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VibGuy~5

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I’ve worked lots with VSDs and was vaguely aware that it could monitor torque, but recently heard there are some VSDs that monitor temperature by looking at a combination of current, speed and torque (i.e. they calculate what they think is the temperature by using a combination of these 3). This means they get rid of the need for wiring thermistors back to a panel or VSD (and save money)

Question-is this a proper torque reading? If so, why would anyone use zebra tape, strain gauges with radio transmitters or SKF Baker equipment?

Couldn’t you just look at the torque trend from the VSD? Or go one step further and sample at a higher frequency and get a spectrum?

John from PA

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Reply with quote  #2 
There is an SKF paper on this technique at http://www.skf.com/binary/68-128048/Instantaneous-Torque-as-Predictive-Maintenance-Tool.pdf.

It should be noted that in the paper there isn't any mention of torsional vibration in the classic sense so whether the technique is a replacement for zebra tape, strain gauges with radio transmitters, etc. remains to be seen.  This may especially be true in an industry that is struggling to reliably measure torsional vibration on machine trains that do not utilize motors.  
OLi

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Reply with quote  #3 
I recently worked with a large VFD that had the software to compensate and active reduce torsion vibration. It was proven during the testing by classic measurement, strain gauges. No extra sensors required for the VFD to do that, only software.
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Curran919

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So calculating the DC torque is pretty straightforward as the VSD will have the electrical power in, and can often have a simplified model of the motor to predict efficiency. With speed, you can get torque from power. Easy stuff.

If you've got an expensive VFD, certainly you can combat torsional vibration with an active control system. For this to be possible without reading the torque and only reading current fluctuations is possible, I suppose. Would be quite difficult. You would have to have a great torsional model of your system, I think, but those VFD producers are smart cookies.

Now the temperature thing is starting to get ridiculous. Obviously yes, they can predict what the temperature can be under perfect operating conditions if they have a good thermal model of their motor, but you don't install thermistors on something to know what the normal operating condition is. You install them to detect a fault before you trash your entire machine. The causes of any possible fault that would cause high stator temps are not all detectable from only the electrical measures. I think its a bit absurd. I would like to be proven wrong though.
RRS_Dave

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Reply with quote  #5 
I fully agree with you curran. There are too many variables for temperature to be calculated and be close over 50% of the time I would say.

D
VibGuy~5

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Thanks Curran,

Agree on the causes of high temp, which are many beyond just current and speed (blocked cowl, broken fan, harmonic distortion, poor power factor, etc.)-it only works for a perfect motor condition.

I’ve got 50° C surface temp from IR, which probably means 70 ° C winding temp, but the inverter has a derived temp of 35 ° C. And they are using this to trip the motor for high temp alarm –make any sense??

Interesting about the active torsional suppression –not much call for it for standard centrifugal pumps, but you never know what they will come out with next.

Curran919

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Quote:
Originally Posted by VibGuy~5

Interesting about the active torsional suppression –not much call for it for standard centrifugal pumps, but you never know what they will come out with next.



Centrifugal pumps probably the least necessary driven to have this on. We do have torsional issues, naturally, but I came into this industry from the recip compressor world, so it really doesn't compare. Very seldom would someone have a recip with electric drive, but when they did, it would have a state of the art VSD.

We do also produce a few compressors here. One which stands out from pretty much everything else in our catalog is a unique rotary positive displacement compressor. Thing had some severe torsional issues, but it was such a strange machine that there was no way we could easily model it. It was small enough and only one size, so we ended up going with the guess and check route of buying a few dozen couplings and seeing which one worked best.

I guess it would have to be a really large critical machine to bother with expensive VSD's.
OLi

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Reply with quote  #8 
2x trains each having a 38MW VFD motor driving gbx and compressors as boosters for a gasfield....... and a export comp at 43MW in the next house...... and a small spare recyc comp.
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