A ZX30 will be very similar to my PDM30, with an induction motor. I'm assuming you have only single-phase power available, probably at 240 volts, 50 Hz. A potentiometer will not control an induction motor. For usable speed control, you need a 3-phase motor controlled by a Variable Frequency Drive. It is easy to find a VFD that takes in single-phase and delivers three-phase. My mill is running off one and I really like it.
BTW, 3-phase motors are generally cheaper than single-phase motors because they are easier to build.
I have VFD's on all my machinery. Until I bought my 1st one for my Bridgeport, I never realized how great these little boxes can be. They are much more than a speed control. They can convert voltage or phase too so you can get rid of that junky static phase converter. How they work is beyond me, but with just a little reading of the manual they come with, you can do all kinds of things to a motor. I just added one to a South Bend heavy 10 I was refurbishing. I was able to eliminate the original Furnas control, still use the original forward/reverse switch the lathe had, power it with 220 single phase and convert it to a 3 phase. I have a preference for Lenze VFD's just because they are a great unit, and their Tech Support is top notch if you need a little help in programming it. I bought my 1st one new from Wolf Automation, which is an authorized distributer, but I found that Radwell International sells used Lenze units, and they guarantee it to function like new. Because of that I'd recommend you stay away from a used Fleabay unit, and I'll pretty much guarantee you'll get a better price from them depending on the brand.