Using an rpm pickup?

[bw77]

Can an rpm pickup like the Fluke RPM80 be used with any dmm by using the frequency setting? I can't find any info on exactly what signal is output.

http://www.tequipment.net/FlukeRPM80.html

Thanks.

 

[bw77]

Answering my own question. According to this page:

http://www.aeswave.com/Inductive-RPM-Pickup-RPM80-p8534.html

the pickup produces a <= 4V dc output. So it should work on dc frequency,
then do the math.

 

[zkling]

Yes, it is what I do. Maybe make a little chart for quick reference that you can keep handy. After a while you will get a sense of Hz reading to RPM. One of the fluke manuals goes through the calculations from Hz to RPM if you need to double check.

 

[dowmace]

A mag pickup also produces ac voltage. When I convert generators to electronic governing I install a mag pickup and I adjust speed by watching the output voltage of the mag pickup before moving to output frequency speed adjustment.

Sent from my SCH-I925 using Tapatalk

 

[bw77]

I am the OP and wanted to update.

I am using a Fluke 179 meter.
I got the Fluke RPM80 and hooked it around the spark plug wire on my
lawnmower. It picks up AC frequency, but shows zero on DC freq.
Shows 48 Hz AC, so 48x60 = 2880rpm which sounds reasonable.

 

[elcom]

I am the OP and wanted to update.

I am using a Fluke 179 meter.
I got the Fluke RPM80 and hooked it around the spark plug wire on my
lawnmower. It picks up AC frequency, but shows zero on DC freq.
Shows 48 Hz AC, so 48x60 = 2880rpm which sounds reasonable.

I am trying to do the same thing with a motorcycle (1983 Honda Nighthawk 450), also with the Fluke 179 meter but not with the Fluke RPM80.

I am using a fluke AC clamp (80i-400), specs at: http://www.fluke.com/fluke/inen/accessories/Current-clamps/80i-400.htm?PID=55353

Clearly, it is picking up (0Hz DC, but picks up Hz AC as expected) and the frequency is varying with the RPMs.

At ~ 600 RPM according to the bike tachometer, I get a reading ~50-60 Hz (varies). How does one interpret that? It doesn't seem to be entirely linear, but perhaps that is just the variability. Also, not sure how accurate the mechanical tachometer is on this bike.

Any ideas re: what the relationship between RPM and Hz AC should be?

I guess that I can take multiple readings, assume that the bike's tachometer is reasonably accurate and fit a curve, but was hoping that someone had a better answer.

Thanks!

 

[bw77]

Are you measuring frequency on one of the spark plug wires?
If so, depending on how the ignition system is designed, there would
be a spark either 1x per each revolution of the engine (waste spark design -
spark on compression and exhaust strokes),
or 1x per each 2 revs of the engine (compression stroke).

In the first case, rpms = freq x 60.
In the 2nd case, rpms = freq X 120.

If you have 50-60 freq, that would be 3000 - 3600 rpms, in the 1st case,
which does not make sense, if your tach says 600.

Maybe someone else can chime in here.

 

[APEowner]

When ever I see a frequency around 60hz I wonder if I'm picking up noise from an AC line somewhere. You said that it's varying with RPM but it still could be that the noise is swamping the signal that you want to measure. What are you getting when the bike isn't running?

 

[elcom]

Thanks for the ideas. Getting 0 when the bike is not on. I did this out in the street. Shouldn't be near another AC line.

The mechanical tach could be off, but I wouldn't think that the bike would be idling at 3000-3600 rpm.