EMC filters for VFDs?

Some VFDs come without EMC filters. I presume I need to fit one before I swamp everyone around with RFI?

Two queries, a lot of EMC filters that I am familiar with have capacitors down to earth. This is likely to trip earth leakage breakers when starting up an electric motor. Are there other types of filter that get around this problem? I don't like operating without an earth leakage trip!

Secondly, the cost of EMC filters seem to be anything from half to several times the cost of the VFD unit! Am I looking at the wrong type of EMC filter? A quick glimpse at the filter fitted to a Siemens 440 VFD appears to be just a very large ferrite ring plus a few turns of heavy duty wire. I have not had the unit apart to see if there is anything else in the filter.

Any experience with these problems or am I worrying over nothing?

Andrew

I appreciate that screened cabling, even for short runs is important.

Andrew.

You wouldn't be able to meet the legislation (EN55011 etc) without caps to ground ("Y" caps) and caps across the phases ("X" caps). Of course, there's no obligation to meet those levels in your own installation but that's the basis of most pukka VFD filters.

You can get any number of different EMC filters, so you have little or no idea what effect most would have, other than perhaps finding that your radio works after fitting one. However, if you buy a matching filter from the manufacturer of the VFD, it's likely to be to some degree appropriate for that product. So my Yaskawa VFDs have Schaffner filters that are specified for those particular models. They are even fitted with threaded inserts so that the VFDs piggy back onto the filter box. Very neat.

Murray

I havent got an EMC filter on my Inverter used daily last 8 years.
Do run screened cable from inverter to the Isolator and then through all the contactors and switchgear, rarely a problem for more than 15 seconds.

The only thing it may upset is certain stations on DAB radio 1 in every 100 days or so.

Did buy a Mitsubishi VFD last year to run a 3hp motor at 220v, still to wire in etc range £270-£340 odd. EMC filter was sub £24.

That is very interesting. I was under the impression that the filter went on the mains input side of the VFD (in my case single phase). So do they go on the three phase o/p side of the VFD? You live and learn!

So Muzzer and Jon, where did you get your relatively cheap filters from? Blowed if I can find a reasonable priced supplier!

Thank you both.

Andrew.

It may be worth taking a shot on running without one, I personally have not fitted any filters and not had any problems or my neighbors as far as I know. If problems do arise I will address the problem. I notice many people just screw their VFD to a wall but a steel enclosure would help to keep any radiated signals under control and using screened cable correctly earthed to the motor. The other thing that VFDs do is send nasty stuff down the supply lines and this can be addressed with a line filter preferably as recommended by the VFD maker. Some drives use ferrite rings on the output wiring as well.

Mike

Hello Mike,

I was thinking of nasties going back along the supply lines to the VDF. Unless I have misconstrued, there is equal problems with the O/P of the VDF. I am beginning to regret asking as it seems that problems are growing like Topsy!

Regards,

Andrew.

P.S. I will try running without EMC filters for a start and then see what is needed to tame the VFD!

I would go with that approach Andrew, I would use the screened cable on the motor connection as it is cheap and simple to do and what gets sent to the motor is some pretty noisy stuff and a long cable makes a nice aerial. This is probably where the radio interference comes from.

Mike

Hi Guys,

Just about every old automatic washing machine will have a suitable filter in it. It will be directly connected to the incoming mains lead. Some will have spade terminals, others will have the wires embedded into the case. The case is normally secured directly to the earthed machine frame. The current capacity varies, commonly 6 to 8 amps, though I've seen some as low as 4 amps.

The older ones look like motor capacitors, the newer ones tend to be potted in plastic cases.

HTH.

Two good reasons for fitting filters:

• To make sure you don't cause your neighbours a problem. Just because you haven't got an issue, doesn't mean you're not causing electronic havoc elsewhere.
• To avoid shooting yourself in the foot! Filters stop spiky muck getting in as well as out. Even if everything works perfectly without filters, you could be reducing the service life of other equipment in your workshop by shooting spiky muck into their electronics. Or upsetting your DRO and causing other minor malfunctions.

Bit like a car engine; they run better if you take the air filter off and bypass the oil filter. The damage doesn't become obvious for quite a long time.

So do you need filters on the input and output? I would imagine that a relatively simple 6 amp or so bog standard filter would do on the input and a fancy 3 phase and neutral style filter for the output? Am I getting this correct? Oh not to mention the screened cable from VFD to motor.

I just remembered the controversy about which end to earth, or both ends? Another thorny problem!

Andrew.

Ideally you should have a filter on the mains input, the primary purpose of which is to stop conducted emissions back into the supply network.

You do not need a filter on the output, and in most cases having one would be pointless, and would reduce the overall efficiency of the drive. However, you do need to screen the output cables to reduce radiated emissions. Although the output PWM signals are not particularly high frequency (usually a few kilohertz) the edges are fairly fast and will radiate.

I'm not going to waste time getting involved in a sterile debate about grounding one end, or both of the screened cable. To some extent it depends upon the set up. Over the years I've used both methods, and passed EMC tests with both.

