Hacker News Reader

Comment by speeder | Hacker News Reader

Once I plugged a toy race track to the outlet (in portuguese we call it "Autorama" I dunno the name in english), I knew enough of physics to expect it to burst in flame...

But instead the car got REALLY, REALLY, REALLY fast, shoot out of the track and smashed into the wall.

Of course, I happily put a second car on it, and changed the track so that the car does not end smashed, and indeed, I could launch cars all over the place. Also as the car raced, sparks came out of the contacts with the track.

Now that I am older and know even more physics, I have no fucking idea on how I made a DC motor work only forward with a AC current.

View on HN

Assuming the track was designed so the cars go forward no matter which direction you place them, the cars likely had a small diode-based circuit to always feed the current into the motor with the same polarity. When you fed an AC current into the track, that circuit in the cars would have acted like a rectifier, turning the AC into poor-quality, high-voltage DC.

The sparks would come from feeding way too much voltage in, enough to jump the gaps caused by momentary losses of contact between the track and a fast-moving car.

Or, they could have used electromagnets for the stator and the rotor. The stator would reverse polarity at the same moment the rotor reversed polarity (modulo differences the transfer functions), so the force would still be in the same direction, despite the alternating current. One could make A/C motors this way, it's just that A/C induction motors are much more efficient.

Diodes! Great idea, I should have thought of that.

I'm guessing your mains supply would've been 230v/5A - in which case I have to say I'm surprised that it didn't blow the diodes out :) I remember plugging a 1N2001 (or similar) into a wall socket, and turning the switch on to watch the diode blow out as a schoolboy.

As a sibling comment mentioned, it's possible there was a diode somewhere in the system.

Another possibility is that the motors in the slot cars had electromagnets for the stationary field instead of permanent magnets. That's what an ordinary power drill, especially one without electronic speed control, has. Such a motor will run pretty much the same on AC or DC. When running on AC, the rotor and stator both change their magnetic fields in perfect sync so the net effect is substantially the same as running to same motor on DC.

Wikipedia has an article on "Brushed DC electric motor" with a section on "wound stator". It doesn't specifically mention running the motor on AC, but there's enough there to show you what's going on.

Reply on news.ycombinator.com