Phase converter vs VFD

I know these questions come up but I need some input. So far I have the Bridgeport mill on a VFD. Now I need to power the lathe and the surface grinder. The question is to keep going VFD or to buy a rotary phase converter. Literature says I need a 15hp VFD for a 7.5hp lathe. I was hoping to get a 10hp RPC and use the existing 40amp circuit its on. If I go RPC, I need to pull another wire and it would be worth it to run a 3phase panel and distribute power.

OR I can just buy 2 more VFD's. Total price for VFD's (Im running 2 chinese ones now) its about $500. The RPC is $900 (10hp...$1300 15hp) and need a panel and misc electrical supplies. If I run VFD's, I can skip the new power pull, panel, etc. I actually may be able to get away with a single VFD for both the lathe and the surface grinder because they are both 7.5hp units.

Any input?
You can use a 10 hp or even a 7.5 hp VFD with your 7.5 hp motor. They are rated for that power. We usually go higher, mostly just "to be safe" or in case we get a bigger machine down the road.

The other question is if you care about the additional abilities of a VFD, like being able to change speeds easily with the turn of a knob. The RPC can handle all the machines, but no variable speed, or other perks.

The rotary phase converter is easier to set up.

Fuses and breakers are there to protect the wiring. The VFD needs to power the machine. If all is workable, then you are good. You did not mention voltage, wire length or sizes, or other details, and the devil can be in the details.

In a one man shop, only one machine will be running at a time, typically.
I disagree with Bob on the VFD sizing. Unless they are specifically rated for given horsepower at single phase input, you should indeed derate the VFD by ⅓, if you are using it to convert single phase to 3 phase (actually not their primary purpose). None of the Chinese units I have seen are, they say to derate for single phase, check the fine print in the ad or look in the manual. A VFD rated for a 7.5 hp 3 phase motor with 3 phase input, should only be used to power a 5 hp motor on single phase input. So a 10 hp VFD will also be light since it should be only used on a max 6.6 hp motor. Since the next size up is 15 hp, that's what you'll need to run 7.5 hp and be in spec. Of course, you may get away with a lesser powered unit but I wouldn't recommend it.

I have traveled down a similar road. I have VFD's on my mill, drill press, bandsaw and air compressor. I just added a RPC for a new lathe and soon will use it to power my surface grinder, I'll likely switch the compressor over as well. I like having both systems. VFD's are fine for machines you really need the speed control, reversing etc. In the case of my lathe it has 16 easy to change speeds so no need for the VFD.
Thanks for the info both of you. The lathe is technically 5.2kw which is closer to 7hp. I doubt I will ever use the 7hp of the lathe. It also runs the coolant pump which I will no longer use. So Im thinking the thing will pull a max of say 6-6.5hp. This will be the biggest motor I have to run and it should soft start essentially. I think Im going to do a 10hp RPC. The surface grinder is actually 5hp spindle 2hp hydraulic pump. Again, I doubt those will run at max load ever. I almost might be better off putting a 5hp motor in the lathe if I need to.

After reading and reading, it seems that a single VFD for the two isnt the best idea. The motors while total close to the same hp, they are not probably the same (different motor speeds). So two VFD's would be in order. I was thinking that the VFD's (two of them) would be close to $500. A larger 15hp one (lots of people say its better to derate) is over $1000. So... RPC it is.
I had a similar problem and went with the rotary phase converter. Both lathe and mill are variable speed (though not as minutely controlled as a VFD would make possible), and using the machine's gearing ensures a bit more power for those heavy cuts.

If you use only one machine at a time, size the RPC for the largest machine, and give it a couple extra HP for the startup surge. This puts you into the 10 HP range, as you expected. The American Rotary people will tell you to double the HP (see Quick Sizing Guide section: ADX RPC) wihch would put you into 15 HP territory, but that seems a bit excessive.

I ran a 50-amp breaker to a (60-amp) disconnect switch, then to the RPC, then to a 3-phase subpanel with room for 4 breakers (i.e. twelve terminals. 3 per beaker). I think it was about $150 for the electrical components. The disconnect is because the RPC draws single-phase power even when it is turned off, so a master off switch is nice to have. Can give you the make and model for these if you're interested.
Well... I was going to go with American Rotary but pulled the trigger on North American instead. For $10 more you get a lifetime warranty vs 5 year warranty on the American Rotary. Also picked up wome 10/4 SOOW wire and a non-fusible switch from Amazon. Sometime next week or the week after, it should be up and running.

Trying to find a 3phase panel and breakers cheaper. Call me cheap but breakers are $40 each!
Yeah the Q340 breakers ran me forty bucks a pop. Forgot about those, that would bump the electrics to over the 200 mark.
Panel was a Siemens P1224L3125CU (rolls right off the tongue, dunnit?) at something like eighty or ninety bucks.

I used 6/3 from the breaker through the disconnect to the converter, and converter to idler. 10/3 from the converter to the panel and from the 3-phase breaker to each machine. The 6/3 is a bit of overkill but it was a short distance (maybe 10' total) and I really don't want that are getting hot.
Heh internet went out due to the snow right when I was editing that post for typos. Hello, half-hour edit timeout.

BTW I used a fusbile safety disconnect, fifty something bucks. A neighbor got his entire house fried by a surge from the power company (no, not lightning) so I figured a bit of protection was in order.
Thanks for the part numbers. I ended up ordering the box and two 40 amp breakers. This whole project is getting more expensive than everything I have in my machines. LOL