How do I run a dc motor slowly?

[Decky]

I'm an electronics noob. and so far started on the simple stuff like LED chaser circuits, etc.

I now want to start a project for a model that requires a small 3v dc motor to run slowly in the order of about 8 rpm, powered by AA batteries. There isn't much room in the model so it needs to be a tiny motor (which I already have but don't have to use it) and there will be negligible load. So as not to ruin the effect it needs to be as silent as possible so using screaming gears to achieve the slow rotation would be no good. I thought about using a PWM speed controller but I don't know whether at that low speed there'll be enough torque to even overcome the tiny amount of static friction in the system (it won't be possible to spin it up to get it going). I also thought about using a small 2 phase stepper motor with a driver circuit I could build but would the motion be too jerky and the current drain be too high for the small batteries to cope with?

On the face of it, it doesn't appear to be anything very complicated but it's proving tricky to achieve. Any advice or ideas please?

 

[KMoffett]

"There isn't much room in the mode...l"
So, what are we to consider?

Ken

 

[Audioguru]

A simple series resistor reduces the current and torque of a DC motor. But PWM delivers full power pulses and lots of torque while reducing the speed.

 

[(*steve*)]

If you're considering a stepper motor, there are a number of geared stepper motors. These are not geared so much that the stepper has to spin it's box off but it will make the rotation much smoother.

Something like this might be suitable

 

[Decky]

Thank you for the replies, it sounds very hopeful. I'll try a PWM controller first, I see there are controllers available on eBay that are a lot cheaper and smaller than I'd be able to make it. If that doesn't work I'll try the geared stepper motor but so far I've not found one that quotes a current rating. Some tiny ones quote a resistance of 10 ohms per phase for example which for 2 phases at 5v would theoretically draw 1 amp (?), just to move the shaft at 8 rpm with no load . Too much for the batteries. There must be something wrong with that assumption somewhere, maybe there's tons of inductance to take account of but I wouldn't be able calculate that.

 

[Minder]

What phases are you talking about?
Not a stepper motor??
Most currently available DC motors are P.M. field so only two conductors for the armature.
The resistance of a DC motor is rather meaningless as the current depends on the load.
The actual resistance only comes into play at zero rpm if the motor is started with full voltage.
This is maximum current draw, if started low via PWM etc then then as soon as the motor starts to rotate it depends on applied voltage, BEMF (generated voltage) and load.
A stepper motor is not suitable for this application due to the low voltage supply and the need for a step/direction controller.
M.

 

[duke37]

I have measured the voltage across a DC motor and if low, supply a pulse of current. The pulses came from a transformed and rectified mains supply, switched with an SCR. This would run a car fan motor very slowly, limited by the motor cogging. Presumably, the higher the inertia, the lower the stable speed.
This is many times bigger than your system but could be run from DC using an op-amp or transistor comparator and a 555.
This is similar to PWM, I do not know if the modules use smoothed voltage feedback. I would think that they have a capacitor across the output which would spoil the speed measurement and control by voltage supply only.

 

[Decky]

Thanks Minder. There are different types of stepper motor, I mentioned a 2 phase type because it's the smallest and simplest. There are some really tiny low voltage ones available but as you say, it will need a controller which I could build as part of the project but I'm sure I can buy a really tiny one. I'm sure there's more involved in calculating the operating current than just the coil resistance and voltage and that the BEMF will affect it greatly. But I don't know why the 'at rest' coil resistance is quoted on some motor specs and not the operating current for a given voltage, that would be far more useful. I don't know how to determine the operating current if it's not quoted but before buying one I need to make sure it won't break the overall power budget. I've read that they do have a higher current drain than the equivalent standard dc motor.

Thanks Duke37, I'm not sure if it's applicable for my application but I've already ordered a PWM module from eBay that has an adjustable duty cycle of 0% to 100% so may just work if I'm lucky. If not then I'll need to pursue the stepper motor option as above.

