VFD Drives

[Gone AwayParticipant @goneaway]

There has been some discussion – I believe here but certainly in other internet forums – about electrical noise caused by VFD drives. It is my impression that those discussions referred to conducted noise on the powerline.

 

Does anyone know if these devices are significant sources of radiated electrical noise?

[Gone AwayParticipant @goneaway]

[John HaineParticipant @johnhaine32865]

It depends what you mean by significant! They will have to meet the EMC directive conditions, but like most modern electronics don’t expect to be able to use a portable radio in the same room! I find for example that a cellphone FM radio or an iPod radio is totally blotted out next to my laptop. On the other hand our kitchen radio works fine on FM and DAB with a wire antenna with two or three computers going in the house and my CNC setup going in the workshop.

[Anonymous]

I doubt that the VFD itself will radiate large amounts of noise, as it is electrically small compared to a wavelength well up in the VHF region, and will presumably have appropriate internal shielding. However, if the cables to the motor are not correctly installed and shielded then it is quite likely that there will be significant radiated noise from said cables. The signals carried by the cables do not have particularly fast edges, but the currents and voltages can be significant so that di/dt and dv/dt are large, and will therefore radiate.

 

Regards,

 

Andrew

[Gone AwayParticipant @goneaway]

Well, the problem is that I have an ICD-Pacemaker and at the last checkup the tech discovered that there had been noise problems on one of the leads going to the heart. That could mean a faulty lead which would mean surgery to have it replaced. However, all the test parameters for that lead were OK and while they still aren’t ruling out a lead problem, they think that an external interference source is more probable and, after some discussion with me and the pacemaker manufacturer, suggest my workshop machinery as the most likely source.

 

It seemed to me that, if the problem is in the shop, the VFD would be a possible candidate.

 

The motor wires are run in flexible conduit which is grounded.

 

Right now this is mostly an academic exercise – I’ve been asked not to run the machines for the next 4 weeks after which they’ll re-check the pacemaker stats. I can’t do the AM radio test.

 

 

[John OlsenParticipant @johnolsen79199]

OK, I am not a specialist in this field, but I would expect the pacemaker to be more suceptable to large (AC) magnetic fields at low frequencies than to high frequency radio signals. So for instance I would be more concerned about the effects of arc welding and the magnetic fields around motors than I would about most ordinary sources of radio interference. (Actual radio transmitters, especially of significant power, would also be a worry because iof the sheer power.)

 

The reason for this is that the pacemaker works at relatively low frequencies, the heartbeat itself being normally just over 1 Hz, so the sensing circuits being used are probably not trying to amplify signals at especially high frequencies. It is much easier to filter out signals that are widely different from the ones you want. It is also harder to separate out signals that are magnetically coupled in than signals that are capacitively coupled in. So I would expect the pacemaker to be fairly good at filtering out high frequency radio signals, especially since they are an obvious risk, but it may not be so good at filtering out low frequency magnetic fields, since this is much more difficult to do.

 

With the VFD, the leads to the motor will all be running together in the shielded cable…this is good because it tends to lead to cancellation of the magnetic fields. The motor itself might leak some magnetic field, but hopefully you are not too close to this when working. But with things like arc welding, you have a large loop of conductors and are just as likely to be in the middle of the loop.

 

So…do you do much welding?

 

regards

John

[Gone AwayParticipant @goneaway]

The manufacturer’s EMC guide allows “Bench-Mounted/Free-Standing Tools with motors of 400 HP or less” at distances 2-Feet or more …. not really very instructive for our kind of machines which have much lower HP motors but generally closer than 2-ft..

 

I contacted the manufacturer (through a third party unfortunately) and they finally advised that if the equipment did not consume more than 15A (at 110V), it should be OK at distances over 6″

 

As far as welding goes, the recommendation is “Avoid using welding and chainsaw equipment”. Period. I guess, fairly obviously, they are talking electric welding (including spot-welding) here. Actually, this is a well-known interdiction for pacemaker users – it’s reinforced at a number of levels.

 

Other limitations are: using a cell-phone on the pacemaker side, ham radios (antenna distance 6″ at 3W, 12″ >3W, 2-ft >15W), On-Star technology, iPod earbuds (don’t put them in the shirt pocket), CAT/CT scans, ultrasound, cordless headphone sending unit, car/motorcycle ignition systems, Airport scanners. The list goes on …

Edited By Sid Herbage on 04/03/2011 14:24:47

[_Paul_Participant @_paul_]

When running my 2.2hp Teco VFD the noise/interference makes the FM radio 6 yards away impossible to listen to.

This may/not be the fault of the VFD but the 30yr old 1hp motor or the unshielded wiring feeding it.

 

Regards

 

Paul

[Gone AwayParticipant @goneaway]

Mine is a 1/2-HP motor fitted brand new about a year ago when I installed the VFD. Makes you think though.

 

I fitted the VFD setup around a year ago (say Jan/Feb 2010). The pacemaker stats are checked every 6 months and there was no problem noted in the March-1 2010 or August-30 2010 checks. It only showed up on my recent (Feb-28 2011 check) so it looks like there was some change after August 2010 (unfortunately, while the pacemaker software keeps date/time records for significant medical events, it apparently only keeps gross statistics – no date/time – for events such as this).

 

Anyway, I was thinking about this. If the problem is indeed in the shop, it suggests that something changed since last August. The only thing I can think of is that when I initially set up the VFD, I went with the default settings for the most part. In particular the default output frequency was limited to the input line frequency (60 Hz). At some point I changed the parameters to double that. I cant recall exactly when I did it but it probably was after August last year. I haven’t used the higher frequency range extensively since then but I have used it a bit.

[JonParticipant @jon]

Paul, something to do with no kite mark a possibility! I took the risk.

Havent tried my supposed advanced Vector yet, not fit for purpose DD want another £550. Still waiting to ring back and answer emails sent last year.

[Author]

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