Author Topic: Measuring rpm using an oscilloscope  (Read 1933 times)

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Capt Slog

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Measuring rpm using an oscilloscope
« on: October 03, 2007, 01:54:08 PM »
I want to measure the RPM / DC voltage relattionship of my turbine.


It's a stadard 12 pole 9 coil wired in star and rectified at the back of the mill so that I get dc coming down the pole.


By my reckoning, I get...



  1. full sine waves per revolution for one phase,  so
  2. rectified half waves (positive humps) per phase per revoltion, so
  3. x 24 half waves = 72 humps per rev for three phases.


Yes?  Or have I missed something?


A separate question, if I may.


I'd also like to know the "resistance" of my turbine, so that I can put this into the blade design calculator program that I've got from Ed Lenz.  Where abouts do I measure this on a three phase wired in star?


Thank you.

« Last Edit: October 03, 2007, 01:54:08 PM by (unknown) »

Boondocker

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Re: Measuring rpm using an oscilloscope
« Reply #1 on: October 03, 2007, 04:03:27 PM »
Rpm can be measured on the AC side of rectifier via frequency:   Rpm = Freq *120/ No. of  poles


Works out nicely on a 12 pole alternator;   Freq * 10 = Rpm


Previously, did some similar measurements and calculations.


http://www.fieldlines.com/story/2007/1/24/233533/669


http://www.fieldlines.com/story/2007/6/10/16819/7312


Do a search using  "test coil".   I suggest reading Flux's replies.  He as commented many times on this subject.


For the alternator resistance, wired star, measure the resistance on 2 of the 3 outs.


Hope this helps.

« Last Edit: October 03, 2007, 04:03:27 PM by Boondocker »

Flux

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Re: Measuring rpm using an oscilloscope
« Reply #2 on: October 03, 2007, 11:57:04 PM »
You will find it confusing and difficult to measure the speed from the 3 phase rectifier ripple even off load with the thing being driven at variable speed with wind.


On load you will not see the voltage ripple so you will need to look at current but again you will struggle. Bring down a pair of light leads from before the rectifier for your frequency measurement.


The ripple of a 3 phase bridge rectifier is at 6 times line frequency, so for a 50 hz supply you will have 300 hz ripple.


For 12 poles the line frequency is rotational speed divided by 10.


To measure the resistance of the alternator you will need the machine on the ground and you will need to measure between any pair of wires. You will need to do it by the volt drop method, multimeters are not capable of measuring these low resistances accurately.


Flux

« Last Edit: October 03, 2007, 11:57:04 PM by Flux »

Capt Slog

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Re: Measuring rpm using an oscilloscope
« Reply #3 on: October 04, 2007, 12:40:06 AM »
Ok. Thanks both.


I know it's difficult to measure from the scope when the frequency changes a lot, so I use a digital camera and take a photo of the trace including the voltage and timebase settings (shame it's not a storage scope!).  I can then look at the photos and work it out from there; much easier.

« Last Edit: October 04, 2007, 12:40:06 AM by Capt Slog »

walsdos

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Re: Measuring rpm using an oscilloscope
« Reply #4 on: October 04, 2007, 04:18:01 AM »
Hello Capt., I have used a standard automotive Tachometer but for higher speeds than you are contemplating. However, it may provide an inexpensive means of constant monitoring.

     Good Luck
« Last Edit: October 04, 2007, 04:18:01 AM by walsdos »

jmk

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Re: Measuring rpm using an oscilloscope
« Reply #5 on: October 04, 2007, 05:50:16 PM »
 How do use an automotive tach?
« Last Edit: October 04, 2007, 05:50:16 PM by jmk »

walsdos

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Re: Measuring rpm using an oscilloscope
« Reply #6 on: October 04, 2007, 06:26:59 PM »
direct coupled to one phase with v limiting circuit will ,I think, read double the speed. Someone here will know better than I.
« Last Edit: October 04, 2007, 06:26:59 PM by walsdos »

laskey

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Re: Measuring rpm using an oscilloscope
« Reply #7 on: October 06, 2007, 01:21:43 PM »
if one phase reads double why don't you try using a diode, so you only get half a phase to the tach.  


Wouldn't it then read right?


Unless, I'm getting old and half of double isn't one anymore. :)


CYa,

Chris

« Last Edit: October 06, 2007, 01:21:43 PM by laskey »