What's the RPM

N

nctoxic

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Is there an easy way for a not-too-electrically-inclined guy to determine the rpms of a motor that is missing the data plate?

I have an old motor without the usual data plate. I've determined it draws appx 5.2 amps when running. That's close to 3/4 hp, but I can't determine the revolutions per minute.

Thanks for any helpful hints.

Tim,,,

Sorry for the double post. I couldn't figure out how to delete the second one.
 
A laser tach is worth having, even if it is only to satisfy a tool craving. Generally, single-phase motors come in two speeds - 17425 and 3450 RPM. Fasten a pulley or something to the shaft and run it up. Compare that to a known motor. You'll be able to guess whether it's the same speed, twice as fast, or half as fast.

In use, the motor will actually turn a bit slower. But this should help.
 
Hi NCTOX, I also bought one of those cheap digital tachometers similar to the link a few posts above, they seem to work well.

Regards

Ed.
 
Hawkeye said:
A laser tach is worth having, even if it is only to satisfy a tool craving. Generally, single-phase motors come in two speeds - 17425 and 3450 RPM. Fasten a pulley or something to the shaft and run it up. Compare that to a known motor. You'll be able to guess whether it's the same speed, twice as fast, or half as fast.

In use, the motor will actually turn a bit slower. But this should help.
Click to expand...

That's a very fast motor, lol. I'm guessing you meant 1745? :p
 
1725, half of the 3450 would be my thinking on that typo. ;)
 
No, HQ, it was supposed to be 1725, like Tony said. Darn laptop keyboard and thick fingers. No space between keys.
 
Hawkeye said:
No, HQ, it was supposed to be 1725, like Tony said. Darn laptop keyboard and thick fingers. No space between keys.
Click to expand...

Thanks Hawkeye I figured that out after seeing Tony's post, I knew there was an extra digit in there somewhere.:)
 
That's always my excuse, and I'm sticking with it. Big fingers; small keys. :)
 
OK, here's a challenge: Find a neon lamp test light. Calculate the circumference of the shaft. Put white marks (whiteout will do) spaced such that they are an even multiple of 60 Hz at either 1725 or 3450. Hold the lit neon lamp near the spinning shaft. If you have calculated it right and marked it correctly, you'll get a stop-action view of the marks. If they don't appear to be stationary, you haven't got the shaft marked for the correct speed.

Anyone remember seeing the turntables with the strobe speed indicators? Works like a timing light on an automobile engine. Another near obsolete tool.

On small shafts, a disc can be afixed to it for marking.
 
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