The Smell of Molten Projects in the Morning
Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
If you measure something often enough, it becomes science
Rolling through the back of the Vassar Campus, watching a murder of crows on the lawn, when all of a sudden:
That squirrel passed about three feet in front of Mary’s bike, running flat out and, at 60 frame/s, touched the ground every 200 ms:
Figuring a squirrel body+tail is 1.5 ft long and it covers 3 of those units with every leap, it’s moving at 22 ft/s = 15 mph. That’s about as fast as we travel…
After 95 minutes on a pleasant ride with temperature around 55 °F, the STK C battery had 0.59 W·h remaining (dark green trace):
The last time around, it had 1.85 W·h after 61 minutes. Subtracting the two (and ignoring that it may have started with slightly different charges and behave differently at different temperatures) says the camera used 1.26 W·h = 76 W·min in 34 minutes, which averages out to 2.2 W.
That’s close enough to the “a bit over 2 W” figured from those partial-to-empty measurements for me.
The discharge tests from early November:
The best STK battery (D) holds just under 4.2 A·h, so its absolute longest run time could be 110-ish minutes. That graph shows the A cell was just about done after 75 minutes, so changing the battery after an hour still makes sense; you never know what will happen during the last few minutes of a ride…
So I can find it again, the way to change the sudo timeout for a particular user (that would be me) involves adding a line to the /etc/sudoers file using sudo visudo, thusly:
Defaults: ed timestamp_timeout=90
# blank line to make the underscore visibleNote the colon! Should you add the timeout to the global Defaults env_reset line, then everybody gets a monster timeout, which may not be what you want.
You can change the default editor (nano in Ubuntu) thusly:
sudo update-alternatives --config editorOr, in Arch / Manjaro, add a stanza:
Defaults editor=/usr/bin/nanoBecause my vi hand is weak:
That’s all I need to insert the proper stanza & move on.
So another knockoff Neopixel started flickering and its blue LED went dark:
Squirting it with circuit cooler brought it back to life, albeit briefly, so it’s a real thermal failure. OK, after I get smacked upside the head twice, I can recognize a problem when I see it.
I removed the top cover and jammed a themocouple into the screw hole in the middle of the pillar:
A folded tissue weighted down with random desktop junk kept the breeze out of the interior:
If the middle of the column hits 50 °C, what’s it like inside the 5050 packages with all those LEDs blazing away? Looks like I’ve been cooking those poor knockoff Neopixels to death.
The temperature is 50 °C with the LEDs running at maximum PWM = 128. Reducing the maximum PWM to 64 reduces the core to 30 °C and that dead blue LED springs back to life.
Figuring each LED package dissipate 250-ish mW at full throttle, that’s 120 mW at PWM 128 / 60 mW at PWM 64. The set of 12 packages dissipates 1.4 W / 750 mW, so, in a 22 °C room, the thermal coefficient is up around 10 to 20 °C/W, which is somewhere between bad and awful. Running the LEDs at full throttle obviously isn’t an option and even half-throttle really doesn’t work.
So, OK, mounting LED strips on a clever 3D printed plastic column with zero air circulation isn’t nearly as smart an idea as I thought: barely more than a watt burns right through the redline.
The Neopixel specs have nothing to say about the thermal coefficient from the LED junctions to the package leads, but cooling the copper conductors in the flex PCB can’t possibly hurt.
No, I do not want to CNC machine an aluminum pillar with little tabs on the platter for better heatsinking. It would be an interesting design project, though.
As part of the power train autopsy, Matt pointed me at the specs for the AmpFlow E30-400 motor they built into the chassis. The Performance Chart (mooched from AmpFlow to forestall link rot) provides useful information:
The Power Wheels Racer rules limit the motor to 1440 W, a tidy 60 A at 24 V. Let’s call it 70 A, which lines up neatly with the second major division up from the bottom: the orange current line hits 70 A with torque = 2.6 N·m.
Draw a vertical line at that point and read off all the other parameters from the scales on the left.
The motor will produce 2.6 N·m at just shy of 4500 RPM; call it 4400 RPM.
The SqWr Racer has 9:40 chain-drive gearing, so the rear wheels turn at:
990 RPM = 4400 RPM x (9/40)
With 13 inch diameter wheels, the racer moves at:
38 mph = 990 RPM x (π x 13 inch) x (60 min/hr) x (1 mile / 63.36x103 inch)
Which is scary fast if you ask me. A higher ratio may be in order.
At that speed the motor delivers:
1.6 HP = 1180 W = 2.6 N·m x 4400 RPM x 2π rad/rev / (60 s/min)
… to the shaft and, minus mechanical losses, to the tires.
