How to Reduce Induction Voltage in Instrument Cables

Dear spaghetti,

Is the signal cable shielded? In a signal cable, the shield should be grounded at only one end, preferrably the enclosure end.

Mike

Dear Mr.Mikerho,

You have referred that "In a signal cable, the shield should be grounded at only one end, preferrably the enclosure end."

What will happen if earthed at both ends.? Pl.explain about this.

DHAYANANDHAN.S

Ground loop effect.

400mm or 40M?

shield wire adds filter can do the trick.

Check your grounding.

Check your cable's shielding.

If the power cable is using a variable frequency drive or is delivering several kilovolts, increase the distance from your signal cable.

Try putting your signal cable inside a metal conduit.

Lastly, do you have to use mV (millivolts)? Try using mA (milliamps). It's more reliable than mV for transmitting signals.

regards,

Vulcan

The signal cable is shielded and it is grounded. The near by power cable is using a variable freq drive. Actually in out rotary kiln, we have "power" slip rings and "signal" slip rings which are 400mm apart. The mV from "signal" slip ring goes to PLC through mV-mA transmitter. When we switch off the power, the signal to PLC is good, But when we switch on the power, the signal gets fluctuated heavily.

As, we can not increase the distance between power and signal slip rings, how can I reduce the fluctuation?

What are you monitoring with the signal slip rings? Are you certain the signal is grounded at only one end of the cable? Sometimes the connector to the slip ring brushes forms a ground, completing an electrical path for ground loops. Moving the mV - mA tranducer close to the slip ring assembly and grounding at the slip ring connection may be a good start. As stated before, mA signals are far less influenced by induced transient voltage.

I believe what you are try to achieve is similar to the difficulties handling low level audio signals (though you may require to measure dc voltages). In audio the twisted pair is the basic solution, though note both conductors must have the same relationship to ground/earth. Normally this means you need to connect to a differential input amplifier (if you don't need dc you can use a transformer to produce one of these). Most professional audio uses screened (shielded) twisted pair or the even better screened (shielded) "star quad" cable (four conductors connect opposite pairs together at both the send and receive ends). Other improvements are possible by using "common mode filters". Improvements in twisted pair cables means that sometimes unscreened (unshielded) twisted pair can be used for even low level signals (talk to your local Belden cable representative if there is one around). Indeed moving to an unscreened (unsheilded) cable can sometimes solve the problem.

LG DAVE, I gave you a GA.

Many engineers add shielding as a precaution rather than doing careful engineering up front. When they encounter noise, they experiment with how to terminate the shield: one end, both ends, ac couple shield, add signal isolation, etc.

When I worked for Disney, I had the task of redesigning the control system when an attraction was put into a new park. The six PLCs and six VME computers all resided in a room with three 2,125 hp DC motors. Just to illustrate the strength of the magnetic fields, images on a CRT monitor placed 3 meters away from the DC cabling would twist and distort until not readable.

I simply made all analog signals differential and matched impedance of signal source, cable, and receiver. I used shielded cabling and connected the shields to power supply common at the computer end. I kept the shielding not as a defense against the magnetic fields generated by the motors, but to keep out the noise generated by the digital computer signals.

All low level (5v) digital signals were made differential and twisted wire pairs without shielding were used. I also matched impedance of signal source, cable, and receiver.

All single ended 24v control signals were common mode (Disney standard practice). Command or sense signal is connected to on one conductor and the +24 power or common (as required) is on the other conductor of a twisted pair.

The original installation had shielded everything and I am told that they had problems. When starting up the system, they had to move cabling, added optical isolators, change cable types and much more. My installation required no changes in the cabling system.

By the way, we used LCD monitors rather than CRTs to control the image distortion, an option not available when the original attraction was installed.

Eliminating the unnecessary shielding was an easy technical decision to make, but a difficult political decision. If there were problems during start-up, management would have asked why the change was made and would have directed the cabling to be changed at a large cost. Of course, all that would have done is delay the project while the cabling was changed. Then the task of the finding the real problem would begin. I am sure that all of you experienced engineers know that management did not recognize the easy start up or cable cost savings.

Hi, spghadei!

Check the entry point of both cables at the PLC cabinet to be certain that they have separate connectors through separate holes (at a distance from one another) and that both are intact.

Mark

Spghadei;

Are you using a grounded DC voltage supply for your mV source? If so, you might want to consider changing to an ungrounded DC supply as the cable shields (drains) to ground coupled to the DC ground reference (loop) will reflect the noise being generated by the VFD back into your control signal. Incidentally, using a grounded DC power source can/will negatively affect mA control signals as well.

