Archive for June 20th, 2016

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.

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