73 Frequency Converters The control algorithm takes two operating conditions into consideration: • No-load state (idle state), see Fig. 2.25a Motor equivalent circuit diagram under “no- load”. In the no-load state, there is no load on the motor shaft. For conveyors the no- load state literally means no products are being transported. It is simply assumed the current drawn by the motor is only needed for magnetisation and compensation for losses. The active current is considered to be nearly zero. The no-load voltage (U L ) is determined on the basis of the motor data (rated voltage, current, frequency, speed). Fig.
2.25a Motor equivalent circuit diagram under “no-load” • Loaded state The motor shaft is loaded, implying that products are being transported, as shown in Fig. 2.25b Motor equivalent circuit diagram under “load”. The motor draws more current when it is loaded. In order to produce the required torque the active current (I W ) is needed. Losses in the motor (especially in lower speed range) need to be compensated for. A load-dependent additional voltage (U Comp ) is made available to the motor: U = U LOAD = U L + U Comp Fig. 2.25b Motor equivalent circuit diagram under “load”