Motor revs?

Anybody know what kind of RPM a stock armature can get up to, for example a stock Tomy Turbo 'red' motor?

Cheers - DEANE

I have no idea what the unloaded max RPM is for a slot car arm. You could try measuring it with a digital laser tachometer, although it may be difficult to get a large enough reflective target on the arm or shaft without interfering with the operation of the motor. A contact tachometer would load these small arms and skew the readings substantially.

Here's some links to laser tachs. They aren't cheap but if you have a buddy who works in plant maintenance you may be able to borrow one.

http://www.kerncoinstr.com/lasertach.htm
http://www.gyroscopes.co.uk/display.asp?product=TACHOMETER
http://www.reliabilitydirect.com/tachometerproducts/EXT-461893.htm
http://www.reliabilitydirect.com/tachometerproducts/ELE-plt-5000.htm

Another possibility would be to use an audio spectrum analyzer and measure the frequencies generated by the motor. You could then figure out how to convert these to motor RPM, taking into account the nature of the generated audio. I must admit that I'm not certain about what the source is of the high pitched whine you hear coming from a slot car motor, but I suspect it is primarily from the interaction between the 2 brushes and the 3 segment commutator.

I often wonder how Aurora came up with the infamous 68,000 RPM advertised for the Super II arm. I suspect it's a theoretical number and nobody ever validated the claim.

I'd love to have a laser tachometer in my tool box.

Seems like it would be easier to do the calculations based on the speed of the car and the gear ratios between the armature and the rear axle. In fact, I know of some software that could help a lot in doing those calculations... We've got several gear gurus at work that no doubt have all the equations already set up. I'll check on Monday.

Seems like it would be easier to do the calculations based on the speed of the car and the gear ratios
But that's not the question that's being asked. He's inquiring about the maximum RPM an armature can attain. Measuring the distance traveled over a specific time interval and working the equation backward through the tire circumference and the gear ratios doesn't answer that question. You must measure the distance and time interval after the car has reached its terminal velocity (i.e., acceleration is zero), with the supply voltage & current at the maximum steady state values, and the back emf generated by the motor at its maximum steady state value.

The hardest part with the on-track method is determining if the car has really reached its terminal velocity. To do this without the aid of an accelerometer would require that you set up two speed traps. If the trap speed is identical for both traps you could assume the car is at its terminal velocity. I think that you would need at least 50 feet of straightaway to do this with a Tomy Turbo. Maybe a lot more.

If you did numerous measurements and the dual trap requirements were met you would be able to approximate the armature RPM under the tested conditions. But the tested conditions would include tire slippage, running gear friction, pickup shoe friction, guide pin friction, and all other forms of drag imposed by the chassis, track, and body. These conditions, like drag, also place a bigger demand on the power supply so you have to make sure the power supply is not limiting the car's peak performance. You still would not have determined the armature's maximum RPM, but only its maximum RPM under a very specific set of conditions.

Check ebay they have one for $49.00 and I have one I bought years ago for $19.00 for RPM on a model airplane.

<http://search.ebay.com/Laser-Tachometer_W0QQsofocusZbsQQsbrftogZ1QQcatrefZC6QQs ojsZ1QQfromZR10QQsacatZ-1QQcatrefZC6QQftrtZ1QQftrvZ1QQsadisZ200QQfposZ9206 4QQfsopZ1QQfsooZ1>