The Little Things I Didn’t Know About Small DC Motors

We’ve all taken apart a small toy and pulled out one of those little can motors. “With this! I can do anything!” we proclaim as we hold it aloft. Ten minutes later, after we’ve made it spin a few times, it goes into the drawer never to be seen again.

It’s all their fault

It always seems like they are in everything but getting them to function usefully in a project is a fool’s errand. What the heck are they for? Where do people learn the black magic needed to make them function? It’s easy enough to pull out the specification sheet for them. Most of them are made by or are made to imitate motors from the Mabuchi Motor Corporation of Japan. That company alone is responsible for over 1.5 billion tiny motors a year.

More than Just the Specs

In the specs, you’ll find things like running speed, voltage, stall current, and stall torque. But they offer anything but a convincing application guide, or a basic set of assumptions an engineer should make before using one. This is by no means a complete list, and a skip over the electrics nearly completely as that aspect of DC motors in unreasonably well documented.

The paint mixers high running speed and infrequent use make it a decent candidate for hooking directly to the motor.
The paint mixers high running speed and infrequent use make it a decent candidate for hooking directly to the motor.

The first thing to note is that they really aren’t meant to drive anything directly. They are meant to be isolated from the actual driving by a gear train. This is for a lot of reasons. The first is that they typically spin very fast, 6,000 – 15,000 rpm is not atypical for even the tiniest motor. So even though the datasheet may throw out something impressive like it being a 3 watt motor, it’s not exactly true. Rather, it’s 3 N*m/s per 15,000 rotations per minute motor. Or a mere 1.2 milliwatt per rotation, which is an odd sort of unit that I’m just using for demonstration, but it gives you the feeling that there’s not a ton of “oomph” available. However, if you start to combine lots of rotations together using a gear train, you can start to get some real power out of it, even with the friction losses.

The only consumer items I can think of that regularly break this rule are very cheap children’s toys, which aren’t designed to last long anyway, and those powered erasers and coffee stirrers. Both of these are taking for granted that their torque needs are low and their speed needs are high, or that the motor burning out is no real loss for the world (at least in the short term).

This is because the motors derate nearly instantly. Most of these motors are hundreds of loops of very thin enameled wire wrapped around some silicon steel plates spot welded or otherwise coerced together. This means that even a small heat event of a few milliseconds could be enough to burn through the 10 micrometer thick coating insulating the coils from each other. Practically speaking, if you stall a little motor a few times in a row you might as well throw it away, because there’s no guessing what its actual performance rating is anymore. Likewise, consistently difficult start-ups, over voltage, over current, and other abuse can quickly ruin the motor. Because the energy it produces is meant to spread over lots of rotations, the motor is simply not designed (nor could it be reasonably built) to produce it all in one dramatic push.

Making Contact

Pololu has the clearest picture of the different kind of brushes inside these small motors.
Pololu has the clearest picture of the different kind of brushes inside these small motors.

This brings me to another small note about these tiny motors. Most of them don’t have the carbon brushes one begins to expect from the more powerful motors. Mostly they have a strip of copper that’s been stamped to have a few fingers pressing against the commutator. There’s lots of pros to these metal contacts and it’s not all cost cutting, but unless you have managed to read “Electrical Contacts” by Ragnar Holm and actually understood it, they’re hard to explain. There’s all sorts of magic. For example, just forming the right kind of oxide film on the surface of the commutator is a battle all on its own.

It’s a weird trade off. You can make the motor cheaper with the metal contacts, for one. Metal contacts also have much lower friction than carbon or graphite brushes. They’re quieter, and they also transfer less current, which may seem like a bad thing, but if you have a stalled motor with hairlike strands transferring the pixies around the last thing you’d want to do is transfer as much current as possible through them. However, a paper thin sheet of copper is not going to last very long either.

So it comes down to this, at least as I understand it: if bursts of very fast, low energy, high efficiency motion is all that’s required of the motor over its operational life then the metal strip brushes are perfect. If you need to run the motor for a long stretches at a time and noise isn’t an issue then the carbon brush version will work, just don’t stall it. It will cost a little bit more.

