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ECE320_Chapter_7

# ECE320_Chapter_7 - ECE 320 Energy Conversion and Power...

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1- 1 ECE 320 Energy Conversion and Power Electronics Spring 2009 Instructor: Tim Hogan (Notes from Prof. Elias Strangas) Chapter 7: Synchronous Machines and Drives (Textbook Chapter 5) Chapter Objectives For induction machines, as the rotor approaches synchronous speed, the frequency of the currents in the rotor decreases, as does the amplitude of these currents. The reason an induction motor produces no torque at synchronous speed is not that the currents are DC, but that their amplitude is zero. It is possible to operate a three-phase machine at synchronous speed if DC is externally applied to the rotor and the rotor is rotating at synchronous speed. In this case torque will be developed only at this speed, i.e. if the rotor is rotated at speeds other than synchronous, the average torque will be zero. Machines operating on this principle are called synchronous machines, and cover a great variety. As generators they can be quite large, rated a few hundred MVA, and almost all power generation is done using synchronous machines. Large synchronous motors are not very common, but can be an attractive alternative to induction machines for some applications. Small synchronous motors with permanent magnets in the rotor, rather than coils with DC, are rapidly replacing induction motors in automotive, industrial and residential applications since they lighter and more efficient. 7.1 Design and Principle of Operation The stator of a synchronous machine is of the type that we have already discussed with three windings carrying a three-phase system of currents. As we saw in Chapter 5, this results in a stator magnetic field that spatially rotates around the stator at a constant angular speed, ω s . Unlike induction machines, synchronous machines have zero slip, and the rotor maintains the same angular speed, ω s , as the stator generated field. In a synchronous machine, the rotor windings carry DC current, or are composed of permanent magnets as discussed next. 7.1.1 Wound Rotor Carrying DC In this case the rotor steel structure can be either cylindrical, that that in Figure 1(a), or salient like the one in Figure 1(b). In either case, the rotor winding carries DC, delivered through slip rings, or through a rectified voltage of an inside-out synchronous generator mounted on the same shaft. In this handout, discussion is limited to cylindrical rotors. 7.1.2 Permanent Magnet Rotor Instead of supplying DC to the rotor, the rotor contains permanent magnets in configurations such as those shown in Fig. 2. The effects of permanent magnet rotors include: The rotor flux can no longer be controlled externally. It is defined by the magnets and the geometry The machine becomes simpler to construct, at least for small sizes.

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1- 2 Sinusoidally distributed winding Concentrated winding (a) (b) Figure 1. Wound rotor configurations of synchronous machines.
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ECE320_Chapter_7 - ECE 320 Energy Conversion and Power...

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