Semiconductor soft starter 3RW3 Star-delta-starter After a certain changeover time, the motor windings are switched over from a wye (star) motor connection to a delta motor connection. In the case of wye starting, the motor current only amounts to around 1/3 of the delta- starting current (in the wye (star) motor connection, the motor torque is also reduced to about 1/3 of the delta torque). Disadvantages:
6 motor leads are needed Changeover jerking occurs (in the current and in the torque profile) Adaption of starting to the situation of the motor is not possible Relatively complex and time-consuming installation Large space requirement in the control cabinet I I A d Starting current in the case of d M M A d M Motor in the case of d M Load Starting current in the case of * / d -Start 1/3 I A d I A * = M Motor in the case of * M A * =1/3 M A d n n n Syn * n Rated n n n Syn * n Rated Fig. 8-3:
Current and torque profile in the case of star-delta starting Frequency converter When a frequency converter is used, the AC voltage of the line is converted to a DC voltage which, in turn, can be converted to a voltage of any fre- quency. The following graphic shows the operating principle of a frequency con- verter: U 0V t Fig. 8-4: Operating principle of a frequency converter Disadvantages: Relatively complex when it comes to keeping to the RFI suppression reg- ulations; filters are often indispensable. Only limited motor line length owing to line capacitance; it may be neces- sary to use chokes, sinusoidal filters or even du/dt filters.
High price Owing to the large number of operator control parameters, commission- ing is often awkward and time-consuming. Rev. 08/99 8-7