Inside delta SS connection 2

[MikiBg]

Hi all!

I hope someone can tell me more about "Inside delta"
connection of softstarter. I read an ABB softstarter
handbook, and according to it, u can insert a SS and
contactor in series with motor phase windings (if
accessible) and downsize their rated current by sqrt(3). Three line diagram attached. Principle seems to be clear, SS copes with phase instead of line current, but "only few types of SS can be used this way" ABB says. I think that's because of different mutual position of SS, motor windings and grid, which requires different SS working logic. The question is: Assuming there are "inside delta friendly" motor and SS, can I use it regardless to working performances/reliability? What else should I have in mind? Anyone done this? Is there something similar with VFDs?


Thank you.

 

[LionelHutz]

Inside delta requires a different SCR phase angle firing range when using the line voltages as a reference. We have supplied 1000's of soft-starters for inside delta chiller use without using a contactor and have supplied inside delta starters for motors rated up to 2900A.

There is an issue that a soft-starter connected inside delta will allow current flow with a single shorted SCR. This isn't a real deal breaker. If something occurs, such as a voltage transient, that can cause a shorted SCR then it's just as likely multiple SCR's will fail. We install an under-voltage trip in the breaker so that line power can not be applied unless the soft-starter logic is working. This ensures the breaker can be immediately tripped if there is a shorted SCR. The contactor that ABB recommends could also take care of this problem.

 

[MikiBg]

Lionel, are you trying to say that with one SCR shorted, and no additional protection, there will be a current flow for Inside delta, and no current flow for standard, In line connection? Are there any types of SS that can be used in both ways, by proper mode selection and settings?

 

[jraef]

The soft starters that can be used inside the delta can also be used in line. But the opposite is not always true. The problem, as LionelHutz said, has to do with the way the soft starter senses the line voltage sine wave and determines the firing angle. To do that with an inside the delta configuration, the soft starter must be designed for it up front. Not all of them are.

There is no equivalent configuration for VFDs. That would not make any sense anyway. The task that a VFD does compared to a soft starter is very different. With a soft starter, you WANT reduced torque, so the inside the delta configuration just allows that to happen with smaller components. With a VFD, you want CONSTANT torque, not reduced torque. S hooking it up in a way that reduces torque is counter to why you would use it in the first place.

My other addition as to "why not" comes from a lot of personal experience. If you are in a part of the world or an industry where Y-Delta motor starting is common and electricians are well versed in the proper connection, no problem. But if you are in the US and not in a select few industries (such as refrigeration or elevators) where Y-Delta s used a lot, the average "Sparky the "
'Lectrician" is not going to understand, and if he has to work on t, he will screw it up and cost you more money than the little bit you originally saved in doing it.


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[LionelHutz]

Yes, look at the connections and you'll see that each motor coil is connected between 2 lines with one end directly connected and the other end having a pair of anti-parallel SCR's isolating it. So, short one SCR and current flows. A standard connection has a pair of anti-parallel SCR's in each line so there has to be two shorted SCR's in two different phases to allow current flow.

Apparently the ABB soft-starter you were looking at can be inside delta connected. The Benshaw MX series starters can be - our bypassed starter line >100A comes with top and bottom connection pads which could be used, for example, line top and motor bottom inside delta bottom connections.

I don't bother memorizing the features of our competitor's starter models so I'm not able to list off other models that are also capable.

 

[Marke]

Hello Mikibg

In addition to the replies from Jraef and LionelHutz,
you need to be careful with inside delta, but properly used, provided that the ss is designed correctly, and the installation is correct, it will give trouble free servec for a long time.

1) You can only install soft starters designed for inside delta connection in this manner.
2) Some ss require very careful connections to ensure correct operation. The output must pass through a motor winding to the proceeding phase. Others are not critical.
3) The SCRs are subjected to much higher transient voltages and there is only one in series, so are more vulnerable to transients than inline connections.
4) Voltage is permanently on the motor windings. I always recommend an upstream contactor for off state disconnection giving improved safety and transient immunity.
5) some ss auto configure for inside delta, others do not.

Best regards,

Mark Empson
L M Photonics Ltd

 

[MikiBg]

Thank you all for being helpful.
Understood your explanations and tips. Just want to bring out my opinion why there's no inside delta VFD. I agree with jref's constatation about different role of SS and VFD, but I think that frequency changing is a major reason why VFDs can't be connected inside delta. In that case each motor winding would be connected to grid's freq. at one end, and to VFD's freq. at the other.

 

[LionelHutz]

3) The SCRs are subjected to much higher transient voltages and there is only one in series, so are more vulnerable to transients than inline connections.

I've never really believed this. Look at the capacitance to ground on the motor side. The SCRs in each phase of a standard connected soft-starter will just appear grounded on the motor side during a high speed transient. There can be voltage sharing during slow over-voltage events but not during a transient (or high dV/dt) type of event.

MikiBg - Yup, you've got it now. The VFD is creating a new 3-phase source and the motor must be connected only to this new source.