Questions and Answers
What causes the induced emf in the stator or armature conductors of a synchronous machine?
What is a significant advantage of having a stationary armature in a synchronous machine compared to a rotating armature?
What determines the frequency of the alternating emf induced in the armature conductors of a synchronous machine?
Why is it preferable to avoid using slip rings in a synchronous machine's armature?
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In a polyphase synchronous machine, why is a stationary armature advantageous when delivering power to an external load?
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What type of currents are present in the rotor windings of synchronous machines?
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Which component of a 3-phase synchronous machine primarily forms the electrical circuit?
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What advantages does a 3-phase system offer for synchronous machines?
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How many coils are necessary to generate a set of three voltages phase-displaced by 120 electrical degrees?
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Which part of a synchronous machine is responsible for minimizing losses due to hysteresis and eddy currents?
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Study Notes
Induction of EMF in Synchronous Machines
- Rotor rotation by the prime mover causes the stator conductors to be cut by DC magnetic flux.
- Induced EMF is generated in the armature conductors due to alternating N and S magnetic poles.
- Direction of induced current follows Fleming’s Right-Hand Rule, alternating as the poles change.
Advantages of Stationary Armature and Revolving Field
- Absence of commutators in synchronous machines allows the armature to remain stationary while field poles rotate.
- Rotating armature would necessitate multiple slip rings for polyphase power, complicating insulation at high voltages (30 kV+).
- Stationary armature avoids risks such as arc-overs and short circuits associated with rotating slip rings.
- Synchronous machines traditionally operate with three-phase systems, leveraging advantages in generation and heavy power applications.
- Three coils, spaced 120 electrical degrees apart, are required to produce three-phase voltages.
Constructional Features of Synchronous Machines
- Synchronous machines have AC in stator windings and DC in rotor windings.
- Essential components include:
- Stator: frame/yoke and armature coils.
- Rotor: rotor core and field windings.
- Slip rings or collector rings, brushes, and bearings.
- The magnetic circuit is formed by the stator frame, pole-cores, rotor core, and air gap.
Stator Specifications
- The stator, the machine's stationary unit, comprises a frame and armature coils.
- Stator core features laminated construction to reduce losses (hysteresis and eddy currents).
- Laminations are insulated, allowing for cooling air circulation through designated spaces.
- The three-phase windings are designed with 120-degree spatial displacement to achieve balanced operation.
Magnetomotive Force (MMF) Contributions
- MMF for each phase is defined mathematically, with each phase contributing to the overall field.
- Phase MMF equations:
- Fa = Fapeak cos(θ)
- Fb = Fbpeak cos(θ - 120°)
- Fc = Fcpeak cos(θ - 240°)
- Resultant MMF is a combination of individual phase contributions adjusted over time.
Time Variation of MMF Amplitudes
- MMF amplitudes vary with current through the phases, expressed as functions of time.
- Time-dependent equations:
- Fapeak = Fa max cos(ωt)
- Fbpeak = Fb max cos(ωt - 120°)
- Fcpeak = Fc max cos(ωt - 240°)
- Current displacement in time creates periodic variation, critical for synchronous operation.
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Description
Explore the principles of EMF induction in synchronous machines, focusing on rotor rotation and the generation of induced EMF in armature conductors. Learn about the benefits of having a stationary armature and revolving field in synchronous operations, including the advantages for three-phase systems in heavy power applications.
