to play detective to tell if the design is reasonable. After all, there's a reason I picked an Omron relay over one from Panasonic (and an ESP32 over a Arduino), but it doesn't make any difference to how the circuit works.

author: james_s
author_rank: Super Contributor
timestamp: January 12, 2023, 05:33:10 am
content: Engineers like to understand the big picture, and they are trying to help, there are a lot of people in this forum that are seasoned professionals, if they see something done in a way that looks unusual they'll often offer advice.The circuit looks to me like it will work, although it does look a tad unusual. I would probably use a large SSR to control the motor directly and dispense with the mechanical contactor completely.

author: Vincent
author_rank: Regular Contributor
timestamp: January 12, 2023, 01:39:24 pm
content: If you want feedback that strictly doesn't go beyond an answer to the question "Will this circuit work?", then yes, it'll work.Now if you're rather looking for a more elaborate reply, it'd be beneficial to this discussion to have more information about your choice of parts. Not to question those choice, but simply to understand your thought process. Maybe there's a very specific reason why you chose a mechanical relay. but otherwise a solid-state relay looks indeed like a more simple approach. With one that can be driven from a 5V signal, the entire 12V circuit can go, along with the mechanical relay and its necessary freewheeling diode. Plus that SSR would directly provide that essential electrical isolation between mains and low-voltage parts.

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 12, 2023, 01:46:24 pm
content: I agree the circuit would work and I don't think there is a problem having the relay on for very long time. However, I think it's best to use somethng like an SSR with output of 10A AC and with input voltage from 3 to 24VDC. They make SSR like that.

author: Benta
author_rank: Super Contributor
timestamp: January 12, 2023, 07:57:46 pm
content: Quote from: tidnull on January 12, 2023, 05:06:55 amYou asked for "feedback" and "Any other pitfalls I haven't thought of?"And you being a newcomer to this forum, I'm astounded at your rudeness. If you keep on like that, helpfulness here will drop to a very low level.

author: james_s
author_rank: Super Contributor
timestamp: January 12, 2023, 08:31:07 pm
content: I'll also mention that this is a discussion forum, not a free engineering answers machine. If you come here with questions, you're generally expected to engage and discuss. People are going to want to know about your project, they're going to want to offer suggestions and advice, especially if you share something and ask for feedback. What do the rest of us get out of just answering questions for people? We want to chat about engineering and learn from each other, not just answer questions.

author: tidnull
author_rank: Newbie
timestamp: January 12, 2023, 10:39:21 pm
content: Quote from: BeBuLamar on January 12, 2023, 01:46:24 pmThanks for the suggestion. I did consider using a 3V SSR but it was cost-prohibitive. A suitable SSR with an integrated heatsink (e.g.: Sensata DR2224D20U) costs $81 CAD, which is about double the cost of a small SSR and a mechanical relay. Lower cost SSRs (e.g. Idec RSSDN-25A) tend to require extremely large and expensive heatsinks.

author: james_s
author_rank: Super Contributor
timestamp: January 12, 2023, 10:45:37 pm
content: Those are valid considerations. I'm kind of shocked that a decent SSR at that rating is so expensive though, 1/2HP should be under 10 amps even if the power factor is abysmal. You could build your own SSR right on the PCB, triac drive optocoupler, power triac and a few passives.One issue that is easy to forget with SSRs is the dissipation, they're ideally suited to applications that require frequent or rapid cycles but at some threshold the dissipation across the triac will exceed the power consumption of the coil in a mechanical contactor.

author: tidnull
author_rank: Newbie
timestamp: January 12, 2023, 10:54:51 pm
content: Quote from: Benta on January 12, 2023, 07:57:46 pmI find this response bizarre. You asked a question, then you posted a belligerent, rude, response when I posted an answer. I'm not the one being rude here.Being rude to people whodiscuss their points of view, as is the intended purpose of most forumsis not the way to encourage people to discuss their questions.Quote from: james_s on January 12, 2023, 08:31:07 pmPlease see above. I came for a discussion, discussed my viewpoint, and got sniped at by someone who seemingly prefers hostility over engagement.

author: tidnull
author_rank: Newbie
timestamp: January 12, 2023, 11:05:55 pm
content: Quote from: james_s on January 12, 2023, 10:45:37 pmThe thing I've noticed is that SSR motor current ratings are way, way lower their nameplate ratings. Most 25A SSRs I've looked at are only rated for ~5 amps continuous in motor use. The exceptions to this (such as the Sensata I mentioned, or DIN-rail mounted solid state contactors) tend to be very expensive or require massive heatsinks. For instance, the RSSAN-25A datasheet calls for 12" x 12" heatsink surface area to avoid derating.SSRs are a really neat technology but the devil is absolutely in the pricetag and the details.

author: james_s
author_rank: Super Contributor
timestamp: January 12, 2023, 11:19:31 pm
content: Quote from: tidnull on January 12, 2023, 10:54:51 pmRelax, I don't think he was trying to be hostile. Engineers tend to be a bit blunt, and he's in Germany so quite possibly not a native English speaker. There's no point in getting at anyone's throat, everyone here is trying to help you. Something I've always been advised to do with online forums is lurk for a while and "read the room" before you jump in and start posting. This helps you get a feel of the sort of discussion that takes place and the individual personalities of the various members.At any rate welcome to the forum, I hope you're able to get whatever answers you're looking for to make your project a success.

author: themadhippy
author_rank: Super Contributor
timestamp: January 12, 2023, 11:21:05 pm
content: QuoteAs are many electromechanical relays and contactors.

author: james_s
author_rank: Super Contributor
timestamp: January 12, 2023, 11:22:49 pm
content: Quote from: tidnull on January 12, 2023, 11:05:55 pmI think this is due to the inductive nature of motor loads, and also the LRA, or locked rotor amps. Inductive loads are a challenge in general and due to the mechanical inertia of a motor it initially behaves like it is locked up and draws an enormous current briefly until it gets up to speed. This is especially challenging for solid state devices because of their very small surface area of the actual silicon die, it can be difficult for them to tolerate large overloads, even for very short periods of time.I'm sure you're well aware that there are a lot of inexpensive SSRs out there with... let's just say "highly optimistic" ratings.