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Posting here because unsure where to post it and it involves a generator.
LONG STORY but ---------------- large Dairy farm with wind turbine /some solar ....... Single phase ...
Replaced 2x vacuum pumps (dairy milking machines -each 3kw) with 2 new variable speed pumps for energy saving. These are 2 x 4kw three phase motors fitted with Yaskawa single to 3 phase inverters. These work really well on mains power with low speeds on a vacuum on demand controller.
(for info -old vacuum pumps run flat out and vacuum level is maintained for admitting air via a vacuum regulator )
(new variable speed pumps run according to demand -typically 40-60% energy saving cos they run slow )
Mains power = no problems.
Recently had a power cut and connected tractor driven generator -result =new variable speed vacuum pumps will not run ......... Either generator inbuilt RCD80A-100ma trips or Wall socket (generator is plugged into) trips (100ARCD-100ma).
Visited farm today .........ran Generator and @ 230v / 50hz with <10amps load -engage variable speed pumps (even 1 only) and one trip will trip-(no pattern -may be 80A or 100A).
Running "old " vacuum pumps (2 x 3kw single phase) no problem.
Not believing its a "power" issue and may be related to RCD ???
I disconnected the earth cable to the "new" vacuum pumps and the generator will start 1 pump only -not 2 at same time....
I am not a qualified electrican BUT cannot work out why mains power starts and runs both variable speed pumps with no tripping/power issues with correct earthing installed.
Generator rated @ 80A will not start even one pump with low background load and trips.
Removing earth lets it start ? wtf??
Need to add this farm has multiple CU's and generator is fed through 2 x change over switches -first change over switch( other farm buildings ??) if not in off position generator trips always .........??????
Unsure if more than one electric supply ?? and totally out of depth BUT .........
Old vac pumps run on mains and generator.
New vac pumps run on mains only -trip generator .
Feeling lucky ............. lol

Not really enough information- you'd be better getting a local sparky to have a look - but my 1st thoughts are:-

1. Does the grid supply also have rcd protection (my reading of your description suggests both rcds are on the generator connection)?

2. Does the generator have a Neutral-earth bond? (It's lack could upset rcds, depending on the loads)

In my experience, multiple series RCDs, particularly of the same trip current / timing can cause nuisance trips for no reason.

Having said that, it ran fine before. You now have single to 3 phase drives which possibly have increased the transient voltages in the system without the whole of the grid to dampen this noise. In fact, you actually have 2x inverter drive feeding another inverter drive (in each vac pump).

As with @Marcus, first check when on generator you have neutral tied to earth.

I suspect the generator Neutral is most certainly not tied to earth .
I have spoken to someone much more qualified than me who said on fixed equipment with inverter drives it was occasionally necessary to fit rcds with 300ma limit
Interesting that the farm rcd is unaffected and only when the generator is used it trips the generator rcd .it's not really my problem but trying to help.

What’s the quality of the back up power supply?
Ie is it 230 vac 50hz? Some good multimeters out there for under £70 have a small screen to let you see the sine wave under load

As other have said neutral earth link on generator that's presuming generator has been earthed as it should be.

I have passed on everyones comments to the farmer and suggested he ask a registered sparky........
The generator is a "cheap" tractor driven model (not even bolted down) and gives a nasty "twitch" when the power take off is engaged !!
I suspect only generator earth is via plug/socket and neutral not linked ... but hey it was cheap !!!
thanks all for your help.....

I'm also not an electrician..

But, a few thoughts on top of creating a new MEN link (Main Neutral Earth) bond - This needs serious consideration though.

1. The generator quite likely has a nasty output, and inverter drives can be funny about it, check the genny output for frequency and voltage, it could be way out of spec and maybe just needs a minor adjustment if voltage is sagging under load, or if possible get a spark etc with an oscilloscope.

2. With Inverter drives being finickity, it's also important to make sure they're installed to the instructions, my ABB inverter drive for our crane winch has very specific instructions on the wiring with regards to the type of RCD that can and can't be used and it also has a special EMF / earth / shield connection which can be connected or disconnected. AFAIU that connection screw also allows the inverter drive to dump EMF noise onto the earth which can cause tripping. So again make sure a spark or someone with the time to read the instructions and make sure it conforms to them can have a look.

3. Last option maybe try putting a decent resistive load on the genny (Electric heaters etc) to get it working before the inverter drives kick in.

Used to use a lot of SY cable and glands for inverter drives for that reason..