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,

I guess I could write a book .. but you seem not to read it nor follow the recommendations.

I got 35V 50hz at the VFD with a disconnected motor, which trips the RCD as soon as I power the VFD.

This simply is not true!
The voltage does not trip the RCD. The RCD trips on current.

But since post#2 we ask several times to do a measurement with a 100 Ohms resistor. (so we are able to calculate the current) But you refuse.
You just repeat what we know since your first post: 35V / 40V when Earth is not connected.

Thus it´s impossible to help you.

Klaus

 

[ETforInd]

The high dv/dt of the inrush could cause emission through stray capacitance and to earth like that.
Or just the general "electromagnetic loudness" of inrush could have gotten coupled into the mains wiring and then noise_bombed the electronics in the RCD....making it trip spuriously.

--- Updated Feb 11, 2025 ---

So you are saying you have significant 50Hz ripple on the VFD input..?
This 35v presumably comes from that switch mode transformer with the yellow tape round it.
So now i am wondering if it is overheating and has insulation damage inside it...

Taking it out to hipot test it doesnt sound like a great idea though.

I suppose the other way would be to get a separate isolated 35V supply and solder wire that in after the yellow transformer...having disconnected the yellow transformer from its ouput ...eg desolder the diodes what have you.

It does look a very small transformer for the power you mention. I dont see a PFC inductor either on the board you show.

Thanks for explaining that would make sense.

I got 50Hz 35V on the Earth of the VFD when disconnected from the main earth.

Heat damage would be sensemaking as these machines sit 8 hours in the Australian sun with roughly getting used every 1-2 minutes so almost consistent use.
VFD is in an enclosed metalbox so overheating would be near. I'm gonna try to find some info in the transformer so I can message it through.

Supplying separate 35V to chase it down?



Lenze is saving some money there on transformer sizing, Is the PFC not the coil below the transformer on the blue mounting bracket?

--- Updated Feb 11, 2025 ---

Hi,

I guess I could write a book .. but you seem not to read it nor follow the recommendations.


This simply is not true!
The voltage does not trip the RCD. The RCD trips on current.

But since post#2 we ask several times to do a measurement with a 100 Ohms resistor. (so we are able to calculate the current) But you refuse.
You just repeat what we know since your first post: 35V / 40V when Earth is not connected.

Thus it´s impossible to help you.

Klaus

Hey Klaus,

Sorry for repeating and refusing your advice, wasn't working on that problem for a month and just brought it back up including the 35V having 50Hz. Gonna get the resistor tomorrow.

Cheers
Jan

 

[D.A.(Tony)Stewart]

The VFD's high-frequency switching and motor torque's high levels of current can radiate high levels of noise that would be audible on an AM radio. Given that RCDs measure leakage current by the imbalance of current or imbalance of Y caps from RF switching voltage, I suggest that your VFDs are prone to generating resonant switching noise under heavy motor loads. As an experiment, you may find this hypothesis audible as harmonics of 50 Hz using an AM radio between channels, if the noise increases under load. More than 10% imbalance of large Y caps in the VFD may also influence this as well as unbalanced stray capacitance in the windings to frame ground.

Additional line filtering seems necessary out of the VFD or find out if RF commutation noise voltage occurs under high high motor current. If the motors are synchronous, I wonder if this triggers, more commutation noise voltage from the VFD under load which triggers more unbalanced current.

 

[ETforInd]

Hi,

I guess I could write a book .. but you seem not to read it nor follow the recommendations.


This simply is not true!
The voltage does not trip the RCD. The RCD trips on current.

But since post#2 we ask several times to do a measurement with a 100 Ohms resistor. (so we are able to calculate the current) But you refuse.
You just repeat what we know since your first post: 35V / 40V when Earth is not connected.

Thus it´s impossible to help you.

Klaus

Hey Klaus,

Getting a scope next week.

I got a 100 ohm resistor which I put in line with earth.

0.482V on the resistor which seems a bit low considering I got 35V when disconnected or might be just a capacitive charge due to missing earthing?

I triple-checked both voltages and seem to be right. Rcd tripped only once while reconnecting to power with 100ohm in line, after power tripped VFD started up just fine still 0.482V on the resistor.

Thanks
Jan

 

[cupoftea]

That would mean you have 48.2mA in the Earth. Are you sure the reading is accurate?....if you connect both probes together do you measure zero volts?
So does the VFD still trip when the earth is open circuit? (Sorry i recomend unsafe things)

Is the PFC not the coil below the transformer on the blue mounting bracket?

..no that is the common mode filter choke.


Just checking the power level of the Lenze 1550 VFD.....

M1550E Lenze - Variable Frequency Drives (VFD) | Powermatic Associates

A Powermatic Sales Pilot instance

www.powermatic.net

the little yellow transformer must be for the housekeeping circuitry as its too small for the 3.7kW.
It looks like this is for line-line input.
I was thinking that a proper 3 phase common mode choke would be needed but of course it only takes in the two phases.
Cant see a diode bridge, must be in the module, the silver rectangle thing on the RHS of photo.

