VFD wiring

Hi Guys,

I fitted a VFD to my mill some few years ago as per instructions [I think], since then as soon as I switch it on I can hear a massive amount of buzzing on the radio.

Just an inconvenience but in layman's terms what is occurring & what is the simplest fix?

Tony

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Tony,

The good ones are much better these days, but ... VFD basically synthesise their "sinewave" so it looks something like this: **LINK**

https://goo.gl/images/3xhJNF

... and all those square rising edges generate broad-spectrum signals

[ look-up Fourier Analysis if you want more ]

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The quick fix is to add a filter to the output, which smooths-out the ragged edges.

... or buy a better VFD

MichaelG.

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Edit: clarified the proposed filter location.

Edited By Michael Gilligan on 19/06/2017 08:07:06

VFD may or may not already contain an input filter (not always easy to find this out, but cheaper ones often don't). Fit the whole thing in a metal enclosure (unless it is specifically designed for use without a cabinet), if adding a filter on the mains input it should be mounted to the backplane of the cabinet, close to the VFD. Make sure cabinet and motor are earthed. Cable between VFD and motor should be screened 4 core (see the document I linked to earlier for typical recommendations about connecting the screen to earth) EMC rated cable gland may be helpful especially at the motor end to ensure good contact of screen to earth.

Earthing and screening is important for the filtering to work correctly.

That's odd, I was under the impression that a VFD output is PWM, with the pulse width varying sinusoidally. So nothing like the waveforms shown in the link. That way you don't need a linear (inefficient) output stage.

The radio problem is almost certainly caused by radiated emissions from the VFD to motor cable. If the radio is battery powered, and still suffers from interference, that would confirm radiated, rather than conducted, emission.

Andrew

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Andrew,

I defer to your wisdom

My suggestion was promted by Tony's opening comment ^^^ and his request for 'layman's terms"

MichaelG.

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P.S. ... even with a PWM output, isn't it the fast rise-times that cause the problem. ?

Edited By Michael Gilligan on 19/06/2017 08:50:41

The voltage on the output will be swinging through 340V or so in as little as tens of nanoseconds - and back again 10,000 times per second in this case. The limits for the allowable voltage ripple sent back up the mains is of the order of mV, so you can imagine how easy it would be for a poor design to exceed that. I've spent hundreds, possibly thousands of hours of my life working on EMC compliance and it's difficult work - a lot more than just adding caps and chokes here and there.

Generally, radio emissions (through the air) are likely to arise from the VFD to motor wiring and conducted emissions from the mains connection. However, the limits are very tight so you usually need to address all areas to stand a chance of compliance.

It's possible to get an "output choke" for connection between the VFD and the motor. This will limit the voltage dV/dt and help emissions but you must be careful not to fit a "normal" EMC filter complete with X and Y caps or you may blow the VFD or cause a fire in the filter.

Most VFDs don't seem to come with EMC filters although you can often buy them as options. So you can imagine the EMC sh**storm caused by many of the cheaper ones used without one.

I believe that many of these home workshop machines are in some way exempt from the mainstream EMC regulations although I have no experience or understanding of how that comes about. But there is no law stopping you splicing a Chinese VFD onto your own machines and making a right pig's ear of it.

Murray

Hi Tony,

I too am looking for a 'simple fix' and as Muzzer points out EMC can be tricky.

In my case the VFD is already inside an earthed metal enclosure (with ventilation and a fan to stop it overheating). That's the first step. See Bob Brown's photo for a good example; you can also see he's used shielded cables.

What I've found on my lathe is that the cabling between the VFD and the motor is not shielded as well as it could be. This is by far the most likely cause of my problem. As per David Jupp's advice I'm going replace the cabling with a shielded type (link thanks to Michael Gilligan) and make sure the shield connects to metal as soon as it enters the enclosure. Another point, the wires connecting the VFD to the motor should be as short as possible. The Mitsubishi guidelines mention a maximum of 50cm.

A third possibility is that a filter is needed on the mains input to stop VFD generated has escaping into the mains. John Fletcher suggested a Washing Machine type, which is what I'll try if shielded cable to the motor doesn't work.

Quick fix for your radio: replace it with a Digital type. Mine cost £20 from a supermarket. It's a selfish cure - don't forget your neighbours have radios too.

Dave

Correct. The harmonic spacing will be set by the basic PWM frequency, normally a few kilohertz. The amplitude of the harmonics will be determined by the speed of the edges and the exact characteristics of the PWM signal.

Andrew

The 50cm max in the Mitsubishi document is Input Filter to VFD cable length. Long motor cables are best avoided, but shouldn't become a huge problem until quite a lot beyond 50cm.

Thanks one & all, plenty of good advice to mull over.

Tony

That's right, but the waveforms aren't so neat in the real world. Just for interest, here's some screenshots of the waveform output by my VFD on dead slow.

As you can see the waveform is a very rough approximation of a 50 Hz sine-wave.

