Safe DC Motor Spacing for Enclosures

I'm looking at some very small n20 DC motors for use of holiday decorations and making small props come to life.

The goal is to be able to seal the motor enclosures so they're weather proof. Casings will be 3D printed and made as small as possible (with safe spacing being the limitations)

I was wondering how much space the motors need to "breathe". Only the shafts will be exposed.

Are there specific guidelines that should be followed when placing motors near plastic for proper thermal dissipation? Plastic used is polypropylene.

Brushed motor specs:
2.7-4.2V power range from a single 3.7V 18650
125mA load current
500mA stall current

Potential problems :

  • Heat dissipation
    How far should the motor be away from the walls of the enclosure? The N20 brackets available lay ontop of the motor so I'm not sure if that is even an issue

  • Enclosure temperatures
    Same issue regarding spacing. I'm looking to put these in a very small enclosure.

  • Dust build up from brushes
    How would I deal with this issue in a sealed setting? There will be a ton of small motor enclosures so won't feasible to open and clean them all out.

The only solution I have currently is to ensure everything is properly spaced. But what would be considered proper spacing?

The only solution I have currently is to ensure everything is properly spaced. But what would be considered proper spacing?

Practically speaking, internal spacing in a sealed enclosure does zip, nada, zilch. The magic number you're asking about doesn't exist.

It's a hobby motor. There are no specifications for these devices. You're putting them outside. They will survive longer sealed than open to the outdoor environment. End of discussion.

If you think I'm being harsh, I'll provide this simple challenge. Select your motor and then find its continious allowable running power output along with a maximum allowable ambient temperature. Find those two values and then we can have a meaningful discussion. Until then, it becomes a game of buy more than you need and replace when they fail, because they will, something that will happen even when they're used indoors in the best of conditions.

BTW, the heat radiated from a sealed enclosure is a function of the exposed area of the enclosure, the thermal conductivity/resistance of the enclosure material and the differential temperature between the inside and outside of the enclosure. So, the bigger the box, the better.

Just remember, plastic is a lousy heat conductor, the thinner the better.

Does it need to be a completely sealed box? You will have a lot less problems with heat build up if it is rain proof but not sealed.

If you really need it to be sealed then you will need to measure the internal temperature when the system is working to see if it is within an acceptable limit.

I think I would be inclined to mount the motors in a metal enclosure if I was worried about heat-dissipation. Sealed metal boxes are widely used in the electrical trade and should be available from most electrical supply businesses.

With those small motors I probably would not worry about heat dissipation. As @avr_fred says - buy a few spares. A complete set of spare motors might be cheaper than a sealed metal enclosure.

...R

ericfragola:
I'm looking at some very small n20 DC motors for use of holiday decorations and making small props come to life.

The goal is to be able to seal the motor enclosures so they're weather proof. Casings will be 3D printed and made as small as possible (with safe spacing being the limitations)

Weatherproof a tiny motor? Good luck with that, never heard of shaft seals that small (because
the friction would prevent the motor turning I suspect). I fear you will need to replace the motors
each season as moisture is bad news for electric motors and bearings.

Here is an excerpt from it's data sheet regarding the temperature ratings. Shows up to 50 deg.C , which seems reasonable so long as it's not bloody hot out.

1.2 Ambient Temperature: -10-+50C (14-122F)

3.2.2 Load Current : 0.125A (0.19A Max)

  1. Test Conditions:
    2.1 Voltage: DC 3.0v
    2.2 Ambient Temperature: 20C (68F)
    2.3 Ambient Humidity: 44RH

Okay, now I'm ready for a meaningful discussion :wink:

I'm looking to use plastic to utilize the 3D printer as opposed to a metal box. Thank you for the suggestion Robin. The metal boxes would be easier but I'd like to learn more through this project.

Another possibility to consider is adding a temp sensor and having it shut down the motors once they reach max operating temp. They will be shaded and only operate a few hours at a time. (Not a 24/7 running project)

The weatherproofing is as best as I can, not waterproof such as shaft seals would be. The motor enclosure will sit underneath, so not in direct rain/sunlight.

Okay, now I'm ready for a meaningful discussion :wink:

Place the motor into a dynamometer test stand. Affix thermocouples to all exposed sides of the motor. Program the dynamometer test chamber to maintain a temperature of 50 deg C. Setup the data collection system to acquire all temperatures from the motor and the local ambient which should be as close as possible to the 50C target.

Run the motor continuously with 3.0 volts input and a motor current at 0.125 amps, adjusting dyno load as required to maintain 0.57 watts output.

Observe the temperature rise of the motor and allow the temperature to stabilize, this may take ten minutes or more, it depends upon the thermal mass of the motor.

Once the motor has stabilized, note the maximum motor temperature and the local ambient. Subtract the two and you now have what you really needed, the maximum allowable motor temperature rise above ambient at full power.

Epoxy an LM75 onto the motor case (Or LM35 or DS18B20 or whatever you want to use). If the temperature of the case exceeds the maximum rise temperature, shut down the motor and wait for it to cool down to a lower ambient.

You can do all the above - but you'll still just chuck the dead motor in the bin and replace it when it stops working.

Or, you can forgo all the fancy, rather expensive motor heat rise studies and still chuck the dead motor in the bin and replace it when it stops working. Much lower end cost :wink:

avr_fred:
Or, you can forgo all the fancy, rather expensive motor heat rise studies and still chuck the dead motor in the bin and replace it when it stops working. Much lower end cost :wink:

+1

...R

The OP gets karma for at least finding the specifications... I would have lost a bet on them even existing.

Hopefully, readers could see from the first sentence that my test procedure was intended as comic relief. It was a rip on the old Steve Martin comedy bit about how to become a millionaire which begins with "First, get a million dollars".

While it may sound impractical, it is an accurate description of what designers and manufacturers do when testing motors for temperature rise, the only real difference is I didn't mention the numerous thermocouples buried inside the motor and viewports for checking commutation on DC machines.

Every wound coil, all bearings, everything that matters get measured. It takes a lot of time and lots of cash, something that would never be done for a hobby motor. Once the test is setup, it becomes a case of absolute boredom waiting for the temperature to stabilise as big machines can take a half-day or more. Of course there are the rare cases when a bearing seizes out of the blue due to lack of lubrication, or other equally silly reasons. Those events are really a noise to behold when the motor power is 750kw and spinning at 7000 rpm.

For those so inclined, building a mini dyno would be quite easy. Two PM DC motors connected end-to-end, one as the prime mover and the other as the absorber. Connect the absorber to a mosfet electronic load and you're off to the races!