Another way to address the issue would be to interrogate the mV signal with an oscilliscope, identify the fequencies, and then build or buy a filter that will eliminate the noise. (tank circuit)

Millivolt temperature signals on kiln slip rings? Crikey!

How about using non-contact methods of temperature measurement, like infra-red, instead?

Is there any chance to put an amplifier close to the signal source to boost it up to volts level first instead? Apart from this are you using twisted pair or coaxial for the signal lines? These should both work the best, but there is still some chance of magnetic induction if you have heavy currents flowing in nearby cables. It might be possible to provide magnetic shielding, by covering the signal line with ferrite sleeves or beads.

What frequency is your fluctuation (mains, or higher?). It might be possible to do some filtering at the destination. For example, if you're only intererested in slow variations in your signal (e.g. over seconds), maybe you can filter it (e.g. using a RC filter).

Our rule of thumb is that as far as possible avoid use of power cables (particularly those carrying heavy switching loads) parallel to instrument cables -- we have noticed that even normal AC 240V 50 Hz single phase supply running parallel & close to instrument cables causes pick-up -- in an analog meter the needle continuously vibrates -- the vibration may not be noticed in a digital meter.

Alternatively run the heavy power cables parallel but minimum one meter from the instrument cables, along a separate cable tray or trench.

Trust this works

Aloysius D'Souza

1) Is it possible to lower the switching frequency of the frequency converter to 4 kHz ?

The motor is going to make more noise, but the emitted noise energy will be less.

2) Put a sinus filter or coils in the motor cables. so the higher harmonics will be filterd out.

3) Put an RFI filter in the means of the frequency drive, this decreased the harmonics to the power supply.

4) Take all the grounding wires to one central point. Take the wire surface as big as possible.

Here in the big leagues (power plant mv level instrument cables and power cables carrying 10's of thousands of amps) we follow a strict policy of voltage level separation, that is the signal cables are carried in conduits and trays seperated by several feet from any power cables. Then we try to use differential inputs on instruments whenever possible. But the differential inputs are often not possible due to instrument availability. Pay close attention to the grounding on your shield, use iron not aluminum conduits for the instrument circuits. (Gosh, Skippy do you meant to say that aluminum is not a magnetic material?)

Good morning, all,

I'm pressed for time this morning and just skimmed through the thread so maybe I missed something. But, OP, is this a new problem or has it been like this since the very beginning? Maybe something got changed somewhere and your current problem is only a symptom and not the root cause.

Anyway, good luck, gotta run.

Logan

You avoid induction problems by avoiding parallelism between signal cables and other cables. Another method is to filter the induced signal using ferrite rings at te ends of the cable, just like computer data cables. You may pass the wires directly thru the ring or by winding onto the ring (torrus).

With your slip ring arrangement and the VFD as the noise source, you have a challenge.

Have you considered using an RF transmitter, or blue tooth, or wireless Ethernet, to transmit the signal? Bypass the slip rings all together.

If this is a new problem, then something has changed, such as bad grounding, bad connections at the slip ring (or power slip ring), etc. Something is generating more noise than before and must be found. Prevention of noise generation is easier than getting rid of noise after the fact. If this is an original problem with installation, then it was improperly installed.

Specifically, how is the signal cable installed (beyond spacing from power cable), what are its hookup details? Where is it grounded and how are the signal carrying wires connected? Is the slip ring grounded or is it 'floating' with the signal ground at the receiving end? These details are needed for a better diagnosis of your problem, other wise, we are all guessing.

Signal cable conductors should be twisted pairs for best rejection of outside noise with a separate shield. If they are not twisted and the 'shield' is being used as part of the signal loop return, that is bad. If a cable shield is being used as part of the signal path, then a second shield outside the cable itself should be used and grounded, not the internal shield. This is sometimes referred to as Triax cable in RF applications. Instrumentation cable should be available with a second shield as well.

Using the same 'shield' for both signal and shielding is not good practice and should definitely be avoided in all low signal conditions.

another option would be fiber optics transmission because there is no induced voltage into the glass fiber. of course it will get complicated a litlle with the overall hardware but it may be an option.

Dear Mr.spghadej,

Pl. Increase the distance by another 1200 MM OR 1500 MM. We faced similar problem, and by increasing the distance to 1200 MM, the problem was solved.

Pl. post the information, how you have solved, as there are good numbers of suggestion here.

DHAYANANDHAN.S