Abstract
This paper establishes a mathematical model of induction machine connected to a frequency inverter necessary to adjust the electric motor drive. The mathematical model based on the Park's theory allows the analysis of the whole spectrum (electric car – frequency inverter) to drive the electric trolley bus made on ASTRA Bus Arad (Romania). To remove higher order harmonics, the PWM waveform of supply voltage is used, set in the general case. Operating characteristics of electric motor drive are set to sub-nominal frequency (f < 50 Hz) and for different angles of the control switching elements in the construction of the inverter. Tensions in the stator of the equivalent machine are determined by three components: the fundamental component; the second component corresponding to the higher order harmonics, which rotate in direct connection; and the appropriate third component of higher order harmonics, which are rotated in reverse. The mathematical model developed will help to determine the harmonics which have a negative influence on the performance of the electric engine and also determines the control angle of the switching elements to remove these harmonics, (Bele 2007).
Highlights
The use of three-phase induction machine in the electric traction trolley is determined by: design simplicity, low cost price, high operational safety, and high technical performance, (Bele 2007).To improve the performance of the induction machine to change the drive speed is necessary to change the voltage and frequency according to system requirements traction.Static frequency converters cause the voltages or currents, besides containing the fundamental harmonic and higher harmonics
At low speeds, when a supply voltage of the traction motor is less than 20 Hz, pulsating torque caused by higher harmonics of the rotor produces a jerky motion, practically confined to the lower speed power tool that can be used; 2
Improving the performance of electric traction motor, it can be achieved by eliminating the higher harmonics or higher harmonics of groups in the number of pulses and control mode of inverter switching elements; 3
Summary
The use of three-phase induction machine in the electric traction trolley is determined by: design simplicity, low cost price, high operational safety, and high technical performance (starting torque, high efficiency, etc.), (Bele 2007). Low harmonic content of output voltage, output current respectively, and high dynamic performance obtained from static frequency converters, based on the principle of inverters in the pulse duration modulation (PWM) as a sine law, promote the use of traction converters electricity. With the system of equations (1) and equation (3) determine the variation of the stator and rotor currents (id (t), iq (t), idr (t) and iqr (t)) and temporal variation rotor angular velocity ω (t). Based on these sizes, it can be determined the real current of induction machine stator (Müller 2005):.
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