Motor Soft Start Testing Modbus Communication Failure

There are two factors here:
1. electronic soft starters are current chopping thyristor devices that work at up to 400% motor rated current. The chopping of such large current in this way generally sends large harmonic currents and voltages into your power system that can disturb sensitive communications devices by voltage disturbance, currents in the earthing systems and harmonic neutral currents. Soft starters are relatively crude and noisy devices compared to other methods of soft starting motors such as VVVF, autotransformer, etc.

2. Communications such as Profibus, modbus, etc are reliable only when the screen earthing and other noise mitigation methods are properly installed. These communication medias are quite prone to interference from electronic soft starting. So make sure your communications earthing and screens are installed as per the book.

A good test for any electronic communications is to start a motor electronic soft starter nearby.