For the lowest EMC I would use a fixed speed three phase induction machine directly connected to the grid (No VFD), and then use a CVT transmission to vary the speed, I would also push the machine as far away from your load as humanely possible with a belt or chain To connect it to the transmission box
If you NEED an all electric solution (which is the worst in term of noise) there is no avoiding a VFD, and in that case it’s all degrees of bad, steppers and DC machines are god awful piles of junk spewing EMI from DC to light (sometimes literally)
BLDCs are less awful but still bad, the best are PMSM and Induction machines (avoid slip rings though, as they might arc with vibration and wear). Now the problem is the VFD, first and foremost buy the best one you can afford (bonus points if it is a CSU inverter, if you manage to find it small enough) put it as far away from anything sensitive as possible (realistically at least in another room, better still if in another building) use the best, most expensive shielded cables you can find and afford, you will still need to filter the crap out of the VFD output, I would say allocate a sizeable chunk of the budget to it (let’s say 25 to 50% of the overall budget, skimp on the machine instead if you really need to) You will want a very large series inductor for low pass, tens to hundreds of millihenry at least, it will need to be custom designed to minimise parasitic capacitance, otherwise high frequencies will pass straight through, so it will be physically large, heavy and very expensive, this will also smooth out the pwm to sinusoids, then you follow it up with as many stages of traditional common and differential mode filtering as needed to get the required performance. The efficiency will be atrocious, however there is not much you can do about that without compromising on EMI
Now when it’s all said and you will have a system that is worse off in pretty much any way imaginable to a hydraulic one
- more expensive
- bigger
- heavier
- not much more efficient
- noisier(in emi terms)