Take Care of Your Tiny Motors

Here is one of these can motors being restrained properly. Only torque on the case itself is restrained. The motor is otherwise free to move.
Here is one of these motors being restrained properly. Only torque on the case is restrained. The motor is otherwise free to move.

To touch one other small mechanical consideration. They are not designed to take any axial load at all, or really even any radial load either. Most of them have a plastic or aluminum bronze bushing, press-fit into a simple stamped steel body. So if you design a gearbox for one of these be sure to put as little force as possible on the bearing surfaces. If you’ve ever taken apart a small toy you’ve likely noticed that the motor can slide back and forth a bit in its mounting. This is why.

Lastly, because most of these motors are just not intended to run anywhere near their written maximum specifications it is best to assume that their specifications are a well intentioned but complete lie. Most designs work with the bottom 25% of the max number written on the spreadsheet. Running the motor anywhere near the top is usually guaranteed to brick it over time.

These are useful and ubiquitous motors, but unlike their more powerful cousins they have their own set of challenges to work with. However, considering you can buy them by the pound for cheaper than candy, there’s a good reason to get familiar with them.

Become Very Unpopular Very Fast With This DIY EMP Generator

Taking a break from his book, “How to Gain Enemies and Encourage Hostility,” [FPS Weapons] shows us how to build our own handheld EMP generator which can be used to generate immediate dislike from anyone working on something electronic at the hackerspace.

The device is pretty simple. A DC source, in this case an 18650 lithium battery cell, sends power to an “Ultra High Voltage 1000kV Ignition Coil” (as the eBay listing calls it), when a button is pressed. A spark gap is used to dump a large amount of magic pixies into the coil all at once, which generates a strong enough magnetic pulse to induce an unexpected voltage inside of a piece of digital electronics. This usually manages to fire a reset pin or something equivalent, disrupting the device’s normal operation.

While you’re not likely to actually damage anything in a dramatic way with this little EMP, it can still interrupt an important memory write or radio signal and damage it that way. It’s a great way to get the absolute shock of your life if you’re not careful. Either from the HVDC converter or the FCC fines. Video after the break.

Continue reading “Become Very Unpopular Very Fast With This DIY EMP Generator”

Google Contest Builds More Efficient Inverters

A few summers ago, Google and IEEE announced a one million dollar prize to build the most efficient and compact DC to AC inverter. It was called the Little Box Challenge, with the goal of a 2kW inverter with a power density greater than 50 Watts per cubic inch.

To put this goal into perspective, the DC inverter that would plug into a cigarette lighter in your car has a power density of about 1 or 2 Watts per cubic inch. Very expensive inverters meant for solar installations have a power density of about 5 Watts per cubic inch. This competition aimed to build an inverter with ten times the power density of what is available today.

Now, the results are in, and the results are extremely surprising. The best entry didn’t just meet the goal of 50 W/in³, it blew the goal out of the water.

The winning entry (PDF) comes from CE+T Power, and comes in a package with a volume of 13.77 in³. That’s a power density of 143 W/in³ for a unit you can hold in the palm of your hand. The biggest innovations come from the use of GaN transistors and an incredible thermal management solution.

Other finalists for this competition include Schneider Electric Team from France that managed a 100 W/in³ and a Virginia Tech team that managed a power density of 61.2 W/in³.

Thanks [wvdv2002] for the tip.

FAA Bans Drones For More Than Six Million People

In recent weeks, the FAA has solicited input from hobbyists and companies in the ‘drone’ industry, produced rules and regulations, and set up a registration system for all the quadcopters and flying toys being gifted over the holiday season. Whether or not the FAA is allowed to do this is a question being left to the courts, but for now, the FAA has assuredly killed a hobby for more than six million people. The FAA has introduced an updated Temporary Flight Restriction (TFR) for a 30-mile radius around Washington, DC.

staticmap
The 30-mile TFR area

Previously, there had been a blanket ban on drones, UAS, and model aircraft within a 15-mile radius of a point inside Reagan National Airport. This point covered the District of Columbia proper, and the suburbs of Bethesda, College Park, and Alexandria – basically, everything inside the beltway, and a mile or two beyond. The new flight restriction for drones covers a vastly larger area – all of the DC metro area, Annapolis, half of Baltimore, and all of northern Virginia. This area encompasses a population of more than six million people.

