Why the angle δ, rotor angle is famously called as the load angle in the equation of rotor dynamics?

In rotor dynamics, the angle δ, known as the rotor angle or load angle, represents the angular displacement between the rotor magnetic field and the stator magnetic field in synchronous machines. It is termed the “load angle” because it indicates how much the rotor lags behind the stator magnetic field due to the electrical load on the machine. As the load increases, the rotor angle δ increases to maintain synchronism between the rotor and stator fields. A larger load angle generally indicates that the machine is operating under higher load conditions, which impacts the stability and performance of the rotor dynamics.