The influence of eddy currents on the excitation winding impedance of solid and laminated salient pole synchronous machines

Abstract

This work investigates the establishment of steady-state eddy currents in solid and laminated salient poles and rotor rim of synchronous machines due to a periodic excitation voltage. It shows that the presence of eddy currents in the rotor magnetic circuit has the double effect of increasing the excitation winding AC-resistance and decreasing its magnetizing AC-inductance. According to that a simple analytical model is presented in here which allows a rapid rough estimation of the excitation winding AC-resistance when little information is available about the machine geometry and its electric/magnetic materials properties. The model is then verified by reproducing in frequency the excitation winding AC-resistance and the related power loss measured in two synchronous generators. Finally, the limits of reliability and applicability of the model are discussed. The model has implications for periodic field winding current control and voltage regulation in synchronous machines.

An experimentally determined field winding model with frequency‐dependent parameters

“…It can be observed that the measured values, as well as the regression straights, superpose for both test conditions. This means that the presence of the armature does not change substantially the damping properties of the rotor winding, which supports a theoretical prevision formulated in [33]. Moreover, a clear common asymptotic tendency of the measured values of R p starting from the kHz range ( σ ≅ 0.7) confirms the suitability of () for representing the winding parallel resistance in the range of the winding resonance frequencies and beyond.…”

“…Table 8 shows that the values of L p and L' p decrease while the frequency increases. This is due to the degaussing effect of the eddy currents in the solid poles mainly [30–33] and to the skin‐ and proximity‐effect in the winding secondarily. In order to catch how strongly the winding inductance and the frequency are negatively correlated, the following simple inductance law has been introduced where K and γ are constants to be found using the inductance values corresponding to the series resonance frequencies.…”

“…Moreover, equation (15) shows that the field winding inductance is weakened by the presence of the stator around the rotor. When the armature surrounds the rotor the enhanced demagnetising effect of the eddy currents in the rotor core [27,33] produces a steeper inductance decrease with the frequency. Therefore, when the rotor is inserted into the stator bore an improved field winding rejection of voltage surges can be expected.…”

“…According to the literature, the most used circuits models for studying transients in electric circuits are based on Transmission Line Model (TLM) or Lumped Element Model (LEM) [25][26][27]. Some models assume the circuit parameters to be constant [28,29] even though the winding resistance and inductance show a strong dependence on frequency [30][31][32][33]. Other models assume a constant wave propagation speed in the windings, resulting in an easy extraction of the inductance values from the winding capacitances [24,34].…”

the influence of material electromagnetic properties to the electric motor performance

“…On the other hand, using a non-laminated magnetic core would be possible only when the magnetic field is constant, e.g. in the rotor of an electrically excited synchronous motor (EESM) [8,9]. Apart from the magnetic core, such rotor contains also an excitation winding as the other active part, which is supplied with direct current (DC) that excites constant magnetic field.…”