Great idea for a lamp dimmer but most motors will not work with that type of speed control. A triac can be used to replace relay(s). You can get approved AC relays for just a few dollars, that would be much safer for you. What experience do you have in working with mains power? If you do not have the experience get somebody to help you that has the experience or abandon the project, we want to keep you around for more projects.
Thanks for your concern, but I do have a 1A fuse, although it's not shown in the above setup.
Yes, I've read that, and I'm a bit puzzled, so I'm asking whether I need it or not. Triac - BT136, Load - 75W, PF - 0.90.
As mentioned in the original post, I have been using this setup for six months. I replicated the design, modeling it after a traditional fan regulator. However, instead of manual input with rotary switches, I replaced them with relays. Now, I am considering replacing the relays with triacs to make the project more compact.
I want to clarify that I have experience only as a hobbyist and am not a certified electrician. For safety, I use a test board with both an RCCB and an MCB. I always keep myself isolated, avoiding any contact with the setup while it is powered. Additionally, I make sure to discharge capacitors.
Your load is not just a motor, it's a complex R, L, C circuit. I'm not 100% sure your design would work without or with a snubber but snubbers are a good idea for inductive loads.
You may go through several Triacs before you find a snubber that works.
Instead of damaging your attic fan motor, not sure how much those cost now, you can get a variable frequency drive for a reasonable cost. Then you can control the speed with a 1 to 10 vdc or 2 to 20 ma input. The frequency drive controls the frequency, amperage and voltage and has all of the necessary protections to keep your motor from burn out. You can find them on Ebay, etc. You just need to know the hp, amps, and volts rating of the motor. a 3/4 hp unit is less than $100.00, less than the cost of a motor. I have used those for many projects and they work well. They even have models that can use single phase in and provide three phase out. Also keep in mind not all motors can work with these. For example many psc (Permanent Split Capacitor) motors don't like it.
3 TRIACS should work just like 3 relays. With the MOC3042 you are building your own solid state relay. I've done that several times but I've only used them for lights, not for motors, and once I made a dimmer.
There's nothing "wrong" with electro-mechanical relays. Although they are mechanical, they usually last a long-long time as long as they are operated within their specs. I think I've only seen a relay fail once and it was in the engine compartment of an old car. (Cars have lots of relays.)
I added a "fan" relay to my home heating system many years ago so I could use the fan switch on my thermostat to turn-on the fan without the heat. I've never had any trouble. (Of course it only gets switched-on occasionally, mot even once a week on-average.)
Your load is not just a motor, it's a complex R, L, C circuit.
I don't think anyone here can say with 100% confidence that your circuit is safe and will work.
If you want to go further you will need to do some experimentation. Time to use your digital storage scope if you have one.
I've had great results controlling the speed of a pump (and during testing my angle grinder) using phase cutting. That were ~700W loads. This were small universal motors; induction motors will behave rather differently.
Using the BT136-600 I recall, and I added an RC snubber as well to help it switch off properly.
Yes that would work fine. Universal motors are actually AC/DC and have brushes to change the field/armature magnetic phase relationship. Here is a link that will explain it better then I can. Johnson Electric
