measuring 300 cpm load variations
measuring 300 cpm load variations
(OP)
We will need to evaluate the possibility of "instantaneous" load variation a 100 HP 1200 rpm motor experiences while driving a ~ 100 rpm agitator with 3 blades. Are there commercial instruments well suited for this ?
thanks
Dan T
thanks
Dan T





RE: measuring 300 cpm load variations
There are a variety of ways you could estimate instantaneous torque from 3 sets of voltage and current measurements based on a model of the machine from simple to complicated models. I'm not sure quite how Baker does it.
I think in another thread you mentioned FFT of load current looking for sidebands at torque oscillation frequency. That would be a pretty good simple start imo.
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(2B)+(2B)' ?
RE: measuring 300 cpm load variations
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I would suspect you could get a fairly similar picture to the torque timewaveform by just plotting an envelope of the current time waveform. Not identical but good enough to tell that one machine is much different than the next.
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(2B)+(2B)' ?
RE: measuring 300 cpm load variations
The problem is that most transducers do not have the bandwidth you need. Three hundred cpm translates to 6 Hz and that is beyond most standard transducers.
But, since the power variation probably is moderate, you can use three CTs and rectify their output to obtain a six-pulse current signal, which closely reflects the power variation.
The reason this works is that the power factor changes very little when the load variation is small. And, if needed, you can brake your motor to calibrate the set-up.
If you smooth the output with 10 - 20 ms, you will get a clean trace where you can see real small variations.
Gunnar Englund
www.gke.org
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RE: measuring 300 cpm load variations
The only problem I can see with Gunnar's approach is that the rotor of the motor will act as a flywheel which will tend to dump energy in to the load as it slows, thus having an averaging effect when viewed from the electrical supply side.
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If we learn from our mistakes I'm getting a great education!
RE: measuring 300 cpm load variations
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(2B)+(2B)' ?
RE: measuring 300 cpm load variations
Note that many teeth are cracked, and there are missing teeth all around the gear, with a few curious patterns of single and 5 or 6 skipped teeth thrown in to confuse me.
RE: measuring 300 cpm load variations
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(2B)+(2B)' ?
RE: measuring 300 cpm load variations
CTs with current output is actually better because their current source characteristics compensate for the diode bridge forward voltage drop. And you save on burden resistors, too. Just one instead of three.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: measuring 300 cpm load variations
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RE: measuring 300 cpm load variations
We each have our comfort zones based on what we have done before.
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(2B)+(2B)' ?
RE: measuring 300 cpm load variations
An interesting example on the forum was:
thread237-249262: Low power factor, recip pump
In that case the oscillation in torque was much more than the oscillation you would have guessed just looking at hte current envelope (the direction of power and torque reversed during each mechanical oscillation cycle). I saw a presentation at LEMUG where a very similar waveform was obtained under similar circumstances.
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(2B)+(2B)' ?
RE: measuring 300 cpm load variations
Swedish company, BTW, and I know their people quite well. So, I shall ask tomorrow. If the bandwidth is OK, then you have the OTS solution there. Stand by!
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: measuring 300 cpm load variations
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: measuring 300 cpm load variations
Bill
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Jimmy Carter
RE: measuring 300 cpm load variations
Preliminary, the reaction time is specified at 100 milliseconds, but that is not the analogue output. It may be faster - but probably slower. Most analogue outputs are slow in order to make the display easier to read.
BTW, these guys are Mark Empson's friends.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: measuring 300 cpm load variations
Measurement & Analysis
The Datum Electronics Torque Trials Kit does more than just measure Torque; it can be used to verify power outputs from engines and motors. The following data can also be measured and analysed:
• Power Transmission
• Torque Trials
• Vibration and Torsional Acceleration
• Power Transients
• Peak Torque Levels
• Power Delivery
• Shaft Vibration
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If we learn from our mistakes I'm getting a great education!
RE: measuring 300 cpm load variations
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: measuring 300 cpm load variations
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: measuring 300 cpm load variations
You might consider posting this on the mechanical engineering page...
Although this could be a blade-passing resonance, it would seem more likely to be a pure mechanical problem; over-hardening of the gear steel, improper mesh set-up, the wrong steel, inadequate gearbox strength, or any of a number of the other mechanical problems. I suspect that your investigation is attempting to blow away the smoke screen to prove just that.
Would a contact audio recording of the running gearbox provide any clues?
RE: measuring 300 cpm load variations
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...