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measuring 300 cpm load variations

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

RE: measuring 300 cpm load variations

The Baker Explorer plots a parameter they call torque. Somewhere on their website they have a case study about detecting pump problems based on erratic torque.

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

The case study I mentioned starts just after slide 40 here:
http://www.vibration.org/Presentation/Joe%20Geiman%20-%20Baker%20Instruments.pdf

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.

=====================================
(2B)+(2B)'  ?

RE: measuring 300 cpm load variations

You can use any three-phase power transducer with high enough bandwidth and analogue output and feed the analogue signal to a recorder.

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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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: measuring 300 cpm load variations

Shaft-mounted direct torque transducers are available. They use some sort of SAW-based technique with a shaft-mounted transmitter as far as I know. For example, http://www.datum-electronics.co.uk/torque-transducers-torque-sensors-and.aspx

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.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: measuring 300 cpm load variations

Just a note on terminology - I think you're talking about rectifying the voltage output of current clamp-on probes, not the output of CT's (which would be current sources).  

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(2B)+(2B)'  ?

RE: measuring 300 cpm load variations

(OP)
I probably should explain what we're after.  Some low speed gears have been failing. Most recently a unit rebuilt in their shop with new gears lasted just a few weeks. The GBox manufacturer claims to have designed the gears to AGMA and ISO standards for the HP and service. The mfr says they suspect overload. The motors are near but do not exceed FLA, so I'm speculating >>if<< the gears are designed and made correctly an "overload" would have to be a pulsation or something that does not show up as excessive amps for long enough to appear in the averaged reading fed to the control room, etc.
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

Another thought for Tmoose - since you have a vibration data collector, it can be used as a single-channel digital recording device. You just have to select a current probe for the range you want with the proper frequency response characeristics. Then you can view time waveforms and spectra using same software tools that you use for vibration.  That's how we do our current signature monitoring for rotor bar defects.  It's not quite the same picture that you'd get from simultaneous collection of 3 channels of voltage and currrent, but good enough for most purposes I think.

=====================================
(2B)+(2B)'  ?

RE: measuring 300 cpm load variations

Doesn't matter which way you choose, Pete.

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

Use a [link http://www.emotron.nl/Documents/Documents/Products_Solutions/Products/Shaft_power_monitors/Emotron_M20/EMOTRON_M20_DATA_SHEET_01-4134-01_EN.pdf?epslanguage=en]shaft power transducer[\link] that provides an analog motor output proportional to torque. Any short duration load transient would be indicated by a sharp rise in torque output as the slip increases. This would be better than looking at current alone, which may vary with applied voltage.

 

"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

Yes, I see it would work.  Personally if CT secondary is available I would just stick the clamp-on probe on it because it's just too easy not to.   On the other hand if I were inserting a full wave bridge rectifier and output single loading resistor I would have to stop and study carefully: what happens if it it is miswired, if it open circuits, if it's undersized, if an error is made during the shorting/switching sequence required to place it in the circuit and remove it again... what relays may be fed from the circuit and what happens if the CT gets pushed into saturation.  May be all easily answerable questions but I'll take the easy safe way myself.

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

Also although I suggested currrent magnitude I agree with jraef there's a lot more that could be gleaned that might not be apparent from just he current magnitude.

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.


 

=====================================
(2B)+(2B)'  ?

RE: measuring 300 cpm load variations

Emotron's transducer has the bandwidth problem I mentioned earlier. At least, they had around 100 ms time constant last time I looked at their devices. OK, it was several years ago, so it may have changed.

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

Gunnar knows everybody important... thumbsup2

"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

And everybody important knows Gunnar.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: measuring 300 cpm load variations

The guy that knows the details is not in the office. I'll send him an e-mail and hope for an answer soon.

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

The link I posted earlier seems to be capable of doing what you are looking for. From their website,

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

   

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: measuring 300 cpm load variations

Oh, and you won't have to learn Swedish. tongue
 
   

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If we learn from our mistakes I'm getting a great education!
 

RE: measuring 300 cpm load variations

No, boring mainstream English will do. Swedish is for a select few. smile

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

Based on the pictures, I'd have to wonder if the pinion driving the ring gear shown is out-of-round.  Although the manufacturer claims that the gearing is per AGMA, I would also wonder about the gearbox itself; if not strong enough it will flex and allow the gears to drive apart just enough to put the loads on the tooth tips rather than on the flanks.  In the pictures, it does look like the tooth tips are spalled.

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

Got an answer from Emotron. They say that the M 20 can't be used. And they do not have any othher suitable device.

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

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