In a 3-phase ac power transmission system using synchronous generator

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In a 3-phase ac power transmission system using synchronous generator
1. The steady-state power limits of both round rotor and salient pole machines are reached at θ = π/2 of their respective power angle characteristics.
2. The steady-state power limit of round rotor machines occurs at a much smaller angle θ as compared to that of salient pole machine power angle characteristics.
3. The steady-state power limit of salient pole machines occurs at smaller angle θ as compared to that of round rotor machine power angle characteristics.
4. The transient state power limits of synchronous generators do not depend on initial load just before the large change in load or on 3-phase fault.

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Correct Answer - Option 3 : The steady-state power limit of salient pole machines occurs at smaller angle θ as compared to that of round rotor machine power angle characteristics.

• For cylindrical rotor machine at δ = 90°
• For salient pole machine at δ < 90°

where δ is the power angle

Important Points:

Cylindrical rotor:

• They are cylindrical in shape having parallel slots on it to place rotor windings.
• They are smaller in diameter but has a longer axial length.
• Cylindrical rotors are used in high-speed electrical machines.
• Windage loss, as well as noise, is less as compared to salient pole rotors.
• Their construction is robust as compared to salient pole rotors.
• The number of poles is usually 2 or 4.
• Damper windings are not needed in non-salient pole rotors.
• Flux distribution is sinusoidal and hence gives a better emf waveform.
• They are used in nuclear, gas and thermal power plants.

Salient pole rotor:

• They consist of a large number of projected poles (salient poles) mounted on a magnetic wheel.
• Salient pole rotors have a large diameter and shorter axial length.
• They are generally used in lower speed electrical machines.
• Flux distribution is relatively poor than a non-salient pole rotor, hence the generated emf waveform is not as good as the cylindrical rotor.
• Salient pole rotors generally need damper windings to prevent rotor oscillations during operation.
• Salient pole synchronous generators are mostly used in hydropower plants.