 

[Minder]

A stepper motor is designed to operate at the rated plate current at all times , this will tend change due to the increase in inductive reactance change as rpm increases, therefore a drive is required that maintains the rated current regardless of rpm, this involves a P.S. that is well above the motor rated voltage this is in order for the drive to maintain the proper (plate) current.
One down side of a stepper, especially when ran on battery, is that the current is present regardless of load.
Even when stationary, unless steps are taken to remove power.
If you are referring to bipolar steppers, these typically are the highest rated current type.
M.

 

[Decky]

Thanks again Minder, that all makes sense and why stepper motors have high current ratings. Yes I am talking about bipolar steppers (4 wire). From what I've been reading (though not in the detail you explained it), I always suspected that I would have problems meeting the current requirement with AA batteries so I hope the PWM control option works out.

 

[Alec_t]

8rpm is a challenge. Unless you have gearing with a large reduction ratio, I don't see how PWM will do what you want with a standard DC motor.

 

[Minder]

You probably could use something along these lines?
http://www.romanblack.com/onesec/DCmotor_xtal.htm
M.

 

[Alec_t]

Even that arrangement would need modest gearing because, as Roman found, "The motor would not run any slower than approx 70 RPM. "

 

[Bluejets]

There are small dc permanent magnet gear motors on Ebay for around Aus$8.00. Maybe 20mm long and 14 wide x 10 high. 6v one should run ok on 3v. For xtra one can even get an encoder already fitted. Lovely stuff.... I saw a query for size earlier but no answer was forthcoming.

 

[Decky]

My PWM controller module arrived today, I tried it on several sizes of motors and it worked very well but as you say Alec_t and as I suspected in my original post, there isn't enough torque to turn it below about 100rpm from rest (a bit lower when already turning but nothing like what I'm looking for). Many thanks for the suggestion Minder but as Alec_t pointed out, the tests didn't achieve less than 70rpm (though managed to achieve 30rpm under certain conditions).

Thanks Bluejets but I suspect the motors with encoders will perform about the same.

As Alec_t suggested, I think my only option is to use a geared motor (with some soundproofing perhaps) but I'm wondering whether combining it with the PWM would work at low speed and therefore lower noise. The problem I think is that as low speed = low torque, there may not be enough power to overcome the added friction of the gears. I think it will be a matter of trial and error getting a geared motor to turn at low speed, e.g. a 80rpm geared motor running at tenth speed may not work and may need a 30rpm higher geared motor running at quarter speed or even a 15rpm motor running at half speed (assuming such things exist) .

 

[Bluejets]

Thanks Bluejets but I suspect the motors with encoders will perform about the same.

.
No it will not..........If you read the suggestion they are "gear motors".......miniature units. Many different voltage ratings and gear ratios available.

 

[Audioguru]

When I saw your 8 RPM I wrongly thought about audio that is cycles per second and 8 RPS is pretty slow but doable for a motor with no load when powered from PWM.

 

[Decky]

Sorry Bluejets, I misread it and thought you were describing a dc pm motor with a speed control encoder included.

Yes Audioguru. To be clear, it's about 8RPM = one revolution every 7.5 seconds.

Anyway, I think I have my solution now - I managed to find a tiny micro servo that I had salvaged from an old crashed RC helicopter. I modified it by cutting out the end stops from the output pot so that it rotates freely and connecting my PWM module directly to the servo motor I can vary the output between 6RPM and 60RPM. And at low revs it's virtually silent (or will be silent when it's installed inside the model) . If I need to replace it they're so amazingly cheap for what are, just a couple of pounds / dollars.

Many thanks all for the help and suggestions, I really appreciate it. I don't think I'm out of the water yet, I still have other challenges with the model and as an electronics noob I may need to raise other posts.

Cheers.

 

[BobK]

Which amountd to a gear motor, which everyone was suggesting and you were rejecting. Oh, well.

Bob

 

[Decky]

No, that was my next option BobK, as I explained in my post #15 above (combining a geared motor with the PWM controller). I think I was lucky that the gearing was just right with not too much friction to turn at very low revs.