One filter for multiple VFD's?

Simple question. If you had one mains input filter sized for a 7.5kw VFD, would it be suitable to provide input filtering for three 1.5kw VFD's?

If not - why not?

If the current rating is OK, then yes that can work. The three may sum up to a bit different current than the single one, and there are some details, but in general it can work fine. Better than no filter.

The three should not have any trouble with interference among them, especially if they are all the same type.

Cheers Jerry, that's exactly what it is, three identical vfd's. Thanks for the input.

Excuse my ignorance, would you please explain to me what a "mains input filter" is? I'm gonna learn something here.

Thanks
Gus

It's a filter on the power line to the VFD.

Dirty Power: What It Is and Why You Need to Clean It Up - Schneider Electric Blog

Sometimes fixed by line reactors: FAQ: What is a VFD line reactor?

"Filter" could mean any number of things though; EMC filter, harmonic filter, line reactor etc.. If you are wanting to re-use a simple line reactor, then absolutely yes, so long as the total input amps of the separate VFDs is lower than the LR rating. But if it's an EMC (EMI/RFI) filter, or a passive harmonic filter, maybe not.

I think it's a line reactor, I had one apart out of curiosity. Donut-ring things inside with heavy turns of wire around them. Some other square blue components.

Peter. said:

I think it's a line reactor, I had one apart out of curiosity. Donut-ring things inside with heavy turns of wire around them. Some other square blue components.

Click to expand...

A line reactor is ordinarily a close cousin in appearance to an ignorant transformer. Toroids have minuses as well as advantages.

You may have a line reactor, but your description of components better fits the "Corcom" tribe of EMI/RFI filter.

Those are more essential on the line side of rude-bugger Thyristor-class of direct-off-the-line DC drives - a full-court-press "drive Isolation transformer" better-yet.

VFD rudeness points more to the load side than the line side.

Well, whatever it is it was originally on the line side. Who knows, perhaps it won't be necessary at all.

Peter. said:

Well, whatever it is it was originally on the line side. Who knows, perhaps it won't be necessary at all.

Click to expand...

Can't hurt. They work both directions. I've been using Corcom and clones for ages. Everywhere I go has been infested with 'puters and comms gear since the S-100 era. If it was not a source of rudeness, it was vulnerable to it from outside plant.

OTOH, the heaviest I have now are only 20A rated. KB-Penta branded, but I suspect they buy them in, badged.

The heavier Dee Cee drives are on drive isolation transformers anyway as the 10EE needed boost. Last VFD have been scrapped already. Too much like work.

The Phase-Perfects seem to only act-up due to ageing. So far, they are tamed "enough" to not mess with the Panasonic Inverter Microwave or the WiFi routers. Covered well-enough by NEMA /NFPA 70 listed Square-D loadcenter add-on filter that covers several "sins within" in general. Stock items, several ratings, all major switchgear makers, those are, and not even very dear.

I'm a "former" experimenter gone lazy enough to prefer bog-standard store-bought gear rather than DIY.

Even where DIY might be better? It dasn't have the approvals, and I remember what they used to cost when I had to have those done.

Okay, so if a guy was going to use a VFD, what do you test for to see if you need a device like this? I've never run into an issue.
Are we concerned about the VFD side of things, or going back somehow to the other equipment in the shop and/with interfere(?)

Again, just trying to learn something new here.
Thanks
Gus

gusmadison said:

Okay, so if a guy was going to use a VFD, what do you test for to see if you need a device like this? I've never run into an issue.
Are we concerned about the VFD side of things, or going back somehow to the other equipment in the shop and/with interfere(?)

Again, just trying to learn something new here.
Thanks
Gus

Click to expand...

First-off, it isn't necessarily worth any more of your effort but reading-up.

There's nothing new to it. VFD manuals and MANY independent sources have it all covered, many times, and in as much "depth" as you care to chase.

One can get a good idea of what WILL "be there" without ever taking any measurements - or the RISK of attaching the goods with which to take the measurements - at all. The cheapest of AM radios you can still find will capture the NOISE of rude-bugger behaviour. May even be one hiding in your cell-phone

Just add the recommended filtering if there is any doubt, and go USE the goods, as millions of others do.

Now.. if one has been pushing electrons - and had them pushing back - for.. I guess 65 years already since I made that new sidetone coil for a hand-cranked telephone? Or wants to take-on the learning, NOW?

