Synchronous Machines

Synchronous Machines - 1 EE 4420 Electric Machine Analysis...

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1 EE – 4420 Electric Machine Analysis Fall 2005 SYNCHRONOUS MACHINES 1. CONSTRUCTION AND PRINCIPLE OF OPERATION A 3-phase synchronous machine is shown in Fig.1. It consists of two parts: stator and rotor. Both, stator and rotor have windings. The stator winding is a 3-phase winding and is sometimes called the armature winding. The rotor winding is called the field winding, which is connected to a dc supply through the slip rings and brushes. There are two types of rotors: Salient-pole rotor (Fig. 1.a) – for low-speed machines (e.g. hydro-generators) Cylindrical rotor (Fig. 1.b) – for high-speed machines (e.g. turbo-generators). (a) X X A 1 A 2 B 1 C 1 B 2 C 2 salient-pole rotor slip rings brushes rotor field winding 3-phase stator winding f f (b) X A 1 A 2 B 1 C 1 B 2 C 2 cylindrical rotor slip rings brushes rotor field winding 3-phase stator winding f f X X X X Fig.1 Construction scheme of synchronous machine with: (a) salient-pole rotor, (b) cylindrical rotor
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2 The cylindrical rotor has one distributed winding and an essentially uniform air gap. The salient pole rotors have concentrated windings on the poles and a nonuniform air gap. 1.1 Synchronous generators When the field current flows through the rotor field winding it establishes a sinusoidally distributed flux in the air gap. If the rotor rotates the rotating magnetic field induces voltages in the stator windings. Since the three-phase windings are shifted by 120 o angle from one another the induced, so called, excitation voltages are shifted in time from one another by the angle of 120 o (Fig.2): sin( ) sin( 120 ) sin( 240 ) A fm Bo fm Co fm eE t t t ω = =− (1) e A e B e C e t Fig.2 Waveforms of 3-phase voltages induced in the armature winding of the synchronous generator The rms excitation voltage in each phase is E E fN K f fm fw == 2 444 . Φ (2) where: Φ f is the magnetic flux due to the excitation current, N is the number of turns in each phase, K w is the winding factor. The frequency of the induced voltage is related to the rotor speed by:
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3 n f p = 120 (3) f n p = 120 (4) where: n is the rotor speed in rpm, p is the number of magnetic poles 1.2 Synchronous motors If the synchronous machine operates as a motor the 3-phase armature winding is connected to a 3-phase ac supply. The stator currents produce the rotating magnetic flux as in 3-phase induction machines. The field winding connected to dc source produces the magnetic flux, steady with respect to the rotor. To produce the torque these two magnetic fluxes cannot move with respect to one another. It means, that the rotor should rotate with the same speed as the rotating flux produced by the stator (Fig.3). When the machine operates as a generator the rotor is driven by the external machine and the stator rotating field follows the rotor being shifted with respect to the rotor by the angle δ (Fig.3a) .
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Synchronous Machines - 1 EE 4420 Electric Machine Analysis...

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