The performance analysis of a 3-phase machine is always conducted on a per-phase basis since three phases are identical except the 120° phase difference.
If the ship generator is supplying a load current I lagging the terminal voltage VT by phase angle θ, the phasor diagram of Equation, all with per phase values, which ignores the armature resistance.
The power generated by each phase, P, is VT I cosθ.
Stator on ship 3 – phase generator
The 3-phase ship generator stator currents produce a magnetic field that rotates exactly at the same speed as the rotor field (called the synchronous speed), and δ is the physical angle between the magnetic center lines of the rotor field and the stator field, the rotor field leading the stator field by angle δ.
The electromechanical torque required from the prime mover to generate electrical power in all three phases is equal to (3 × power per phase) divided by the mechanical angular speed of rotor.
Output power on ship generator
The output power versus power is angle relation. An increase in output power results in an increase in power angle only up to Pmax at δ = 90°. Beyond this limit, the rotor and stator fields would no longer follow each other in a magnetic lockstep and would step out of the synchronous mode of operation, that is, the machine would become unstable and unable to produce steady power.
Therefore, Pmax is called the steady-state stability limit (or the pullout power) of the machine.
It occurs at δ = 90°
