Power Line Filters - When Are Appropriate?

[Mecha RobZilla]

I'm having some difficulty figuring out when an AC line filter would be appropriate for an application. What exactly is being filtered, the amplitude? How does the filter protect against low-level transients as well as high-level transients... does it have an internal capacitor or battery to store a charge?

It sounds very similar to a UPS... how does it differ?

We have a customer who is convinced that the line power coming in is "dirty" and therefore we require an AC Line Filter ahead of our PLC and temperature controllers.

How do you determine if a customer has "dirty line power". Some sort of meter or device which can monitor a power line for an extended period of time?


Is it:
1) Based on the quality of the power coming in and the type of hardware being controlled? (i.e. if you have a microprocessor/PLC and your incoming power is off by "x" Volts, then you need a line filter?)

2) Based on the requirements of the equipment being powered? (i.e. the manual specifically recommends installing with the use of a line filter, and because of this you use one).

 

[bill4807]

A power quality analyer can monitor power for an extended period of time.
There is a limited amount of memory depending on what you are measuring or trending.
Fluke 435 is what i have used before.
Mostly when i was doing this we were responsible to supply the study to another engineering firm that did the correcting. Most infeeds to machine tools or bus ducts for a series of equipment.

Maybe someone else can explain the specifics of correcting, based on your readings.

 

[jraef]

The concept of "dirty power" is over used and hyperbolic; nobody as an official definition, so marketing people jump on that to sell people unnecessary hardware.

YEARS ago, when PLC power supplies were of the linear design, most PLC suppliers required that you use a "CVS", a Constant Voltage Source, at the input of the power supply, because linear power supplies would pass any voltage variation through them as changes in the control bus voltage and that might cause errors in execution. A CVS was a ferroresonant transformer (Sola transformer) that put out a consistent AC RMS voltage with really wide swings of incoming line voltage. That entire issue went away when power supplies all became "Switch Mode Power Supplies" (SMPS) and incorporated output voltage regulation as standard. SMPS technology is one of the reasons why everything got smaller/cheaper/faster starting about 20 years ago. Now you can feed the power supply anything from 100-270V and get a rock solid 24VDC output. It adapts all by itself.

There are lots and lots of scam artists on the internet now that are trying to convince people that 'dirty power" is the root of all evil, and then selling them "line filters" that supposedly fix everything from component damage to arthritis (literally, I saw one that said that!) and even claim to reduce energy costs by 20-40% or more. It's all "snake oil". Most of those are just an MOV or an R-C (resistor-capacitor) snubber of some sort, often put behind a small inductor and MAYBE a capacitor to make it look like it is improving power factor (assuming you don't have a good meter to read it). 99% of these things are basically useless, but since what they claim to DO cannot be proved or disproved, they tend to get away with it. Occasionally their BS claims are SO egregious that the FTC will prosecute them, but that rarely happens.

If someone is genuinely concerned, get a good quality surge protective device, one that INDICATES when it has sacrificed itself in the performance of its duty so that you know when to replace it (most do not, so you don't know that it's no longer there). But for the most part, SMPS power supplies ARE the "line filter" now.