Soft start bypass 1

[glowz]

Our customer request to have bypass on soft start allowing for motor starting accross-the-line (full voltage) on soft start failure. I think to comply with this requirement, line and load contactors are required to fully isolate (faulty) soft start. These contactors will open when bypass (shorting) contactor is closed. It seems to me that all contactors will have to be fully rated (NEMA FVNR). Any thoughts on this.

 

[asekhar]

What I understand is that your customer wants a provision to do DOL start incase of softstarter failure. Is the motor rated for a DOL start & is the power system designed to support DOL start? That is the first issue which needs to be established.

Incase both the above requirements are okay, then, provision to isolate the faulty starter & then a separate contactor / breaker to operate the motor suitable for full duty needs to be selected. The softstarter needs to be opened before the bypass is closed, not after bypass closes as mentioned by you.



Anand Sekhar
Power Systems Consultant

 

[rohra]

I fully agree with Anand Shekhar,Capability /Stability of System should be first analysed for DOL start.
The deisgn could be such that in case of any fault in Soft starter ,the softstarter contactor should get opened & give command fot Bypass contactor to Close.
is that the thing Anand.
In VFD's there is also a Bypass given ,do the bypass closes in Auto when there is a problem with VFD or it is manually done.
We have a Softstarter Compressor Motor 5.4 MW , but there is no provision of Bypass

Thanks

 

[asekhar]

Rohra,
I do not think that the VFD would be having a Auto changeover to bypass, it must be manual changeover.

Anand Sekhar
Power Systems Consultant

 

[glowz]

Thank you all for response. We are sizing power service and standby generator to allow for DOL start of motors. For now, we sized generator allowing less than 15% voltage dip during starting of motors. This value is recommended as well in this Forum under FAQ. However, we were told by one of the generator manufacturers that 25% instantenous voltage dip would be acceptable. As well, I found that at least some of the starters (e.g. C-H) have app. 75% voltage pickup and 50% drop-out. This could imply that dip up to 25% wouldn't cost coil to chatter.

 

[jraef]

OK, now that you have determined that DOL is possible, you need to know that NO, it is NOT necessary to isolate the soft starter from the line or load when the bypass is energized. There is no problem with line or load power being in contact with the soft starter terminals when the bypass is energized, in fact most soft starters have bypass contactors included now anyway. the bypass acts as a simple power shunt around the SCRs, providing a path of least resistance for the motor power.

The only important thing to remember is that if the soft starter was providing solid state overload protection and it goes down, so does the overload protection. So your DOL bypass must be a complete starter (contactor + overload relay) not just a DOL sized contactor.

What most people do in this situation is that they have a selector switch for the contactor control circuit. In "Normal" operation, the contactor is controlled by the soft starter at-speed contact and the soft starter is providing overload protection, so the separate OL relay aux. contact is not in the contactor control circuit in order to avoid confusion as to which device has tripped. In "Manual Bypass" operation, the selector switch places the contactor control directly in the manual start-stop circuit and adds in the OL relay aux. contact to drop it out in case of an overload.

If by chance your soft starter never had OL protection or it is some low cost system that you want to override anyway, then get a good quality solid state OL relay and just place it down stream of the tie point of both controllers so that the motor power is always going through the same OL relay.

 

[asekhar]

jraef,

My understanding of glowz's requirement is that he needs a bypass to take the contigency of soft start failure.

To take care of the contigency of soft start failure, I think it is essential to isolate the softstarter.

The bypass which you have mentioned is basically the energy saving & softstarter component life enhancing bypass which is now a days a normal practice.

Anand Sekhar
Power Systems Consultant

 

[LionelHutz]

There are 2 cases where isolation is necessary.

#1 - the soft starter itself has a insulation failure to ground.

#2 - the soft starter has shorted SCR's on 2 or 3 phases causing a single phase condition or not allowing the motor to be stopped.

So, for these cases, there must be line and load isolation to allow a full-voltage starter to be used without doing any other work.

 

[jraef]

I'll buy on those 2 scenarios, but they are rare IMHO. They would fall under the same reasoning for using in-line isolation contactors for any soft starter, bypassed or not. Expensive, but worthwhile if the risk is greater in any given area, i.e. where lightning strikes are common. A shunt trip on the circuit breaker feeding the soft starter is cheaper.

My point is, many people assume it is absolutely necessary "so they can service the soft starter while the motor continues to run", but they don't think that through all the way. Under the new Arc Flash rules (here in North America anyway), are you going to have a technician standing in front of that open cabinet in the full type 3 or 4 PPE regalia trying to replace SCRs with arc resistant gloves on? Or are you going to shut down, remove the soft starter, and turn the motor back on with the bypass starter after you shut the door?

 

[LionelHutz]

jraef, what you're saying is perfectly valid. I just wanted to point out that there are 2 scenarios where the isolation contactors are required. It's up to the customer to decide which way they want to go.

Lots of places have operational staff and electrical staff. The maintenance staff is allowed to throw a switch on nights and weekends to get equipment to run again but they have to call an electrician in to actually open the cabinet and do any real troubleshooting. Full isolation can make sense in these cases because an electrician (or the manufacturers service tech) can be scheduled to shut it down and fix it sometime during normal work hours.