Is line contactor not required. what is his purpose? 1

[Askarel27]

In the attached file

, line and bypass contactor are provided. One operation engineer from the client side commented to remove the line contactor. Is it ok to remove the line contactor, if it is not remove thus it give additional protection. I want to know also what is the purpose of the line contactor.

Link

 

[Drivesrock]

Hi Askarel27.

If the softstarter is thyristor based and not transistors as I understand the symbol then the line contactor seems to me to be extra to requirements.
But, that might depend on the type of softstarter - are there three phases of thyristors or is one phase a link (this means a cheap and nasty one)?
What is the make and model of 1SSD1?

Perhaps the project has been copied from a variable speed drive circuit? I could see where in bypass we wouldn't want the output terminals with mains power - the drive could still be pre-charging through it's input, for example.

Or the line contactor and the bypass contactors could have external interlocks to actually open them and prevent the motor from starting/operating in softstarter or bypass mode?

There could be project, customer or other rules and specifications that require this?

What is the application?

 

[Askarel27]

The soft starter used in this project is Siemens 3RW40, the application is for pump for lift station.

 

[jraef]

Reasons for line contactors on soft starters:

SCRs used in soft starters are subject to shorting when subjected to severe voltage spikes on the incoming line, such as nearby lightning strikes or downed power lines, etc. When running the motor, the spikes are passed through and often have little immediate effect, but when the soft starter is connected, but not "on", the SCRs are subjected to all disturbances on the line, so some people prefer to have a line isolation contactor ahead of them to ensure that when not being used, they are safe from line disturbances.

In addition, the "snubbers" used across the SCR pairs on each phase will have a measurable "leakage current" and some safety protocols insist that there is ZERO measurable energy on a circuit not in use.

Some soft starters do not use SCRs in all 3 phases, they only control 2 phases (4 SCRs), the 3rd phase is a piece of bus bar that goes straight through. This reduces the cost, but represents an immediate risk to the motor. In a normal soft starter (with 6 SCRs) if one SCR shorts, there is no current flow path through the motor, because the other SCRs will block it. You need TWO devices to short in different phases in order for there to be a problem, which is the same risk as having two contacts weld in a contactor; very low. In the 4SCR design, 1/2 of the circuit is ALWAYS energized, so if just ONE of the remaining SCRs short, there is no way to disconnect power from the motor and it usually results in a burned up motor. The isolation contactor then is tied to a shorted SCR detection circuit and will open to interrupt the current flow. The 3RW40 units fit this category, they only have 4 SCRs, not 6. This of course results in actually costing MORE than a 6 SCR starter, but if you insist on the isolation contactor for other reasons, then it makes more sense.

Lastly, some units are DESIGNED as "soft start controllers", meaning their electronics are NOT designed to do the actual On-Off control of the motor, they are designed to simply begin ramping when line voltage is applied, as in a situation where you retrofit one behind an EXISTING starter. So if the unit is designed for that purpose, but you do NOT have an existing starter, you must add a contactor ahead of the soft start unit. The 3RW30 fits that description, it is NOT a "soft starter", it is a "soft start controller"; in requires that something else, a line starter or contactor, perform the On-Off control. The 3RW40 can be used either way.


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