978-1-4244-9074-5/10/$26.00 ©2010 IEEE 2010 Annual IEEE India Conference (INDICON) Modeling and Estimation of Core Losses for Doubly- fed Wound Rotor Induction Machine Krishna Roy #1 , Debashis Chatterjee #2 , A. K. Ganguli #3 # Electrical Engg. Department, Jadavpur University, Kolkata, India 1 krishna_004@sify.com 2 dchatterjee@ee.jdvu.ac.in 3 akganguly@ee.jdvu.ac.in Abstract— Wound-rotor induction motor drive fed from inverters on both the stator and rotor side is discussed. The sensorless control scheme for the motor requires a V/f-type direct frequency control preferably on the rotor side, with either vector control or direct torque control on the stator side. Selection of any frequency for the rotor side inverter keeping the rotor flux constant is possible. This rotor frequency will decide the selection of the stator side frequency. In this paper, a study on core loss at different rotor injected frequencies is carried out at different loads. Also to operate the motor at an optimum efficiency during any loading condition, a method for correct selection of stator and rotor frequency is studied. It is also shown that core losses constitute a considerable amount of the total losses and hence should not be neglected for the sake of efficiency. Experimental results on a real machine are presented in support of the proposed concept. Keywords— Doubly fed induction motor, sensorless control, core loss, wound rotor machine, stator flux and rotor flux. I. INTRODUCTION ENSORLESS rotor flux oriented control (RFOC) of induction machines requires correct estimation of stator and rotor fluxes over the entire controlled speed range. At lower input frequencies estimation of stator through normal integration method leads to saturation and DC offset problems for the integrators [1]-[3]. Moreover, estimation of the stator flux at lower input speeds is very much affected by motor parameter variation problems [4]- [6]. A number of works have already been carried out to avoid this problem [1]-[3], [8]. But most of these methods involve extended computational efforts and also are not completely error free. The wound-rotor induction motor with control on both stator and rotor sides of the machine can be employed as one of the solutions to the problem [7]- [11] with an additional advantage of more flexibility of the drive. In this scheme both the stator and rotor can be operated at higher frequencies for both sub-synchronous and super-synchronous operation. When operating in the super-synchronous mode, 2.0 p.u. power can be extracted from the machine. Thus the above mentioned problem to operate the machine at lower frequencies can be avoided as the input frequencies for both the rotor and stator are high in this mode of operation. Also the rotor-side inverter can be used to supply the full magnetizing current, so that the grid-connected stator side inverter always operates at unity-power-factor mode. Thus, this drive can operate at unity power factor throughout its operating region at the stator side. For this drive, the rotor side inverter frequency will decide the slip frequency when operated in the sub-synchronous or super-synchronous region. Thus, higher frequencies will introduce increased amount of core losses in the rotor of the machine. This will cause over heating of the rotor during its operation necessitating the requirement for derating the machine. In this paper a modeling for core losses with variable rotor frequencies is presented and motor efficiency is plotted as a function of rotor speed. Based on the developed model, a modified selection of rotor speed with rotor frequency is proposed for reducing rotor core loss and extending the motor output capacity upto 2.0 p.u. for super-synchronous operation. Experimental results are presented to validate the proposed concept. II. DOUBLY-FED WOUND ROTOR INDUCTION MACHINE MODELING The stator and rotor voltage equations in synchronous d-q reference frame can be written as, sd sd s sd e sq d V ri dt ψ = + -ω ψ (1) sq sq s sq e sd d V ri dt ψ = + +ω ψ (2) rd rd r rd sl rq d V ri dt ψ = + -ω ψ (3) rq rq r rq sl rd d V ri dt ψ = + +ω ψ (4) Alignment of the synchronous reference frame with the rotor flux axis gives, rq rd r 0 ψ = ψ = ψ (5) r rd r rd d V ri dt ψ = + (6) S