VFD & Noise

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ELA

ELA

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Electrical Test Engineer
gar said:
100117-1636 EST

ELA:

The transducer almost certainly has internal amplification and thus I expect an AC small signal output impedance is very close to zero from the negative feedback of the amplifier.

There are limitations by the available drive current. The AD524 can drive a 2 k load to +/-10 V.

The wiring from the amplifier may have a higher impedance than the amplifier itself.

If internally the output impedance was 100 ohms with no feed back, the gain was 1,000,000, and 100% negative feedback, then output impedance is about 0.000,1 ohm. A lower gain amplifier and it is about 0.001 ohm.

See following references:
http://www.electro-tech-online.com/general-electronics-chat/35701-op-amp-cct-output-impedance.html
http://zebu.uoregon.edu/~rayfrey/431/notes9.pdf

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Gar,
I understand basic op-amp theory and the concept that an op-amp output may be considered to be low impedance.

I am talking real world relative circuit impedances. Most 0-10V circuits I have worked with are relatively low current (relatively speaking - high impedance).
If you are only driving a low current in a cable then signal to noise ratio can be low. The suggestion was to raise the signal level in relation to the noise being induced.

Consider that you may be able to drive 2k minimum but in reality you are only driving 1-10Meg (scope input impedance). As I am sure you know this is a very noise sensitive situation.
Consider that an analog voltage output module may be able to drive
10v/2k =5 ma absolute maximum.
When the output is at 1V it is only sourcing 0.5ma. This is not a real large current level in a noisy environment.

Yet is is a lot better than when you are not terminating the cable and using only the O'scope input impedance.

My suggestion was to terminate the transducer in as low of an impedance as possible.
Even if a device may be able to source a relatively higher current does not mean that it is doing so in the circuit in question. It depends upon the circuits termination. That is what was talking about.

I am not talking about absolute op-amp theory.

Consider that many analog (0-10V) input modules have input impedances >1Meg ohm.

I have had situations where someone mistakenly designed a 0-10V drive signal to control devices in a noisy industrial environment. Control was erratic.
Replacing the transducer with a 4-20ma output solved the problem.
Granted that there may be other ways to address low signal to noise issues but 0-10V analog voltage modules are not the best choice for a noisy industrial environment.


The point was that many 0-10V voltage signal circuits are way more suceptable to noise than current loops. This is especially true if the voltage output is terminated in a high impedance. Thus, once again, the suggestion to terminate in a lower impedance,
 
weressl

weressl

Esteemed Member
Location
Texas by the Gulf, geat crab, great shrimp
Besoeker said:
Yes, I'm old enough to remember them too! ;)
Most of our guys now use a Tektronix TDS 2012 which is convenient to carry around and has a decent performance. I routinely carry one around in my box of kit in the car boot (trunk) I also have an old TDS 360. I like it because it has a built in 3.5 inch drive and you can store several hundred waveforms in *.csv format on one disk. And you don't have to connect your PC to the scope - one of lads has managed to fry three laptops doing just that :(
On the down side, floppy disks are not so common these days.
Click to expand...

What is a 3.5" drive......:D

Naaah, I'm just kidding, still remember Heathkit and Tandy....
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
100118-1057 EST

ELA:

I was not assuming that you were not aware of basic op-amp theory, but others may not be.

If we look at the op-amp at its output terminals and with an external source of current attempt to change its output voltage within the current drive capability of the op-amp, then with 100% negative feed back it looks like a very low output impedance at low frequencies.

At any voltage level output of the op-amp within its operating range it is capable of sourcing its maximum current. In fact at lower voltages than it maximum output voltage it can source more current.

Up to 5 MA the op-amp looks like an output impedance of 0.001 ohms for example. As the current gets greater the output impedance grows toward about 100 ohms.

I think the big difference with respect to noise between a low impedance voltage source and a current loop is that the voltage source is generally referenced to common (ground) at the source end and there may be voltage added to the signal in the ground path between the voltage source and the other end of the cable. If the voltage source is a differential line driver, then the ground path noise voltages generally are not a problem.

As a rough approximation the current loop is a two wire signal source with no ground path current flowing in either signal wire. In this way it is similar to the differential line driver.

.
 
T

TxEngr

Senior Member
Location
North Florida
PWM based VFDs have a nasty habit of creating circulating shaft currents on the motor which can be passed to everything around them if all of the metal is continuous (e.g. motor -coulpling-pump-discharge pipe all tied together). This could be creating some 'stray' voltages that you're picking up on the o'scope. You might try grounding the shaft of the motor. A simple piece of braded copper drapped over the shaft and tied to ground for the system might work. The motor manufacturers also make brush kits specifically for this issue. It certainly wouldn't hurt to do this.
 
ohmhead

ohmhead

Senior Member
Location
ORLANDO FLA
Well have ya seen this ?
Coupling.gif


It isolates motor from drive no more bearing problems they claim its a speed control itself .


Its a new air gap kinda magnetic coupling for motors its controlled by a micro processor and there is no physical connection between motor and equipment drive meaning no shaft mechanical connection only a tiny air gap .

Its a magnetic clutch kinda.

It eliminates eddy currents by harmonics of VFD eliminates vibration & alignment issues problems and you can control motor speed with it also if a motor locks out or jams it releases automatically the motor from the faulted equipment they say you can fix a jam and it resets itself they claim it a motor protection kinda thing .

Whats next ?
 
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T

TxEngr

Senior Member
Location
North Florida
I've used isolated couplings (destroyed the bearings on a gearbox once) but never that device. I had heard about it but never seen one nor even a picture of one. Who knows what's next - it gets more and more interesting doesn't it!
 
ohmhead

ohmhead

Senior Member
Location
ORLANDO FLA
Well we kinda know that switching in a VFD causes all the problems with bearings pitting by eddy currents and harmonics are a bigg problem so if one can eliminate a connection why not . These kinda work like the levitation theory thats on a magnetic high speed train the more it pulls in or magnetically grabs speed then increases on shaft also motor can run at any speed it controls the speed by the air gap between .

Just a thought were always talking harmonics and VFD scr switching problems so there is other ways to overcome sometimes but yes i like new stuff and i can not stop looking this may be a benefit to a industrial plant or power plant if used .
 
Besoeker

Besoeker

Senior Member
Location
UK
ohmhead said:
Its a new air gap kinda magnetic coupling for motors its controlled by a micro processor and there is no physical connection between motor and equipment drive meaning no shaft mechanical connection only a tiny air gap .

Its a magnetic clutch kinda.
Click to expand...
Sounds a lot like eddy couplings that I first met in the sixties.
 
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