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Q-talk 94 - Measuring RPMs

Steve Whiteside Liberty, KS

Just a note about the Westach errors. I used to work for a company that built permanent-magnet tachometers. These looked like small high-quality dc motors. They were all calibrated from the 60 Hertz power line using hysteresis synchronous motors. Hyst. Sync, motors run at exactly 1800 rpm or 3600 rpm and require a small capacitor across one set of the two windings. You used to find them in old reel-to-reel tape decks. Anyway, if you cannot find such a motor, almost any permanent magnet motor can be calibrated with it. I just calibrated a Radio Shack 273-256 motor. It produces 0.688 volts per 1000 rpm being driven clockwise and 0.720 volts per 1000 rpm going counterclockwise. Now that motor can be used with a digital voltmeter to measure almost any turning shaft. Rubber or vinyl tubing makes good temporary couplings.

The accuracy of that calibration is on the order of 0.1% for the power line and 0.3% for the voltmeter. If you measure with the same voltmeter that you calibrate with, then you can achieve nearly 0.1% accuracy near your calibrate rpm. Overall, this calibration would probably be good up to about 8000 rpm. Above that RPM or so, brushes begin to bounce off of the commutator and you lose output voltage.

This leaves the problem of coupling the motor to the aircraft engine. You will have to figure that out. If you use magnetic materials for mounting, it will change your calibration. Some commercial permanent magnet tachs have a speedometer cable shaft attachment and screw onto the tachometer cable mounting of an aircraft engine.

Another method would be to get a model aircraft proptach. Tower Hobbies lists one of these for $22. It may only measure to 100 rpm, I don' know. I also see optical pickup tachometers advertised in the tool catalogs for about $80. And at the high end is the PropTach by Cardinal Electronics of Michigan ($400).

I modified a JC Whitney Tach from 8000 to 4000 rpm full scale for one of the fellows here. My recollection was that it stayed within about 50 rpm, but that has been a while back. The scale is much bigger than the Westach meters. Gee, should I make a kit tachometer available?

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