Ron
If you use too large an RPC it will just consume too much power. Remember that your max breaker size in your panel will ultimately limit the total HP you can operate.
I put in the 7-1/2 HP by 22 HP unit. The largest motor this will handle is 7-1/2 HP in a class 1 Non-CNC application, and you can run multiple motors up to a total of 22 HP. The largest Class 2 motor is 5 HP and Class 3 is 3 Hp. If going CNC it will handle a 3 HP largest motor and a total of 3-3/4 HP max load
Now let's talk class ratings.
The heavier loaded the motor is at starting is its class. A compressor running in the freezing cold is somewhere between a Class 2 and a class 3, I know as my 5 HP 3 Phase compressor, starts fine in the summer, but in -15 Deg C weather it gets really hard to start.
A motor starting with no load on it is Class 1
Then we throw CNC into the mix and that further reduces the capacity of a RPC.
My preference for a small home shop system, whether for wood working or metal working is the 7-1/2/22 or the 10/30
This gives room to grow into.
a 3/9 like I sent you the PM quote for limits you to smaller machines, not that 3 Hp is all that small.
Now as you get larger capacity of machines, if you run a really small motor on them, they are not that sensitive so can fluctuate the voltages a bit too much for the little motor. In that case we add a VLS (Voltage Level Sensor) or Voltage Regulator, this way small motors do not get harmed by the larger system. without the VLS, yes they have a lower limit. But say you have a lathe like mine which is 3 HP drive motor and 1/8 HP coolant pump motor, that VLS is now very important to the coolant pump motor.
Also any system 7-1/2 Hp or larger should have the soft start feature which helps to prevent browning out the neighbourhood when you start up the RPC.
So to sum up, 3/9 would be the minimum system requirement for you today, but no room to grow into larger toys.
3/9 minimum, 5/15 OK, 7-1/2/22 better or even better 10/30 would be my recommendations. If you plan to CNC your machine then these recommendations need further review.
As I said, I got the 7TZ, 7-1/2 / 22, but I have wished that I had gone with the 10 TZ - 10/30 as there are times when I have found the other a bit small, such as mid winter starting the 5 Hp compressor which I got for "cheap" as the seller was having a hard time finding a buyer. 3 phase is not very popular in this area and even commercial shops avoid it because hydro charges big bucks to install it.
If you go 5 or larger add the VLS
If you go 7-1/2 or larger add the Soft Start
On all af them have the TLC factory installed.
There are a number of brands of RPC's out there, I have found that the PhaseMAXX has been the best quality and has the best availability of add-ons like VLS, Soft Start and TLC, so that is the brand I have been using and selling to my clients.
Avoid static converters like the plague!
See here for more info on the systems we supply
http://www.electram.com/Advertising/brochure2010.pdf
Walter