Created Date: March 12,2016

Created by: pressgrove

I think I found the answer. From Rockwell Automation Publication 150-WP007A-EN-P......

Soft Starter

Soft starter harmonics are typically less than 10% in starting or stopping modes when SCRs are turned on and provide partial voltage amplitudes, producing partial sine waves. With the motor at full speed, the SCRs are fully conducting, there are virtually no harmonics. In bypass condition, there are almost no harmonics generated. Long cable/wire runs with the soft starter product typically do not need any special treatment other than having properly sized cable/wire to compensate for the voltage drop. The Allen-Bradley soft starts typically have runs up to 762 meters (2500 feet) based on the capacitance of the cable, which was factored into the design. No special wire or wire type is needed. Soft starters do not typically require EMC mitigation to meet IEC harmonics requirements. IEC requirements pertain to the full-on running state of the soft starter.

Drive

Long cable/wire runs from a drive to a motor can create reflected wave issues. It is recommended to use line reactors to prevent harmonics from feeding back onto the power source and causing voltage distortions harmful to other equipment. Other devices that are used to help reduce harmonics in drives are DC link chokes, passive filters, 12-pulse converter with phase-shifting transformer, active filter, active (regenerative) converter and 18-pulse converter. You must also consider wire type when you install drives.

Created Date: March 10,2016

Created by: pressgrove

I have an existing application where there are two Danfoss MCD3185-T7-C20-CV2 soft starters powering a pair of 250HP centrifugal vacuum blowers. Over the past 6 years, we have had to replace one of the motors twice due to shorts in the windings. In both cases the motor required a complete rebuild and we elected to replace it with a new motor each time. The motors are inverter duty and well maintained. The system is grossly oversized for the application so the motors only run at about 160 amps @ 480 VAC. The system runs at full speed 24/5 with a constant load. Only one blower at a time is used (the other is for a backup), and there is an alternating relay in the controls to select the opposite blower each start cycle so each blower has roughly the same hours. This means that on average, each blower has less than one start cycle per week. The motor that has failed each time is the farthest from the starter (about 75 ' linear wire run). The closer blower has run with no issues (about 50 ' linear wire run).

Since there are no bypass contacts in these particular starters (they run on the SCRs all of the time), my first thought is that maybe there is some sort of reflective wave creating voltage spikes and that I should install line reactors on the load side between the starter and motor (Specifically, Automation Direct model LR-4250). That said, I don't see any info in the manual showing this. In installations with VFD's and long wire runs I 've always used line reactors and have never had any issues (albeit with much smaller motors).

Does this make sense? Is there something I am missing? Has anyone ever installed a line reactor on a soft starter?

I 've sent this same request to Danfoss. I'll share their response in this thread.