Power Conditioners: Adding to PLC circuit

My customer has an automated saw that has given me many "learning opportunities!"

It has a ControlLogix PLC with an L5555 processor, one 24vdc input card, one 120vac output card, and one M02AE analog servo card. (I'm at home and don't have the other card P/Ns- probably not relevant anyway)

I've replaced the M02AE twice, and the output card once since September. The saw is acting up again...

It's like the program is corrupt. The outputs are firing at the wrong time, but the problem is intermittent. It seems to happen more often the longer the saw operating.

After watching the inputs (prox switches), and card output signals, plus the ladder program, I suspect the problem is a bad processor.

The only thing I can imagine that would cause that many PLC problems is bad power. I've never been in a situation like this before, so I'm trying to go through the learning curve on it. The power coming into the saw panel is 480vac. There is a transformer on the saw panel, stepping it down to 120vac for the PLC circuit and the 24vdc power supply. Of course there is a circuit breaker on that circuit, and the Cutler-Hammer rep just gave them a surge protector last week. But I suspect there needs to be more than that, something like:

http://www.solahevidutysales.com/cvs_hardwired_series_power_conditioner.htm

Do you think I'm on the right track? I need to give a recommendation to my customer, and I'd like to feel some confidence that we finally have a handle on what is causing these issues. Anyone have similar experiences?

I did find a good article on power conditioners, written by the same folks that manufacture the conditioner in the above link. Here is that link:

http://www.automation.com/library/a...onditioning/power-conditioning-for-automation

If we do get the conditioner, I need to decide if I add it in addition to the transformer (120v to 120v), or replace the transformer with this (480v to 120v). Any thoughts on this?

Thanks!

What ideally you should have:

480VAC to 120VAC step-down transformer 120VAC isolated with a main feed fuse (fused appropriately)

Separate fused branches for the 120vac output card, 24VDC power supply, and the PLC itself

The fuse for the PLC should go to a line filter (corcom line filter for example) to allow the CPU/PLC to have a steady power source. not as important for the output card The type of output card also makes a difference.

The product you linked to would probably be sufficient... the most important thing is isolation from the rest of the saw and proper branched and fused circuits.

If I were you I'd get a good quality meter and take some measurements while the saw is running.. should be able to find your culprit.

If the voltage is fluctuating badly you will get outputs turning on/off, depending on the type of output card.

There are a lot of factors but starting with steady power is key.

Sorry if my reply is a little rambling I'm on-site and in the middle of doing other things!

1. Check that the chassic of the processor has a good large sized earth wire attached to the bolt supplied and then on to a known earth source.

2. Some Output Cards have a terminal to attach a neutral, or negative wire too, if it has that terminal and I think your card quoted does, then make sure that wire exists and is attached as it really is required to do some snubbing of outputs.

Good point, I had forgotten to mention anything about grounds.

Are there any variable frequency drives for this automated saw? If your control circuits are run in the same raceway as the VFD power cables without any shielding, that can cause a lot of electrical noise problems.

Follow-up: I installed a power filter before the PLC and all the problems went away. There was a small VFD running the saw blade in the same panel. I dialed back the speed from 60hz to 48ish and it seemed to decrease the PLC errors. Once I installed the filter, all errors went away.

I have been using these a lot in my systems http://ab.rockwellautomation.com/Power-Supplies/Voltage-Sag-Protector#/tab6

They are a great filter and act as a small UPS giving you about 7-10 seconds of ride thru which is all I need and you don't have to change batteries every year.

They make these large enough to run an entire MCC and as small as 2 amp din rail mount units.

This is what I installed: http://ab.rockwellautomation.com/Surge-Protectors-Filters/DIN-Rail-Filter-and-Surge-Protection

does the VFD had any sort of incomming filter or directly connected to the same 480v feeding 120v TFO ?

Your symptoms and solution point to a larger issue of interference from the VFD, and from my experience, the output wiring from the VFD to the motor. Most likely I would suspect that the VFD output wires are mixed in with input power wires and maybe even control wires within the panel or on the machine, without proper shielding. Most of the time people get away with it inside of panels because the lengths are short, but that is relying on luck, not design. You may have cured the symptom, but not the cause.