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Mcgyver
12-23-2010, 12:25 PM
I understand slowing an AC motor is basically accomplished by either gearing or changing the frequency.....a vfd is the obvious answer for larger motors but I want a diy solution for a small motor at a fixed speed

I have on my diy central coolant tank/system a diy skimmer using a small gear reduction motor, kindly donated by Duffy. Its revolving an old saw blade at about 8 rpm. Said saw blade passes through a couple of shim stock blades that clean it.

Everything is working very well, but, the problem is the skimmer is collecting way too much coolant....it'll fill the small reservoir in an hour. The solution probably is that I have to use material that the water doesn't really stick to....but it might also improve with by slowing the motor (AC, 110v) and doing so got me thinking....

Two ideas i had....1) pulse the motor, on for a second every so many seconds - make a timing circuit that controls relays (I forget what IC is good for longer timing apps, 555 don't like long periods do they?) ....or 2).... create a DC source and use a 555 to supply a lower the frequency of AC going to the motor....sort of FFD (Fixed frequency drive) at say 15 hertz. Problem is its only + that I have to work with....hmmm, will that work?

Mechanical is out, the motor is geared already and drives timing belts that reduce it further. There's not space or budget to start changing that out

1) (pulsed) might do the trick, but I'm wonder the feasibility of 2) (diy FFD) - do the motors care that they're getting a square wave instead of sine? could you set up a 15 hertz square wave and drive a small ac motor at 1/4 its regular speed? Curious minds want to know :)

Thoughts?

Peter.
12-23-2010, 12:40 PM
I've hear of people using simple light dimmers to control the speed of small motors, but it has to be a particular type. Would that not be worth a try?

mechanicalmagic
12-23-2010, 12:48 PM
Check your local home center for a "fan speed control". They work just like your #1 option. Current is turned on for a few cycles, then off. Be sure to get one sized large enough to run your skimmer motor.

macona
12-23-2010, 02:39 PM
Put it on a timer. It dosnt need to run all the time. I have found they work best running for a short time then stopping to allow the film to spread over the surface.

12-23-2010, 03:58 PM
I understand slowing an AC motor is basically accomplished by either gearing or changing the frequency.....a vfd is the obvious answer for larger motors but I want a diy solution for a small motor at a fixed speed

Thoughts?

How small is small, is 120vac 72rpm work?
AC synchronous stepper ebay #230410947488
Max.

The Artful Bodger
12-23-2010, 06:41 PM
Hi Mcgyver

I think a timer is the way to go for this application, a timer and a stop switch on the disk shaft.

My thoughts are that the disk should turn for one revolution then stop while the excess water drains back into the tank, then after a while the disk turns again wiping off the oil into the catcher and picking up another load from the tank.

The disk shaft would have a switch arranged to open the circuit briefly each revolution of the disk and a timer would momentarily short the switch at intervals. This is the same thing as happens in a vehicle windscreen wiper system where the wipers run through one cycle then stop when the auto park switch opens, the intermittent wipe circuit shorts the park switch circuit briefly, just long enough for the motor to start another cycle.

John

Duffy
12-23-2010, 06:59 PM
Macona is right on the money. A motor like this one should easily handle any reasonable duty cycle. I would expect that 1 minute on/2 minutes off would show no undue heating. It is a shaded pole motor, I think, so there arent any moving parts but the rotor.

Jpfalt
12-23-2010, 07:10 PM
I don't think the rpm is the issue. The issue is that the disk is picking up too much coolant.

You will probably want to polish the disk surface and apply something that will stay put, but make water bead up and fall off. Grease is one thing that comes to mind. Something like a teflon cookie sheet for the disk might also work well if you switch to a non scratch squeegee blade. Something else that came to mind is a spray and bake teflon paint from Brownells used for gun finishes would also work. The key is that water beads off and oil flows on to the surface.

kf2qd
12-23-2010, 07:55 PM
If you are using an induction motor the only way to slow it down is to have a VFD. If you are using a brush type Universal motor then a light dimmer will work if it is rated high enough (in watts)

Starting and stopping and induction motor will kill the relay because of the high starting current.

Try a smaller disk so you have less in contact with the coolant and see if that helps.

Mcgyver
12-23-2010, 09:15 PM
Macona is right on the money. A motor like this one should easily handle any reasonable duty cycle. I would expect that 1 minute on/2 minutes off would show no undue heating.

yeah, not sure, I believe you that the motor will be ok, but I think what it needs is to just go a lot slower to allow the coolant to drain....the timer is still a good idea, run it so many minutes an hour or something...but when its running i want it picking up less coolant than it is.

I was playing with tonight, and by flicking the switch on say 1/10 a turn, there is a lot of coolant that would drain to the tank if it had more time before hitting the blades. I think it should go 1/4 the speed (2 rpm) or stop every 1/10 of a turn for a a few seconds. That's based on seeing how much coolant is trying to drain off but can't before it hits the blades. Pausing the wheel after each revolution will still pick up a lot of coolant.

Starting and stopping and induction motor will kill the relay because of the high starting current

what if i used a SCR..... or maybe with an thermistor to limit current initially?

Jpfalt, agreed, a material that doesn't pick up the water would be great. That the saw blade was there was a big part of its selection, any disk of similar size could be used....or chrome or otherwise coat/plate the saw blade? I'd like to either treat the saw blade or achieve the functionality through electronics, just to minimize alterations/searching for a better disk.

J Tiers
12-24-2010, 12:26 AM
Small gear motors, which this sounds like (8 RPM) are often PSC induction motors, or sometimes shaded pole motors, which can be slowed in some cases with dimmers etc..

In this case, running essentially unloaded, dimmer switches may not work, because they essentially vary torque, but with litle load, torque requirement is small.

Dimmer types work great on fans, where the load increases faster than RPM.

I like the timer.

macona
12-24-2010, 01:23 AM
yeah, not sure, I believe you that the motor will be ok, but I think what it needs is to just go a lot slower to allow the coolant to drain....the timer is still a good idea, run it so many minutes an hour or something...but when its running i want it picking up less coolant than it is.

I was playing with tonight, and by flicking the switch on say 1/10 a turn, there is a lot of coolant that would drain to the tank if it had more time before hitting the blades. I think it should go 1/4 the speed (2 rpm) or stop every 1/10 of a turn for a a few seconds. That's based on seeing how much coolant is trying to drain off but can't before it hits the blades. Pausing the wheel after each revolution will still pick up a lot of coolant.

what if i used a SCR..... or maybe with an thermistor to limit current initially?

Jpfalt, agreed, a material that doesn't pick up the water would be great. That the saw blade was there was a big part of its selection, any disk of similar size could be used....or chrome or otherwise coat/plate the saw blade? I'd like to either treat the saw blade or achieve the functionality through electronics, just to minimize alterations/searching for a better disk.

Is this a commercial unit or a home made unit? If its home made I would try a disc out of HDPE or something like that.

You dont want to use a scr. SCRs are for DC switching. You could use a triac, but to make it simple I would just use a solid state relay. This should be a pretty tiny motor so I wouldnt worry about inrush even with a regular relay.

dewat
12-24-2010, 04:14 PM
My 2¢, sounds like it would be easier to get a windshield wiper motor and a light dimmer, then as Macona suggested make the disk from plastic.

derekm
12-24-2010, 05:33 PM
How long are you leaving this on for? My skimmer runs at only 2rpm yet it manages to remove most of the oil in about 5 minutes. leaving it on longer means more water is transferred.