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ECE320_Chapter_6 - 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 6: Induction Machines (Textbook Sections 6.1-6.5) Chapter Objectives The popularity of induction machines has helped to label them as the ‘workhorse of industry’. They are relatively easy to fabricate, rugged and reliable, and find their way into most applications. For variable speed applications, inexpensive power electronics can be used along with computer hardware and this has allowed induction machines to become more versatile. In particular, vector or field-oriented control allows induction motors to replace DC motors in many applications. 6.1 Description The stator of an induction machine is a typical three-phase one, as described in the previous chapter. The rotor can be one of two major types – either (a) it is wound in a fashion similar to that of the stator with the terminals connected to slip rings on the shaft, as shown in Figure 1, or (b) it is made with shorted bars. Shaft Slip Rings Rotor Figure 1. Wound rotor, slip rings, and connections. Figure 2 shows the rotor of such a machine, while the images in Figure 3 show the shorted bars and the laminations. The bars in Figure 3 are formed by casting aluminum in the openings of the rotor laminations. In this case the iron laminations were chemically removed.
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1- 2 Figure 2. (a) Cutaway view of a three-phase induction motor with a wound rotor and slip rings connected to the three-phase rotor winding shown in Figure 6.1 in your textbook [1]. (b) Cutaway view of a three-phase squirrel-cage motor as shown in Figure 6.3 in your textbook [1]. (a) (b) Figure 3. (a) The rotor of a small squirrel-cage motor. (b) The squirrel-cage structure after the rotor laminations have been chemically etched away as shown in Figure 6.3 in your textbook [1]. 6.2 Concept of Operation As these rotor windings or bars rotate within the magnetic field created by the stator magnetizing currents, voltages are induced in them. If the rotor were to stand still, then the induced voltages would be very similar to those induced in the stator windings. In the case of a squirrel cage rotor, the voltage induced in a bar will be slightly out of phase with the voltage in the next bar, since the flux linkages will change in it after a short delay. This is depicted in Figure 4. If the rotor is moving at synchronous speed, together with the field, no voltage will be induced in the bars or the windings.
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1- 3 B g 1 2 3 4 5 6 7 13 19 -1 -0.5 0 0.5 1 0 50 100 150 200 250 300 350 e(t) ω t bar 3 bar 1 bar 2 bar 7 (a) (b) Figure 4. (a) Rotor bars in the stator field and (b) voltages in the rotor bars. Generally when the synchronous speed is s s f π ω 2 = , and the rotor speed ω 0 , the frequency of the induced voltages will be f r , where o s r f ω ω π = 2 . Maxwell’s equation becomes here: g B v × = r r E (6.1) where E r is the electric field and v r is the relative velocity of the rotor with respect to the field: ( ) r v o s ω ω = (6.2) Since a voltage is induced in the bars, and these are short-circuited, currents will flow in them. The
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