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A mathematical model of a doubly-fed asynchronous generator is defined in the paper, using spatial vector theory, in relation to the reference axis associated with the stator. Expressions for characteristic quantities are derived, and operational characteristics for the torque, active and reactive power, power factor, and efficiency are plotted, from which generator operation is analysed at different rotational speeds. Based on this, possibilities and advantages of using doubly-fed asynchronous generators in wind turbines, for obtaining electrical energy from wind energy, are deduced.

Belt conveyor drives at the open pit mines consist of motors with large installed power. Each analysis of their energy efficiency presents an important factor for proper control to achieve better financial effects. The paper considers efficiency of each belt conveyor in the V ECS (excavator-belt convey or spreader) system at the open pit mine 'Drmno' as a function of time, determined based on measured values for the observed period of time. This paper analyses variations of efficiency depending on the load at constant speed. Conversion of electrical energy into mechanical by means of electrical motors and mechanical transmission is performed on belt conveyors. Therefore, the efficiency of drive station driven by electrical motors of multi motor drive is defined as a relation between the output mechanical power of multi motor drive and its input electrical power. Information about the motor torques is obtained from frequency converters, within integrated models which estimate the value of motor torque. Information about the belt speed is achieved by measuring the number of drive drum revolutions in certain period of time. Electrical power for supplying all consumers at the belt conveyor drive station is gained by measuring the power in the supply medium voltage cubicle.

The subject of this paper is the harmonics asynchronous torqueses which appear due to the presence of high time harmonics in the voltage and current of wound rotor induction motor whose speed is regulated by frequency converter in the rotor circuit. Theoretical and experimental analysis was carried out in the case when the waveform of voltage, obtained from frequency converter, is rectangular.