Regulating speed of a 220vac Induction motor - Page 1
Forum section: electronics / beginners

author: caegear
author_rank: Contributor
timestamp: January 16, 2016, 07:26:50 pm
content: Hi all,I have an old Dremel 1731 "Moto Sander" that is a great little machine, but that is somewhat brutal for the fine tasks I want it to help me with (I want to sand down nylon gears from 5 to 3.7mm thickness). It has a motor marked "180w" and it runs on 220vac. I found an old ad describing the motor as an Inductive motor, and some testing indicates this is the case (ran it of a current limiter dimmer made for halogen lights and it reached same rpm as without, but it sounded as if it took a little longer to get there and as if it was lacking power / torque).As far as I understand an induction motor will not slow down if voltage is limited (might get "weaker"), but it will if the frequency is lowered at the same time as the voltage is increased by some algoritm unbeknownst to me (to have same torque)? Anyway, I understand that this is too complex for someone of my limited knowledge to tinker to succsess, and I am considering buying a dedicated device to regulate the motor. Another option would be to replace the motor with a Servo motor (have a sewing machine motor that I could fit to the belt sander and disable the sanding disk).So. I have this question: What on earth would I google or search for on eBay if I want a 220v thing to regulate the speed on my old 180w induction motored Dremel MotoSander? There has got to be better options than the 1.5kw vfd thingy i found on eBay, and the only other place on earth that I found describing this kind of conversion (Oldfoundry.com:http://www.oldfoundry.com/catalog/product_info.php?products_id=144) said they used a "VariAC". I tried googling that too, but my searches yields no conclusive results on eBay or Google (other than som antique 120v thing resembling a ceramic fuse on stereoids).Any help would be greatly appreciated. Links to specific devices available for intl purchase online even more so

author: Simon
author_rank: Unknown
timestamp: January 16, 2016, 07:29:53 pm
content: uh, be careful with variable frequency, even 50 and 60 herz motors are not interchageable because even that 20% change can have drastic results. as far as I know and I'm no motor expert inductions motors are just a nuisance for control. I'd just buy a modern machine that can be controlled.

author: caegear
author_rank: Contributor
timestamp: January 16, 2016, 10:11:54 pm
content: Thanks for replyingI guess the regulation will be more difficult than switching the motor to some Servo motor. And yes, I would consider buying another machine if there were any machines out there that had regulation or even selectable speeds. My options are MicroMill from Proxxon (to mill off the 1.3mm) or filing by hand (I am planning to do this on 100+ nylon gears...). Proxxon MF70 is like 3-500 USD. Servo motor and custom fitting would be some 150 USD.Anyone have one of the "clones" of the Dremel MotoSander from Harbour Freight or china direct that could check if they still are made with induction motors?

author: Simon
author_rank: Unknown
timestamp: January 16, 2016, 10:44:11 pm
content: what type of speeds do you need ? A servo motor will have a huge amount of control as you just drive it at the right frequency and it goes round at that speed.

author: amyk
author_rank: Super Contributor
timestamp: January 16, 2016, 11:13:53 pm
content: What type of induction motor is it exactly? Chances are if it's single-phase, it won't be controllable with a VFD - those are usually for 3-phase motors. However, if you are willing to consider swapping the motor with a 3-phase one...

author: rich
author_rank: Frequent Contributor
timestamp: January 16, 2016, 11:27:26 pm
content: I think Simon is right, you're going to get into a world of pain trying to slow that motor from what you describe in your original post as 'brutal'. That sounds significantly more than 20% reduction needed.Does nylon sand well when removing that amount of material? In my head I've got a picture of a stringy finished texture like molten cheese on a pizza slice, but I haven't worked with nylon before.One other thought is could you make up a holding jig for the part and rotate it in a drill against a stationary cutter/tool? Effectively make your own mini lathe.

author: rich
author_rank: Frequent Contributor
timestamp: January 16, 2016, 11:31:20 pm
content: @amyk I looked this up out of curiosity, its single phase with 4 wires. 2 for Live/Neutral and 2 for a big starter cap.

author: caegear
author_rank: Contributor
timestamp: January 17, 2016, 07:27:46 am
content: Quote from: Simon on January 16, 2016, 10:44:11 pmThe original motor is rated with a "magic" 3450/min, and Im guessing this is the motor rpm. The belt is probably some 90-ish mm long, and the drive wheel on the motor shaft is 93mm in diameter. This could mean the belt moves one round pr 10 motor rpm / roughly 340 rpm on the belt.I could probably manage with half or 1/3rd of the original speed. That would eliminate vibrations and give me sub-mm control. I would prefer really slow speeds for the nylon gears. Like 10th or 20th of this speed whatever it is. My sewing machine motor (servo) comes to mind as it can be set to operate between 0 and 15 rpS (second), and has torque like an elephant even at 1/2 rps / 30rpm. But then again I do use this motosander at full speed to do minor surface work on harder materials.

