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SquidWrench Power Wheels Racer: Power Train Autopsy

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:

Motor Controller - smoked housing

Motor Controller – smoked housing

That smudge came from a rank of detonated MOSFETs:

Motor Controller - exploded MOSFET

Motor Controller – exploded MOSFET

Other MOSFETs had unsoldered themselves:

Motor Controller - unsoldered MOSFETs

Motor Controller – unsoldered MOSFETs

Explosively:

Motor Controller - solder ejecta

Motor Controller – solder ejecta

I brought along an ancient Sears starter-motor ammeter to measure the motor current:

Sears 244-2145 Starter Ammeter - front

Sears 244-2145 Starter Ammeter – front

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:

Sears 244-2145 Starter Ammeter - wire guides

Sears 244-2145 Starter Ammeter – wire guides

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:

  • 12 V: 1600 RPM @ 40 A
  • 24 V: 2400 RPM @ > 100 A

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:

Motor - charred windings

Motor – charred windings

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…

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  1. #1 by Red County Pete on 2015-12-07 - 09:54

    Ouch!

    Thanks for the tip on DC motors. I’ve rebuilt a few AC ones (bearings), and got away with rebearing a lawn mower motor (12V cordless), but I suspect I was lucky.

    compulsive wire neatness

    May I also suggest: http://www.solar-electric.com/fb-110t.html The holder’s shroud will help contain the plasma when Murphy gets his way. $19.00 or so on a new fuse would hurt, but a lot less so than replacing all the rest of the stuff. I’m using this one on the off-grid power system in the garage. I have a similar fuse on the 300W system for our camping trailer, though that holder doesn’t have a shroud. I’ll probably upgrade it when I replace the circa 2000 batteries (AGM FTW) in the trailer. Real Soon Now.

    • #2 by Ed on 2015-12-07 - 10:43

      The rules recommend installing the matching fuseholder, which fully encloses the fuse and provides robust 5 mm termination bolts. I think that got overlooked in the first pass, but you really don’t want mechanical stress on the fuse terminals; there’s enough going on inside already.

  2. #3 by Vedran on 2015-12-07 - 12:11

    “never disassemble a working DC motor, because you’ll demagnetize the stator”

    I’ve read about this somewhere but always fought it was just an urban legend… what mechanism causes the demagnetization?

    Oh, and somewhat off-topic, I also read somewhere that Neodymium magnets were poisonous. Not the nickel plating itself, but the inner magnetic material. It didn’t say how toxic they are, but it did say to dispose of magnets that are broken or have damaged plating. I wasn’t able to find any mention of this anywhere else, and not from lack of trying. Do you know if this is true and how toxic they actually are?

    I’m mean yes, you probably shouldn’t eat them but that applies to almost everything on my workbench :)

    • #4 by Ed on 2015-12-07 - 17:13

      what mechanism causes the demagnetization?

      As I understand it, the stator magnets run just shy of saturation (maximum flux density B), because otherwise they’d use smaller = cheaper magnets. The field in the stator-to-rotor gap depends on the rotor position, so when you (try to) remove the rotor and it slams against the stator, the field increases dramatically at that point: the increasing H pushes B further out along the saturated part of the curve. When you reassemble the motor, the flux density B doesn’t return to its previous value, because that’s what the hysteresis curve means.

      The motor still works, in the sense that the rotor still goes round and round, but the reduced magnetic field means it can’t generate the same torque.

      Sorta-kinda handwavy, but it makes sense to me.

      Neodymium magnets were poisonous

      Wikipedia sez neodymium is mildly toxic, but breathing oxidized dust isn’t at all good for you. I have seen what happens when the nickel plating flakes off, which means I’m so screwed…

      • #5 by Vedran on 2015-12-08 - 09:53

        “when you (try to) remove the rotor and it slams against the stator”

        Interesting. So as long as you removed the rotor along the central axis without touching the stator it would be fine? Some sort of nonmagnetic spacer sleeve comes to mind.

        I have a couple of NEMA23 steppers internally wired with central taps (6 wires) that I want to run in parallel winding setup (8 wires). I rewired one of them some tome ago and didn’t notice (a negative) change in torque. Maybe more precise measurement is in order before I rewire others.

        “Wikipedia sez neodymium is mildly toxic”
        But magnets are actually NdFeB alloy so it’s that doesn’t mean much, right? Most of us had mercury amalgam filling in our teeth at some point, and mercury is definitely toxic…
        NdFeB dust is classified as combustible but the amount of dust produced when the magnet chips can’t be that significant. Machining or sanding the stuff is a different matter, but that’s nor here nor there as you say :)

        • #6 by Ed on 2015-12-08 - 13:48

          without touching the stator it would be fine?

          Without the rotor, the B field would roll way down the hysteresis curve, then back up again when you (ever so carefully) reinsert the rotor. Might work, but …

          A book on the Power Electrics shelf of the Basement Library describes how The Old Masters “charged” stator magnets in situ with a gadget that, in different context, would be a head-on collision between the irresistible force and the immovable object, but I don’t know if that’s how it gets done these days.

          For sure, you can’t handle neodymium magnets with precision, unless you’re got some pretty fancy tooling.

          Most of us had mercury amalgam

          Every dentist I’ve ever seen wants to drill out the amalgam filling in one of my incisors that’s been there since I was a pup. I make growling noises, tell them to put down the probe, keep their hands in view while they step back, and nobody will get hurt. At some point it’ll wash out and then we’ll fix it, but I’ve been waiting for decades and it’s still going strong.

          • #7 by Kuba Ober on 2015-12-15 - 08:37

            I’d leave that amalgam alone. There’s no point to replacing it if there’s no decay behind it and no obvious problems such as looseness. An old time dentist told me that amalgams work themselves slowly over time, but composites don’t. Mechanically then, an amalgam is self-healing at the interface to the dentin and enamel. That’s in contrast to composite that, once cracked at the interface, will gladly set itself free but not before letting decay destroy the what’s behind.

            • #8 by Ed on 2015-12-15 - 09:15

              Aye! In this case, if it ain’t broken, I’m definitely not looking for trouble…

  1. SquidWrench Power Wheels Racer: Motor Musings | The Smell of Molten Projects in the Morning