Abstract: The paper studies some aspects regardingasynchronous motor electric drives. In connection with these categories of electricdrives, it is proposed to develop a modern electric drive scheme, based on aconverter made with the I.G.C.T technology. With such an approach, we aim toincrease the functional parameters of the electric drives so that we canachieve a reduction of the electricity consumption and a higher reliability ofthe installation.
Key words: Electric drives, asynchronous motor, automatic adjustment, converter, I.G.C.T.
technology, power losses. 1. INTRODUCTION The asynchronous motor with a squirrel cage rotor arefrequently used in electrical drives. The operation of the three-phaseasynchronous motor is based on the existence of a spinning magnetic field inthe machine’s air gap created by the stator windings powered by a three-phasevoltage system.The rotation speed called synchronousspeed is determined by the stator current frequency, 5: (1)where: f1- frequency of stator currents; p – numberof pairs of poles.
Definesthe sliding: (2) Frequency of rotorcurrents: (3) Theelectromagnetic torque will be, 5: (7)where: and the total leakage reactance is: Thecritical torque is given by the equation: (8)The critical slip is: (9) Fromthe relation (3) result the speed adjustment methods: variation of rotorcircuit resistance, sliding variation, variation of supply voltage, frequencysupply voltage variation, etc. 5.The most convenient and cheap method would be toadjust the supply voltage. A very good method for speed adjustment a.m. is the variationof the frequency of the supply voltage correlated with the variation of itsamplitude, U/f = const.The induction motors with squirrel cage rotor areclassified according to IEC 60034-30:2008 in three efficiency classes:a.
Standard Efficiency IE1;b. High Efficiency IE2;c. Premium Efficiency IE3.In the conductors of the stator winding appearprincipal and supplementary losses. Starting of T equivalent scheme (fig1.
1),parameters are reported to the nominal impedance, and are expressed for slide sthrough the below relations: so noting r1 – the stator resistance; r2- the rotor resistance (reported to stator); r0 – the iron lossesresistance; x1 – the stator leakage reactance; x2 – therotor leakage reactance and x0 – the magnetization reactance; pw1- Joule power losses instator; pw2 – Joule powerlosses in reported rotor; p0- the iron losses; pmv – ct., the mechanical and ventilation losses, 4. Fig. 1.
1. Asynchronous motor T scheme (11) (12) (13) Where: (14) (15) (16) The applied voltage is equal with 1 in the equivalentelectrical scheme, written in the relative units, and the current is i1(sn)= 1 and the power p2(s) = 1 which involve the equality (9). Theequation solution drives the load of motor to a nominal load, 4.The motor efficiency and the power factor can bedetermined: (17) (18) (19) (20)Usually, the asynchronous motor with a squirrel cagerotor requires relatively big starting moments and low starting currents.
Thisrequires a certain shape of the rotor notch, which must have a pronouncedrefutation of the current from the rotor bar, in order to increase the apparentresistance, 4.The supplementary losses in the core appear both at noload and full load status of the machine. According to the literature, thesupplementary losses can reach up to 8-25% from the total losses.