author: caegear
author_rank: Contributor
timestamp: January 17, 2016, 07:33:02 am
content: Quote from: amyk on January 16, 2016, 11:13:53 pmI am guessing it is single phase. It is completely enclosed in a metal holder that is at the center of the machine, and I havent taken it completely apart yet. Dont know if it would help either because I understand motors arent readily marked too well regarding type aso. I have opened up the housing for the on/off switch and see no resistors or any other electronics. I cant completely rule out that it has some electronics inside the steel casing surrounding the motor itself. Guess Ill find out

author: pmbrunelle
author_rank: Regular Contributor
timestamp: January 17, 2016, 07:46:27 am
content: Maybe you can find a 4 or 6 pole motor (with the same format) to replace your motor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 17, 2016, 07:50:53 am
content: Yes, a VFD. This is just fine for single phase motors, but beware the limitations: they only deliver torque during the waveform peaks, and torque is proportional to RPM, so you get very little power indeed, even at just half speed. And the force will be pulsating palpably. And the maximum torque is lower, so it's that much easier to stall. Stalling is particularly unfortunate for single phase motors, because there is no torque at all at 0 RPM (indeed, it can be started in either direction from zero, assuming no phased or starter winding is active).Also, centrifugal start switches are very common, which means below a certain RPM, it'll kick in the start winding, which isn't designed for continuous operation, even at reduced V, F.So, probably not very practical for a sander.Tim

author: caegear
author_rank: Contributor
timestamp: January 17, 2016, 07:59:28 am
content: Quote from: rich on January 16, 2016, 11:27:26 pmYou are right. 20% is not enough. Im looking into replacing original motor with a servo sewing machine motor for complete control.Nylon sands ok for this application. Its a dual gear (two gears cast as one) made for speed reduction, and Im going to have to remove 0.4mm from outside of the large gear, and 0.9mm from outside of the small one.You are right again about cutting with some other tool. I do have an old semi homemade micro metal lathe (powered by a 180w brushed AC sewing machine motor and regulated with a cheap chineese current limiter), but Ill have to dig it out from its storage, and the clamping of the nylon gears worries me. Dont want the gear inside that chuck without protection. Ill look into maybe casting a small holder for the gear in polymer clay if polymer clay doesnt eat nylon. Might even be able to mold it directly in the chuck.(But i still want a variable speed Moto-sander

author: caegear
author_rank: Contributor
timestamp: January 17, 2016, 08:03:49 am
content: Quote from: rich on January 16, 2016, 11:31:20 pmI knew I would find people with magical powers in here. Thank you

author: caegear
author_rank: Contributor
timestamp: January 17, 2016, 08:04:51 am
content: Quote from: T3sl4co1l on January 17, 2016, 07:50:53 amThank you so much. I guess Ill go another route than regulating this thing

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 17, 2016, 08:05:20 am
content: Another option is to insert a pulley reduction stage, if you can open up the room for it, and don't mind a bit of mechanical handiwork.Tim

author: Simon
author_rank: Unknown
timestamp: January 17, 2016, 09:54:43 am
content: You sewing machine motor is likely a brushed "universal" motor that can be PWM controlled.

author: BradC
author_rank: Super Contributor
timestamp: January 17, 2016, 11:37:36 am
content: Just to add to the VFD info. I have a cheap single phase drill press. I control the speed on this using a VFD. I grabbed a cheap 230v 3 phase VFD from China and just connect active & neutral on the drill press to 2 of the phases on the VFD.T3sl4co1l is dead right, torque is woeful at low speeds. I did it so I could briefly double the speed of the machine for high speed but low torque drilling, and for that it works a treat.I've also experimented with speed control on single phase pumps, but more than +/- 20% is asking too much.I have a VFD & little 3 phase motor on my lathe, and torque is still usable down to almost a dead stop, but on a single phase motor it's not much good.

author: nowlan
author_rank: Frequent Contributor
timestamp: January 17, 2016, 02:18:42 pm
content: Is a normal dremel and grinding disc not suitable. They are speed controlled these days.I have a single phase vfd, but is over kill for what you want.http://www.invertekdrives.com/variable-speed-drives/optidrive-e2-single-phase/specifications.aspxNot sure about 3 phase vfd. I always thought the waves were 120' apart. Also would fault with no load on 3rd phase?.Variac tend to be expensive unless your lucky.

author: Fank1
author_rank: Regular Contributor
timestamp: January 17, 2016, 02:23:58 pm
content: Most likely a series connected brushed motor.Even a 2 pole motor, fastest frequency dependent type, will run at 3500 PRM on 50 HZ.Most all AC motors running faster than 3500 RPM on 50 Hz are series connected.The speed of series motors is then LOAD dependent, any type of "speed" control is actually torque control.

author: Simon
author_rank: Unknown
timestamp: January 17, 2016, 03:42:13 pm
content: Quote from: Fank1 on January 17, 2016, 02:23:58 pmI think your 3500 rpm is for a 60Hz motor that will run at 3600 rpm no load and 3500 would sound about the right amount of drag.dremels are usually brushed motors. Mine goes up to 33 Krpm so the slow setting is still a few thousand rpm

author: amyk
author_rank: Super Contributor
timestamp: January 17, 2016, 03:44:57 pm
content: If you can find a 3-phase motor of the right dimensions then you could fit that and a VFD.