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Lesson 37 AC Generators II

Lesson 37 AC Generators II. Learning Objectives. Use the power conversion diagram to describe power flow for a three phase generator. Find line voltages and current for a Y-connected three phase generator. Large AC generator.

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Lesson 37 AC Generators II

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  1. Lesson 37 AC Generators II

  2. Learning Objectives • Use the power conversion diagram to describe power flow for a three phase generator. • Find line voltages and current for a Y-connected three phase generator.

  3. Large AC generator • Unlike our generator model with a fixed magnetic field and rotating armature, it is more practical to fix the armature windings and rotate the magnetic field on large generators. • Brushes and slip rings pass EXCITATION voltage to the field windings on the rotor to create the magnetic field • Minimizes current flow through brushes to rotor windings

  4. Review DC Power Conversion Diagram Electrical Mechanical

  5. AC Generator power conversion diagram Mechanical Electrical PIN = Trotor=746*hp POUT Pelectr loss Pmech loss NOTE: ω is the speed of the rotor, not the angular velocity of the AC current.

  6. Example Problem 1 Consider a 3-phase, 4 pole, 60Hz, 450 V synchronous generator rated to supply 1687.5 kVA to a ship distribution system requiring a 0.8 lagging power factor. • If this machine was operating at rated conditions, what would the real (P) and reactive (Q) power and the current being supplied? • If the generator has an efficiency () of 95%, what torque does the prime mover provide? • What is the speed of the rotor (rpm)?

  7. XS RS A Ia  + EAN - Einduced N Single-Phase Equivalent Circuit • Just like 3-phase loads, it is useful to look at just a single phase of the generator. Single-phase equivalent 3-Phase Generator

  8. XS RS A + EAN - Ia Einduced N Single-Phase Equivalent Circuit • EAN is the phase voltage of the a-phase Ia is the line current • Einduced is the induced armature voltage. • RS is the resistance of the generator’s stator coil. • XS is the synchronous reactance of the stator coil.

  9. AC Generator Power Balance • Mechanical Input Power can be calculated: • Electrical (Armature) Losses can be calculated (notice 3 sets of armature windings, so must multiply by 3) • Electrical output power can be calculated • The total overall power balance:

  10. Solution steps • Determine the rms value of IL • Determine phase angle of IL from the given power factor FP (using phase voltage as the reference)

  11. Solution steps • Determine Electrical losses (zero for a “negligible stator resistance”). This is PER-PHASE, so must multiply by 3 when adding to other power. • Determine PIN • Determine torque supplied to the generator if needed

  12. Example Problem 2 A submarine has a 3-phase, Y-connected, 2-pole, 60Hz synchronous generator rated to deliver 1687.5 kVA at a FP= 0.8 lagging with a line voltage of 450-V. The machine stator resistance RS = 0.004 Ω. The synchronous reactance is Xs=0.08 Ω. The actual system load on the machine draws 900 kW at FP = 0.6 lagging. Assume that a voltage regulator has automatically adjusted the field current so that the terminal voltage is its rated value. Mechanical losses are 100 kW. • Determine the reactive and apparent power delivered by the generator. • Find the current delivered by the generator • What is the overall efficiency? The rated voltage (here, 450 V) is always a line voltage (this is the voltage we can measure between any two cables in a 3-phase system)

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