It's not hard to keep it in phase. Once it's synchronized it is actually rather hard to not keep it in phase. Generator spins at 50hz Motor spins at 50hz Add

It's not hard to keep it in phase. Once it's synchronized it is actually rather hard to not keep it in phase. Generator spins at 50hz Motor spins at 50hz Add load to the motor, it starts slowing down both itself and the generator. Generator governor increases input energy, frequency goes up. Both are in phase all the time. Remove load, both start spinning faster. Reduce governor to regulate it to 50hz again. It's a bit harder with inverters but the idea is similar, you follow the grid phase and if you want to send energy to the grid you will be slightly early to the grid phase and if you want to take energy from the grid your phase will slightly lag.