After I start the 10 hpT-T3 = 230
T1-T2 = 233
T2-T3 = 252 volts
That seems about right when idling unloaded. Adding the load should bring it in pretty close.
It really looks like your line voltage is sagging quite a bit when running, 243 down to 233. I would expect more like a 2 or 3 volt drop, that would indicate that the wiring is too small.