Start the 2HP per usual, then start the 7HP in
series with the 2HP, (feed the 7HP with the 2HP, 3Ph) then switch to
parrallel when 7HP is up to speed, (a moment). This would shunt the single phase current feed to the 7HP, around the the 2HP feed, to be rejoined as 3Ph output from both.
Inexpensive industrial relays on ebay. There are some, say 30 to 40A relays with many poles and possible configurations that can be had reasonably. Just in case you have limited experience at this, NO means normally open, (no power through) and NC means normally closed, (power through). When you send power to the coil, those reverse. Cool huh?
The coil:, watch this, they are replaceable but you can get them with the ideal coil too. Both 120 and 240 would work well for you. Low voltage, (12V, 24V) requires a transformer, and probably not useful in your case. Long control runs in damp locations would suggest low voltage for safety as an example.
Not too hard to work out a diagram.
The 2HP start would need to remain separate of course.
Making the 7HP start up optional, (manual switching), would allow a nice option for efficient 3Ph power to smaller machines. A signal conductor in the circuit, could feed an indicator light at each high HP outlet. Say signal off the 7HP relay. Light on = sufficient power.
Much easier than it sounds, might be an interesting capacitor balance job, or not. I've never balanced my 10HP RPC but my biggest machine is 5HP.
OK, enough from the carpenter, next? Bob
Oh lord, miss read OP, please substitute 5HP for 7HP above.
