how to wire 120v power feed on 240v 3 phase shaper.

The title says it- I was thinking about pulling one leg of the three phase supply (TI, T2, T3 and Ground) and using the shaper ground for the neutral and power feed ground, but wonder about the safety of this approach. I did do this on a three wire 240v single phase shaper with a 120v feeder, but now am wondering about that as well. A 120v cable could be run to power the feeder and leave the ground and neutrals separate back to the panel. Advise, please.

In my (carpenter's) opinion, there is nothing strictly wrong with it, but it lowers the ampacity of the whole system.

Basically, you are bringing Line power to the disconnect, and distributing it from that to the loads.
If the lines are properly protected, that one, leading back to the panel/load center will trip when it reaches capacity from combined loads of shaper and feeder before the other 2 reach capacity from their share of the shaper load. If all the wires to the split are oversized (all sized to accommodate both the feeder and shaper FLA) and protected appropriately, and the motors all have separate protection, it is not a safety issue. This is true for the single phase system, as well.

OTOH, questions like this are not woodworking related, and really should be posted in the electrical forum.

smt

I'd run the separate 120V extension cord or cable back to the single phase load center.

Your 3-phase machine does not have a neutral going to it, and by using a ground as a neutral you are in violation of code. So at a minimum you would need to run a separate neutral line.

Additionally, the current draw from the power feeder will create a current imbalance on the 3-phase feed to the shaper.

Presuming that the power feeder is 1 hp, a 12 gauge extension cord or dedicated circuit would work fine. It would not be much more costly to install the dedicated circuit versus running a new neutral.

^^ Thanks, that is what I was wondering. There are 120v 20A outlets to plug the feeder into.
The thing that bothers me is losing the shaper power for some reason, like having the rpc drop off, and still having power to the feeder. Maybe it is remote enough risk to just not worry about, especially as it is just to wood and cutter- not skin and bone.

Your shaper is a 4 wire delta load (3 phases and safety ground). Your power feed is a single phase load requiring one phase and neutral. The correct way is to bring into the shaper a 5 wire feed (3 phases, ground and neutral.) Your safety ground must never carry return current, it could endanger electronic loads. Neutral and ground must only connect once at the building's power entrance box. To do this, I normally install a barrier strip as a neutral bus in the shaper. It can then be used as a connection point for other single phase loads, like machine lamps. Very often on older machines you will see secondary transformers to create 120V from 208 or 24V AC for a control voltage in which case building neutral is not required.

Yes, what steve-L says above.

If you want to have the feeder shut off should the shaper loses power, perhaps you could have a relay in the 120v circuit, energized by a hot leg of the shaper motor feeder. But it sounds like a pretty remote risk, altho I have no experience with RPC's.

I had a somewhat similar issue with my widebelt sander. If the machine shut down, the feed would stop immediately, while the abrasive belt would coast to a stop. Sometimes from forgetting to turn the compressor on or bumping into the emergency stop bar, and it left a big divot in the work. So when I was sanding the tape off veneered panels that I couldn't afford to lose, I would disconnect the feed motor from the machine and use a different 3 phase outlet with an extension cord. Had a guy standing right next to that plug to disconnect if anything went wrong, but certainly not OSHA kosher.

A question - If stoneaxe were to install a transformer, is there any issue with a higher load on 2 of the 3 legs?

I did the same thing to run an electric pump on a saw for the last 25 years, no issues. It is against code, but I do not give a hoot.