Wiring/DIY-CNC guys, a question...

Joined: 12:00 AM - Jan 01, 1970

7:23 AM - Oct 10, 2018 #1

As I was working on Wednesday's comic, I've been watching- well, listening to- This Old Tony's build of his gantry router mill.

Gantry Router Mill build part 3.

About ten minutes in, he mills up a gigantic heat sink for some little Gecko 210 stepper drivers, and then mounts the drivers with some thermal paste to it.

Is... is that necessary? I mean, I know the controllers move some power, and those Geckos don't have the attached heat sinks my drivers do, but do those things get THAT hot? Or is that giant heat sink more of TOT's silliness?

I'm curious as I've been worried about the control box I made for my Logan- the stepper drivers I used have built-in heat sinks, and I have them mounted for the best/most open-air exposure given the confines of my little enclosure. But I have yet to really task the machine, so I still don't know just how hot n' nasty it's gonna get in there.

The plan was to add a vents and a fan or two if things started getting really hot- I'm thinking 150F might be a good place to start thinking it's getting pretty hot.

Also, his wiring shows some large capacitors on the connections to the drivers- I'm not sure if that's the power out to the motor or in from the power supply connection. Is something like that necessary? Or recommended, to clean up power spikes or whatever? Damp some noise from the motors?

Doc.
Quote
Like
Share

Joined: 11:52 PM - Oct 04, 2018

10:44 AM - Oct 10, 2018 #2

knowing electronics as i do (imma programmer not an electrician, jim!), and being a darned big fan of ToT I can semi-confidently say that a heatsink is necessary for those gecko controlers, if not for running the rig than for longevity of the stepper controllers (the hotter electronics get the faster they break, especially if they've got cheap lead free solder inside as that breaks down at a lower temperature than proper solder)

the heatsink he built is probably OTT  for what he's doing, so that's either for the camera, planing ahead, or my back of the envelope calculation rather underestimates the heat dissipation needed on those controllers.

as for the caps they'll do several things, possibly the most important is that they'll help correct the power factor of the whole mess by adding an capacitive load to counteract the inductive load of the motors. It'll also act to snub power spikes caused by the collapsing or building magnetic fields when the motors wind down/up. I highly doubt they're necessary (especially considering that basically no residential utility measures and charges based on powerfactor) but.... they are cool as hell and will be a net positive to the build (if a small one)

buuuuut.... you should probably get someone who does more than dabble in the matters of angry wall pixies before deciding. :-)
Quote
Like
Share

Joined: 8:57 PM - Jan 11, 2016

11:30 AM - Oct 10, 2018 #3

It's common to put capacitors on the DC input to the stepper drivers for noise / transient suppression, as the controllers are switching significant DC currents to the motors and may produce power rail noise, They also provide a reservoir for sudden large loads - a surge of 'big amps' may cause the power supply to go into current limit, or drop volts briefly, so a few thousand microfarads as close as possible to the input terminals helps overcome that problem. (Steppers are DC - power factor problems are an AC only thing.)

The heatsink is almost certainly OTT, and because there's so little space for air to convect between the fins, it won't be particularly effective. The large mass of metal will provide a high thermal inertia, meaning long heatup and cooldown times, but their design isn't really all that good for long term cooling unless he adds a fan and forces air between the fins. Look at the relative metal-to-air distances on a commercial 'convecting' heat sink for an idea of 'good' air spaces.

Having said that, good design says that the cooler you keep the controllers, the less likely they are to die or otherwise misbehave. 150 F / 70 C is probably about as hot as I'd like to run the heatsink on something carrying large spikes of current, as the silicon itself will be quite a bit warmer than the heatsinks - and if it gets hot enough that a fan proves necessary, I'd get it running at a lower heatsink temperature - 120 F / 50 C unless its noise is a problem.
A little knowledge is..... the default setting.
Quote
Like
Share

Joined: 2:49 PM - Oct 01, 2014

1:39 PM - Oct 10, 2018 #4

In my experience running a CNC mill that was basically built using DIY type designs, yes.