I have not HAD to operate without at least one, if not two (have three at the moment) Oscilloscopes since the mid 1950's.

And basically "will not". They became my "eyes" to see what otherwise invisible electricity was getting up to under the bedclothes, even as a kid fixing-up old Dumont, Eico, Heathkit, similar primitives, to salvaging abused Tek scopes as resembled a whole B-36 ECM suit.

Otherwise, no 'scope? Blind is blind, as there is no second-place winner on "what to look for".

Capture and display is cheap and easy. Nearly all this "power related" stuff falls within the audio passband and we have lots of "graphic display" freebies or cheap-be's even on handheld cell phones.

Before retiring, nothing so cheap as a Rigol 'scope was even on the menu. It was Fluke. Tektronix. H-P. Or better.

MUCH more "value for money" is out there, present era. Also even more garbage.

Do your own due diligence.

CAVEAT, DOWNSIDE, S T R O N G -- W A R N I N G ! ! !

Off-the shelf 'Scopes (and meters) OFTEN have test leads and attachment "systems" For those leads that are NOT SAFE for the Voltages involved!

Most especially when switching spikes or other transients are factored-in.

My four US-made, high-grade scalable HV leads for the very USEFUL but cheap Rigol 4-channel 'Scope cost considerably more than the 'scope itself.

Apply the same standard to even any "basic" meter you might acquire as well. Proven reliable maker, safe goods, safe leads, fancier "features" never the primary selector.

You'll let out less "magic smoke". Your own fried or steamed meat at the head of the "value" list.

I checked again. It a RFI filter. The point is entirely moot now however because I have discovered that the suffix on the model number of the small VFD's indicates that they have built-in filters anyway so this one's headed for the bin.

Peter. said:

I checked again. It a RFI filter. The point is entirely moot now however because I have discovered that the suffix on the model number of the small VFD's indicates that they have built-in filters anyway so this one's headed for the bin.

Click to expand...

In the past, VFDs didn't come with EMI/RFI filtering because it added cost and people were already shy of spending the money. But now, regulatory agencies have begun requiring that EMI/RFI be controlled better, and the benefits of using VFDs are better understood. So as the cost to manufacture them has dropped, some of that savings was plowed back into adding the required filtering.

I wouldn't toss it out just yet though. Every now and then you will come across a situation where the inherent filtering is not good enough for some sensitive piece of gear on the same circuit, and you might be able to solve the problem with that added filter.

Then again, I can't park my car in my garage because of all of the "just in case" stuff I save...

The filters in the VFDs are usually the most rudimentary ones possible, which MAY satisfy EC requirements for "industrial locations".

Putting an appropriate actual EMI filter before the VFD MAY be able to bring the interference down to "residential" limits, or at least much closer to them.

EMI filters are not all the same..... they vary in their target range of frequency, their dB loss vs frequency, etc. If you HAVE TO meet a requirement, you need to "design" the installation. But if you are just trying to cut the noise to radios, eliminate odd interferences , etc, then you may get away with nearly any decent filter that is good for the current.

With EMI filters, especially in combination with VFDs........ physical size matters. VFDa tend to have a large output at fairly low frequencies, where other devices such as computers may have considerably higher frequency hash coming from them.

The larger the EMI filter, generally the larger the inductors in it, and the more effective it is likely to be at the lower frequencies that VFDs tend to put out.

I would surely not suggest binning a decent EMI filter.... they tend to be expensive.

Peter. said:

I checked again. It a RFI filter. The point is entirely moot now however because I have discovered that the suffix on the model number of the small VFD's indicates that they have built-in filters anyway so this one's headed for the bin.

Click to expand...

Please not. Big ones are dear.

If you happen to run a Dee Cee Drive that has no isolation transformer, it could be very useful for reducing the fast-rise edges of its switched Thyristors getting all over your "local grid". May even be of advantage where the drive IS isolated, but has arc-ey spark-ey contactors on its output, rather than logic control back of 4Q pass-elements.

This, I have not checked, as I generally ceased using such goods in the 1970's.

Yep, the mains filter is primarily to keep noise from going BACKWARDS... up the power line.

