I have an installation with six 10kW Quattros, wired 3-phase.
It is operating a 7.5kW induction motor, and whenever the motor runs (regardless of load) the lights (which are on one of the three phases) flicker at about 5Hz.
Any suggestions as to what might be the cause or a remedy?
Is it just a coincidence that 5Hz is about the difference between rotation speed and synchronous speed?
If 5Hz is the difference between rotation speed and sync speed it is indeed tempting to look in that direction first. It could be that the EMF generated by the induction motor interacts with the AC frequency used by the Quattros. Other than that, it might be some weird oscillation, or there is a 5Hz ripple in the load current drawn by the motor (although that seems unlikely). Keep in mind that inverters have a higher output resistance than the main power grid so that current fluctuations may result in voltage fluctuations.
This might be tricky to troubleshoot. Measuring current ripples requires proper equipment, and typically one doesn’t have different motors or power sources on hand to swap and see what happens. 
Could you replace the motor with three resistive loads in the same configuration (star or triangle) to see if that makes a difference? Heating elements would do fine. If the flicker goes away then the rotation/sync speed differential deserves further investigation . If not, then the problem is on the Quattro side.
The motor has a star-delta starter, and the flickering is only observed after it switches to delta.