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RPM measurements with multimeter?
By gotwind2, Section Mechanical Posted on Wed Aug 29, 2007 at 06:57:57 PM MST
Hugh Piggotts excellent book

Hi.
In Hugh Piggott's 'How to build a wind turbine' book (Page 42) and under DC voltage tests, it is suggested - 'if your multimeter has frequency testing ability's that the alternators rpm can be measured'?
e.g 190rpm would read as 19 Hz
I'm positive this must be correct if Hugh suggests it, I wasn't aware DC had a frequency,I know AC does - I am obviously no expert.
Also I notice such 'frequency equipped' multimeter's are pretty expensive. Would be a handy tool though, if it can be afforded.
Ben.

RPM measurements with multimeter? | 6 comments (6 topical)

Re: RPM measurements with multimeter? (3.00 / 0) (#1)
by DanB on Wed Aug 29, 2007 at 01:18:07 PM MST

You would measure the frequency on the AC side of the rectifier... and yes, for 12 pole alternator 190 rpm would read as 19hz. Meters that can do that are not expensive - lots of cheap meters can read frequency these days.

Re: RPM measurements with multimeter? (3.00 / 0) (#2)
by gotwind2 on Wed Aug 29, 2007 at 01:35:54 PM MST

Thanks DanB.
That makes more sense.
It was the 'DC test section' that threw me.

Cheers
Ben.

www.gotwind.org

[ Parent ]

Re: RPM measurements with multimeter? (3.00 / 0) (#3)
by Flux on Wed Aug 29, 2007 at 02:10:16 PM MST

On no load using frequency from the ac input is perfectly simple.

If you try using ac before the rectifier while battery charging you will almost certainly have to devise some form of filter for most of the cheap multimeters to read sensibly, the commutation spikes from the rectifiers usually screw up the answers otherwise. Some meters are fairly good but others are an absolute pain.

Flux

[ Parent ]

Re: RPM measurements with multimeter? (3.00 / 0) (#4)
by kell on Thu Aug 30, 2007 at 10:33:42 AM MST

What do you use, a series resistor with a capacitor shunt?

[ Parent ]

Re: RPM measurements with multimeter? (3.00 / 0) (#5)
by Flux on Thu Aug 30, 2007 at 10:39:15 AM MST

Yes.

Most meters are 10m ohm input impedance and work with a low input voltage so a series 47k and shunt .1 or .22 Uf is often good enough . Use the largest capacitor you can without reducing volts below the operating point.

Flux

[ Parent ]

Re: RPM measurements with multimeter? (3.00 / 0) (#6)
by elt on Wed Oct 03, 2007 at 06:00:01 AM MST

Thank you, that worked great! - Ed.

[ Parent ]

RPM measurements with multimeter? | 6 comments (6 topical)

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