VFD wiring

Has anybody actually experienced any problems with interference from their VFD if they have not followed recommended guidelines?

Mike

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Given that I am reliant upon my pacemaker, Mike, I decided not to risk it.

MichaelG.

I've looked into a case where the VFD's installed made equipment unusable - to be fair that was in a lab environment which was very sensitive to RFI (NMR measurements). There were a number of issues with that particular VFD installation.

Much of the time I suspect (as many here have suggested), most of us probably wouldn't notice any issues in a workshop setting. Might be a different story if there is sensitive equipment nearby (radiated RFI) or connected to the same mains supply (conducted RFI).

In your situation Michael I would be very careful as well, our weld control inverters all carried a pacemaker warning and they were installed in proper cabinets and properly wired. I note that Many people just screw their VFD to the wall which does nothing to minimise any radiated signals or offer any protection from swarf ingress or coolant if used. a friend has a built in defibrillator and you wouldn't want that firing off at the wrong time.

Mike

Yes: The Wi-Fi router doesn't work while I'm using the VFD driven lathe.

Cheers,

Rod

Just spent a happy couple of hours conducting an EMC review of my workshop.

I used an SDRPlay RSP1 covering 10kHz to 2GHz. The receiver is connected via a USB cable to an Acer Aspire V15 laptop running Windows 10. The RSP1, which is a Software Defined Radio, was managed by SDRConsole v3 set to decode Amplitude Modulation (AM). The antenna was 10m of wire looped around my dining room and it is about 8 metres distant from the lathe as the crow flies.

I make no claims for calibration not least because the antenna is a complete lash-up. Therefore my assessment of my lathe's EMC as 'very poor' does not mean that it is out of specification, only that I'm not impressed.

Results:

• The laptop's switch mode power supply lifts the radio noise floor by 10dB, the racket gradually fading away at about 150Mhz. (Poor) Therefore the workshop tests were run with the laptop on battery only.
• I can just detect 6 tubes of fluorescent lighting around 125kHz. (Very Good)
• I can detect an impulse when the grinding wheel is started, but it is otherwise fully suppressed. (Very Good) Ditto the band-saw.
• My cheap pillar drill is nearly as good as the grinding wheel (impulse start detected), except for minor noise centred around 500kHz. (Very good)
• My small belt-sander is nearly as good as the grinding wheel (impulse start), except for some hash centred around 3.6MHz. (Very good)
• Best of all - no interference detected - was my Milling Machine. It has an 1100W brushed DC motor. (Very good)
• In comparison the 1500W VFD in the lathe is very poor, see pictures and comments below.

When the lathe's electronics are switched on but the motor is off, a series of moderately strong signals appear. They are unstable, about 80kHz apart and peak in strength at about 1MHz . This is inside the medium wave broadcast band. The strongest spur measured -54dB compared with a nearby radio station at -48dB. (ie the radio station is only twice as loud as the lathe. There is potential for interference even before the motor starts.)

In the picture above the two strong white lines are interference from lathe and there is an AM broadcast station between the spurs. By luck the lathe is not interfering with that particular broadcast, phew!

Unfortunately the interference gets much worse when the lathe's motor is switched on.

The picture shows heavy interference. It starts at 10kHz with harmonics every 10kHz. Although the signals peak in the medium wave band, they remain powerful up to about 20MHz and can still be just detected at 50MHz. Bad news. The interference is considerably strong than broadcast radio in the medium waveband and all stations are wiped out.

The signals are far from pure being modulated with bursts of noise every second or so.

Being inside the house the antenna emphasises near source radio interference. Next step is to see how far away from the house I can detect lathe generated interference on an AM portable. If the interference is being carried by the mains cabling rather than radiated directly it could be quite a long way. Like miles! I'm hoping that I'm picking up short range interference and that the VFD filter is keeping most of the muck out of the mains wiring.

My workshop DAB radio is not interfered with. DAB signals are broadcast at about 240Mhz which is too high to hear the lathe's VFD. There is no problem with TV. I haven't tested the effect on the Internet yet.

Before throwing rocks at the lathe and it's VFD I need to do more checking. One obvious problem is that in the workshop the lathe is on the end of a longish lead, and that the workshop itself is in an extension somewhat distant from the consumer unit. The actual earth could be 30 metres away from the lathe, fine as a safety earth, but total poo when it comes to grounding radio frequencies. Or it might not be that at all.