In fact i am just wondering if adding a decent eg schaffner EMC filter box at the front end of the VFD is needed. Pick one with a decent bit of common mode filtration in it aswell.
Also, i am wondering if it still trips if you add a series inductor in the earth wire...(though i appreciate the safety concerns about this).
That 0.482V that you measure, i am wondering about the waveform of that....is it a nice sine wave, or is it a load of voltage spikes?....i suppose you have the scope coming.
____________________________ -______________________________--
Summarizing

1.....So from what you say it doesnt trip when the VFD earth is disconnected? (but the motor earth is connected)
2.....It trips less frequently when you have the 100 Ohm in the VFD Earth?
3.....These faults have never been seen with a brand new VFD? ...typically happen after 6 months or so, and then happens more when VFD heavily loaded?
_______________________________ _ _____________________________

There dont look to be many electrolytic caps in there for a 3kW VFD that gets used so much in hot conditions.
I wonder if theyve gone high-ESR and the resultant high dv/dt due to that is causing emissions via stray capacitance to earth. I mean, replacing the electrolytic caps with equivalent ones (or better quality ones) could be a thing to consider...

What is the brand name and C and V rating for those electro caps?

If we had the VFD schem then we could look at places to connect a Y capacitor from the metal enclosure to the circuit. (as you know, y capacitors are needed to bring emissions from the metal case back into the product so that the emissions go back down the earth wire, whereby they get nicely squashed by the common mode filtering....if no y cap (from metal case to circuit) then the emissions go into the surrounding "literal" earth where they cause emissions problems).

Could also try replacing the Y caps on the VFD board. Maybe they have got killed by mains transients and are just open circuit now.

 

[KlausST]

That would mean you have 48.2mA

0.482V / 100 Ohms is 4.82 mA.

I guess this could be normal.

And again: 35V or any other voltage (I´ve seen more than 130V) at unconnected GND is nothing to worry about ... as long as the current is limited.
***
Currently I can´t find your input conditions. Thus I´m just assuming 230V AC 50Hz.

If this all is fully capacitive and 50Hz, than we talk about Xc of 47.7kOhms and a capacitance of 67nF.

Currently I expect the black capacitor marked X201 as an Y capacitor and/or the other black part next to it. We can´t see the printing to verify the values.


Klaus

 

[cupoftea]

Woops yes my apologies, 4.82mA
The VFD is here...

https://www.edaboard.com/attachments/20250211_150029-jpg.197288/

https://www.edaboard.com/attachments/20250211_150040-jpg.197289/

The electro caps are 290uF 400V, but the "Lenze 1550 VFD" advert claims it is for line-line input 480VAC.
So if running it from 230VAC, then it will draw a lot more current....hmmmm..
The input filter looks pretty small for the power level.
Also no sign of a mains diode bridge. Is there something else fitted to this PCB upstream?
Must admit i am unable to find a datasheet for this VFD, do you have a link to it at all?

Anyway, yes replacing the Y caps on the VFD board sounds like a cheap and easy thing to do for a start.

 

[KlausST]

The electro caps are 290uF 400V, but the "Lenze 1550 VFD" advert claims it is for line-line input 480VAC.
So if running it from 230VAC, then it will draw a lot more current....hmmmm..

You still think about the input current / inrush current. None of them should trip the RCD.

The RCD trips when line current and return current are not the same: Either leakage current to EARTH or Y capacitor current to EARTH.

Klaus

 

[cupoftea]

The RCD trips when line current and return current are not the same: Either leakage current to EARTH or Y capacitor current to EARTH.

Yes i agree, but inrush often triggers RCD trip. Due to complex stray interactions.
I have worked in many places where every morning the company's power transformer is powered up and the inrush into it trips the RCD. Again, there is no outwardly obvious path in earth involved...but the RCD still gets tripped.
Because its a stray path its difficult to find and so every day, when the transformer is powered, you spend the next 10 mins un-tripping the RCD until you can power it without RCD trip.....lots of company's have that situation and just pay someone to keep re-applying power until they can get it powered without RCD trip.

I wonder if this VFD has a floating heatsink on that IGBT module.....that certainly could be a factor adding to the malaise.

ETforInd says it trips on power up, so maybe inrush could be an issue here....though confessedly ETforInd also says that tripping happens when the VFD is heavily loaded.

 

[KlausST]

Yes i agree, but inrush often triggers RCD trip. Due to complex stray interactions.

If so, then replace it. ;-)
I mean, every current causes stray field. And in case of high current, the fuse should trip, but the RCD should stay ON.

Just function wise.

Klaus

 

[cupoftea]

Thanks, and RCD's are severely expensive for what they actually are. -Because of the safety regs saying that only "compliant" companys can make them...so i bet there is a big black market in RCDs....and its likely that these will give loads of false negatives so they appear to be "super safe"......

I know one place that did a dirt cheap RCD using a fluxgate current sensor without bothering to do the feedback bit with it....they were tripping like goodness knows what!

What about a big inductor in the earth?.....just to stop the RCD tripping perhaps. High inductance as you can on an iron powder torroid. Obviously able to take the fusing current for a few seconds aswell. I appreciate its not legal but maybe just so that they can finish the job etc etc.