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


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…