Squidwrench Power Wheels Racer: Motor Current

With a new motor and controller in the reconfigured SqWr Power Wheels chassis, I made a few measurements under somewhat less than controlled conditions, with the butt end of the chassis on jack stands. The general idea was to find out what the “lightly loaded” condition looked like in terms of motor current; after some mechanical and electrical improvements, we’ll be in a better position to determine the battery load & suchlike.

Preliminary measurements:

  • Motor DC resistance: 0.7 Ω (meter lead resistance 0.2 Ω, so don’t trust it)
  • Motor winding inductance: 128 µH
  • Motor shaft key: 1/8 inch (keyway chewed by pulley setscrews, needs matching shaft flats)
  • Twist-grip throttle applies nonzero voltage when released: possibly damaged

With everything in position and the Tek 6303 probe set for 10 A/div, this is what happens when you push the deadman switch:

Out V I 10 A - start transient

Out V I 10 A – start transient

Obviously, the motor controller takes much too long to wake up & sense the current.

The initial slope of that current waveform looks like 80 A/360 µs = 220 kA/s. The upper trace gives the motor voltage, so 23 V / (220 kA/s) = 104 µH, surprisingly close to the measured 128 µH.

Deploying the might Tek CT-5 (with an enclosed A6302), cranking the gain to 50 A/div, and poking the deadman again:

Out V I 50 A - start transient full

Out V I 50 A – start transient full

During that initial pulse, the controller connects the battery directly to the motor, so you’re looking directly at 200 A of battery current. For reasons that aren’t relevant here, the mandatory 60 A safety fuse isn’t present, although it should be able to withstand a millisecond or two of moderate overload without blowing.

With that out of the way and the motor running at a few hundred RPM, due to the nonzero twist-grip output voltage with no throttle applied, the controller actually does PWM pretty much as you’d expect:

Out V I 10 A - run low speed

Out V I 10 A – run low speed

It’s not clear what caused the small dent just before the middle pulse; perhaps the motor commutator switched from one winding to the next.

The battery current will be much lower than the motor current in this mode, roughly (motor current) * (PWM fraction). We haven’t verified that, but for 30% PWM it should be around 5 A = 15 A * 0.30. The actual battery current looks smoother than I expected, although I have no traces to show for it; more study is needed.

Eks once again graciously loaned me his Tek current probes; this whole Power Wheels mess motivated me to get off my ass and accumulate my own collection, about which more later.


  1. #1 by madbodger on 2016-06-20 - 09:30

    I do love quality test gear. I don’t have any of those nice current probes either, but I do have an ancient Tek P6013 1000X high voltage probe which comes in handy once in a while when nothing else will do (I used it recently to determine that an ordinary CCFL power supply will produce an 800V sine wave output from a 1VDC input).

    • #2 by Ed on 2016-06-21 - 08:46

      Not only do those CCFL supplies produce astonishingly high voltages, they can give you a savage RF burn.

      I recall a note by one of the gods of analog design (Pease? Williams?) about optimizing a CCFL supply, including repeated warnings to not use a standard scope probe, because the combination of high voltage and high frequency will destroy it almost instantly.

      Rummages through Internet, finds it again: Linear Tech AN49 by Williams. Page three says you’ve got the right probe for the job!

      • #3 by madbodger on 2016-06-21 - 10:51

        I got the idea from a TubeTime design, which referenced the Williams design, so I read AN49 and a dozen or so of the related app notes. Williams knew what he was talking about, so I took those warnings to heart. That probe is so old it has a UHF connector on it, but it works fine with my most modern (1990s) scope.

      • #4 by madbodger on 2016-06-21 - 10:56

        I realized I had written up that project if you’re curious: http://bodger.dreamwidth.org/22750.html

        • #5 by Ed on 2016-06-21 - 14:05


          Makes my blinky LEDs look downright tacky…

  2. #6 by david on 2016-06-21 - 01:58

    Only slightly related, I have some Tek gear (a thm565 scopemeter and some random tm500 modules) I’d like to get officially calibrated in advance of a big LiFePO4 project. Do you have a vendor to recommend?

    • #7 by Ed on 2016-06-21 - 08:28

      officially calibrated

      Close enough is good enough around here; I know nothing of real calibration… [sigh]

      • #8 by david on 2016-06-21 - 20:16

        Normally i would too. But it turns out that in the world of kWh lithium cells, millivolts Matter…

        • #9 by Ed on 2016-06-22 - 08:14

          Well, only if you want the batteries to work… [sigh]

          The ancient battery packs for the bike radios have started calling for replacements. The least-awful alternative may be rebuilding them with new cells, although, by the time I finish cutting all the cases open, building new packs from scratch might not be all that much more work.

          • #10 by david on 2016-06-22 - 20:29

            Well, “work” would be nice but I really have my heart set on “not explode in unquenchable fire and kill me and everyone nearby”.

            • #11 by Ed on 2016-06-22 - 22:01

              Sounds like a good intermediate goal: one worthy of your effort, but not insuperably difficult. The “unquenchable fire” thing is definitely to be avoided.

              Says the guy who punched a hole in a cylindrical lithium cell with a screwdriver, of course.