The DC metro area has, since 9/11, become some of the most complex airspace in the entire country. There are several military bases, Aberdeen proving grounds, the US Naval academy, and of course the White House, Capitol building, and the Pentagon. Even commercial airliners are subject to some very interesting regulations. For the same reason general aviation shuts down in southern California every time the president visits LA, you simply can’t fly model aircraft within the beltway; it’s a security measure, and until now, flying clubs in the DC area have dealt with these restrictions.

The new TFR has effectively shuttered more than a dozen flying clubs associated with the Academy of Model Aeronautics. DCRC, a club with a field in the middle of some farmland in Maryland, has closed down until further notice. The Capital Area Soaring Association has also closed because of the TFR.

Although called a Temporary Flight Restriction, this is a rule that will be around for a while. The FAA says this restriction is here for good.

Scope Noob: Bridge Rectifier

Welcome back to this week’s installment of Scope Noob where I’m sharing my experiences learning to use my first oscilloscope. Last week I started out measuring mains frequency using an AC-AC wall wart adapter. Homework, for those following along, was to build a bridge rectifier and probe the signals from it. Let’s take a look.

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Hackaday At MakeDC

makedc

Last Wednesday, our Hackaday travels took us to the Washington, DC area for a visit to NOVA Labs near Dulles and a yet-to-be opened Metro stop. Also on our itinerary was a visit to MakeDC, an informal get together for people around the nation’s capitol to show off their latest projects and builds.

The highlight of the evening was a pair of talks from [Julian] and [Taylor] on a project they did for work: a social cooler, or a locked box holding cool drinks that will only open when enough people send a text to a certain number. We’ve got [Julian]’s talk on video, but despite our fancy new camera gear for this sorta thing, [Taylor]’s demo of what an Electric Imp can do was lost to the digital wastes.

Aside from [Julian]’s talk on APIs and [Taylor]’s talk on the Electric Imp, there were a few impromptu presentations from the attendees. One of the most thorough was the duo from Shiny & Jackal Cosplay, crafters of EVA foam and LEDs. Truth be told, Hackaday doesn’t see many of these ‘softer’, cosplay and prop making builds in the tip line, and that’s a shame; the amount of skill that goes into these costumes is at least as equal as a woodsmith that can build fine furniture using only hand tools.

Perhaps a little premature, but TechShop is opening a new location in Arlington, VA at the end of the month. The GM [Addam Hall] was there scoping out the hacks and letting the attendees know there’s going to be a huge, awesome shop that’s down town in Crystal City. Close enough to public transportation, anyway, because anyone who drives in DC is certifiable.

The last item of note isn’t a build yet, but it’s shaping up to be pretty cool. It’s BRWRY – pronounced, ‘brewery’ – and will be a semi-automated beer making machine. Robots and beer, what can’t you love?

We’d like to thank [Zach], [Julian], [Taylor], and all the other guys from iStrategyLabs for putting together a nice evening of hanging out, drinking beer, eating pizza, and talking about what you’ve built. We had a great time, and we’re looking forward to the next one, as well as any other similar get together in other cities.

Searching For Makers In Washington DC

makeDC

Despite there being an inordinate amount of techies and tech companies in the Washington, DC/Northern Virginia area, there aren’t really that many hacker/makerspaces, or really anywhere else for tinkering, building, and generally futzing around with a soldering iron. [Zach] thought it was time for a change and is now organizing the second Make DC an informal get together to show off your latest projects and builds. Here’s the best part: Hackaday is coming, and we’re bringing some sweet swag.

Right now [Matt] has two talks lined up focused on bringing APIs into the physical world. There’s space for plenty more speakers, so if you have something to show off be sure to sign up.

The event is scheduled for Wednesday, March 19, 6:30 PM, half a block away from the Dupont Circle Metro station. Be there. You’ll get a sticker at least.