Slowing down AC syncronous motors - Page 1
Forum section: electronics / projects

author: sacherjj
author_rank: Frequent Contributor
timestamp: June 15, 2011, 01:57:41 pm
content: I'm looking at the feasibility of slowing down small AC synchronous motors. I'm talking about small little geared down motors like this:I'm looking at a 5 RPM motor on a laminater feed and trying to get down to 1-2 RPM. I'm trying to use the laminater for toner transfer style of PCB etching.The motor is 120VAC 4W motor. So I'm thinking that I could make a crude sine wave oscillator (like the 50% duty cycle alternating polarity square waves) to drive a 9-12V secondary back to primary voltage and run the motor. It will probably be easier to do the hardware work involved to put in a stepper or similar, but this is a section of electronics I haven't done much with and it seems like a cool learning experience.What I'm looking for are any gotchas and if there are any problems with my assumptions:1 - Small motors like this would have permanent magnet rotors and only run power to the poles.2 - Reactant would not vary much with a change from 60 to 15 Hz driving frequency.3 - Fairly crude sine wave signal will get the job done.Looking at Xl = 2 pi f L makes me think that I'll have 1/4 of the inductive reactance in the poles at 15 Hz that I would at 60 Hz. So it makes me think that #2 is bogus and I would have to drive at reducing current to keep from burning up the pole windings. This would lower my torque and at some point I really couldn't get lower frequency, due to lack of power to supply the torque needed.If that isn't a problem, I figure there is no issue with running a sine-ish wave between 0-12V to drive the secondary backwards, because the DC offset will disappear in the transformer. So either an H-bridge or just power MOSFET driven output stage of an oscillator would get the job done.What am I missing?(The other wacky idea I came up with last night is driving this with AC directly, but using zero crossing to do AC modulation. Full speed is always on. Then next step down is cut out every other cycle, to half speed. Next, cut out full cycles between upper and lower sections for 1/3 speed. This also seems like it would be fun to play with.)

author: GrumpyDave
author_rank: Contributor
timestamp: June 15, 2011, 03:21:40 pm
content: I'm not sure what these look like inside but could you possible reduce the number of poles inside?Might be too fiddly and it assumes that there are enough poles in the first place otherwise you may not get smooth operation...

author: sacherjj
author_rank: Frequent Contributor
timestamp: June 15, 2011, 03:38:43 pm
content: Quote from: GrumpyDave on June 15, 2011, 03:21:40 pmYeah, I think the poles are generally spaced to get enough torque and reducing that would most likely result in a motor that will stall.I've looked into ordering a compatible motor with lower speed, but they only seem to be available in manuf. quantities. I'm not sure if they use a fixed motor and alter the gearing for the 1-10 RPM motors, but I would assume that is easier than maintaining different pole windings.I'm worried if I pull apart the motor to modify, I will end up with a non-working one. Due to the number of posts requesting these motors for replacement that go unanswered for fake fireplaces, laminators and many other consumer goods, getting a new one isn't an easy task.

author: sacherjj
author_rank: Frequent Contributor
timestamp: June 15, 2011, 03:58:17 pm
content: I think I might play around with my wacky idea first. I don't think my initial thoughts about speed will work, as the motor will still be spinning due to inertia when no power is supplied. So I might have to be creative with the cycles of AC let through. But it would be fun to play with.With a protection diode and a current limit resistor for 170V (peak voltage in US 120V power), I should be able to drive an optoisolator directly, to make a power line frequency clock. Then use an Uno board to play around with turning on and off a zero-crossing capable optoisolator with triac output capable of running the motor. I would just need to be accurate enough for the signal to change during the piece of the cycle prior to the next off or on point, which is easy with a 16 MHz micro.I should have some 60 RPM clock synchronous motors in the parts bin to mess around with. Just need some parts from Digikey.

author: bilko
author_rank: Frequent Contributor
timestamp: June 15, 2011, 04:20:45 pm
content: An interesting article here if you haven't seen it already.http://www.allaboutcircuits.com/vol_2/chpt_13/2.html

author: Zero999
author_rank: Super Contributor
timestamp: June 15, 2011, 04:38:49 pm
content: Yes, you need to reduce the frequency to slow the motor down. The trouble is, you also need to reduce the voltage, otherwise the motor will probably overheat because the current will increase.An industrial variable frequency drive rectifies the mains voltage then converts is back to DC using an inverter. PWM is used to make a sinewave which also makes reducing the voltage easy.As it's only 4W, you could reduce the mains voltage using a transformer to power the rectifier and inverter connected to another mains transformer connected in reverse to boost the voltage.

author: sacherjj
author_rank: Frequent Contributor
timestamp: June 15, 2011, 04:48:35 pm
content: Quote from: yachtronics on June 15, 2011, 04:20:45 pmYep. Found that while researching this last night. Definitely a good refresher for my time since I looked at this in University.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 16, 2011, 04:06:35 am
content: V/Hz control is very easy and would work nicely for your application. Use a microcontroller to drive a H bridge that in turn drives a transformer.Your cycloconverter idea is used in some good quality ceiling fans. (The best ones are inverter drive.) It is cheaper than an inverter (not much in your case, though) but is much less flexible in control. As you've stated, there are problems with the motor trying to run on a harmonic unless the load is enough to prevent that. In a ceiling fan, the aerodynamic drag would be enough to prevent the motor from speeding up to a harmonic speed.Have you considered replacing the actual motor with a DC motor, reusing the gearbox? That would probably be the cheapest and easiest. (A related idea is to see if rotating an external magnet near the motor can turn the shaft, then spin the magnet with a DC motor.)

author: sacherjj
author_rank: Frequent Contributor
timestamp: June 17, 2011, 02:27:17 pm
content: Quote from: NiHaoMike on June 16, 2011, 04:06:35 amI have considered that. Trying to determine if I want to go hardware route with another AC or DC motor or adding external gears to the current, or learn something new and play with the inverter idea. The one nice feature is with a DPDT switch, I can toggle the laminater back to stock and then have an external input for my drive. I think the inverter will be this largest learning experience, so I think that is the route I'm taking.