Noisy loop!

This week I ran into a most bizarre situation.

The x-mitters (2 of them) are Celesco PT800 serie LVDT. There range is 0-6" but we use them as 0-20mm (our display is calibrated for 0-20mm=0-20.0)

They where setup individually with there own 24Vdc supplys. They emit 4-20mA and with resistors we interface them with a 0-10V input dispalys.

All is OK on this setup and it's been working with satisfactory precision for a few years.

I connected them in parallel into an Fx2N-4AD extension module with the main 24Vdc power supply.

When one of them is connected its OK. As soon as I connect the second one the signal goes wild. It varies by about 1 Volt up and down when I start the process line .

We have tryed using them with there own isolated supply. Same thing.

We have tryed using them with ungroundes sheilded cables run on the goround floor for testing purposes. Same thing.

We have tryed using them with grounded sheilded cables run on the goround floor for testing purposes. Same thing.

We have tryed using them in a mA loop. Same thing.

We have tryed capacitors. Same thing.

We have tryed using them on 2 different seperated extension module (4AD). Same thing.

We have tryed all those with central and seperated 24 volts supplys.

Lord we have tryed hitting the control panel on the side with a hammer. Well, not really but we liked too

What is most bizarre about this is that no noise is apparent when we run the procces line with any one of them. Or any two of them with the 6 last motors stopped.

The noise is there ONLY WITH 2 x-mitter AND the last 6 motors all running AND when they are connected to the AD modules. Running any of those motors individually does not induce noise.

All those motors have Inverters. (5 AC, 1 DC)

We have connected them with the central PS and not connected to the AD modules and we have no voltage variation.

Any ideas?

Have you tried putting a signal isolator between the sensor output and the input modules? Without isolating the output signal, it doesn't matter much if you isolate the power supplies.

This indeed is one of the other things I will have to do. Since I don't have one in hand right now I am trying a little black magic.

maybe try this

It may help, try reducing the carrier frequency of the variable speed drives it has worked wonders for vsd noise related issues for me, lower carrer frequencies tend to induce less emf into surrounding cables other than this can you isolate it down to one particular motor? or is it any one of them? Good luck
Tom

Ah! Noise problems! What has caused me more sleepless nights than noise problems?!

A good candidate for the cause of your troubles is common mode noise from the inverters on the motors. I doubt that the DC drive is involved.

Common mode noise is caused by the high-frequency drive output pulses capacitively coupling to the building ground system. Once coupled into the ground, the current is looking for a way to return to the drive DC bus from where it originated.

Usually, this return path is thru a grounded neutral in a substation transformer secondary which can be a long way away. If this ground path is not near zero impedance or if the substation secondary is ungrounded, these remarkably high current pulses may look for another lower impedance path. This often ends up being instrumentation shielding or some other equally unsuspected path. The resulting noise in these normally quiet shields can create havoc and often occurs in equipment randomly far removed from the source inverters.

The solution is to provide a ground current return path to the inverter DC busses right at the inverters so the problem currents don't have to search all over your facility for one. I have used an isolation transformer to feed the inverters with good success. The primary must be delta and the secondary configured wye with the center of the wye grounded. Getting one with an electrostatic shield between primary and secondary is adviseable too. You can use one transformer to feed multiple drives as long as they are located close to the transformer.

I haven't used them but some suppliers like MTE and TCI are offering common mode filters that are somewhat less expensive than transformers. They might be worth trying.

While this equipment isn't cheap, its only a matter of time before this problem rears its ugly head. The only other solution I know of is to install a premium all-copper ground bus network throughout your facility. This should be done on all new construction but, in an older building, it isn't practical.

Probably time to spring for a transformer!

We've used TCI & Mirus filters when dealing with noise issues. They both have pretty good products. I think Mirus filters are made in Canada?

http://www.clrwtr.com/TCI-Trans-Coil.html
http://www.ctiautomation.net/Mirus-International.htm