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EPS Lorenzo Spinelli July 19, 2016 Part I Electric Machinery 1 Synchronous machine The synchronous machine is usually devised as a generator in electric systems. Its design features rotor windings in direct current configuration and stator windings in 3-phase AC. The preferred connection type for these windings is usually Y , since ∆ absorbs three times as much corrent for any given operating condition. Necessarily , the number of poles must be the same. 1.1 No-load conditions We want to consider conditions in which the machine is disconnected from the network. In the rotor windings, which we suppose moving at a rotational speed Ω ̸= 0 , we have a DC current which we call excitation current IE = VDC=R. This will produce a time-constant magnetic field which has a sinusoidal space distribution at any time t; because of this, no flux linkage will be present in the rotor loops, and no electromotive force will be applied there. We can modify the magnitude of this magnetic field by simply modulating VDC , and consequently IE. Switching focus to the stator windings, we’ll have a flux linkage ϕc = Nsbϕ cos ( ) which will generate on each winding an e.m.f. equal to eb = dϕc dt = Nsbϕ d ( ) dt
sin ( ) = = Nsbϕ (! !r) sin [(! !r) t + 0 0] (1.1) each shifted in phase by 120◦. If we were to connect the stator to the network, these would generate a set of balanced three-phase AC Ib = Eb=Rb, which would in turn fulfill Galileo Ferraris theorem’s conditions: another RMF would be borne. 1.2 Stator only We’ll now consider a condition where only the stator’s RMF is present (which is never really the case in syn- chronous machines) and apply the superposition principle. Once the stator windings are connected to the network, their magnetic field will have speed N0 = 60
f p=2 [rpm]: this…