I was planning on buying one of the pre-made rotary converter panels from ebay that go for around $150. Would that need tuning still?
Not if purchased to match the idler motor HP, no. The builder does that for you.
Will the "tuning" be perfect? Never quite. It is
always a compromise because it changes with the power demanded by the LOAD motor.
But it isn't all that BAD of a compromise, so mostly RPC JFW even with a wide range of loads.
My Phase-Craft, Jim even tuned for the slight difference as to whether the idler motor to be used with was to be a 2 pole, 4-pole, etc.
As to HP:
1.5 times load motor HP is about the
minimum, and expects the load motor will start UNLOADED. Eg has a clutch or at least a low/no initial load. Really HARD starting loads can call for an RPC idler HP MANY times as large as the load.
Even so, one of the nicest things about an RPC is that you can ADD ON to the idler after it is up and running.
So if your 5 HP has trouble starting the load? Just start another unloaded 3-P motor with it first. That can be a dedicated idler OR just some other 3-P tool motor that can run unloaded. A 2 HP added, your effective idler HP goes from 5 HP to 7 HP, 5+3 = 8, etc.
More test lab than "shop", my 'array' has a total of
four idlers 3 HP to 28 HP "summed", many combinations in-between. I can pick and choose to suit a wide range of starting and running loads and tests.
Once past the heavy starting load, the supplementary idler(s) can be shut-off.
The Phase-Craft was ordered to start the largest (10 HP), it has NO problem starting one of the 7.5 HP or the 3 HP.
Supplementary idlers do not need their own starter/controller - just a 3-pole contactor or the ordinary mag-starter switch as if they were ordinary load motors. As they can be. Lot of folks start their RPC, then a table saw, then a mill, then switch-off the saw. Or similar.
VFDs work. But.
... two machines need two VFD, four machines need four VFD. six need six, etc.
And each VFD has to be wired directly to its ONE load motor, so the controls must change, too.
One RPC can power
many machines. No change to their controls. No extra wiring or switches.
Not a lot of advance planning needed as your machines and loads change by the hour or by the year.