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### Study of the behaviour of a doubly-fed induction machine supplied with scalar open-loop control DOI:10.15199/48.2019.05.06

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Introduction Doubly-fed induction machine (DFIM) is usually used in large-power variable-speed drives with limited range of speed in the vicinity of the synchronous speed using bidirectional power converter [1-5], e. g. wind turbines, large pumps etc. Stator is connected directly to the public network (400 V and 50 Hz in Europe), whereas rotor is connected to the same network via power converter with adjustable voltage and frequency (Fig. 1). Fig.1. Power flow in the drive system with DFIM and power converter If power converter (rotor voltage source) allows only unidirectional power flow (Fig. 1), i. e. from the network to the machine (diode rectifier at the input side instead of the transistor bridge), then the operational area is limited to the supersynchronous speeds [6-7], assuming that the machine operates as a motor (Fig. 2). The power rating of the rotor voltage source is a fraction of the total power and it increases with increasing speed range [8-10]. In the steady-state motor operation, the rotor rotates in the supersynchronous area with the mechanical speed, which is defined as the sum of the rotational frequency of the stator and rotor rotating magnetic field, divided by the number of pole pairs: (1) p ω ω Ω 1 2 + = where: Ω - mechanical speed in [1/s], ω1/ω2 - angular frequency of the stator/rotor voltage in [1/s], p - number of pole pairs. Fig.2. Supersynchronous motor operation area (grey) Simulated drive system The two-dimensional finite-element-method (FEM) model (Fig. 3) with analytically calculated resistances and end-winding leakage inductances was built in order to obtain as accurate results as possible. The machine data are given in Table 1. The scalar open-loop control with variable voltage-to-frequency ratio (U/f control) has been used for the rotor voltage source in the simulations. Namely, the U/f control it is one of the simplest, easy-to-design and low-cost m[...]

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