Andrew

So without expensive test gear and no obvious effects e.g. DAB radio being even more useless than normal, how do you know if your VFD is putting shash back down the mains?

I have four VFDs in the workshop and have no obvious problems.

A medium wave AM radio tends to be a better detector than a DAB radio.

Is one supposed to check them for filters? I have two, a Jaguar Cub and a Hitachi, these I bought and just connected up, mains in and motor out, I haven't checked but I doubt if there's space to add even something as small as an X2 or Y2 cap'. I would just assume that the manufacturer would have put them in if necessary.

John

Hi Guys,

Assuming that the VFD meets UK/EU legislation it should already have mains filtering built in !

As far as screened cables on the output side, yes I would use them. In Industry they would have armoured cables to the machines anyway, which are earthed and would automatically screen them. Output filters are not advised, since the relatively high frequency content could become distorted and cause other problems particularly with vector control drives.

No, not that simple. They are designed for industrial applications, to be built into cabinets etc. The requirement is for the final, completed system to be compliant and the degree of filtering required to achieve that will be very specific to the particular installation. That's why most VFDs are sold without filters. However, if you buy the specified filter (as I mentioned above), it will generally get you close to the "Class A" limits as a standalone unit, (these are are the limits that apply to industrial equipment), if used correctly.

Not a good idea to fit a general purpose EMC filter to the output unless you want to generate lots of smoke. There are filters you can buy for the outputs but they should be correctly specified for that application, ideally by the VFD manufacturer. The output waveform is a rectangular PWM voltage of ~340V amplitude and typically 5-10kHz or so, so a capacitive filter won't go down at all well.

Murray

As most have intimated, its not simple..

Also, as said, your best chance of any solution is to use the filter designed or specified by the VFD manufacturer.

If you have the equipment you could measure the radiated and conducted emissions and then choose from the myriad of filters commercially available, and get close to a solution.

You cannot fit just any filter on the output of the VFD either (certainly NOT a 50hz 3phase mains capacitive Y filter!) , The VFD manufacturer will specify a filter type for a specific VFD, and also for the specific output PWM frequency the VFD operates at. Generally these are inductive filters only on the output, but most applications do not use output filters and have well screened supply cables between VFD and motor.

In addition, filtering relies heavily on the grounding methods employed and how effective they are - Again, using a manufacturer supplied filter helps take care of a lot of those issues - they generally recommend and give wiring methods and practices for said filter connections, grounding, etc...

If you really need to fit filter, pay the money and have some peace of mind - else you may throw some cheap cash at the 'problem' and either make it worse, or no better, but have no idea of any other issues since to measure is to know. If no improvement or worse , more cheap cash is required..

Joe

In respect of annoying your neighbours with interference on their radios and televisions I remember back in the 80’s one of my daughters was seriously into CB radio and would be on it all hours of the day and night. One of our neighbours three doors down came round bitterly complaining that she was ruining their tv viewing with her transmissions. I had ensured that her equipment was compliant with the current standards so not to cause interference but the neighbours got really nasty and insisted that her equipment was defective, I said well there is only one way to resolve your complaint and that is make a complaint to the GPO Telecoms, who at that time were the regulators for transmission interference. They duly complained and the GPO man came round and thoroughly tested my daughters equipment, it was fully complaint so he visited the neighbours and tested their aerial arrangement and found that it was faulty and was picking up on the CB transmission and converting to interference, don’t ask me the technical details I don’t understand radio transmissions, I am ok with electrons while they are rattling around wires but I am lost with radio. So it would appear there are many ways that emissions can interact with other equipment.

Dave W

Mains borne interference is very hard to identify even with specialist equipment. I have had a couple of problems with mysterious faults that have turned out to be caused by the supply. One instance was a robot that would occasionally crash into its colleagues, after a long elimination process of changing boards, power supplies and filters and eventually the whole control system the problem persisted. A mains monitor detected no issues. I happened to have a UPS laying around and decided to supply the robot from this, the problem never occurred again. On another occasion a monorail system that used a data signal super imposed on one of the phases to control the sling motor became unstable to the point of being impossible to continue production and 2500 people sent home. One again a long process of elimination brought our focus to the mains supply. As we did not have access to power analysis equipment we moved the supply to another source and the problem disappeared. The only equipment still running in the building was the heating and ventilation system which had some fairly large inverter drives and this was the most likely culprit. A specialist was flown in to give a report on the supply and found very little to note, some harmonic distortion around the 27th harmonic was the only notable event. Once again the decision was taken to protect the machine with a UPS/power conditioner unit and again no more problems were encountered. With around 1000 inverters installed in a car body in white shop a filter failure can cause havoc and be very hard to diagnose as the faults that are caused do not make you leap to the conclusion that you have a supply problem. Another classic failure is cooling fans that fail and then a hot spell shows up all sorts of problems that do not immediately make you think the processor is over temperature. Control cabinet cooling is a vital maintenance check as hot weather approaches, the payback is an army of happy robots and less down time.

Mike