The steady-state torque equation for a synchronous motor or generator is given by
where
• T = mechanical shaft torque
• P = number of poles
• Ï•SR = air-gap flux (approximately proportional to the stator current)
• FR = rotor field MMF (proportional to the field current)
• δR = mechanical torque angle between rotor and stator field.
• T = mechanical shaft torque
• P = number of poles
• Ï•SR = air-gap flux (approximately proportional to the stator current)
• FR = rotor field MMF (proportional to the field current)
• δR = mechanical torque angle between rotor and stator field.
A plot of torque and rotor angle is shown . For a synchronous motor, the rotor field (FR) lags the rotating stator field (ϕSR) by the angle δR. For a generator, FR leads the stator field.
During a system disturbance or a fault, the power output of the generator is reduced. Because the power input to the prime mover does not change, the excess energy is added to the rotating inertia, which causes an increase in the angle between the internal voltage of the generator and the system voltage. After the fault is cleared, the generator power exceeds the power input to the prime mover as the angle is advanced. This condition brings the angle back to the initial value, and the rotor.
During a system disturbance or a fault, the power output of the generator is reduced. Because the power input to the prime mover does not change, the excess energy is added to the rotating inertia, which causes an increase in the angle between the internal voltage of the generator and the system voltage. After the fault is cleared, the generator power exceeds the power input to the prime mover as the angle is advanced. This condition brings the angle back to the initial value, and the rotor.
Torque vs. rotor angle relationship for synchronous machines in steady state
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