The influence of eddy currents on the excitation winding impedance of solid and laminated salient pole synchronous machi
- PDF / 2,131,363 Bytes
- 14 Pages / 595.276 x 790.866 pts Page_size
- 65 Downloads / 146 Views
ORIGINAL PAPER
The influence of eddy currents on the excitation winding impedance of solid and laminated salient pole synchronous machines Roberto Felicetti1 · Curt Johan David Abrahamsson1 · Urban Lundin1 Received: 4 November 2019 / Accepted: 17 June 2020 © The Author(s) 2020
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 ACinductance. 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. Keywords Eddy current · Current control · Iron losses · Magnetizing inductance · Salient pole rotor · Skin effect · Stability · Voltage control
1 Introduction pe = The occurrence of currents in a metal body moving across a magnetic field or being crossed by a time-varying magnetic field is a well-known phenomenon [1, 2]. Currents induced by that way are mainly responsible for heat generation and electrodynamic force reaction into said body. These effects are sometimes welcomed and pursued in specific applications, such as induction ovens, electromagnetic brakes, metal trash sorters, nondestructive testing devices, just to cite a few. On the contrary, when it comes to electrical machines, the same effects are normally undesired. Eddy currents are therefore avoided or weakened as possible, in order to improve efficiency, reliability and life-time expectation of the electromechanical converters. The well-known specific eddy current loss formula pe for a conductive laminated magnetic core [3] is
* Roberto Felicetti [email protected] 1
Division for Electricity, Department of Electrical Engineering, Ångström Lab, Uppsala University, Box 534, 751 21 Uppsala, Sweden
𝜋 2 B̂ 2 f 2 dl2 6𝜌
∼ dl2 ,
(1)
where B̂ is the amplitude of the sinusoidal flux density, f its frequency, 𝜌 the specific resistivity of the conductive material and dl its lamination thickness. By showing that the specific eddy current losses are proportional to the second power of the thickness dl , Eq. (1) explains why magnetic circuits are usually finely laminated there, where the flux density is varying in time. However, Eq. (1) is valid when the layer is completely penetrated by the magnetic field or, in other words, when the magnetic reaction of the induced eddy current is n
Data Loading...