The VFD's inverter is switching on and off rapidly, and that switching causes noise to propogate towards your power source... backwards through the mains, and it can make it's way all the way to your utility company's pole transformer. When it does that, it turns your overhead feeder into a very interesting transmitting antenna... and because it's RF, it can become quite a handful... the RF component can wind up creating current and voltage maximas at regular intervals along the wire that are really high. When I was working with railway inverters, it was not unusual to see harmonics of the thyristor's switching frequency somewhere in the ballpark of 6x the normal system voltage... like 3500v... on a 600v input system... that's 600v DC coming into the inverter, but the noise coming BACKWARDS was 3.6KV... suffice to say that the 600v feeders' insulation was at genuine risk if the input reactor was omitted...

Because it's switching at a substantially higher frequency than AC mains frequency, typically all the filter needs is a little inductance to yield an effective cutoff frequency of a bit higher than the drive's maximum operating frequency in order to knock out all the 'bad stuff'.

You'll get some good hints from stuff around you... if you run the machine up through it's operating range, and the CNC controls on your lathe freak out... or the computer in your office reboots... or you get RF burns on your hand while laying it on the machne's table... then the filter would be good to have around. Connect it as close as you can to the VFD...

Dave, thanks for that!

Do you have any general thoughts on when to use a line reactor, and how to size it properly for the application? And what about the other forms of industrial power filters, and how can we judge what type is most appropriate?

[Without getting an EE degree, that is]

Milland said:

Dave, thanks for that!

Do you have any general thoughts on when to use a line reactor, and how to size it properly for the application? And what about the other forms of industrial power filters, and how can we judge what type is most appropriate?

[Without getting an EE degree, that is]

Click to expand...

Evidence is all around in layman's eyeshot.

The USEFUL cousin to what Dave mentioned came ages ago as a free bonus to railways and power company grid operators and even natural gas and petroluem pipeline operators.

A carrier signal could carry telemetry data to monitor and operate the <whichever> system over existing POWER wire as a piggyback. It wasn't long before voice and data for other purposes followed, then even leasing-out bandwidth to other users. All without having to buy more wire or such than was already run over massive networks spanning extreme distances.

BUT.. transformers meant to operate at 60 Hz blocked some of those useful signals and needed a bit of bypass gadgetry.

Fast forward to already a rather long while ago, and in-home LAN gadgets ran Ethernet over the house power wires, relying on the powerco transformer to keep it "local" to just the few homes served off the same transformer. Those are still hanging on the display wall at Best Buy and such, and may be among the several reasons you'd like to keep VFD or DC Drive noise to a minimum, your own shop and office or home where 'puters are in use.

Where I am heading with this is that "generally" those Corcom-style EMI/RFI filters can do rather a lot of good towards taming a VFD as well. Especially newer VFD of major makers that are already better behaved than cheaper or older tech ones.

The line reactors, then, are for the tougher challenges where there is more "power under the curve" in the problem zone, and/or the gear is installed in more sensitive environments.

You'd not be allowed NEAR a major telco Data Center with that noise. You be hunted down like a terrorist.

Too many billions of dollars being moved every few seconds by the world's major banks, for one thing, to not react to the detection of anomalies as a possible threat. Too many other folks with comparable interest in not being messed with.

On a more personal scale, I just like my WiFi and TV to work well, the microwave and fridge to not go bug***k.

Your service entrance panel maker - or maker of any OTHER loadcenter - list filtering/ protective devices with the general goal of reducing entrance of EXTERNAL nastiness or reducing the spread of INTERNAL nastiness.

They are not a perfect fit to knocking-down what a DC Drive or a VFD or even a compromised Phase-Perfect may get up to, but they have two MAJOR advantages: That of being Code-Compliant as "listed" or approved goods, and of being more affordable because of their (relatively) higher sales volume.

Start there. It's easiest. It even may be all you need to cover a whole facility.

Then go and ID any truant units and make the spend to put what THEY need right up close and personalized as Dave suggested. Eg: BEFORE their s**t has had time to reach the proverbial fan, broadcasting like a beacon all along the wire on the route as if it were an antenna before it reaches a filter.

"Test gear" to find the noisy ones? AM radio. Older and more cheaply made the better.
Tune it of off the bottom of its band to where there are no stations. Go listen to heterodynes and "stuff.

It's kinda neat even around 'puters. Back when they had only 600 Khz CPU clocks, yes "KILO Herz", not MHz or GHz, we'd set an AM radio next to one chunking away, do something useful. Knew by the sound when it had got hung in a loop or had finished its peas and taters and was ready for the next meal.

20CW