d)Transition times too low

Transition times too low


At star-delta starting and pole changing or reversing of motors transitions between star and delta or low and high speed or between rotation directions it can happen that high peak currents occur. When a motor is switched off, a residual voltage remains in its windings for a period of some milliseconds. If the motor is connected to the network within this period, the summary of residual voltage and line voltage (in unfavorable phase angle) is causing high peak currents which cannot be safely handled by the contactor main contacts anymore. Welding occurs as a consequence. In order to prevent contact welding in these instances we recommend a minimum transition time of 50 ms between star and delta and between pole changing or reversing of motors.


Important Note:


Star-delta-timer relays from Siemens incorporate this transition time in their timing program. If control circuit is operated from a PLC the 50 ms transition time shall be programmed for each rotation direction and as well for each change of rotation speed. This is not only valid above 500 V, even at 400 V it is obligate to obey these rules however one might hear other opinions sometimes