If the racer doesn’t require that much power to roll at breakneck speed, it’ll go even faster, until the motor’s (falling) power output matches the (rising) mechanical load at some higher speed with correspondingly lower current.
With a current of 70 A and a winding resistance of 0.089 Ω (let’s say 0.10 Ω), the motor dissipates 490 W. That’s probably too much for long-term running, even with a 70% (= 1150 / (1150 + 490)) efficiency.
The mandated Littelfuse 60 A fuse has a bit under 1 mΩ of cold resistance and will dissipate 3.6 W at 60 A. The specs say it will blow within 6 minutes at rated current.
The resistance of the wiring / connectors / switches / whatever should be on that same order. Figuring the racer needs 2 m of stranded copper wire, that calls for 2 AWG or larger (0.5 mΩ/m). Right now, the racer uses 8 AWG (2 mΩ/m) and might have 4 mΩ total resistance, although I think it has less than 2 m of wire. Empirically, the motor conductors get really hot at 40 A for about ten seconds, but that’s with a severely defunct motor.
If the conductors + connectors between the battery and the motor introduce, say, 10 mΩ of resistance, they’ll dissipate 36 W at 60 A. That scales linearly with resistance, so a high-resistance connection will incinerate itself.
Using a PWM controller to reduce the speed will reduce the available horsepower, so the racer will accelerate slowly. With the torque limited to 2.6 N·m, the horsepower will vary linearly with the PWM duty cycle: nearly zero for small PWM, up to 1.5 HP for large PWM at 60 A, then upward as the RPM increases with decreasing load. Yeah, you get more torque when you need it least.
I could make a case for a three-speed transmission in addition to higher gear ratio, although that seems overly complex.
A less beefy motor will be in order and The Mighty Thor suggests a torque converter as a low-budget transmission. Sounds good to me; I should learn more about electric traction motors…
The Power Wheels Racer taking shape at SquidWrench let out The Big Stink at the Mini Maker Faire a few weeks ago, so I brought some test equipment to the regular Weekly Doing and helped with the autopsy.
The PWM motor controller purports to do 60 A at up to 50 V, but removing the cover showed it wasn’t going to do any more controlling:
That smudge came from a rank of detonated MOSFETs:
Other MOSFETs had unsoldered themselves:
Explosively:
I brought along an ancient Sears starter-motor ammeter to measure the motor current:
The magnetic field around the wire directly drives the meter movement, with two guides for the 75 A and 400 A ranges, and none of that newfangled Hall effect nonsense to contend with:
Yeah, that says FEB 79; I’ve been collecting tools for quite a while…
I slapped the motor connectors directly on the battery terminals, holding them with small locking pliers after discovering that the wires got way too hot, way too fast. A snippet of retroreflective tape on the motor sprocket and a laser tach gave us the speed:
The AmpFlow E30-400 motor data sheet confirmed that those numbers were grossly wrong. Unloaded, it should spin at 5700 RPM at 24 V while drawing 3.2 A (thus, 2800 RPM at 12 V & 1.6 A).
Diassembling the motor showed it hadn’t escaped the carnage:
Those windings should be the usual amber enamel-over-copper, not charred black. The excessive current and reduced speed suggests many shorted turns inside the rotor.
Protip: never disassemble a working DC motor, because you’ll demagnetize the stator. The motor should still run when you put it back together, but the reduced magnetic field will wreck the performance.
As nearly as we could tell, one of the motor wires shorted to the frame when it got pinched under the seat; that’s an easy mistake to make and shows why compulsive wire neatness pays off big time. Shorting the controller output blew the transistors and, after raising the seat to look underneath, the motor would cook itself without generating much torque while you figure out what happened.
As far as I’m concerned, if you’ve never blown up anything that severely, you’re not building interesting stuff and definitely not trying hard enough.
The next iteration should work better!
Thanks to Dragorn of Kismet for stepping into the stench with phone camera in hand…
APRS tracks for my rides around Poughkeepsie in early November 2015:
Turning on the topo data and squinting at the Red Oaks Mill area:
The topography isn’t in my favor, with two ridgelines between Red Oaks Mill and the two APRS nodes near Poughkeepsie. APRS coverage southwest of Red Oaks Mill along the Mighty Wappingers Creek (basically, Vassar Road) ranges from spotty to nonexistent, because that route has even worse topography.
Seems to me an APRS iGate in Red Oaks Mill, running Xastir (perhaps headless) on an RPi, conjured from my heap (perhaps with a shiny new TNC-Pi atop the RPi, rather than an ancient Kantronics KPC-9612), and using a vertical VHF antenna in the attic (because lightning), might improve the situation.
That whole project continues to slip into the future, but at least I have more motivation and linkies…
The Smell of Molten Projects in the Morning 