VFD started tripping RCD

[ETforInd]

Hey Guys,

I'm troubleshooting a steel cable pusher and need some advice.

The 2.2kW motor is driven by a Lenze I550 VFD. It is powered via a 10a plug, as it is used in different locations. (Tripping RCD on different Locations)

The pusher was running fine for half a year it started tripping the rcd when either plugging it in or when the motor got too much mechanical pressure back. ( If I shut the lead off, plug in, and turn the lead live it's fine and will then only trip when too much mechanical pressure)

So far I disconnected the motor but rcd still tripping. I disconnected the Main Earth to determine how much voltage the VFD got against Earth and It's over 40V.

If I attach a second lead to increase the resistance against Earth rcd won't trip. Earth connections seem to be all right.

Was thinking EMF may be faulty? If so how can I determine that?


Thanks in advance for your input!!!

 

[KlausST]

Hi,

indeed I think this is a bit complicated.
(And to me it sounds as if there is something defective)

There is the HF current that will go via EARTH ...
And there will be mains frequency current.

The HF current may be caused by output filters, motor cables (especially shielded ones) and EMI reduction capacitors.

The best I can recommend is to put a 100 Ohms 0207 resistor in series with the EARTH line.
SAFETY FIRST: ALWAYS expect high voltage at any conductive part. Don´t touch directly. Don´t let any other people touch it.
In worst case the resistor may get hot, overheat or even explode !!! Put it on a non flammable surface.
Then try to switch ON the VFD.
If the RCD does not trip: use a voltmeter (AC, true RMS) to measure the voltage across the resistor.
If the voltage is above 5V --> Switch it off immediately I expect a technical failure inside. It needs to be repaired
If below 5V: you could use a scope to see the waveform and frequencies. Show us the scope picture (with all necessary informations)

Klaus

 

[cupoftea]

Could be a faulty Y capacitor in the VFD. Maybe its gone a little short, though they should fail open...but thats assuming the makers used a "proper" y capacitor, and not some cheap alternative.

Of course, maybe they used a too big value Y capacitor.
Maybe they didnt use a common mode choke, or not enough common mode choking inductance....that would pertain to high frequency currents flowing to earth though.

Maybe some or many of the installations where you work have other devices where they actually have too much y capacitance inside them...and your VFD is then adding to that and tripping out.......so that wouldnt necessarily be a problem with your vfd...but a problem with other peoples kit.

I lived in a flat block where the rcd kept tripping, and an electrician had to go through everybodys room to see who had the dodgy thing with too much y capacitance (or an insulation fault to earth) that was causing the overall earth current to be high anyway, and then often it tripped because it was so near the tripping threshold........

Also, could be the insulation from one of the power semiconductors has overheated or something and so is breaching its insulation to its earthed heatsink.

Something in that VFD is connecting, either directly , or capacitively, to the earth somehow.

But also, as discussed, insulation faults to earth are commonplace...so it might not actually be your VFD thats at fault....maybe the earth current in many installations is already well high, and then when you plug your thing in it just goes over that threshold....and trips the rcd.

Also, some places use cheap RCDs which trip at very low earth current....hooky RCDs from some cheap supplier.
Its no surprise you get dodgy RCDs because they are so expensive (at least the proper standards regulated ones are)...so some places skip out and use cheap ones that are trash.

 

[Easy peasy]

arching / tracking on the pcb - usually around the IGBT's or rectified mains to earthed heatsink or bracket - or other earth point . . .

 

[cupoftea]

Even with good RCDs and good VFDs...you can still trip an RCD if just too many items containing y capacitors are plugged into the phase overall.

I think somehow you are going to have to (like Klaus said) measure that earth current...so a sense resistor sounds good.

 

[FvM]

If I attach a second lead to increase the resistance against Earth rcd won't trip.

Sorry, I don't understand what you are doing here, please explain. How can an additional wire increase resistance?

My general thoughts about VFD tripping RCD: RCD is usually not sensitive to high frequent currents. Motor cable should always include a low inductance earth connection between motor chassis and VFD earth ground, preferably use a shielded cable with respective EMC cable glands at both ends. This way PWM frequent VFD output can't spread into mains power supply.

Most common RCD problem with VFD is combination of too sensitive RCD with large Y capacitors.

 

[cupoftea]

Most common RCD problem with VFD is combination of too sensitive RCD with large Y capacitors.

Very true, and its amazing because there is no way of knowing how much y capacitance is connected to any mains phase at any particular time.....with all the kit plugged into the mains. Nobody knows.....it could be well high earth current before you plug your vfd in....then you plug it in and the overall earth current has gone above 15 to 30mA or whatever is the threshold.

I worked in one place where they had made their own simple open loop fluxgate RCD's for a couple of quid in cheap parts.....(the "proper ones are £80 plus) .they installed hundreds of thousands of them and they were false tripping left right and centre.......we had literally tens of thousands of returns.

--- Updated Today at 9:25 AM ---

So it could also just be noise from the VFD....going down the mains cable and noise tripping the RCD circuitry.

So you could all told..

Measure earth current
Add Klaus's 100R resistor in series with the earth wire.
Measure Y capacitance at the plug from L-E and N-E
Wrap the earth wire round a torroid
Clasp a ferrite clamp round L,N of the VFD.
Even just a biggish X2 cap from L to N input wires can sometimes work. (as near to the vfd mains input terminals as poss)
Buy an RCD and power it through that and see if it trips. (on a mains cct where theres no other kit plugged in)

Others will think of more