The shit gets hot, fast. We would regularly have to clean our heat sinks and air filters on the CNC mill in question when it would start doing strange things. Once airflow was restored, it would behave as expected.
Quote
Like
Share

Joined: 2:05 PM - Oct 08, 2014

1:57 PM - Oct 10, 2018 #5

Beejay5169 wrote: (Steppers are DC - power factor problems are an AC only thing.)
Well, kind of. The inductance of motor windings which causes poor power factors in AC circuits also does funny things to DC when it's switched on and off. When you switch on the power the current doesn't come up instantly, it ramps up over time. That causes the torque to change until the current levels off. Also, as the rotor turns it influences the inductance of the windings, which further affects the current and torque.

When I worked at a printer manufacturing company torque ripple used to drive our mechanical designer nuts. Our printers marked a single row of dots at a time on label stock at high speed, so any ripple in the feed made for unever character height.
If it ain't broke, I'll fix it!
Quote
Like
Share

Joined: 4:37 PM - Jan 24, 2017

4:47 PM - Oct 10, 2018 #6

  On heat dissipation, sometimes you just have to make your best guess, and plan for worst case. I currently have a drive panel with 20 servo drives in it being built, but the duty cycle or idle current draw on most of the axes is (should be...) low. Engineering calculations say that the surface area of the enclosure will provide enough heat dissipation, but I provided a fan and vent on general principles, and a temperature switch for the panel as a further backup warning. The drives themselves also have temperature monitoring, so if one of them has a problem we'll know it. The difficulty is, in this case we really won't know all of the actual loads until the complete system is in operation.

 If necessary, we can always add more internal circulating fans or a larger vent fan. At least we won't require a monster air-conditioner as we've needed on a couple large server racks to meet the theoretical max loads...
AD ASTRA, AUT VIAM INVENIUM AUT FACIUM
Quote
Like
Share

Joined: 1:06 AM - Oct 28, 2014

6:59 PM - Oct 10, 2018 #7

The rule-of-thumb I've heard for electronics heating is literally the rule-of-thumb.  If you can leave your thumb, (or finger of course) against the component, continuously, it wasn't "hot".  But if it was too hot to touch continuously, then it was too hot.   Yes, it's a very crude test, but one that anyone can do, almost anywhere, with the usual precaution: "If you shock yourself, or burn yourself, or pinch yourself, or cut yourself, you're not qualified to run the equipment".
Quote
Like
Share

Joined: 12:00 AM - Jan 01, 1970

7:13 PM - Oct 10, 2018 #8

The enclosure I used was about 25% smaller than I wanted, but it's what was cheap and available.

In this pic...



My drivers are at the top. With the unvented front cover on, the only air circulation inside is either convection or the small fan at the bottom left in the power supply.

The first controller I used was this homebuilt one I bought off of Craigslist:



Which had a lot more room inside, as it didn't have the actual controller in there, and only had one power supply, not two.

But, the guy who built it clearly put a couple good sized fans on the case- one pushes, one pulls. I never ran it long enough to see how warm it got, but always figured if mine started overheating, I could put similar vents and fans in the case.

Doc.
Quote
Like
Share

Joined: 11:09 AM - Nov 17, 2014

8:34 AM - Oct 11, 2018 #9

Bang a decent fan directly onto the front lid, so that it blows directly at the drivers.    
That's what I do... I also bought 4 of them, and a dozen cheap foam filters to mount on top of them. 
(These filters are designed for PC cases in geek rooms and aren't up to handling anything worse than a slightly irritable dust bunny. Workshops have dust bunnies made of steel chips...  )
Add air holes at the bottom for the air to escape to help cool the rest of the doodads.
Quote
Like
Share

Joined: 2:02 PM - Jul 10, 2016

4:44 PM - Oct 11, 2018 #10

Heat sinks and capacitors bigger than you ever thought you needed are almost mandatory on small CNC mills.  Both are a dramatic help in getting the rest of your machinery last longer, and the capacitors in particular prevent all kinds of nastiness with spikes and sags and motor feedback. 
Quote
Like
Share