Product features
5.3 Designing motor starters in conjunction with electromechanical switching devices
Motor starter (3RK1308‑0**00‑0CP0)
Equipment Manual, 11/2019, A5E34821005002A/RS-AC/003
41
5.3
Designing motor starters in conjunction with electromechanical
switching devices
NOTICE
Use of motor starters in series with mechanical switching devices, e.g. line contactors
When using motor starters in series with mechanical switching devices, e.g. line contactors,
ensure that voltage peaks are prevented by means of the mechanical shutdown of small
motors (high inductance).
You can achieve this with a corresponding shutdown sequence or through the use of RC
elements.
One advantage of the hybrid motor starters is, among other things, the shutdown in the
current zero point, which has the advantage that no voltage peaks or arcing occur in the
power circuit. Thus, electromagnetic interferences no longer exists due to the shutdown.
Switching an inductive load, in particular of motors with an output of < 1 kW with
correspondingly high inductance, e.g. with a contactor, can cause an inductive voltage of
several 1000 V with steep voltage edges, depending on the switching time. This can also
occur multiple times due to the bouncing of the contactor contacts and the resultant arcs. For
a contactor, this leads to increased contact erosion (wear) and to high electromagnetic load
on other components due to coupling into other electric lines.
If a line contactor is also connected upstream or downstream of a motor starter, the
shutdown of the line contactor when the motor is running generates a counter-acting
self-induction voltage, which can reach a voltage of several 1000 V with a very steep voltage
rise.
The motor starter is protected against overvoltage by internal varistors
.
If, however, the
voltage peaks are significantly higher and/or they occur more frequently or in rapid
succession (due to contactor bouncing), the varistors become overloaded and can no longer
limit the voltage peaks. The voltage peaks are then present directly on the electronic
switching element of the motor starter and can ultimately lead to damage and a defective
device.
