Creating a three phase current system with e.g. 70Hz instead of 50Hz
You could use a motor-generator set, but you'll go nuts if you're anywhere near to it - those are true acoustic nuisances (I have some experience). You could use a 3-phase induction motor to drive the generator, and use a VFD to control the speed to get 70Hz on the generator output. And if you move it away sufficiently not to hear it, then you won't have the problem with magnetic field interference from the motor or the generator itself either. If you can do it with components you can scavenge from &quot;junk&quot; or secondhand, it can probably be very cheap, although I do suggest that you rent a hi-pot tester or a megger and test the insulation on both the motor and the generator to make sure it's functional.
But I also think that you don't really need all this complication - it seems that you're doing some EMC measurements, like hum pickup etc. You don't need to use a 70Hz supply for that.

avoid confusion as to what caused my measurements

Such measurements require some care and understanding what's going on, so if you don't know how to set it up to get valid results, then I doubt very much that 70Hz will be of much help.

Are there other, simple, cheap and robust ways?

If your time has any value at all, then the cheapest and simplest way of doing it is to rent a 3-phase power supply that is designed for device testing. Those provide output frequencies up to 1kHz at least, and would be the simplest option.
There are many such supplies out there, and you would rent one. That's how it's typically done in development labs that don't need that functionality non-stop.
You should look into test&amp;measurement equipment rental houses in your area.