Zooming in on the detail reveals how it's been built up

The 50Hz 'sine wave' seen by the motor is synthesised from a 10kHz square wave. Square waves are wonderfully efficient because the semiconductor switches inside the VFD are either fully on or fully off, which means they don't get hot. But those very fast on-off transitions create radio frequency energy. While DC and AC tend to stay with the cable RF has no such limitation. Rapidly alternating currents produce mutually supporting electric and magnetic fields that expand to infinity. Although most of the power produced by a VFD behaves like 50Hz AC, a proportion of it is RF that will radiate if it can. A good way to stop it is with earthed metal shields and perhaps a filter..

Sharp spikes produced by VFDs might cause grief inside older motors (pre 1980, allegedly). Although most old 3 phase induction motors 'just work', there are reports on the web of insulation failures and unwanted currents flowing in the bearings.

RF currents produce counter-intuitive effects. If, for example, you connect a 50Hz power source to the base of a large air-cored inductor, nothing much happens. A 100kHz power source connected to the same coil will produce spectacular effects like a Tesla Coil. A Tesla coil can be made by accident. If your motor/VFD combination starts producing big sparks, suspect a loose connection. It's important to fix it quickly; although RF created voltages are unlikely to cause much damage in themselves, they can start an arc that ordinary AC will sustain with thoroughly bad results.

Dave

Edit: checked it twice and still didn't see the typos before hitting save.

Edited By SillyOldDuffer on 19/06/2017 13:50:23

By definition an RF signal is AC. They're one and the same thing, not different things. Any AC signal will radiate an electromagnetic wave, unless of course Maxwell got it wrong.

I'm afraid you're getting confused about Tesla coils. They are a resonant transformer, not a coil as such, and it is the Q of the resonant circuits, and the step up ratio, that allows very high voltages to be generated.

By the way the waveform you 'see' from the output of the VFD will depend upon whether you measure phase to phase or phase to 'neutral'.

Andrew

Yup. True that AC is RF, I have no argument with Maxwell.

What I was trying to say was that the tendency for a wire to radiate is much lower at 50Hz than at radio frequencies. An efficient antenna (half-wave dipole) at 50Hz would be 3000km long. In comparison domestic electrical wiring is much too short and close to the ground to radiate much. Radio Frequency AC is not nearly so constrained, an efficient transmitting antenna at 30MHz being only 5 metres long. Light is also AC and that doesn't behave much like 50Hz electricity at all.

Though I understand Tesla Coils to my own satisfaction I plead guilty to confusing everyone else. I started by saying 'like a Tesla Coil', meaning only that shoving a spike of energy into an inductor is liable to up the voltage at the other end. But then I said 'A Tesla coil can be made by accident' which is definitely misleading. You're right, the way a car ignition coil makes volts is not quite the same as a transformer, nor the same as a resonant transformer. I still suggest that: odd things happen when RF is about.

Good point about looking at the waveforms phase to phase, which is what the motor actually gets as input. I'd have put my oscilloscope across two of the phases except I couldn't think of a quick way of doing it without accidentally shorting one of them to earth. It's too hot for that kind of excitement today.

Thanks for helping me clarify.

Dave

Edit , typos...

Edited By SillyOldDuffer on 19/06/2017 15:21:07

Being concerned that my invertor did not back radiate any unpleasant RF into the mains (did not want to mess up anything in our house or any other house connected to the same phase), I fed it through a (secondhand) suppressed mains socket. After nearly fourteen years, there seem to have been no problems.

This is UK, running off a 13 Amp 240 volt socket, 50 Hertz, with the feed coming from the house through a RCD.

Elsewhere in the world, things may be different.

Howard

some picture of the Star wiring that i need to make Mesh/Delta so i can then connect the Inverter which i believe connects from U,V,W to any of ABC on the motor.

Any helps would be great

Thanks

Need a better picture.....

If that is a Star wired motor, you need the star point accessing to change to Delta....

What motor is it?

There are threads on here giving advice in how to change from Star to Delta...

Hi John

i have seen a few posts on it but no simple explanations i.e remove all wires and connect Colour/Wire to A,B,C,N etc

The white cable in the picture is going to the electrical control unit on the lathe itself so i presume that will be disconnected altogether and the remaining wires i are to the motor which then need connecting Brown to A, White to B and Black to C with nothing on N to make it Mesh/Delta

Thanks

Edited By Lee Goulding on 20/06/2017 15:32:44

He was talking about loudness actually... which can be interpreted as voltage or power but is neither.

Neil;

You can set the carrier frequency of an IMO Cub anywhere between 0.75 and 15Khz, which should provide a way of reducing any interference left after improving the screening to avoid your favourite radio station (or target the one your neighbour listens to).

Do read the manual on the pros and cons cf changing the frequency:

Lowering the carrier frequency increases the ripple components (harmonic
components) on the output current waveform so as to increase the
motor's power loss and raises the temperature of the motor. If the carrier
frequency is set at 0.75 kHz, for example, estimate the motor output
torque at 85% or less of the rated motor torque.
On the contrary, raising the carrier frequency increases the inverter’s
power loss and raises the temperature of the inverter. The inverter has a
built-in overload protection function that automatically decreases the
carrier frequency to protect the inverter. For details about the function,
refer to function code H98.

See the video someone posted a while ago who thought they had a bad VFD, showing exactly this behaviour.

N.