《《7_ Inverter operated induction machines》.pdf

7. Inverter operated induction machines 7. Inverter operated induction machines Source: Siemens AG, Germany DARMSTADT UNIVERSITY OF Dept. of Electrical Energy Conversion 7/1 TECHNOLOGY Prof. A. Binder Inverter-fed induction machine • Frequency converter (inverter) generates three-phase voltage system with variable frequency f and variable amplitude U (rms). Hence synchronous speed is continuously s s variable. With that induction machine is continuously variable in speed. • Reversal of speed = Changing of two phases of stator winding. Changing of energy flow (motor / generator) by decreasing / increasing phase shift between voltage and current : motor ϕ π/ 2 generator ϕ π/ 2 • Voltage amplitude Us must be changed in proportion to fs to keep the flux in the machine constant. Thus torque will stay constant, if the the same current is used. For R = 0: Us j ω L I s +j ω L (I s +I ′r ) s s sσ s h U s ˆ ˆ ˆ jL sσI +jL h (I +I ′ ) j (Ψ +Ψ ) / 2 jΨ / 2 const. s s r sσ h s ω s U ~ ω Rule for controlling the inve