75 Frequency Converters Voltage vector control is particularly useful for low speeds, where the dynamic performance of the drive can be signifi cantly improved (compared with U/f control) by means of appropriate control of the voltage vector angle. In addition, steady-state behaviour improves, since the control system can make better estimates for the load torque on the basis of the vector values for both voltage and current than it would be able to on the basis of the scalar signals (amplitude values). Table 2.3 explanations of symbols used in: Fig. 2.23 Structure closed loop fl ux vector control Fig.
2.24 Basic principles of current standard frequency converters from Danfoss Fig 2.25a Motor equivalent circuit diagram under “no-load Fig. 2.25b Motor equivalent circuit diagram under “load” Fig.
2.26 Basic principles of Danfoss VVCplus control f Internal frequency fs Reference frequency set Δ f Calculated slip frequency I SX Reactive current (calculated) I SY Active current (calculated) I , I SXO SYO No-load current of x/y axis (calculated) I , I , I u y w Measured phase current (U, V, W ) R s Stator resistance R r Rotor resistance θ Voltage vectors angle θ 0 “No-load” theta value Δθ Load-dependent angular compensation T c Heat-sink temperature (measured) U DC Intermediate circuit voltage U L No-load voltage vector U s Stator voltage vector U Comp Load-dependent voltage compensation U Motor supply voltage X h Reactance X 1 Stator leakage reactance X 2 Rotor leakage reactance ωs Stator frequency L s Stator inductance L Ss Stator leakage inductance L Rs Rotor leakage inductance