It's too early to suggest a fix, or even to conclude the bad results matter much in practice. Interesting though...

Dave

Would be interesting to know a few basic details about the lathe and its VFD. Is this part of the original machine or was it a retrofit? Does it actually have an EMC filter on the input? How is the machine grounded? How is the VFD housing itself grounded and ideally bolted to the machine? Have you got shielded motor cables?

Murray

Yes - our fridge / freezer was pretty unhappy on the other side of the wall from my milling machine. A large ferrite on the mains input sorted it out.

Yes, no problem. I've just retreated from the garden where I'm pleased to report that the interference appears to be much lower. But, having wasted a lot of time looking for a portable radio and suitable batteries I find it's too hot and humid to walk far. Also got hay-fever so I'm going to look inside the lathe next.

Is this part of the original machine or was it a retrofit? The lathe is a Warco WM-280 VS fitted as delivered with a Delta VFD. The motor is 1500W.

Does it actually have an EMC filter on the input? Yes according to the circuit diagram - I'm going to check that it's actually connected.

How is the machine grounded? Through the mains plug. (I think this is a problem.)

How is the VFD housing itself grounded and ideally bolted to the machine? It's enclosed in a metal box that forms the rear of the lathe, i.e built-in around the headstock. The VFD box is mounted on a metal rail system that I believe to be screwed to the headstock but I need to check. (Difficult to see.) All the metal parts of the lathe are earth strapped.

Have you got shielded motor cables? I suspect not. Should be easy to confirm once I have the back off.

Dave

I've opened Pandora's box and looked at my lathe's wiring:

• A 3-neon ring-main tester shows L,N and E correct
• There are 0 volts between N and E
• DC resistance from Neutral to Earth at the lathe is a little under 1 ohm
• the mains input cable is unshielded
• the wires to the interlock switches, main on/off switch and to the cooling fan mounted on the motor are all unshielded.
• the cable to the front panel control potentiometer is shielded
• the removable front panel has an earth wire.
• all the earth wires connect to a common copper strap about 4mm thick. The connecting bolts clamp the strap to the headstock
• All exposed metal parts read 0 ohms back to the mains earth
• The filter isn't connected as a filter! The filter board has mains input and provides power feed (probably 5V) to the tachometer. Nothing is connected to the mains voltage output terminals. The input-side capacitors will provide basic suppression, but that's all. I doubt that the filter components are man enough to take the 6.9 Amps needed to run the lathe. Nor is the board shown as an input-output filter by design on the circuit diagram.
• The 3 VFD to motor wires are unshielded except where they pass through plastic conduit from the headstock to the motor's terminal box. The plastic conduit has an internal metal shield but it's not confirmed that it's connected. Poking at the gland with a multimeter failed to find a ground. I think the conduit shield is floating.
• The VFD to motor wires, which should be short, are about 200mm long. (Good)

So, nothing outrageous and a couple of obvious fixes to try. It will be easy to replace the motor wires with a shielded cable, and I have a beefy mains filter that can be put in-line with the lathe provided I can box it up safely.

I'm concerned about the radio earthing though. My house has PME which I've never understood. Any suggestions?

Thanks,

Dave

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This is a useful start, Dave: **LINK**

https://www.tlc-direct.co.uk/Book/5.6.1.htm

MichaelG.

Muzzer & John S.

Definition of an 'Expert'. An 'ex' is a has-been, and a '(s)pert' is a leak under pressure. QED.

Life can get too serious!

Chris

> -54dB compared with a nearby radio station at -48dB. (ie the radio station is only twice as loud as the lathe.

6dB difference is four times

10kHz sounds about right for the switching frequency of teh inbverter, in which case it is almost certainly the motor supply wires radiating the interference.

The pot connection is screened to stop spurious command signals getting in to a high impedance connection. I wouldn't expect the other control wires to be shielded.

N.

6dB is actually a factor of 2, given that we are talking about voltages. For power, 6dB would be a factor of 4 (= 2 squared).

The heat is getting to you!

Murray

2 x 2 = 4? Oh no, it's just like being back at school...

I'm overheating too. We are indeed talking voltages. Now I've made a complete decibels of it I might take a cold shower.

Talking of filters, has any one tried an ex washing machine filter unit, there are plenty about and ok up to 3 KW. For inverter control circuit I use a length of ex computer screened multi core, again miles of it finds its way the local recycle centre. John

I have an EMC filter fitted in the box along side the VFD to the mill