Soft starter unbalances

[MNEE]

Some soft starters have back-to-back SCRs in all three phases legs (i.e., 6 SCRs total). Others have back-to-back SCRs in only two phase legs (i.e., 4 SCRs total). Motor has delta-connected windings. Do the soft starters having SCRs in only two legs create unbalanced currents or voltages (i.e., negative sequence currents in the motor windings)?

 

[Marke]

Hello MNEE

Yes, where two phase controllers are used, there is a current vector imbalance resulting in negative sequence currents during start.
At least one such manufacturer has "balanced' the start currents by modifying the firing angles, but this balances the magnitudes only, the angles of the vector are further out of balance. I am not sure whether this improves or degrades the performance, but it makes them feel comfortable.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jraef]

Yes they do, and it is done becuase it is cheaper and smaller. The issue they (the mfgrs of this design) use to "justify" this practice is that it is only unbalanced during ramping, and the duration is typically so short that no real motor damage occurs as a result. This has some validity, but you will notice that the design practice is usually limited to smaller frame sizes only, indicating their acknowledgement that at some point the risks outweigh the financial benefits.

That design comes with other limitations and dangers however. You can't accomplish current limiting for any extended amount of time as a result of the unbalance issue, and some protective relays may pick this up causing nuisance trips. In addition, with a 6 SCR design if one SCR shorts it is not automatically a danger for the motor because there is no path for current to flow. You need to have at least 2 shorted SCRs in opposing phases before it becomes a problem. In the 4 SCR design, there is always 1/2 of a valid current path available, so if even 1 SCR shorts, you can damage the motor winding from unrestricted current flow in short order, requiring additional protective devices and/or isolation contactors, which then invalidates the original cost / space savings! Lastly, realize that one phase is essentially a direct feed-through bus from the line, with no control or isolation. If the motor lead or winding goes to ground it is an immediate short circuit, protected only by the SCPD. All they really do is shift the burden of equipment and personnel protection to the installation rather than the device.

They tend to play down these issues. At one time, one of the larger suppliers of this 4 SCR design had an FAQ website posting about this. One of the "questions" they used was about personnel safety, and the "answer" stated that it was acceptable because there was no regulation outlawing it! The arrogance must have cost them some customer comfort level however, and that website is no longer available.

"Venditori de oleum-vipera non vigere excordis populi"

 

[busbar]

 
It would seem that the semiconductor peak-inverse voltage ratings would have to be increased on the order of 73% for four-SCR compared to six-SCR configurations. Is that the case?