Trying to build a custom step down transformer for a small a.c. motor. - Page 1
Forum section: electronics / repair

author: tehmagicsmoke
author_rank: Contributor
timestamp: November 07, 2018, 02:14:01 am
content: I'm using a small motor, squirrel cage and hepa filter from a furnace as a solder fume filter.The motor just spins TOO fast. So I'm trying to slow it down to reduce noise.I have lots of transformers and lots of extra enameled wire.What I'm wondering is if I will need to put a capacitor on it somewhere, either in parallel or series for the emf.What gauge of wire I should use.And any other suggestions.I pulled this motor apart and greased the old bearings. I really need to replace them. I just don't want to take it apart a second time, at least yet... I stabbed myself in the base of my thumb and got a nice scar from it, so I'm not looking forward to doing that again.

author: tehmagicsmoke
author_rank: Contributor
timestamp: November 07, 2018, 03:48:58 am
content: The motor is a fasco 7002‑2941, 2 amp, 3000rpm, 120volt ac.https://www.ebay.com/p/Fasco-7002-2941-Furnace-Draft-Inducer-Blower-Motor-3000-RPM/2121372919

author: coromonadalix
author_rank: Super Contributor
timestamp: November 07, 2018, 10:54:46 am
content: Why dont you use an ac variable speed motor controller scr/triac based ? your setup is radical loll unless you arc weld, mig weld ?

author: tehmagicsmoke
author_rank: Contributor
timestamp: November 10, 2018, 11:52:05 pm
content: Well, I am noob when it comes to electrical enginerding, but I think I have the simplest motor there is.I think it's a synchronous drive 120v a/c motor.I'm trying to build it as cheaply as possible. I see some motor speed controls on eBay. They all just look like ceiling fan speed controls/just a big potentiometer. I have some like that.I'm not finding results that seem to be what you're pointing me toward. Nothing with frequency modulation. If you know of a good low budget option I'd love to see a link. Thanks!

author: coromonadalix
author_rank: Super Contributor
timestamp: November 11, 2018, 01:50:30 am
content: the problem is when your motor "kick from start to spin", it will be more than 2 amps of current spike. An scr based controller cost pretty much nothing, you have tons of schematics on the web, for me its a very basic motor.

author: james_s
author_rank: Super Contributor
timestamp: November 11, 2018, 02:21:44 am
content: That's a shaded pole motor, they generally respond reasonably well to inexpensive thyristor speed controllers, very similar to light dimmers with some modifications to deal with the inductive load.

author: tehmagicsmoke
author_rank: Contributor
timestamp: November 12, 2018, 02:29:18 am
content: Thanks guys. I was big into dc motors in my teens.I never knew there were so many types of a.c. motors!I found a fan speed control in my box of spare parts.I've been working on my parents house electric alot. I knew I should save that for something!No idea if it'll handle the amps but I think it's for a ceiling fan, which should be higher amps than my little motor.I really need to get a degree in electrical engineering.If you have time to point me toward some info about what modifications I should do to make that fan speed controller compatible with my motor, I'd love to study up on it.

author: Circlotron
author_rank: Super Contributor
timestamp: November 12, 2018, 03:07:06 am
content: I did this with an exhaust fan. Put several microfarads of capacitance in series with the motor. Motor inductance protects cap from mains borne spikes so doesn't have to be an X2 cap, ordinary film caps seems okay. 630V rating and you're good to go.

author: james_s
author_rank: Super Contributor
timestamp: November 12, 2018, 04:31:07 am
content: If it's already a motor speed controller there is no modification necessary, just hook it up. My mention of modifications was referring to the basic light dimmer circuits. A ceiling fan motor will draw much more than that little blower so you should be fine.

author: 6PTsocket
author_rank: Regular Contributor
timestamp: November 12, 2018, 04:37:34 am
content: Quote from: coromonadalix on November 07, 2018, 10:54:46 amTriac speed controls only work with universal (brushed) motors. Anything from a furnace would run far too much for that type of motor. The brushes would be gone in no time. They use induction motors that are not easily speed controlledSent from my SM-G900V using Tapatalk

author: james_s
author_rank: Super Contributor
timestamp: November 12, 2018, 04:48:49 am
content: Quote from: 6PTsocket on November 12, 2018, 04:37:34 amThis is incorrect, triac speed controls are very commonly used with shaded pole type induction motors, I have one right here on my bench that works perfectly. Virtually all ceiling fans use induction motors, it is a broad category, some of which can be controlled this way and some can't.

author: tehmagicsmoke
author_rank: Contributor
timestamp: November 12, 2018, 10:44:24 pm
content: I just wanted to double check this. The one on the left had the fan symbol on the knob. The right has a light symbol. I just wanted to double check that they didn't get switched. I am skeptical, because I don't see why a light would need the coil.Thanks for all your time and replies. The time I spend here has been really invaluable in helping to speed up my learning and information intake.

author: james_s
author_rank: Super Contributor
timestamp: November 12, 2018, 10:53:03 pm
content: I've seen light dimmers with coils for EMI suppression so that may not be a reliable way to tell.One thing that might tell you is that motor speed controllers normally turn on at 100% then dial down, while light dimmers normally turn on at 0% and dial up. You could test both with an incandescent bulb as a load since that will work fine on either type of control.

author: tehmagicsmoke
author_rank: Contributor
timestamp: November 12, 2018, 11:10:50 pm
content: Thanks! I did try it on the motor. Then just did an ohms test. Seems like the knobs were on the correct pots. I took the pot apart and cleaned the dust out. It was acting inconsistently and I smelled a little smoke-hopefully just dust.I'm gonna build a custom mount. Maybe I have an enclosure I can use. I'm looking forward to getting this done and using my solder fume filter.

author: coromonadalix
author_rank: Super Contributor
timestamp: November 13, 2018, 02:09:01 am
content: add an heatsink for the scr to help with long uses .......

author: tehmagicsmoke
author_rank: Contributor
timestamp: November 13, 2018, 03:14:58 am
content: Thanks. I plan to just use the aluminum plate it was on. I cut it in half and bent it.I ran it for a couple minutes and didn't feel any heat.